PRE-COURSE SELF-STUDY MATERIAL Prepared by: Airways Engineering
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Foreword
The Communications, Navigation, Surveillance / Air Traffic Management (CNS/ATM) course is an introductory course that will benefit all Airways Engineering personnel. In order to accomplish this task trainees must go from the known to the unknown. The problem is knowing where we should start and how much material can be taught. Even if we knew the background of all the trainees on a particular course it would take too many resources to tailor a specific course to the specific knowledge level determined to exist. So this self-study material provides an overview of what the CNS/ATM system concept is, and an insight into on how computers work and communicate. It is important that you know the material well before attending the CNS/ATM course. The terms and concepts are used throughout the CNS/ATM course and you will have difficulty following the certain systems description without a basic understanding of the principles and definitions included in this pre-course study package. A pre-course threshold knowledge test will be administered to ensure an understanding of this self-study material. You must pass this test with an average of 50% to attend the course.
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SECTION - 1 1.
AIR TRAFFIC DEMAND
1.1 Air traffic demand is increasing in all parts of the world. Although rates of growth may differ between regions, significant increases in air traffic demand are expected to continue. The current demands have already increased the pressure on air traffic service providers and users, straining airspace and airport resources. Without change, the result will be further congestion and delays due to the capacity limitations of today’s system, which together with environmental considerations, could have significant economic consequences. 1.2 The average annual growth rate for the period 2000-2005 was expected to be 5.2%. For the period 2005 -2010 it was projected at 4.8%. These rates of growth have fallen off and it still remains to be confirmed if the rate of growth in Air Traffic will return to the levels that existed prior to the events of September 11th, 2001. 2.
SYSTEM PROBLEMS 2.1
2.2
System shortcomings in the early 1980s amounted to essentially three factors:
Propagation limitations of existing line of sight systems;
The difficulty, caused by a variety of reasons, to implement current Communications, Navigation and Surveillance systems and operate them in a consistent manner in large parts of the world; and,
The limitations of voice communications and the lack of digital air-ground data interchange systems to support automated systems un the air and on the ground.
These shortcomings were reflected in problems, such as:
Schedule delays due to insufficient Air Traffic Control (ATC) capacity to meet the traffic demand in particular during peak hours.
Differences in operating concepts and procedures and the lack of co- ordination between regions and Flight Information Regions (FIRs) causing increased workloads for both ATC and flight crew.
Air Traffic Flow Management (ATFM) which prevented matching available capacity with demand over the entire route, causing the need for flying holding patterns in the sectors with the highest capacity constraints.
The inflexibility of fixed route structure systems preventing the most efficient use of airspace and most economical conduct of flight operations.
The inability to expand to meet future traffic growth, in an evolutionary fashion.
The inability to fully ’exploit the capabilities of advanced airborne equipment such as flight management systems.
The increasing operating costs of the present Air Traffic Services (ATS) system 3
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associated with the need to increase capacity. Without advanced automation, increases in capacity can only be achieved by decreasing the size of existing control areas and increasing the number of controllers. 2.3 Unless there were improvements to the old system, international aviation would experience a continuing increase in airport and airspace congestion, which would become progressively worse as air traffic increased. This would result in higher operating costs and a stifling of the market for the aviation industry. 3
FUTURE AIR NAVIGATION SYSTEM (FANS) COMMITTEE - FANS Phase I
3.1 In the early 1980s, the International Civil Aviation Organization (ICAO) recognized the increasing limitations of the existing air navigation systems and the need for improvements to take civil aviation into the 21st century. In 1983, ICAO established the Special Committee on Future Air Navigation Systems (FANS) with the following terms of reference: 3.2 "To study technical, operational, institutional and economic questions, including cost/benefit effects, relating to future potential air navigation systems; to identify and assess new concepts and technology, including satellite technology, which may have future benefits for the development of international civil aviation including the likely implications they would have for users and providers of such systems; and to make recommendations thereon for an over-all long term projection for the co-ordinated evolutionary development of air navigation for international civil aviation over a period of the order of twenty five years……"
3.3 The committee presented to the ICAO Council and the international aviation community, a consolidated proposal for a future global air navigation system. 3.4 The Committee concluded that the application of satellite, communications and computer technology was the only solution that would enable international civil aviation to overcome the shortcomings of the present CNS system and fulfill the needs and requirements of the foreseeable future 4
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on a global basis. In arriving at this concept of FANS the Committee was guided by the Objectives that a new CNS system should provide for: Global communications, navigation and surveillance coverage from (very) low to (very) high altitudes, also embracing remote, off-shore and oceanic
Digital data interchange between the air-ground systems to fully exploit the automated capabilities of both.
Navigation/approach service for runways and other landing areas which need not be equipped with precision landing aids.
3.5 The FANS I Committee completed its work in May 1988 and recommended that the ICAO Council urgently establish a new committee to advise on the overall monitoring, co-ordination of development and transition planning to ensure that implementation of the future CNS system takes place on a global basis in a cost effective manner, and in a balanced way between air navigation systems and geographical areas. 4
FANS PHASE II
4.1 In July 1989, the ICAO council, acting on the recommendation of the FANS Phase I Committee, established the Special Committee for the Monitoring and Co-ordination of Development and Transition Planning for the Future Air Navigation System (FANS Phase II). 4.2 In October 1993, the FANS Phase II Committee completed its work. It recognized that implementation of related technologies would not arrive overnight, but would rather evolve over a period of time, depending on the existing infrastructure in the various States and Regions, and the over-all requirements and needs of the aviation community. 4.3
The Tenth Air Navigation Conference - September 1991
4.3.1 The Tenth Air Navigation Conference held in Montreal in September 1991, endorsed the FANS concept which then became known as the Communications, Navigation, Surveillance / Air Traffic Management system (CNS/ATM). CNS/ATM involves a complex and interrelated set of technologies depending largely on Satellites and including new communications and computer technologies. CNS/ATM is a vision developed by ICAO with the full co-operation of all sectors of the aviation community to accommodate future needs of international air transport. The results of the conference encapsulated a set of agreed recommendations covering the full spectrum of CNS/ATM activities that continue to offer guidance and direction to the international civil aviation community. 4.4
Global Planning
4.4.1 The FANS PHASE II Committee was tasked to develop a plan of action which was included in their report as an appendix (FANS Phase II - Doc 9623). In 1996 the ICAO Council directed the ICAO Secretariat to revise the Global Plan as a living document comprising technical, operational, economic, financial, legal and institutional elements. The intention was to offer guidance and advice to regional planning groups and States on implementation strategies, which include technical cooperation aspects. 4.4.2 The Secretariat on the first page of the revised Global Plan included the following information on CNS/ATM:
4.2
Definition
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4.2.1
Communications, navigation and surveillance systems, employing digital technologies, including satellite systems together with various levels of automation, applied in support of a global air traffic management system.
4.3
Strategic Vision
4.3.1 To foster implementation of a global air traffic management system that will enable aircraft operators to meet their planned times of departure and arrival and adhere to their preferred flight profiles with minimum constraints and without compromising agreed levels of safety.
4.4
Mission
4.4.1 To develop a seamless, globally co-ordinated system of air navigation services that will cope with world-wide growth in air traffic demand while:
Improving upon the present levels of safety; Improving upon the present level of regularity; Improving upon the over-all efficiency of airspace and airport operations, leading to increased capacity; Increasing the availability of user-preferred flight schedules and profiles; and minimizing differing equipment carriage requirements between regions.
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OVERALL DESCRIPTION OF THE NEW CNS/ATM SYSTEMS
5.1
Communications - Navigation - Surveillance
5.1.1
The main features of the global concept of the CNS systems could be summarized as follows:
5.1.2
EXISTING COMMUNICATIONS
- Existing
Figure 1 6
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5.1.3 The present communications environment is based on the use of Very High Frequency (VHF) and High Frequency (HF) voice transmissions with all the problems of language, slow rate of information transfer, high workload, possibility of errors etc. Due to its propagation characteristics the use of VHF is limited to line-of-sight communication and world-wide coverage is clearly not possible. Mobile HF communications were the only ones available for over-the horizon communications. Such communications have reliability limitations as a result of the variability of propagation characteristics. Though data-link can and will solve the majority of these problems it has been recognized that there is a need to compensate for the "situational awareness" that the pilot has in a voice based environment. Some of the problems with communications included:
• • 5.1.4
VHF spectrum saturation in many areas of the world. The lack of digital air-ground data interchange systems to support automated systems in the air and on the ground. Voice communication has slow rate of information transfer. Voice communication problems arise due to language skill or accent of controllers and pilots. Possibility of errors of transmission or comprehension. High workload of a controller. FUTURE COMMUNICATIONS
Future Communication Environment Satellite Data and voice
Data Data and voice VHF data and voice HF data and voice
Air Traffic Service
Satellite dish
DATA
VHF Radio
AERONAUTICAL TELECOMMUNICATION NETWORK
HF Radio
SSR Mode S
Neighboring Center
Figure 2 5.1.5 Data and voice communications are made available through direct satellite/aircraft/ground links. Initially, high frequency (HF) voice may have to be maintained in the transition period and over polar regions until such time as satellite communication is available. HF data is considered for data link 7
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coverage in the polar regions and remote continental areas as a backup or possible alternative to mobile satellite communication. 5.1.6 Very high frequency (VHF) will remain in use for voice and data communication in many continental and terminal areas. 5.1.7 The secondary surveillance radar (SSR) Mode S data link will be used for air traffic services (ATS) purposes in high density airspace. 5.1.8 The aeronautical telecommunication network (ATN) will provide the interchange of digital packet data between end-users over various air-ground and ground-ground communication sub-networks. 5.1.9 5.1.10
Data communication using ARINC 622 standard is available as an interim system BENEFITS
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Table 1
BENEFITS OF NEW COMMUNICATION SYSTEM N
Current limitations
Benefits
1
The propagation limitations of current lineof-sight VHF voice communication systems (curvature of the earth; attenuation along the propagation line; other geographical features).
A single aviation network connecting all users
2
No full coverage of VHF voice communication especially for oceanic airspace.
Global coverage including polar and oceanic regions
3
HF communication is not restricted to lineof-sight because the radio waves are reflected off the ionosphere. However HF voice communication is a subject to severe fading and interference.
Improved air - ground linkage, reduced errors more reliable communication due to data links
4
VHF spectrum saturation in many areas of the world.
Reduced channel congestion, safety enhanced
5
The lack of digital air-ground data interchange systems to support automated systems in the air and on the ground.
Interoperability guaranteed, safety enhanced
6
Voice communication has slow rate of information transfer.
More fast data transmission, more reliable, more accurate, safety enhanced
7
Voice communication has the problems concerning to language skill or accent of controller
Problems eliminated due to print messages transmission and standard forms of the messages, safety enhanced.
8
Possibility of errors of transmission or comprehension.
Errors are detected and corrected. More accurate data, safety enhanced.
9
High workload of a controller.
Reduced workload
5.2 NAVIGATION
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Current Air Navigation Environment Omega/Loran-C INS,IRS Barometric Altitude
En-route
ILS Landing VOR/DME, NDB
FIGURE 3 5.2.1
LIMITATIONS
5.2.1.1 Very High Frequency Omni-Directional Radio Range (VOR) Limited coverage; Decreasing accuracy at increasing distance from beacon; Extensive flight inspection measurements required to reassure and maintain required accuracy. 5.2.1.2 Distance Measuring Equipment (DME) Limited coverage; Decreasing accuracy at increasing distance from beacon; Limited number of users (at reaching maximum, coverage decreases); Sometimes coverage adjustment required to prevent interference; In order to meet Required Navigation Performance (RNP) 1 requirements with multi DME, the geometry of the location of DMEs is a constraint. 5.2.1.3
Non Directional Radio Beacon (NDB) Limited range; Limited accuracy.
5.2.1.4
Inertial Navigation System (INS) / Inertial Reference System (IRS) Navigation information derived from INS decreases in accuracy with time (in general less than 2 NM/hr).
5.2.2
FUTURE NAVIGATION
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Figure 4
5.2.3
Progressive introduction of area navigation (RNAV) capability in compliance with the required navigation performance (RNP) criteria.
5.2.4
Global Navigation Satellite System(s) (GNSS) will provide world-wide coverage and will be used for aircraft navigation and for non-precision-type approaches and, with appropriate augmentation, Category I approaches. With adequate augmentation systems Category II and Category III approaches may be available in the future.
5.2.5 The global strategy for the introduction and application of non-visual aids to approach and landing is as follows: i) continue Instrument Landing Systems (ILS) operations to the highest level of service as long as operationally acceptable and economically beneficial; ii) implement Microwave Landing System (MLS) where operationally required and economically beneficial; iii) promote the use of multi-mode receiver (MMR) or equivalent airborne capability to maintain aircraft interoperability; iv)GNSS, with such augmentations as required, to support approach and departure operations, including at least Category I operations, and implement GNSS for such operations as appropriate; v) Category II and III operations, based on GNSS technology, with such augmentations as required [e.g. Differential GNSS (DGNSS), Space Based Augmentation Systems (SBAS) such as the Wide 11
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Area Augmentation System (WAAS), Ground Based Augmentation System (GBAS) such as Local Area Augmentation Systems (LAAS)], where operationally acceptable and economically beneficial; and vi)enable each region to develop an implementation strategy for future systems in line with the global strategy. vii) Non-directional radio beacon (NDB) and VHF omnidirectional radio range/distance measuring equipment (VOR/DME) will be progressively withdrawn. 5.3
BENEFITS
5.3.1 "The global navigation satellite system will provide a high-integrity, highaccuracy, world-wide navigation service for the en-route, terminal, and non-precision approach phases of flight, and possibly for Category I precision approach and landing operations as well as making it possible to achieve capacity improvements at limited cost throughout the world. 5.3.2
Three- and four-dimensional navigation accuracy will be improved.
5.3.3 Aircraft will be able to navigate in all airspace in any part of the world using a single set of navigation avionics. 5.3.4 Provider States will realise cost savings as existing ground-based navigation aids are no longer needed One of the attractions of GNSS is that it may lead to the removal of some or all ground-based radio navigation aids, such as NDB, VOR, DME etc. If Local Area Augmentation System (LAAS) meets CAT III requirements then GNSS could eventually replace ILS and MLS as a landing aid. However, this is not going to happen overnight and GNSS must prove itself before anything will happen. Nevertheless, the removal of "classical" navigation aids was recognised by FANS as one of the drivers behind the move to satellite technology.. 5.3.5 The new system can be used in conjunction with other systems, such as inertial navigation systems, to support operations through all phases of flight. N 1
Limitations The propagation limitations of current line-of-sight ground-based VHF navigation systems.
Benefits Global coverage, world-wide navigation service is available. Single set of navigation avionics will allow to navigate in any part of the world.
2
No full coverage of VOR/DME beacons and others as well in many regions of the world.
Global coverage, world-wide navigation service is available. Single set of navigation avionics will allow to navigate in any part of the world.
3
Accuracy limitations, that does not allow to use flexible routes and area navigation (RNAV).
High-accuracy navigation for all phases of flight.
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Table 2 5.4
SURVEILLANCE
Figure 5
5.4.1 In areas of high traffic density, Secondary Surveillance Radar (SSR) Modes A and C currently provide the main method for surveillance and control of air movements backed up by primary radar and voice reports on VHF. As these are "line of sight" systems, for oceanic operations, remote land areas, and areas where primary and secondary radar cannot be justified economically, voice reports on HF are used for a procedural service which demands wide separation standards to ensure adequate safety.
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Figure 6 5.4.2 The key feature of the FANS surveillance concept is Automatic Dependent Surveillance (ADS), a means of extending surveillance service to oceanic airspace, remote land areas, and other areas where radar coverage is not available. Instead of having to rely on voice position reports, an aircraft operating in these non-radar areas will automatically transmit its position (and other relevant data, such as aircraft intent, speed and weather) to the air traffic centre via satellite or other communication links. The aircraft position can then be displayed in a manner similar to that of present radar displays. Used in conjunction with complementary two-way pilot-controller communications, ADS will serve as the basis for the provision of tactical air traffic services. 5.4.3 SSR will continue to be used for surveillance in terminal areas and high-density continental airspace. Enhancing SSR with Mode S will provide selective address and data link capabilities to extend further the benefits of SSR for surveillance purposes. The resulting system will be characterised by reduced interference and high accuracy 5.4.4
BENEFITS
5.4.4.1 Automatic dependent surveillance (ADS) services will be the basis for potentially significant enhancements to flight safety by reducing position report errors. 5.4.4.2
ADS will provide significant early benefits in oceanic and other non-radar areas.
5.4.4.3 employ of ADS, supported by direct pilot-controller communications, ill allow these non-radar areas to evolve to the point where air traffic services are provided in the same manner as in today’s radar airspace. 5.4.4.4 ADS will support reductions in separation minima in non-radar airspace. These reductions will alleviate delays, minimise necessary diversions from preferred flight paths, and reduce flight operating costs.
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5.4.4.5 ADS will support increased air traffic control (ATC) flexibility, enabling controllers to be more responsive to aircraft flight preferences. With or without reductions in separation minima, this flexibility will contribute to cost savings for flight operations. 5.4.4.6 Mode S in combination with ADS will facilitate uniform surveillance services world-wide. It will provide high-accuracy, interference-protected surveillance in high-density airspace. 5.4.4.7 Cost savings to provider States will be realised through the gradual elimination of various ground systems. 5.4.4.8 5.5
Mode S provides the ability to selectively address aircraft through the 24 bit Mode S code. Air Traffic Management (ATM)
5.5.1 The main beneficiary of the new CNS systems will be the ATM system. The new CNS systems will enable the direct transfer of digital information between the ground and the air during all phases of flight. The deployment of the new CNS infrastructure will additionally facilitate the exchange of information between the main ATM functions, i.e. Airspace Management (ASM ), Air Traffic Flow Management (ATFM ) and Air Traffic Control (ATC ), resulting in an integrated ATM service to users from gate-to-gate.
Figure 7 5.5.2 Increased Use of Automation The future ATM system will make increasing use of automation to reduce or eliminate constraints imposed on operations by current systems, and to derive the benefits made possible by implementation of the new CNS systems. The flexibility afforded by the new CNS systems will allow for the introduction of automation capabilities from the simplest to the most advanced as required by individual States, but in a globally consistent yet evolutionary manner. For this reason, it is expected that the use of ATM automation will be most visible in the areas of: flow management tactical control oceanic operations en-route/terminal operations and 15
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5.5.3
airport operations
Improved Flow Management
Flow Management will in future be based on sophisticated models and databases describing the current and projected levels of demand and resources. This new level of automation will make it possible to predict the possible sources of congestion and delay and formulate real-time flow management strategies to cope with demand. 5.5.4
Tactical Control
Automation will allow rapid negotiation between the service provider and aircraft to enhance tactical control. Improved tactical control will permit the accommodation of changes in a user’s preferred trajectory in three or four dimensions while satisfying any ATM constraints, resolving conflicts and scheduling the use of scarce resources, such as runways. 5.5.5
Oceanic Operations
International air traffic is growing much more rapidly than domestic operations. This area of ATM stands to benefit significantly from the new technologies and will experience significant improvements through the next decade. Extensive use will be made of ADS, Satellite Communications, GNSS, weather system improvements etc. to integrate ground-based ATM and airborne Flight Management Systems. The goal is to develop flexible oceanic operations which accommodate the users’ preferred trajectories to the maximum extent. 5.5.6
En-route and Terminal Operations
The automated flow management will monitor available capacity and demand at airports throughout the en-route and terminal airspace, and will implement strategies to prevent the development of congestion. The ATM functions will be integrated to provide smooth traffic flow into and out of terminal areas. 5.5.7
Airport Operations:
Traffic flows at airports will be structured to ensure maximum utilisation of approach and departure capacities. Curved approaches will eliminate some of the current constraints on approach capacity, and the use of new aids will permit independent Instrument Flight Rules on parallel runways spaced as closely as 2,500ft thus reducing the estate required for enlarging airports. Improved surface guidance systems at congested airports will increase capacity still further. 5.5.8 The new ATM capabilities and more accurate data will make it possible to enhance safety, reduce delays, and increase airspace and airport capacity. 5.5.9 Oceanic ATM operations will become much more flexible, resulting in a greater capability to accommodate user-preferred trajectories. 5.5.10
Improved flow management will prevent excessive levels of congestion.
5.5.11 Data link will transmit a variety of information from appropriately equipped aircraft to the ground, and provide enhanced information to the cockpit. It will dramatically reduce the communicator’s workload, and reduce the channel congestion and communications errors that characterise the current voice environment 16
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5.5.12 New capabilities will make it possible to permit flexible routing, as well as dynamic modifications to aircraft routes in response to changes in weather and traffic conditions. 5.5.13 Terminal and en-route ATM functions will be integrated to provide smooth traffic flows into and out of terminal areas. 5.5.14 Air traffic controllers will be able to establish efficient approach streams for parallel and converging runway configurations. 5.5.15 Single-runway capacities in instrument meteorological conditions (IMC) will increase to a level approaching current single-runway capacities in visual meteorological conditions (VMC). 5.5.16 Independent instrument flight rules (IFR) operations on triple and quadruple parallel runways will become routine in high density environments. 5.5.17
Conflicts among departure and approach operations involving adjacent airports will be reduced.
5.5.18
Flexibility in controlling the noise footprint of airport traffic operations will be increased.
5.5.19 The over-all benefits of the new ATM system will derive from the combined benefits of the new communications, navigation, and surveillance systems, together with increasing use of automation. 5.5.20
CONCLUSION
5.5.20.1 This section of the CNS/ATM self-study material has given the trainee a broad overview of what CNS/ATM is and defined some of the terms they will encounter on the course. To provide a realistic view of the systems now under consideration or being actively implemented a look at one of the CNS/ATM Charts from the Middle East Regional Plan is appropriate. 17
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5.5.20.2 The future CNS systems from the Middle East ICAO CNS/ATM Plan are presented in a condensed form in the CNS evolution table below. Included is the function they perform and the conventional systems they will eventually replace:
Oceanic continental en-route airspace with low density traffic
FUNCTION
Conventional System Elements
Communications
VHF voice HF Voice
Navigation Surveillance
Communications Continental Airspace with highdensity traffic Navigation Surveillance Communications
SSR Mode A/C or S ADS / ADS-B AMSS data/voice (Note 4) HF data/voice (Note 4) ATN router/end system 1. GNSS 2. Barometric altimetry 3. GNSS Altitude (Note 2) 4. INS/IRS ADS VHF voice/data (notes 3&4) SSR Mode S data link ATN router/end system GNSS ILS/MLS/DGNSS (Note 5) Barometric Altimetry INS/IRS SSR Mode A/C or S ADS / ADS-B
End of Section - SECTION I - Review Exercise Answers found at page 24 Q1
What does the abbreviation CNS/ATM stand for? a) A term agreed on at the ICAO 10th Air Navigation Conference? b) Central Navigation System / Air Traffic Movements c) Communications, Navigation, Surveillance / Air Traffic Management d) Communications, Navigation, Surveillance / Air Traffic Movements
Q2
One of the System shortcomings identified in the early 1980s was? a) The propagation limitations of existing line of sight systems. b) The ease with which new Communications, Navigation and Surveillance systems could be implemented in large parts of the world. c) The capability of voice communications and the availability of digital air-ground data interchange systems to support automated systems in the air and on the ground. d) There were none.
Q3
The shortcomings identified in the early 1980s were reflected in problems, such as? a) Air Traffic Control (ATC) needed more air traffic demand. b) Air Traffic Flow Management (ATFM) which had the available capacity over the entire route, causing no need to fly holding patterns in the sectors with the highest constraints. c) The flexibility of fixed route structure systems which allow the most efficient use of airspace and most economical conduct of flight operations. d) The inability to fully exploit the capabilities of advanced airborne equipment such as flight management systems.
Q4
What does FANS mean? a) It includes all Committees of ICAO dealing with FANS b) It relates to the Future Aeroportable Nexus System (FANS) c) Future Air Navigation System d) Nothing it is just a term coined by ICAO Staff
Q5
In arriving at the concept of FANS the ICAO Committee was guided by the Objectives that a new CNS system should provide for? a) VHF communications, navigation and surveillance b) Digital data interchange between the air-ground systems to fully exploit the automated capabilities of both. c) aircraft to maintain their own separation d) satellite tracking and ephemeris
Q6
Due to its propagation characteristics the use of Very High Frequency (VHF) radios is limited to? a) world-wide coverage b) line-of-sight communication and world-wide coverage is clearly not possible c) line-of-sight communication and world-wide coverage is possible d) line-of-sight communication and oceanic areas
Q7
Mobile HF communications have reliability limitations as a result of the variability of propagation characteristics and: ……..? a) over the horizon communications are not available b) were the only ones available for over-the horizon communications c) the system has the ability to communicate far out into space d) a High Fence (HF) must be broken down and rebuilt to make it more reliable 20
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Q8
Some of the problems with communications identified by the ICAO FANS Committee were: a) VHF spectrum saturation in many areas of the world. b) no automated systems in the air and on the ground. c) Voice communication has rapid rate of information transfer. d) No possibility of errors of transmission or comprehension. e) Low workload of a controller.
Q9Data and voice communications are made available through direct satellite/aircraft/ground links. HF data is considered for data link coverage in the polar regions and remote continental areas as a backup or possible alternative to mobile satellite communication. Name another planned data link service? a) Very high frequency (VHF) data communication will be used in many Oceanic and Remote Areas b) (SSR) Mode S data link may be used for air traffic flow management purposes c) The aeronautical telecommunication network (ATN) will only be the airborne data service d) Data communication using ARINC 622 standard is available as an interim system Q10
What are some of the limitations of VORs? a) Broken coverage above Flight Level (FL) 400 b) Decreasing accuracy at increasing distance from beacon c) Few flight inspection measurements required d) Increasing accuracy at increasing distance from beacon
Q11
Which statement is NOT true about Distance Measuring Equipment (DME) a) Limited coverage b) Decreasing accuracy at increasing distance from beacon c) Unlimited number of users (at reaching maximum altitude, coverage decreases) d) Sometimes coverage adjustment required to prevent interference e) In order to meet Required Navigation Performance (RNP) 1 requirements with multi DME, the geometry of the location of DMEs is a constraint.
Q12
The accuracy of INS/IRS degrades at how many nautical miles per hour? A) 1 NM B) 3.5NM C) 2 NM D) .5NM
Q13
What do the letters RNP stand for? a) Rollex Navigation Precision b) Rollover Nightime Point c) Required Navigation Precision d) Required Navigation Performance
Q14
The FANS Committee strategy for Navigation included the following sentence? a) Global Navigation Satellite System (GNSS), without augmentations b) Only Category II and III operations c) Enable each region to develop an implementation strategy for future systems in line with the global strategy d) INS/IRS will be progressively withdrawn e) All of the above
Q15
One of the advantages of GNSS is that it may lead to: 21
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a) b) c) d) e)
the removal of some or all ground-based radio navigation aids, such as NDB, VOR, DME etc. the installation of ILS and MLS as a landing aid less accurate navigation resulting in less accidents the installation of more classical navigation aids such as NDB,VOR,DME etc. All of the above
Q16
In areas of high traffic density the main method for surveillance and control of air movements is? a) Primary & Secondary Surveillance Radar (SSR) b) primary radar alone c) voice reports on VHF d) HF Voice Reports e) All of the above
Q17
The key feature of the FANS surveillance concept is Automatic Dependent Surveillance (ADS), a means of extending surveillance service to oceanic airspace, remote land areas, and other areas where radar coverage is not available. Instead of having to rely on voice position reports, an aircraft operating in these non-radar areas will: a) automatically transmit its position (and other relevant data, such as aircraft intent, speed and weather) to the air traffic centre via satellite or other communication links b) Not be displayed in a manner similar to that of present radar displays c) Not use controller-pilot-data-link-communications (CPDLC) d) never use ADS as the basis for the provision of tactical air traffic services. e) All of the above
Q18
SSR with Mode S will provide? a) selective address and data link capabilities to extend further the benefits of SSR for surveillance purposes. ADS will support increased air traffic control (ATC) flexibility, enabling controllers to be more responsive to aircraft flight preferences. This flexibility will contribute to cost savings for flight operations. The resulting system will be characterised by reduced interference and high accuracy b) Automatic dependent surveillance (ADS) services will be the basis for potentially significant enhancements to flight safety by reducing position report errors. ADS will provide significant early benefits in oceanic and other non-radar areas. Implementation of ADS, supported by direct pilot-controller communications, will allow these non-radar areas to evolve to the point where air traffic services are provided in the same manner as in today’s radar airspace. c) ADS will support reductions in separation minima in non-radar airspace. These reductions will alleviate delays, minimise necessary diversions from preferred flight paths, and reduce flight operating costs. Mode S in combination with ADS will facilitate uniform surveillance services world-wide. d) Cost to provider States will be realised through the gradual elimination of various ground systems. Mode S provides the ability to selectively address aircraft through the 24 bit Mode S code. It will provide high-accuracy, interference-protected surveillance in high-density airspace. e) All of the above
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Q19
The main beneficiary of the new CNS systems will be the ATM system. The new CNS systems will enable the direct transfer of digital information between the ground and the air during all phases of flight. The deployment of the new CNS infrastructure will additionally facilitate the exchange of information between the main ATM functions, which are: a) Airspace Management (ASM ) and Air Traffic Services (ATS) b) Surveillance, Automation, Airways and Approach c) Airspace Management (ASM ), Air Traffic Flow Management (ATFM ), and Air Traffic Control (ATC ) d) None of the above
Q20
The future ATM system will make increasing use of automation to reduce or eliminate constraints imposed on operations by current systems, and to derive the benefits made possible by implementation of the new CNS systems. The flexibility afforded by the new CNS systems will allow for the introduction of automation capabilities from the simplest to the most advanced as required by individual States, but in a globally consistent yet evolutionary manner. For this reason, it is expected that the use of ATM automation will be most visible in the areas of: a) Airspace management, Air Traffic Flow Management, Maintenance, Statistical Analysis b) Personnel Management, Pay Services, Tower Operations, ACC Operations c) Flow Management, Tactical Control, Oceanic Operations, En-route/Terminal Operations and Airport Operations d) All of the above.
Q21
Flow Management will in future be based on sophisticated models and databases describing the current and projected levels of demand and resources. This new level of automation will make it possible to predict: a) The number of aircraft that will cross a point in space which divided by two results in the mean average air traffic flow b) The number of near miss incidents that will occur over the total possible flow of air traffic c) The possible sources of congestion and delay and formulate real-time flow management strategies to cope with demand. d) How many computers the civil aviation authority will need to make automation possible. e) All of the above
Q22
Automation will allow rapid negotiation between the service provider and aircraft to enhance tactical control. Improved tactical control will permit the accommodation of changes in a user’s preferred trajectory in three or four dimensions while satisfying: a) potential safety violations of aircraft and aircraft algorithms b) required separation minima and preferred profiles of ATC c) resolving conflicts and giving the aircrew a happy flying feeling d) any ATM constraints, resolving conflicts and scheduling the use of scarce resources, such as runways. e) All of the above
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Q23
International air traffic is growing much more rapidly than domestic operations. This area of ATM stands to benefit significantly from the new technologies and will experience significant improvements. Extensive use is made of ADS, Satellite Communications, GNSS, weather system improvements etc. to integrate ground-based ATM and airborne Flight Management Systems. The goal is to: a) transition to satellite and OMEGA systems as quickly as possible b) develop flexible oceanic operations which accommodate the users’ preferred trajectories to the maximum extent c) terminate the use of INS/IRS operations d) use data communications exclusively e) All of the above
Q24
The automated flow management will monitor available capacity and demand at airports throughout the en-route and terminal airspace, and will implement strategies to prevent the development of congestion. The ATM functions will be integrated a) to assist the air traffic controller approach supervisor b) to allow smoother landings c) to provide smooth traffic flow into and out of terminal areas d) to enhance straight in approaches and landings e) all of the above Please indicate whether the following statements are true or false? The new ATM capabilities and more accurate data will make it possible to enhance safety, reduce delays, and increase airspace and airport capacity. Oceanic ATM operations will become much more flexible, resulting in a greater capability to accommodate user-preferred trajectories. Improved flow management will prevent excessive levels of congestion.
Q25
True
False
Data link will transmit a variety of information from appropriately equipped aircraft to the ground, and provide enhanced information to the cockpit. It will dramatically reduce the communicator’s workload, and reduce the channel congestion and communications errors that characterise the current voice environment. New capabilities will make it possible to permit flexible routing, as well as dynamic modifications to aircraft routes in response to changes in weather and traffic conditions. True
False
Terminal and en-route ATM functions will be integrated to provide smooth traffic flows into and out of terminal areas. Air traffic controllers will be able to establish efficient approach streams for parallel and converging runway configurations. Single-runway capacities in instrument meteorological conditions (IMC) will increase to a level approaching current single-runway capacities in visual meteorological conditions (VMC). True
False
Independent instrument flight rules (IFR) operations on triple and quadruple parallel runways will become routine in high density environments. Conflicts among departure and approach operations involving adjacent airports will be reduced. Flexibility in controlling the noise footprint of airport traffic operations will be increased. The over-all benefits of the new ATM system will derive from the combined benefits of the new communications, navigation, and surveillance systems, together with increasing use of automation. True
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ANSWERS TO REVIEW QUESTIONS - SECTION I A1 A2 A3
c) Communications, Navigation, Surveillance / Air Traffic Management a) The propagation limitations of existing line of sight systems. d) The inability to fully exploit the capabilities of advanced airborne equipment such as flight management systems. A4 c) Future Air Navigation Systems (FANS) A5 b) Digital data interchange between the air-ground systems to fully exploit the automated capabilities of both. A6 b) line-of-sight communication and world-wide coverage is clearly not possible A7 b) were the only ones available for over-the horizon communications A8 a) VHF spectrum saturation in many areas of the world. A9 d) Data communication using ARINC 622 standard is available as an interim system A10 b) Decreasing accuracy at increasing distance from beacon A11 c) Unlimited number of users (at reaching maximum altitude, coverage decreases) A-12 c) 2 NM A13 d) Required Navigation Performance A14 c) Enable each region to develop an implementation strategy for future systems in line with the global strategy A15 a) the removal of some or all ground-based radio navigation aids, such as NDB, VOR, DME etc. A16 a) Primary & Secondary Surveillance Radar (SSR) A17 a) automatically transmit its position (and other relevant data, such as aircraft intent, speed and weather) to the air traffic centre via satellite or other communication links A18 e) All of the above A19 c) Airspace Management (ASM ), Air Traffic Flow Management (ATFM ), and Air Traffic Control (ATC ) A20 c) Flow Management, Tactical Control, Oceanic Operations, En-route/Terminal Operations and Airport Operations A21 c) The possible sources of congestion and delay and formulate real-time flow management strategies to cope with demand. A22 d) any ATM constraints, resolving conflicts and scheduling the use of scarce resources, such as runways. A23 b) develop flexible oceanic operations which accommodate the users’ preferred trajectories to the maximum extent. A24 c) to provide smooth traffic flow into and out of terminal areas A25 True, True, True, True,
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SECTION - II 6.0 6.1
“BITS & BYTES AND COMPUTER COMMUNICATIONS DATA LINK
6.1.1 In oceanic areas and remote land airspace with limited ground-based air navigation facilities, surveillance of air traffic is envisioned to be provided by ADS position reporting through satellite communications. Surveillance of low-altitude traffic operations, including helicopters, will be conducted in a similar manner. In continental airspace, surveillance of air traffic may be achieved by ADS/ADS-B reports integrated with ground-based radar systems. Controller Pilot Data Link Communication (CPDLC) and the interchange of ATS messages will be carried out by satellite, Secondary Surveillance Radar (SSR) Mode S, Very High Frequency (VHF), High Frequency (HF) or other suitable data links available. There will also be DATIS, DFIS, DPDC ………. 6.1.2 Whoa! Far too much for some of us and not enough for others. Well let us back up a bit and bring all of us along to the same starting point shall we. First if we know nothing about data communications we need to know some of the very basic stuff like how bits and bytes work and how computers can talk with each other. We need to know enough of this to be able to conceptualize more advanced information. We need to bring all of you from the known (what you already know from school or other learning experiences) to the unknown (what we are trying to make you aware of.) So let us start at the beginning by going to the very basics. If you find that this is too elementary for you then you can skip the reading and go to the practice test at the end of the self study package to see just how much you do know. 6.2
How Bits and Bytes Work
6.2.1 If you have used a computer for more than five minutes, then you have heard the words bits and bytes. Both RAM and hard disk capacities are measured in bytes, as are file sizes when you examine them in a file viewer. 6.2.2 You might hear an advertisement that says, "This computer has a 32-bit Pentium processor with 64 megabytes of RAM and 2.1 gigabytes of hard disk space." And many articles talk about bytes. In this material we will discuss bits and bytes so that later on you can understand some of the issues in CNS/ATM. 6.3
Decimal Numbers
6.3.1 The easiest way to understand bits is to compare them to something you know: digits. A digit is a single place that can hold numerical values between 0 and 9. Digits are normally combined together in groups to create larger numbers. For example, 6,357 has four digits. It is understood that in the number 6,357, the 7 is filling the "1s place," while the 5 is filling the 10s place, the 3 is filling the 100s place and the 6 is filling the 1,000s place. So you could express things this way if you wanted to be explicit: (6 * 1000) + (3 * 100) + (5 * 10) + (7 * 1) = 6000 + 300 + 50 + 7 = 6357 6.3.2 Another way to express it would be to use powers of 10. Assuming that we are going to represent the concept of "raised to the power of" with the "^" symbol (so "10 squared" is written as "10^2"), another way to express it is like this: (6 * 10^3) + (3 * 10^2) + (5 * 10^1) + (7 * 10^0) = 6000 + 300 + 50 + 7 = 6357
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6.3.3 What you can see from this expression is that each digit is a placeholder for the next higher power of 10, starting in the first digit with 10 raised to the power of zero. 6.3.4 That should all feel pretty comfortable -- we work with decimal digits every day. The neat thing about number systems is that there is nothing that forces you to have 10 different values in a digit. Our base-10 number system likely grew up because we have 10 fingers, but if we happened to evolve to have eight fingers instead, we would probably have a base-8 number system. You can have base-anything number systems. In fact, there are lots of good reasons to use different bases in different situations. 6.4
Bits
6.4.1 Computers happen to operate using the base-2 number system, also known as the binary number system (just like the base-10 number system is known as the decimal number system). The reason computers use the base-2 system is because it makes it a lot easier to implement them with current electronic technology. You could wire up and build computers that operate in base-10, but they would be fiendishly expensive right now. On the other hand, base-2 computers are relatively cheap. 6.4.2 So computers use binary numbers, and therefore use binary digits in place of decimal digits. The word bit is a shortening of the words "Binary digit." Whereas decimal digits have 10 possible values ranging from 0 to 9, bits have only two possible values: 0 and 1. Therefore, a binary number is composed of only 0s and 1s, like this: 1011. How do you figure out what the value of the binary number 1011 is? You do it in the same way we did it above for 6357, but you use a base of 2 instead of a base of 10. So: 1 (1 * 2^3) + (1*2*2*2) + 8 +
0 (0 * 2^2) + (0*2*2) + 0 +
1 (1 * 2^1) + (1*2*1) + 2 +
1 (1 * 2^0) = (1) = 1 = 11
6.4.3 You can see that in binary numbers, each bit holds the value of increasing powers of 2. That makes counting in binary pretty easy. Starting at zero and going through 20, counting in decimal and binary looks like this: 0= 0 1= 1 2 = 10 3 = 11 4 = 100 5 = 101 6 = 110 7 = 111 8 = 1000 9 = 1001 10 = 1010 11 = 1011 12 = 1100 13 = 1101 14 = 1110 15 = 1111 16 = 10000 17 = 10001 18 = 10010 19 = 10011 20 = 10100 27
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6.4.4 When you look at this sequence, 0 and 1 are the same for decimal and binary number systems. At the number 2, you see carrying first take place in the binary system. If a bit is 1, and you add 1 to it, the bit becomes 0 and the next bit becomes 1. In the transition from 15 to 16 this effect roles over through 4 bits, turning 1111 into 10000. 6.5
Bytes
6.5.1 Bits are rarely seen alone in computers. They are almost always bundled together into 8-bit collections, and these collections are called bytes. Why are there 8 bits in a byte? A similar question is, "Why are there 12 eggs in a dozen?" The 8-bit byte is something that people settled on through trial and error over the past 50 years. 6.5.2
With 8 bits in a byte, you can represent 256 values ranging from 0 to 255, as shown here: 0 = 00000000 1 = 00000001 2 = 00000010 ... 254 = 11111110 255 = 11111111
6.5.3 In the article How CDs Work, on the WEB you can learn that a CD uses 2 bytes, or 16 bits, per sample. That gives each sample a range from 0 to 65,535, like this: 0 = 0000000000000000 1 = 0000000000000001 2 = 0000000000000010 ... 65534 = 1111111111111110 65535 = 1111111111111111 6.5.4 Bytes are frequently used to hold individual characters in a text document. In the American Standard Code for Information Interchange (ASCII) character set, each binary value between 0 and 127 is given a specific character. Most computers extend the ASCII character set to use the full range of 256 characters available in a byte. The upper 128 characters handle special things like accented characters from common foreign languages. 6.5.5 You can see the 127 standard ASCII codes below. Computers store text documents, both on disk and in memory, using these codes. For example, if you use Notepad in Windows 95/98 to create a text file containing the words, "Four score and seven years ago," Notepad would use 1 byte of memory per character (including 1 byte for each space character between the words -- ASCII character 32). When Notepad stores the sentence in a file on disk, the file will also contain 1 byte per character and per space. 6.5.6 Try this experiment: Open up a new file in Notepad and insert the sentence, "Four score and seven years ago" in it. Save the file to disk under the name getty.txt. Then use the explorer and look at the size of the file. You will find that the file has a size of 30 bytes on disk: 1 byte for each character. If you add another word to the end of the sentence and re-save it, the file size will jump to the appropriate number of bytes.
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6.5.7
Each character consumes a byte.
If you were to look at the file as a computer looks at it, you would find that each byte contains not a letter but a number -- the number is the ASCII code corresponding to the character (see below). So on disk, the numbers for the file look like this: F o u r a n d s e v e n 70 111 117 114 32 97 110 100 32 115 101 118 101 110 6.5.8 By looking in the ASCII table, you can see a one-to-one correspondence between each character and the ASCII code used. Note the use of 32 for a space -- 32 is the ASCII code for a space. We could expand these decimal numbers out to binary numbers (so 32 = 00100000) if we wanted to be technically correct -- that is how the computer really deals with things. 6.6
Standard American Standard Code for Information Interchange (ASCII) Character Set
6.6.1 The first 32 values (0 through 31) are codes for things like carriage return and line feed. The space character is the 33rd value, followed by punctuation, digits, uppercase characters and lowercase characters. 0 NUL 1 SOH 2 STX 3 ETX 4 EOT 5 ENQ 6 ACK 7 BEL 8 BS 9 TAB 10 LF 11 VT 12 FF 13 CR 14 SO 15 SI 16 DLE 17 DC1 18 DC2 19 DC3 20 DC4 21 NAK 22 SYN 23 ETB 24 CAN 25 EM 26 SUB 27 ESC 28 FS 29 GS 30 RS 31 US 32 29
! " # $ % & ' ( ) * + , . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ? @ A B C D E F G H I J K L M N O P Q R S T U 30
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86 V 87 W 88 X 89 Y 90 Z 91 [ 92 \ 93 ] 94 ^ 95 _ 96 ` 97 a 98 b 99 c 100 d 101 e 102 f 103 g 104 h 105 i 106 j 107 k 108 l 109 m 110 n 111 o 112 p 113 q 114 r 115 s 116 t 117 u 118 v 119 w 120 x 121 y 122 z 123 { 124 | 125 } 126 ~ 127 DEL 6.7
Lots of Bytes
6.7.1 When you start talking about lots of bytes, you get into prefixes like kilo, mega and giga, as in kilobyte, megabyte and gigabyte (also shortened to K, M and G, as in Kbytes, Mbytes and Gbytes or KB, MB and GB). The following table shows the multipliers:
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Name
Abbr.
Size
Kilo
K
2^10 = 1,024
Mega
M
2^20 = 1,048,576
Giga
G
2^30 = 1,073,741,824
Tera
T
2^40 = 1,099,511,627,776
Peta
P
2^50 = 1,125,899,906,842,624
Exa
E
2^60 = 1,152,921,504,606,846,976
Zetta
Z
2^70 = 1,180,591,620,717,411,303,424
Yotta
Y
2^80 1,208,925,819,614,629,174,706,176
=
6.7.2 You can see in this chart that kilo is about a thousand, mega is about a million, giga is about a billion, and so on. So when someone says, "This computer has a 2 gig hard drive," what he or she means is that the hard drive stores 2 gigabytes, or approximately 2 billion bytes, or exactly 2,147,483,648 bytes. How could you possibly need 2 gigabytes of space? When you consider that one CD holds 650 megabytes, you can see that just three CDs worth of data will fill the whole thing! Terabyte databases are fairly common these days, and there are probably a few petabyte databases floating around the Pentagon by now. 6.8
Binary Math
6.8.1 Binary math works just like decimal math, except that the value of each bit can be only 0 or 1. To get a feel for binary math, let's start with decimal addition and see how it works. Assume that we want to add 452 and 751: 452 + 751 --1203 6.8.2
6.8.3
To add these two numbers together, you start at the right: 2 + 1 = 3. No problem. Next, 5 + 5 = 10, so you save the zero and carry the 1 over to the next place. Next, 4 + 7 + 1 (because of the carry) = 12, so you save the 2 and carry the 1. Finally, 0 + 0 + 1 = 1. So the answer is 1203. Binary addition works exactly the same way: 010 + 111 --1001
6.8.4 Starting at the right, 0 + 1 = 1 for the first digit. No carrying there. You've got 1 + 1 = 10 for the second digit, so save the 0 and carry the 1. For the third digit, 0 + 1 + 1 = 10, so save the zero and carry the 1. For the last digit, 0 + 0 + 1 = 1. So the answer is 1001. If you translate everything over to decimal you can see it is correct: 2 + 7 = 9. 6.9
Quick Recap
6.9.1
Bits are binary digits. A bit can hold the value 0 or 1. Bytes are made up of 8 bits each. 32
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6.9.2
Binary math works just like decimal math, but each bit can have a value of only 0 or 1. There really is nothing more to it -- bits and bytes are that simple!
6.9.3 A bit is short for "binary digit." It is the smallest possible unit of information; i.e. a bit is to information what an atom is to an element! A bit could be represented by an on-off switch. This is essentially the case for static memory chips, or RAM (random-access memory), where each of the thousands of bits is a transistor, which may be in the on or off state. Dynamic RAM uses charged or uncharged capacitors to store data. [As an aside, static RAM is faster than dynamic RAM, but is also more expensive and power-consuming. Because capacitors discharge over time, dynamic memory must be constantly refreshed.] Magnetic media (floppies, hard drives, tape, even credit cards and ATM cards, use a positively or negatively charged magnetic region for each bit. 6.10
Data lines
6.10.1 Transferring individual bits between CPU and memory would be very time consuming, so it is customary to send a larger chunk of information. For some reason or another, memory is usually divided into eight-bit chunks called bytes. Four-bit chunks are called nibbles - no kidding! Most computers, then, will have at least 8 lines running between CPU and memory for data in and 8 lines for data out. Thus, a whole byte is transferred at once (in parallel). This also means that we only need a unique address for each byte in our memory, rather than having to have a unique address for each bit. Early IBM-type PCs had an 8-line (or 8-bit) data bus running between memory and the 8086 CPU. Later, the 80286 CPU improved on this by have a 16-bit data bus. Computers containing 80386, or 80486 have a 32-bit data bus. Pentium chips have a 64-bit external data bus. 6.10.2 We also have names for larger chunks of information. Since computers work in binary, memory is usually divided up into divisions that are a power of 2. Hence, kilobyte = 1024 bytes (1024 = 210), abbreviated KB (Kb is kilobits) megabyte = 1024 KB = 1,048,576 bytes, abbreviated MB (Mb is megabits) gigabyte = 1024 MB = 1,073,741,824 bytes, abbreviated GB (Gb is gigabits) 6.11
Address lines
6.11.1 Address lines run between the CPU and memory and are used to tell the memory which particular byte we want to read or write. Early microcomputers used 16 address lines. Each line could be [on/off; high/low; true/false; 0/1; etc.], allowing for 216 unique addresses, or 65,536 bytes of addressable memory. 6.11.2 The original IBM PC had 20 address lines. Since 2 20 = 1,048,576, IBM PCs, and the Microsoft DOS which was written for it, could only utilize 1 MB of memory. About 360 KB was reserved for ROM BIOS, video RAM, etc. (more on this later), and the other 640 KB was available for the DOS operating system and user programs. PCs have essentially been stuck with that limitation ever since, although Windows (and other, newer, operating systems like OS/2, LINUX, Windows 95) gets around it somewhat. The CPUs themselves have advanced beyond that stage (the 80286 has 24 address lines, allowing for 16 MB to be addressed; the 80386/486 has 32 address lines to address 4 gigabytes), but in order to maintain "backward compatibility," DOS has remained a 1 MB (640 KB) world. Windows, and to a much greater extent, Windows 95, have crashed the 1 MB barrier by using protected mode, 32-bit addressing 6.12
So what do we put into memory?
6.12.1
Instructions and data!
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6.12.1.1 Instructions 6.12.1.1.1 Instructions are stored as binary numbers, just as data is. For example, 10110010 (decimal 178) might be interpreted by the CPU as "get the byte of data located at the address given by the next two bytes and add it to the number in the accumulator (a part of the CPU that contains numbers to be acted on). 6.12.1.2
Data: ASCII, integers, floating point real and complex, etc.
6.12.1.2.1 Data are usually either numbers or letters. Of course, memory can only hold binary numbers, so we have to agree how to interpret those numbers (a kind of code) when we want them to represent letters (or numbers, for that matter). ASCII stands for American Standard Code for Information Interchange. In this system, 7 bits are used to represent 128 (2 7) different letters, numbers, punctuation, and special codes. When the eighth bit is used (why waste it?), we have Extended ASCII, in which 256 characters are available. The "upper" 128 characters are not as standard as the first 128. Refer to an Extended ASCII chart. 6.12.2 Not all computers use this system. IBM, on their mainframes, stubbornly sticks with EBCDIC, which means something but I have tried very hard to forget what. Unicode is a new system which use 2 bytes, allowing many more characters to be represented (how many, class?), such as Kanji, Cyrillic, Hebrew, etc. Unicode is backwards compatible with ASCII, meaning that operating systems using Unicode, such as Windows 95, can still make sense of ASCII files. 6.12.3 Earlier it was mentioned that 10110010 equals decimal 178. So why do we need a "code" for representing numbers in a computer? Why not just use binary (converted back to decimal when we type it in or read it, of course)? Well, how would we represent negative numbers? What about fractions? What about numbers larger than 8 bits can represent (28 = 256)? You see the problem. The solution, shortened a bit (no pun intended) is to use more than one byte, and, for floating point numbers, to use some of the bits for an exponent and the rest for the mantissa. The table below shows some of the more common data types. The exact details depend on the particular program used (which, in turn, usually depends on the compiler used to create the program).
6.13
Common Data Types
Data Type
Approximate Range (from) logical False (0) character a short integer -128.00 integer -32,768 long integer -2 x 109 single precision -3.4 x 10+38 to -1.2 x IEEE real 10-38 double precision -1.8 x 10+308 to -2.2 x IEEE real 10-308 like single precision single precision real, for both real complex and imaginary parts like double precision double precision real, for both real complex and imaginary parts 7.1 Personal Computers (PC)
(to) True (1) Z (sort of!) 127.00 +32,767 +2 x 109
Significant Bits Digits 8 NA 8 NA 8 2 16 4 32 9
+1.2 x 10-38 to +3.4 x 10+38
32
6
+2.2 x 10-308 to 1.8 x 10+308
64
15
like single precision real, for both real and imaginary parts like double precision real, for both real and imaginary parts
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7.1.1 When you mention the word "technology," most people think about computers. Virtually every facet of our lives has some computerized component. The appliances in our homes have microprocessors built into them, as do our televisions. Even our cars have a computer. But the computer that everyone thinks of first is typically the personal computer, or PC. 7.1.2 A PC is a general purpose tool built around a microprocessor. It has lots of different parts -memory, a hard disk, a modem, etc. -- that work together. "General purpose" means that you can do many different things with a PC. You can use it to type documents, send e-mail, browse the Web and play games. we will talk about PCs in the general sense and all the different parts that go into them. You will learn about the various components and how they work together in a basic operating session. You'll also find out what the future may hold for these machines. Motherboard - This is the main circuit board that all of the other internal components connect to. The CPU and memory are usually on the motherboard. Other systems may be found directly on the motherboard or connected to it through a secondary connection. For example, a sound card can be built into the motherboard or connected through PCI. Power supply - An electrical transformer regulates the electricity used by the computer. Hard disk - This is large-capacity permanent storage used to hold information such as programs and documents. Operating system - This is the basic software that allows the user to interface with the computer. Integrated Drive Electronics (IDE) Controller - This is the primary interface for the hard drive, CDROM and floppy disk drive. Peripheral Component Interconnect (PCI) Bus - The most common way to connect additional components to the computer, PCI uses a series of slots on the motherboard that PCI cards plug into. SCSI - Pronounced "scuzzy," the small computer system interface is a method of adding additional devices, such as hard drives or scanners, to the computer. AGP - Accelerated Graphics Port is a very high-speed connection used by the graphics card to interface with the computer. Sound card - This is used by the computer to record and play audio by converting analog sound into digital information and back again. Graphics card - This translates image data from the computer into a format that can be displayed by the monitor. 7.2
Defining a PC
7.2.1 Here is one way to think about your PC: "A PC is a general-purpose information processing device. It can take information from a person (through the keyboard and mouse), from a device (like a floppy disk or CD) or from the network (through a modem or a network card) and process it. Once processed, the information is shown to the user (on the monitor), stored on a device (like a hard disk) or sent somewhere else on the network (back through the modem or network card)." 7.2.2 We have lots of special-purpose processors in our lives. An MP3 Player is a specialized computer for processing MP3 files. It can't do anything else. A Global Positioning Satellite (GPS) is a specialized computer for handling GPS signals. It can't do anything else. A Gameboy is a specialized computer for handling games, but it can't do anything else. A PC can do it all because it is general-purpose. 7.3
On the Inside
7.3.1
Let's take a look at the main components of a typical desktop computer.
Central processing unit (CPU) - The microprocessor "brain" of the computer system is called the central processing unit. Everything that a computer does is overseen by the CPU. 35
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Memory - This is very fast storage used to hold data. It has to be fast because it connects directly to the microprocessor. There are several specific types of memory in a computer: Random-access memory (RAM) - Used to temporarily store information that the computer is currently working with Read-only memory (ROM) - A permanent type of memory storage used by the computer for important data that does not change Basic input/output system (BIOS) - A type of ROM that is used by the computer to establish basic communication when the computer is first turned on Caching - The storing of frequently used data in extremely fast RAM that connects directly to the CPU Virtual memory - Space on a hard disk used to temporarily store data and swap it in and out of RAM as needed 7.3.2 The computer you are using to read this page uses a microprocessor to do its work. The microprocessor is the heart of any normal computer, whether it is a desktop machine, a server or a laptop. The microprocessor you are using might be a Pentium, a K6, a PowerPC, a Sparc or any of the many other brands and types of microprocessors, but they all do approximately the same thing in approximately the same way. 7.3.3 If you have ever wondered what the microprocessor in your computer is doing, or if you have ever wondered about the differences between types of microprocessors, then read on. In this self-study material you will learn how fairly simple digital logic techniques allow a computer to do its job, whether its playing a game or spell checking a document! 7.4
Inside a Microprocessor
7.4.1 To understand how a microprocessor works, it is helpful to look inside and learn about the logic used to create one. In the process you can also learn about assembly language -- the native language of a microprocessor -- and many of the things that engineers can do to boost the speed of a processor. A microprocessor executes a collection of machine instructions that tell the processor what to do. Based on the instructions, a microprocessor does three basic things: 1
2
3
Using its ALU (Arithmetic/Logic Unit), a microprocessor can perform mathematical operations like addition, subtraction, multiplication and division. Modern microprocessors contain complete floating point processors that can perform extremely sophisticated operations on large floating point numbers. A microprocessor can move data from one memory location to another. A microprocessor can make decisions and jump to a new set of instructions based on those decisions.
7.4.2 There sophisticated things microprocessor its three basic following diagram extremely simple capable of doing
may be very that a does, but those are activities. The shows an microprocessor those three things: 36
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7.4.3
This is about as simple as a microprocessor gets. This microprocessor has: a) An address bus (that may be 8, 16 or 32 bits wide) that sends an address to memory b) A data bus (that may be 8, 16 or 32 bits wide) that can send data to memory or receive data from memory c) An RD (read) and WR (write) line to tell the memory whether it wants to set or get the addressed location d) A clock line that lets a clock pulse sequence the processor e) A reset line that resets the program counter to zero (or whatever) and restarts execution f) Let's assume that both the address and data buses are 8 bits wide in this example.
7.4.4
Here are the components of this simple microprocessor: a) Registers A, B and C are simply latches made out of flip-flops. b) The address latch is just like registers A, B and C. c) The program counter is a latch with the extra ability to increment by 1 when told to do so, and also to reset to zero when told to do so. d) The ALU could be as simple as an 8-bit adder (see the section on adders in How Boolean Logic Works for details), or it might be able to add, subtract, multiply and divide 8-bit values. Let's assume the latter here. e) The test register is a special latch that can hold values from comparisons performed in the ALU. An ALU can normally compare two numbers and determine if they are equal, if one is greater than the other, etc. The test register can also normally hold a carry bit from the last stage of the adder. It stores these values in flip-flops and then the instruction decoder can use the values to make decisions. f) There are six boxes marked "3-State" in the diagram. These are tri-state buffers. A tristate buffer can pass a 1, a 0 or it can essentially disconnect its output (imagine a switch that totally disconnects the output line from the wire that the output is heading toward). A tri-state buffer allows multiple outputs to connect to a wire, but only one of them to actually drive a 1 or a 0 onto the line. 37
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g)
The instruction register and instruction decoder are responsible for controlling all of the other components.
7.4.5 Although they are not shown in this diagram, there would be control lines from the instruction decoder that would: a) Tell the A register to latch the value currently on the data bus b) Tell the B register to latch the value currently on the data bus c) Tell the C register to latch the value currently on the data bus d) Tell the program counter register to latch the value currently on the data bus e) Tell the address register to latch the value currently on the data bus f) Tell the instruction register to latch the value currently on the data bus g) Tell the program counter to increment h) Tell the program counter to reset to zero i) Activate any of the six tri-state buffers (six separate lines) j) Tell the ALU what operation to perform k) Tell the test register to latch the ALU's test bits l) Activate the RD line m) Activate the WR line n) Coming into the instruction decoder are the bits from the test register and the clock line, as well as the bits from the instruction register. 7.5
RAM and ROM
7.5.1 The previous section talked about the address and data buses, as well as the RD and WR lines. These buses and lines connect either to RAM or ROM -- generally both. In our sample microprocessor, we have an address bus 8 bits wide and a data bus 8 bits wide. That means that the microprocessor can address (28) 256 bytes of memory, and it can read or write 8 bits of the memory at a time. Let's assume that this simple microprocessor has 128 bytes of ROM starting at address 0 and 128 bytes of RAM starting at address 128. 7.5.2 ROM stands for read-only memory. A ROM chip is programmed with a permanent collection of pre-set bytes. The address bus tells the ROM chip which byte to get and place on the data bus. When the RD line changes state, the ROM chip presents the selected byte onto the data bus. 7.5.3 RAM stands for random-access memory. RAM contains bytes of information, and the microprocessor can read or write to those bytes depending on whether the RD or WR line is signaled. One problem with today's RAM chips is that they forget everything once the power goes off. That is why the computer needs ROM. By the way, nearly all computers contain some amount of ROM (it is possible to create a simple computer that contains no RAM -- many microcontrollers do this by placing a handful of RAM bytes on the processor chip itself -- but generally impossible to create one that contains no ROM). 7.5.4 On a PC, the ROM is called the BIOS (Basic Input/Output System). When the microprocessor starts, it begins executing instructions it finds in the BIOS. The BIOS instructions do things like test the hardware in the machine, and then it goes to the hard disk to fetch the boot sector .This boot sector is another small program, and the BIOS stores it in RAM after reading it off the disk. The microprocessor then begins executing the boot sector's instructions from RAM. The boot sector program will tell the microprocessor to fetch something else from the hard disk into RAM, which the microprocessor then executes, and so on. This is how the microprocessor loads and executes the entire operating system. Read-only memory (ROM), also known as firmware, is an integrated circuit programmed with specific data when it is manufactured. ROM chips are used not only in computers, but in most other electronic items as well. You will learn about the different types of ROM and how each works. 38
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7.5.5 Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell. 7.5.6 The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order. Question I know my computer uses DRAM (dynamic RAM) for the main memory. I have also heard of static RAM. What is the difference, and why are there two kinds? Answer Your computer probably uses both static RAM and dynamic RAM at the same time, but it uses them for different reasons because of the cost difference between the two types. If you understand how dynamic RAM and static RAM chips work inside, it is easy to see why the cost difference is there, and you can also understand the names. Dynamic RAM is the most common type of memory in use today. Inside a dynamic RAM chip, each memory cell holds one bit of information and is made up of two parts: a transistor and a capacitor. These are, of course, extremely small transistors and capacitors so that millions of them can fit on a single memory chip. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work on the WEB page for more information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second. This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory. Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory. A flip-flop for a memory cell takes 4 or 6 transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes a lot more space on a chip than a dynamic memory cell. Therefore you get less memory per chip, and that makes static RAM a lot more expensive. So static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.
7.6
How ROM Works 39
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7.6.1 Read-only memory (ROM), also known as firmware, is an integrated circuit programmed with specific data when it is manufactured. ROM chips are used not only in computers, but in most other electronic items as well. In this trainee handbook you will learn about the different types of ROM and how each works. 7.6.1.1
Let's start by identifying the different types of ROM.
7.6.2 7.6.2.1
ROM Types There are five basic ROM types: ROM PROM EPROM EEPROM Flash memory
7.6.2.2 Each type has unique characteristics, which you'll learn about in this article, but they are all types of memory with two things in common: Data stored in these chips is nonvolatile -- it is not lost when power is removed. Data stored in these chips is either unchangeable or requires a special operation to change (unlike RAM, which can be changed as easily as it is read). 7.6.2.3
This means that removing the power source from the chip will not cause it to lose any data.
7.6.3
ROM at Work
7.6.3.1 Similar to RAM, ROM chips (Figure 1) contain a grid of columns and rows. But where the columns and rows intersect, ROM chips are fundamentally different from RAM chips. While RAM uses transistors to turn on or off access to a capacitor at each intersection, ROM uses a diode to connect the lines if the value is 1. If the value is 0, then the lines are not connected at all. 7.6.3.2 A diode normally allows current to flow in only one direction and has a certain threshold, known as the forward breakover, that determines how much current is required before the diode will pass it on. In silicon-based items such as processors and memory chips, the forward breakover voltage is approximately 0.6 volts. By taking advantage of the unique properties of a diode, a ROM chip can send a charge that is above the forward breakover down the appropriate column with the selected row grounded to connect at a specific cell. If a diode is present at that cell, the charge will be conducted through to the ground, and, under the binary system, the cell will be read as being "on" (a value of 1). The neat part of ROM is that if the cell's value is 0, there is no diode at that intersection to connect the column and row. So the charge on the column does not get transferred to the row. 7.6.3.3 As you can see, the way a ROM chip works necessitates the programming of perfect and complete data when the chip is created. You cannot reprogram or rewrite a standard ROM chip. If it is incorrect, or the data needs to be updated, you have to throw it away and start over. Creating the original template for a ROM chip is often a laborious process full of trial and error. But the benefits of ROM chips outweigh the drawbacks. Once the template is completed, the actual chips can cost as little as a few cents each. They use very little power, are extremely reliable and, in the case of most small electronic devices, contain all the necessary programming to control the device. A great example is the small chip in the singing fish toy. This chip, about the size of your fingernail, contains the 30-second song clips in ROM and the control codes to synchronize the motors to the music.
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7.6.4
PROM
7.6.4.1 Creating ROM chips totally from scratch is time-consuming and very expensive in small quantities. For this reason, mainly, developers created a type of ROM known as programmable read-only memory (PROM). Blank PROM chips can be bought inexpensively and coded by anyone with a special tool called a programmer. 7.6.4.2 PROM chips have a grid of columns and rows just as ordinary ROMs do. The difference is that every intersection of a column and row in a PROM chip has a fuse connecting them. A charge sent through a column will pass through the fuse in a cell to a grounded row indicating a value of 1. Since all the cells have a fuse, the initial (blank) state of a PROM chip is all 1s. To change the value of a cell to 0, you use a programmer to send a specific amount of current to the cell. The higher voltage breaks the connection between the column and row by burning out the fuse. This process is known as burning the PROM. 7.6.5
You Need Connections
7.6.5.1 No matter how powerful the components inside your computer are, you need a way to interact with them. This interaction is called input/output (I/O). The most common types of I/O in PCs are: Monitor - The monitor is the primary device for displaying information from the computer. Keyboard - The keyboard is the primary device for entering information into the computer. Mouse - The mouse is the primary device for navigating and interacting with the computer Removable storage - Removable-storage devices allow you to add new information to your computer very easily, as well as save information that you want to carry to a different location. Floppy disk - The most common form of removable storage, floppy disks are extremely inexpensive and easy to save information to. CD-ROM - CD-ROM (compact disc, read-only memory) is a popular form of distribution of commercial software. Many systems now offer CD-R (recordable) and CD-RW (rewritable), which can also record. Flash memory - Based on a type of ROM called electrically erasable programmable read-only memory (EEPROM), Flash memory provides fast, permanent storage. CompactFlash, SmartMedia and PCMCIA cards are all types of Flash memory. DVD-ROM - DVD-ROM (digital versatile disc, read-only memory) is similar to CD-ROM but is capable of holding much more information.
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Ports Parallel - This port is commonly used to connect a printer. Serial - This port is typically used to connect an external modem. Universal Serial Bus (USB) - Quickly becoming the most popular external connection, USB ports offer power and versatility and are incredibly easy to use. Firewire (IEEE 1394) - Firewire is a very popular method of connecting digitalvideo devices, such as camcorders or digital cameras, to your computer. Internet/network connection Modem - This is the standard method of connecting to the Internet. Local area network (LAN) card - This is used by many computers, particularly those in an Ethernet office network, to connected to each other. Cable modem - Some people now use the cable-television system in their home to connect to the Internet. Digital Subscriber Line (DSL) modem - This is a high-speed connection that works over a standard telephone line. Very high bit-rate DSL (VDSL) modem - A newer variation of DSL, VDSL requires that your phone line have fiber-optic cables.
7.7
From Powering Up to Shutting Down
7.7.1 Now that you are familiar with some of the parts and workings of a PC, let's see what happens in a typical computer session, from the moment you turn the computer on until you shut it down: i) You press the "On" button on the computer and the monitor. ii) You see the BIOS software doing its thing, called the power-on self-test (POST). On many machines, the BIOS displays text describing such data as the amount of memory installed in your computer and the type of hard disk you have. During this boot sequence, the BIOS does a remarkable amount of work to get your computer ready to run. The BIOS determines whether the video card is operational. Most video cards have a miniature BIOS of their own that initializes the memory and graphics processor on the card. If they do not, there is usually video-driver information on another ROM on the motherboard that the BIOS can load. The BIOS checks to see if this is a cold boot or a reboot. It does this by checking the value at memory address 0000:0472. A value of 1234h indicates a reboot, in which case the BIOS skips the rest of POST. Any other value is considered a cold boot. If it is a cold boot, the BIOS verifies RAM by performing a read/write test of each memory address. It checks for a keyboard and a mouse. It looks for a PCI bus and, if it finds one, checks all the PCI cards. If the BIOS finds any errors during the POST, it notifies you with a series of beeps or a text message displayed on the screen. An error at this point is almost always a hardware problem. The BIOS displays some details about your system. This typically includes information about the following: Processor Floppy and hard drive Memory BIOS revision and date Display Any special drivers, such as the ones for SCSI adapters, are loaded from the adapter and the BIOS displays the information. The BIOS looks at the sequence of storage devices identified as boot devices in the CMOS Setup. "Boot" is short for "bootstrap," as in the old phrase "Lift yourself up by your bootstraps." Boot refers to the process of launching the operating system. The BIOS tries to initiate the boot sequence from the first device using the bootstrap loader. 42
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iii)
iv)
v)
vi)
vii) viii) ix) x)
The bootstrap loader loads the operating system into memory and allows it to begin operation. It does this by setting up the divisions of memory that hold the operating system, user information and applications. The bootstrap loader then establishes the data structures that are used to communicate within and between the sub-systems and applications of the computer. Finally, it turns control of the computer over to the operating system. Once loaded, the operating system's tasks fall into six broad categories: Processor management - Breaking the tasks down into manageable chunks and prioritizing them before sending to the CPU Memory management - Coordinating the flow of data in and out of RAM and determining when virtual memory is necessary Device management - Providing an interface between each device connected to the computer, the CPU and applications Storage management - Directing where data will be stored permanently on hard drives and other forms of storage Application Interface - Providing a standard communications and data exchange between software programs and the computer User Interface - Providing a way for you to communicate and interact with the computer You open up a word processing program and type a letter, save it and then print it out. Several components work together to make this happen: The keyboard and mouse send your input to the operating system. The operating system determines that the word-processing program is the active program and accepts your input as data for that program. The word-processing program determines the format that the data is in and, via the operating system, stores it temporarily in RAM. Each instruction from the word-processing program is sent by the operating system to the CPU. These instructions are intertwined with instructions from other programs that the operating system is overseeing before being sent to the CPU. All this time, the operating system is steadily providing display information to the graphics card, directing what will be displayed on the monitor. When you choose to save the letter, the word-processing program sends a request to the operating system, which then provides a standard window for selecting where you wish to save the information and what you want to call it. Once you have chosen the name and file path, the operating system directs the data from RAM to the appropriate storage device. You click on "Print." The word-processing program sends a request to the operating system, which translates the data into a format the printer understands and directs the data from RAM to the appropriate port for the printer you requested. You open up a Web browser and check out a Web page. Once again, the operating system coordinates all of the action. This time, though, the computer receives input from another source, the Internet, as well as from you. The operating system seamlessly integrates all incoming and outgoing information. You close the Web browser and choose the "Shut Down" option. The operating system closes all programs that are currently active. If a program has unsaved information, you are given an opportunity to save it before closing the program. The operating system writes its current settings to a special configuration file so that it will boot up next time with the same settings. If the computer provides software control of power, then the operating system will completely turn off the computer when it finishes its own shut-down cycle. Otherwise, you will have to manually turn the power off.
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7.8
A Fit for the Future?
7.8.1 Silicon microprocessors have been the heart of the computing world for more than 40 years. In that time, microprocessor manufacturers have crammed more and more electronic devices onto microprocessors. In accordance with Moore's Law, the number of electronic devices put on a microprocessor has doubled every 18 months. Moore's Law is named after Intel founder Gordon Moore, who predicted in 1965 that microprocessors would double in complexity every two years. Many have predicted that Moore's Law will soon reach its end because of the physical limitations of silicon microprocessors. 7.8.2
The current process used to pack more and more transistors onto a chip is called deep-ultraviolet lithography (DUVL), which is a photography-like technique that focuses light through lenses to carve circuit patterns on silicon wafers. DUVL will begin to reach its limit around 2005. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create more powerful chips. Many are already looking at extreme-ultraviolet lithography (EUVL) as a way to extend the life of silicon at least until the end of the decade. EUVL uses mirrors instead of lenses to focus the light, which allows light with shorter wavelengths to accurately focus on the silicon wafer. To learn more about EUVL, see How EUV Chipmaking Works.
7.8.3 As the computer moves off the desktop and becomes our constant companion, augmented-reality displays will overlay computer-generated graphics to the real world. 7.8.4 Beyond EUVL, researchers have been looking at alternatives to the traditional microprocessor design. Two of the more interesting emerging technologies are DNA computers and quantum computers. DNA computers have the potential to take computing to new levels, picking up where Moore's Law leaves off. There are several advantages to using DNA instead of silicon: As long as there are cellular organisms, there will be a supply of DNA. The large supply of DNA makes it a cheap resource. Unlike traditional microprocessors, which are made using toxic materials, DNA biochips can be made cleanly. DNA computers are many times smaller than today's computers. 44
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7.8.5 DNA's key advantage is that it will make computers smaller, while at the same time increasing storage capacity, than any computer that has come before. One pound of DNA has the capacity to store more information than all the electronic computers ever built. The computing power of a teardrop-sized DNA computer, using the DNA logic gates, will be more powerful than the world's most powerful supercomputer. More than 10-trillion DNA molecules can fit into an area no larger than 1 cubic centimeter (.06 inch3). With this small amount of DNA, a computer would be able to hold 10 terabytes (TB) of data and perform 10-trillion calculations at a time. By adding more DNA, more calculations could be performed. 7.8.6 Unlike conventional computers, DNA computers could perform calculations simultaneously. Conventional computers operate linearly, taking on tasks one at a time. It is parallel computing that will allow DNA to solve complex mathematical problems in hours -- problems that might take electrical computers hundreds of years to complete. You can learn more about DNA computing in How DNA Computers Will Work. 7.8.7 Today's computers work by manipulating bits that exist in one of two states: 0 or 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits. A qubit can be a 1 or a 0, or it can exist in a superposition that is simultaneously 1 and 0 or somewhere in between. Qubits represent atoms that are working together to serve as computer memory and a microprocessor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers. A 30-qubit quantum computer would equal the processing power of a conventional computer capable of running at 10 teraops, or trillions of operations per second. Today's fastest supercomputers have achieved speeds of about 2 teraops. You can learn more about the potential of quantum computers in How Quantum Computers Will Work. 7.8.8 Already we are seeing powerful computers in non-desktop roles. Laptop computers and personal digital assistants (PDAs) have taken computing out of the office. Wearable computers built into our clothing and jewelry will be with us everywhere we go. Our files will follow us while our computer provides constant feedback about our environment. Voice- and handwriting-recognition software will allow us to interface with our computers without using a mouse or keyboard. Magnetic RAM and other innovations will soon provide our PC with the same instant-on accessibility that our TV and radio have. 7.8.9 One thing is an absolute certainty: The PC will evolve. It will get faster. It will have more capacity. And it will continue to be an integral part of our lives 7.9
How Modems Work
7.9.1 If you have the capability you can try opening the WEB page and accessing this article on your on the internet on your computer at home. If you do it probably has arrived via modem. 7.9.2 In this pre-course self study package we'll show you how a modem brings you Web pages. We'll start with the original 300-baud modems and progress all the way through to the ADSL configurations! (Note: If you are unfamiliar with bits, bytes and the ASCII character codes, reading How Bits and Bytes Work on the WEB Page "how stuff works" will help make this article much clearer.) 7.9.3
Let's get started with a short recap of how the modem came to be.
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7.10
The Origin of Modems
7.10.1 The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. 7.10.2 The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back. 7.10.3 Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below:
7.10.4 In a configuration like this, a dumb terminal at an off-site office or store could "dial in" to a large, central computer. The 1960s were the age of time-shared computers, so a business would often buy computer time from a time-share facility and connect to it via a 300-bit-per-second (bps) modem. 7.10.5 A dumb terminal is simply a keyboard and a screen. A very common dumb terminal at the time was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user typed a character on the terminal, the modem sent the ASCII code for the character to the computer. The computer then sent the character back to the computer so it would appear on the screen. 7.10.6 When personal computers started appearing in the late 1970s, bulletin board systems (BBS) became the rage. A person would set up a computer with a modem or two and some BBS software, and other people would dial in to connect to the bulletin board. The users would run terminal emulators on their computers to emulate a dumb terminal. 7.10.7 People got along at 300 bps for quite a while. The reason this speed was tolerable was because 300 bps represents about 30 characters per second, which is a lot more characters per second than a person can type or read. Once people started transferring large programs and images to and from bulletin board 46
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systems, however, 300 bps became intolerable. Modem speeds went through a series of steps at approximately two-year intervals:
7.11
300 bps - 1960s through 1983 or so 1200 bps - Gained popularity in 1984 and 1985 2400 bps 9600 bps - First appeared in late 1990 and early 1991 19.2 kilobits per second (Kbps) 28.8 Kbps 33.6 Kbps 56 Kbps - Became the standard in 1998 ADSL, with theoretical maximum of up to 8 megabits per second (Mbps) - Gained popularity in 1999 300-bps Modems
7.11.1 We'll use 300-bps modems as a starting point because they are extremely easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits. 7.11.2 When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare. 7.11.3 Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary (see How Bits and Bytes Work for details on binary). A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over the phone line. 7.12
Faster Modems
7.12.1 In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques (see the links at the end of this article for more information).
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7.12.2
Here's a look inside a typical 56K modem:
7.12.3 All of these high-speed modems incorporate a concept of gradual degradation, meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed. 7.12.4 The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central office and your house are equipped with an ADSL modem on your line, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data. 7.12.5 The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe. 7.12.6 7.13
(For information on the latest DSL technology, see How DSL Works.) Point-to-Point Protocol
7.13.1 Today, no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world. Because of the 48
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relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP. 7.13.2 The standard technique for routing these packets through your modem is called the Pointto-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer. See this page (you need to be on the WEB to see this information) on the internet for additional information on PPP. 7.13.3 If you want to know more about modems, protocols, and especially if you wish to delve into things like PSK and QAM in more detail, check out the links on the WEB! 8.1
How Routers Work
8.1.1 The Internet is one of the 20th century's greatest communications developments. It allows people around the world to send e-mail to one another in a matter of seconds, and it lets you read, among other things, the articles on. We're all used to seeing the various parts of the Internet that come into our homes and offices -- the Web pages, e-mail messages and downloaded files that make the Internet a dynamic and valuable medium. But none of these parts would ever make it to your computer without a piece of the Internet that you've probably never seen. In fact, most people have never stood "face to machine" with the technology most responsible for allowing the Internet to exist at all: the router. 8.1.2 Routers are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways. In this training package we'll look at how these behind-the-scenes machines make the Internet work. 8.2
Routers Keep The Messages Moving
8.2.1 When you send e-mail to a friend on the other side of the country, how does the message know to end up on your friend's computer, rather than on one of the millions of other computers in the world? Much of the work to get a message from one computer to another is done by routers, because they're the crucial devices that let messages flow between, rather than within, networks
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8.2.2 Let's look at what a very simple router might do. Imagine a small company that makes animated 3-D graphics for local television stations. There are 10 employees of the company, each with their own computer. Four of the employees are animators, while the rest are in sales, accounting and management. The animators will need to send lots of very large files back and forth to one another as they work on projects. To do this, they'll use a network.
8.2.3 When one animator sends a file to another, the very large file will use up most of the network's capacity, making the network run very slowly for other users. One of the reasons that a single intensive user can affect the entire network stems from the way that Ethernet works. Each information packet sent from a computer is seen by all the other computers on the local network. Each computer then examines the packet and decides whether it was meant for their address. This keeps the basic plan of the network simple, but has performance consequences as the size of the network, or level of network activity increases. To keep the animators' work from interfering with that of the folks in the front office, the company sets up two separate networks, one for the animators and one for the rest of the company. A router links the two networks and connects both networks to the Internet. 8.2.4 The router is the only device that sees every message sent by any computer on either of the company's networks. When an animator sends a huge file to another animator, the router looks at the recipient's address and keeps the traffic on the animator's network. When an animator, on the other hand, sends a message to the bookkeeper asking about an expense-account check, then the router sees the recipient's address and forwards the message between the two networks. 8.2.5 One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including: Information on which connections lead to particular groups of addresses Priorities for connections to be used Rules for handling both routine and special cases of traffic 8.2.6 A configuration table can be a simple as a half-dozen lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages.
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8.2.7 A router, then, has two separate but related jobs. First, the router ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the connections of "innocent bystanders." 8.2.8 Second, the router makes sure that information does make it to the intended destination. In performing these two jobs, a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. 8.2.9 It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. As the number of networks attached to one another grows, the configuration table for handling traffic among them grows, and the processing power of the router is increased. Regardless of how many networks are attached, though, the basic operation and function of the router remains the same. Since the Internet is one huge network made up of tens of thousands of smaller networks, its use of routers is an absolute necessity. 8.3
Taking Packets from One Place to Another
8.3.1 When you make a telephone call to someone on the other side of the country, the telephone system establishes a stable circuit between your telephone and the telephone you're calling. The circuit might involve a half dozen or more steps through copper cables, switches, fiber-optics, microwaves and satellites, but those steps are established and remain constant for the duration of the call. This circuit approach means that the quality of the line between you and the person you're calling is consistent throughout the call, but a problem with any portion of the circuit -- say, a tree falls across one of the lines used, or there's a power problem with a switch -- brings your call to an early and abrupt end. When you send an e-mail message with an attachment to the other side of the country, a very different process is used. 8.3.2 Internet data, whether in the form of a Web page, a downloaded file or an e-mail message, travels over a system known as a packet-switching network. In this system, the data in a message or file is broken up into packages about 1,500 bytes long. Each of these packages gets a wrapper that includes information on the sender's address, the receiver's address, the package's place in the entire message, and how the receiving computer can be sure that the package arrived intact. Each data package, called a packet, is then sent off to its destination via the best available route -- a route that might be taken by all the other packets in the message or by none of the other packets in the message. This might seem very complicated compared to the circuit approach used by the telephone system, but in a network designed for data there are two huge advantages to the packet-switching plan. First, the network can balance the load across various pieces of equipment on a millisecond-by-millisecond basis. Second, if there is a problem with one piece of equipment in the network while a message is being transferred, packets can be routed around the problem, ensuring the delivery of the entire message. 8.3.3 The routers that make up the main part of the Internet can reconfigure the paths that packets take because they look at the information surrounding the data packet, and they tell each other about line conditions, such as delays in receiving and sending data and traffic on various pieces of the network. Not all routers do so many jobs, however. Therefore, routers come in different sizes. For example: If you have enabled Internet Connection Sharing between two Windows 98-based computers, you're using one of the computers (the computer with the Internet connection) as a simple router. In this instance, the router does so little -- simply looking at data to see whether it's intended for one computer or the other -- that it can operate in the background of the system without significantly affecting the other programs you might be running. Slightly larger routers, the sort used to connect a small office network to the Internet, will do a bit more. These routers frequently enforce rules concerning security for the office 51
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network (trying to secure the network from some sorts of attacks). They handle enough traffic that they're generally stand-alone devices rather than software running on a server. The largest routers, those used to handle data at the major traffic points on the Internet, handle millions of data packets every second and work to configure the network most efficiently. These routers are large stand-alone systems that have far more in common with super-computers than with your office server
8.3.4 Let's take a look at a medium-sized router -- the router used in the How Stuff Works office. In our case, the router only has two networks to worry about: The office network, with about 50 computers and devices, and the Internet. The office network connects to the router through an Ethernet connection, specifically a 100 base-T connection. (100 base-T means that the connection is 100 megabits per second, and uses a twisted-pair cable like an 8-wire version of the cable that connects your telephone to the wall jack.) 8.3.5 There are two connections between the router and our ISP (Internet Service Provider). One is a T-1 connection that supports 1.5 megabits per second. The other is an ISDN line that supports 128 kilobits per second. The configuration table in the router tells it that all out-bound packets are to use the T-1 line, unless it's unavailable for some reason (e.g. - a backhoe digs up the cable). If it can't be used, then outbound traffic goes on the ISDN line. This way, the ISDN line is held as "insurance" against a problem with the faster T-1 connection, and no action by a staff member is required to make the switch in case of trouble. The router's configuration table knows what to do. 8.3.6 In addition to routing packets from one point to another, the How Stuff Works router has rules limiting how computers from outside the network can connect to computers inside the network, how the How Stuff Works network appears to the outside world, and other security functions. While most companies also have a special piece of hardware or software called a firewall to enforce security, the rules in a router's configuration table are important to keeping a company's -- or a family's -- network secure. 8.3.7 One of the crucial tasks for any router is knowing when a packet of information stays on its local network. For this, it uses a mechanism called a subnet mask. The subnet mask looks like an IP address and usually reads "255.255.255.0". This tells the router that all messages with the sender and receiver having an address sharing the first three groups of numbers are on the same network, and shouldn't be sent out to another network. Here's an example: The computer at address 15.57.31.40 sends a request to the computer at 15.57.31.52. The router, which sees all the packets, matches the first three groups in the address of both sender and receiver (15.57.31) , and keeps the packet on the local network. 8.3.8 Between the time these words left the Howstuffworks.com server and the time they showed up on your monitor (if the trainees is connected to the internet), they passed through several routers (it's impossible to know ahead of time exactly how many "several" might be) that helped them along the way. It's very similar to the process that gets a postal letter from your mailbox to the mailbox of a friend, with routers taking the place of the mail sorters and handlers along the way. 8.4
How Routers Know Where to Send Data
8.4.1 Routers are one of several types of devices that make up the "plumbing" of a computer network. Hubs, switches and routers all take signals from computers or networks and pass them along to other computers and networks, but a router is the only one of these devices that examines each bundle of data as it passes and makes a decision about exactly where it should go. To make these decisions, routers must first know about two kinds of information: addresses and network structure.
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8.4.2 When a friend mails a birthday card to be delivered to you at your house, they probably use an address that looks something like this: Joe Smith 123 Maple Street Smalltown, FL 45678 8.4.3 The address has several pieces, each of which helps the people in the postal service move the letter along to your house. The ZIP code can speed the process up, but even without the ZIP code, the card will get to your house, as long as your friend includes your state, city and street address. You can think of this address as a logical address because it describes a way someone can get a message to you. This logical address is connected to a physical address that you generally only see when you're buying or selling a piece of property. The survey plat of the land and house, with latitude, longitude or section bearings, gives the legal description, or address, of the property. 8.4.4 Every piece of equipment that connects to a network, whether an office network or the Internet, has a physical address. This is an address that's unique to the piece of equipment that's actually attached to the network cable. For example, if your desktop computer has a network interface card (NIC) in it, the NIC has a physical address permanently stored in a special memory location. This physical address, which is also called the MAC address (for Media Access Control) has two parts, each 3 bytes long. The first 3 bytes identify the company that made the NIC. The second 3 bytes are the serial number of the NIC itself. 8.4.5 The interesting thing is that your computer can have several logical addresses at the same time. Of course, you're used to having several "logical addresses" bring messages to one physical address. You mailing address, telephone number (or numbers) and home e-mail address all work to bring messages to you when you're in your house. They are simply used for different types of messages -- different networks, so to speak. 8.4.6 Logical addresses for computer networks work in exactly the same way. You may be using the addressing schemes, or protocols, from several different types of networks simultaneously. If you're connected to the Internet (and if you're reading this, you probably are), then you have an address that's part of the TCP/IP network protocol. If you also have a small network set up to exchange files between several family computers, then you may also be using the Microsoft NetBEUI protocol. If you connect to your company's network from home, then your computer may have an address that follows Novell's IPX/SPX protocol. All of these can coexist on your computer. Since the driver software that allows your computer to communicate with each network uses resources like memory and CPU time, you don't want to load protocols you won't need, but there's no problem with having all the protocols your work requires running at the same time.
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8.5
Routers Understand the Protocols
8.5.1 The first and most basic job of the router is to know where to send information addressed to your computer. Just as the mail handler on the other side of the country knows enough to keep a birthday card coming toward you without knowing where your house is, most of the routers that forward an e-mail message to you don't know your computer's MAC address, but they know enough to keep the message flowing. 8.5.2 The chances are very good that you'll never see the MAC address for any of your equipment because the software that helps your computer communicate with a network takes care of matching the MAC address to a logical address. The logical address is what the network uses to pass information along to your computer. 8.5.3 If you'd like to see the MAC address and logical address used by the Internet Protocol (IP) for your Windows computer, you can run a small program that Microsoft provides. Go to the "Start" menu, click on "Run," and in the window that appears, type "WINIPCFG." When the gray window appears, click on "More Info" and you'll get this sort of information: 8.5.4
Windows 98 IP Configuration: Host Name: NAMEHOWSTUFFWORKS DNS Servers: 208.153.64.20 208.153.0.5 Node Type: Broadcast NetBIOS Scope ID: IP Routing Enabled: Yes WINS Proxy Enabled: No NetBIOS Resolution Uses DNS: No Ethernet adapter: Description: PPP Adapter Physical Address: 44-45-53-54-12-34 DHCP Enabled: Yes IP Address: 227.78.86.288 Subnet Mask: 255.255.255.0 Default Gateway: 227.78.86.288 54
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DHCP Server: 255.255.255.255 Primary WINS Server: Secondary WINS Server: Lease Obtained: 01 01 80 12:00:00 AM Lease Expires: 01 01 80 12:00:00 AM 8.5.5 There's a lot of information here that will vary depending on exactly how your connection to the Internet is established, but the physical Routers are programmed to understand the most common network protocols. That means they know the format of the addresses, how many bytes are in the basic package of data sent out over the network, and how to make sure all the packages reach their destination and get reassembled. For the routers that are part of the Internet's main "backbone," this means looking at, and moving on, millions of information packages every second. And simply moving the package along to its destination isn't all that a router will do. It's just as important, in today's computerized world, that they keep the message flowing by the best possible route. 8.5.6 In a modern network, every e-mail message is broken up into small pieces. The pieces are sent individually and reassembled when they're received at their final destination. Because the individual pieces of information are called packets and each packet can be sent along a different path, like a train going through a set of switches, this kind of network is called a packet-switched network. It means that you don't have to build a dedicated network between you and your friend on the other side of the country. Your email flows over any one of thousands of different routes to get from one computer to the other. Depending on the time of day and day of the week, some parts of the huge public packet-switched network may be busier than others. When this happens, the routers that make up this system will communicate with one another so that traffic not bound for the crowded area can be sent by less congested network routes. This lets the network function at full capacity without excessively burdening already-busy areas. You can see, though, how Denial of Service attacks, in which people send millions and millions of messages to a particular server, will affect that server and the routers forwarding message to it. As the messages pile up and pieces of the network become congested, more and more routers send out the message that they're busy, and the entire network with all its users can be affected. 8.6
Tracing a Message
8.6.1 If you're using a Microsoft-Windows-based system, you can see just how many routers are involved in your Internet traffic by using a program you have on your computer. The program is called Traceroute, and that describes what it does -- it traces the route that a packet of information takes to get from your computer to another computer connected to the Internet. To run this program, click on the "MSDOS Prompt" icon on the "Start" menu. Then, at the "C:\WINDOWS>" prompt, type "tracert www.howstuffworks.com". The results looked like this:
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End of Section - SECTION II - Review Exercise Answers found at page 62 Q26
Computers use a: a) base-2 number system, known as the binary number system b) base-10 number system known as the decimal number system c) base-4 number system d) none of the above
Q27
The word bit is a shortening of the words "Binary digit." Whereas decimal digits have 10 possible values ranging from 0 to 9, bits have only two possible values: 0 and 1. Therefore, a binary number is composed of only 0s and 1s, like this: 1011. You figure out what the value of the binary number 1011 by using a base of 2 instead of a base of 10. So 1011 = 1 0 1 1 (1 * 2^3) + (0 * 2^2) + (1 * 2^1) + (1 * 2^0) = (1*2*2*2)+ (0*2*2) + (1*2*1) + (1*) = 8 + 0 + 2 + 1 = 11 You can see that in binary numbers, each bit holds the value of increasing powers of 2. That makes counting in binary pretty easy. Which on of the following answers is correct?? a) b) c) d) e)
0= 1 1= 1 2 = 11 3 = 11 4 = 101
Q28
Bits are rarely seen alone in computers. They are almost always bundled together into 8-bit collections, and these collections are called bytes. Why are there 8 bits in a byte? A similar question is, "Why are there 12 eggs in a dozen?" The 8-bit byte is something that people settled on through trial and error over the past 50 years. With 8 bits in a byte, how many values can you represent? a) 64 values b) 128 values c) 256 values d) 512 values
Q29
In the American Standard Code for Information Interchange (ASCII) character set, each binary value between 0 and 127 is given a specific character. Most computers extend the ASCII character set to use the full range of 256 characters available in a byte. The upper 128 characters handle special things like accented characters from common foreign languages. If you were to look at the file as a computer looks at it, you would find that each byte contains not a letter but? a) a number -- the number is the ASCII code corresponding to the character b) digits, uppercase characters and lowercase characters c) binary numbers d) a one-to-one correspondence between each character
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Q30
When you start talking about lots of bytes, you get into prefixes like kilo, mega and giga, as in kilobyte, megabyte and gigabyte (also shortened to K, M and G, as in Kbytes, Mbytes and Gbytes or KB, MB and GB). The following table shows the multipliers: Name Abbr. Size Kilo
K
2^10 = 1,024
Mega M
2^20 = 1,048,576
Giga
G
2^30 = 1,073,741,824
Tera
T
2^40 = 1,099,511,627,776
Peta
P
2^50 = 1,125,899,906,842,624
Exa
E
2^60 = 1,152,921,504,606,846,976
Zetta
Z
2^70 = 1,180,591,620,717,411,303,424
Yotta Y
2^80 = 1,208,925,819,614,629,174,706,176
You can see in this chart that kilo is about a thousand, mega is about a million, giga is about a billion, and so on. So when someone says, "This computer has a 2 gig hard drive," what he or she means is that the hard drive stores 2 gigabytes, or approximately 2 billion bytes, or exactly? a) 2,147,483,648 bytes. b) 1,125,899,906,842,624 bytes. c) 1,152,921,504,606,846,976 bytes. d) 1,048,576 bytes. Q31
Bits are binary digits. A bit can hold the value 0 or 1. Bytes are made up of 8 bits each. Binary math works just like decimal math, but each bit can have a value of only 0 or 1.
Q32
Since computers work in binary, memory is usually divided up into divisions that are a power of 2. Hence, which of these statements is not true? a) kilobyte = 1024 bytes (1024 = 210), abbreviated KB (Kb is kilobits) b) megabyte = 1024 KB = 1,048,576 bytes, abbreviated MB (Mb is megabits) c) gigabyte = 1024 MB = 1,073,741,824 bytes, abbreviated GB (Gb is gigabits) d) Zettabyte= 1024 GB = 1,152,921,504,606,846,976 bytes, abbreviated ZB(Zb is Zetabits)
Q33
What do we put into memory? a) Instructions b) Data c) Characters d) Instructions and data e) None of the above
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Q34 Write the number of the subject beside the correct definition? 1. Central Processing Unit 2. Memory 3. Random-access memory (RAM) 4. Read-only memory (ROM) 5. Basic input/output system (BIOS) 6. Caching 7. Virtual memory
Q35
Used to temporarily store information that the computer is currently working with A type of ROM that is used by the computer to establish basic communication when the computer is first turned on A permanent type of memory storage used by the computer for important data that does not change Space on a hard disk used to temporarily store data and swap it in and out of RAM as needed The storing of frequently used data in extremely fast RAM that connects directly to the CPU The microprocessor "brain" of the computer system is called the central processing unit. Everything that a computer does is overseen by the CPU. This is very fast storage used to hold data. It has to be fast because it connects directly to the microprocessor. There are several specific types of memory in a computer:
Which one of these things does a microprocessor NOT do? a) Using its ALU (Arithmetic/Logic Unit), a microprocessor can perform mathematical operations like addition, subtraction, multiplication and division. Modern microprocessors contain complete floating point processors that can perform extremely sophisticated operations on large floating point numbers. b) A microprocessor can move data from one memory location to another. c) A microprocessor can make decisions and jump to a new set of instructions based on those decisions d) A microprocessor can build a test register with a special latch that can hold values from comparisons performed in the ALU.
Q36 Read-only memory (ROM), also known as firmware, is an integrated circuit programmed with specific data when it is manufactured. ROM chips are used not only in computers, but in most other electronic items as well. There are different types of ROM and each type has unique characteristics, , but they are all types of memory with two things in common. Which of the following statements about ROM is NOT true? a) Data stored in these chips is nonvolatile -- it is not lost when power is removed and Data stored in these chips is either unchangeable or requires a special operation to change (unlike RAM, which can be changed as easily as it is read). b) Data stored in these chips is volatile -- it is lost when power is removed and Data stored in these chips is either changeable OR requires a special operation to change (unlike RAM, which can be changed as easily as it is read) c) Data stored in these chips is nonvolatile -- it is not lost when power is removed and Data stored in these chips is changeable (like RAM, which can be changed as easily as it is read). d) It is firm and wares well.
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Q37
Similar to RAM, ROM chips contain a grid of columns and rows. But where the columns and rows intersect, ROM chips are fundamentally different from RAM chips. While RAM uses transistors to turn on or off access to a capacitor at each intersection, ROM uses a diode to connect the lines if the value is 1. If the value is 0, then the lines are not connected at all. The way a ROM chip works necessitates the programming of perfect and complete data when the chip is created. You cannot reprogram or rewrite a standard ROM chip. If it is incorrect, or the data needs to be updated, you have to throw it away and start over. Creating the original template for a ROM chip is often a laborious process full of trial and error. But the benefits of ROM chips outweigh the drawbacks. Once the template is completed, the advantages of ROM chips include which of the following statements: a) actual ROM chips can cost as little as a few cents each. b) ROM chips use very little power, ROM chips extremely reliable and, c) ROM chips contain all the necessary programming to control the device d) All of the above
Q38
Match the identified Internet/network connection on the left column to the definition on the right? 1. Modem A newer variation of DSL, VDSL requires that your phone line have fiber-optic cables. 2. Local Area Network This is a high-speed connection that works over a standard (LAN) telephone line. 3. Cable Modem Some people now use the cable-television system in their home to connect to the Internet. 4. Digital Subscriber Line This is used by many computers, particularly those in an (DSL) modem Ethernet office network, to connected to each other. 5. Very high bit-rate DSL Modulator-Demodulator is the standard method of (VDSL) modem connecting to the Internet
Q39
Once loaded, the operating system's tasks fall into six broad categories. Which one of the following answers is correct? a) Processor management - Breaking the tasks down into manageable chunks and prioritizing them before sending to the CPU b) Memory management - Coordinating the flow of data in and out of RAM and determining when virtual memory is necessary c) Device management - Providing an interface between each device connected to the computer, the CPU and applications d) Storage management - Directing where data will be stored permanently on hard drives and other forms of storage e) Application Interface - Providing a standard communications and data exchange between software programs and the computer f) User Interface - Providing a way for you to communicate and interact with the computer g) All of the above
Q40
The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back. Modem speeds went through a series of steps at approximately two-year intervals: 300 bps - 1960s through 1983 or so; 1200 bps Gained popularity in 1984 and 1985; 9600 bps - First appeared in late 1990 and early 1991. 56 Kbps - Became the standard in which year?? a) 1995 b) 1996 c) 1997 d) 1998 59
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Q41
A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits. When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as? a) Quarter-duplex modems b) One third-duplex modems c) Double duplex modems d) Half-duplex modems
Q41
Select the answer with the correct words to insert into the following paragraph at (1), (2) and (3)? "In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to (1) _______and then (2) _______These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems. All of these high-speed modems incorporate a concept of (3)______meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed." a) (1) phase-shift keying (PSK) (2) 3,000 hertz (3) gradual degradation, b) (1) asymmetric digital (2) 9600 bps, (QAM) (3) gradual degradation, c) (1) phase-shift keying (PSK) (2) quadrature amplitude modulation (QAM) (3) gradual degradation, d) (1) Cable Modem
Q42
Today, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP. The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). We're all used to seeing the various parts of the Internet that come into our homes and offices -- the Web pages, e-mail messages and downloaded files that make the Internet a dynamic and valuable medium. But none of these parts would ever make it to your computer without a piece of the Internet that you've probably never seen. What is the technology considered most responsible for allowing the Internet to exist? a) modems b) network c) diode d) the router
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Q43
The router is the only device that sees every message sent by any computer on either of the company's networks. One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including: Information on which connections lead to particular groups of addresses, Priorities for connections to be used, Rules for handling both routine and special cases of traffic. A configuration table can be a simple as a halfdozen lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. A router, then, has two separate but related jobs. First, the router ensures that information doesn't go where it's not needed." Second, the router makes sure that information does make it to the intended destination. In performing these two jobs, a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. A router also? a) protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other b) As the number of networks attached to one another grows, the configuration table for handling traffic among them grows, and the processing power of the router is decreased c) Depending on how many networks are attached, though, the basic operation and function of the router will change d) Since the Internet is one huge network made up of tens of thousands of smaller networks, its use of routers is not really needed.
Q44
Internet data, whether in the form of a Web page, a downloaded file or an e-mail message, travels over a system known as a packet-switching network. In this system, the data in a message or file is broken up into packages about 1,500 bytes long. Each of these packages gets a wrapper that includes information on the sender's address, the receiver's address, the package's place in the entire message, and how the receiving computer can be sure that the package arrived intact. Each data package, is called a? a) Bit b) Byte c) Packet d) None of the above
Q45
Hubs, switches and routers all take signals from computers or networks and pass them along to other computers and networks, but a router is the only one of these devices that examines each bundle of data as it passes and makes a decision about exactly where it should go. To make these decisions, routers must first know about two kinds of information what are they? a) addresses and network structure b) the ZIP code & logical addresses c) physical address & network interface card d) MAC address & logical addresses
Q46
What is a protocol? A set of rules governing the exchange of data between two entities. Protocols are used to speak the same language. The key elements of a Protocol are: a) Data format, coding signal level, error handling, speed matching and sequencing. b) knowing enough to keep the message flowing c) matching the MAC address to a logical address d) the MAC address and logical address used by the Internet Protocol (IP)
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Q47
Physical Routers know the format of the addresses, how many bytes are in the basic package of data sent out over the network, and how to make sure all the packages reach their destination and get reassembled. In a modern network, every e-mail message is broken up into small pieces. The pieces are sent individually and reassembled when they're received at their final destination. The individual pieces of information are called packets and each packet can be sent along a different path, like a train going through a set of switches, this kind of network is called? a) A Ring Network b) A Star Network c) A Tree Network d) A Packet-switched network.
Q48
One of the benefits that Data Communications provides is as follows? a) Allow slower transfer of information between locations; b) Denies access to databases, services and computing facilities; c) Prohibits duplication and mirroring of important information resources; d) Prohibits application load sharing. e) Many data communications requirements can be satisfied by collections of point to point communications links, and this type of interconnection has been used many times in the past.
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A26 A27 A28 A29 A30 A31 A32
a) c) c) a) a) True d)
A33 d) A34 1. Central Processing Unit 2. Memory 3. Random-access memory (RAM) 4. Read-only memory (ROM) 5. Basic input/output system (BIOS) 6. Caching
ANSWERS TO REVIEW QUESTIONS -SECTION II base-2 number system, known as the binary number system 3 = 11 256 values a number -- the number is the ASCII code corresponding to the character 2,147,483,648 bytes Zettabyte= 1024 GB = 1,152,921,504,606,846,976 bytes, abbreviated ZB(Zb is Zetabits) Instructions and data 3. 5. 4. 7. 6.
Used to temporarily store information that the computer is currently working with A type of ROM that is used by the computer to establish basic communication when the computer is first turned on A permanent type of memory storage used by the computer for important data that does not change Space on a hard disk used to temporarily store data and swap it in and out of RAM as needed The storing of frequently used data in extremely fast RAM that connects directly to the CPU
1.
The microprocessor "brain" of the computer system is called the central processing unit. Everything that a computer does is overseen by the CPU. 7. Virtual memory 2. This is very fast storage used to hold data. It has to be fast because it connects directly to the microprocessor. There are several specific types of memory in a computer: A35 d) A microprocessor can build a test register with a special latch that can hold values from comparisons performed in the ALU. A36 d) It is firm and wares well. A37 d) All of the above A38 1. Modem 5 A newer variation of DSL, VDSL requires that your phone line have fiber-optic cables. 2. Local Area Network 4 This is a high-speed connection that works over a standard (LAN) telephone line. 3. Cable Modem 3 Some people now use the cable-television system in their home to connect to the Internet. 4. Digital Subscriber Line 2 This is used by many computers, particularly those in an (DSL) modem Ethernet office network, to connected to each other. 5. Very high bit-rate DSL 1 Modulator-Demodulator is the standard method of (VDSL) modem connecting to the Internet A39 g) All of the above A40 d) 1998 A41 c) (1) phase-shift keying (PSK) (2) quadrature amplitude modulation (QAM) (3) gradual degradation A42 d) the router A43 a) protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other A44 c) Packet A45 a) addresses and network structure A46 a) Data format, coding signal level, error handling, speed matching and sequencing. A47 d) A Packet-switched network. A48 e) Many data communications requirements can be satisfied by collections of point to point communications links, and this type of interconnection has been used many times in the past.
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SECTION - III 9.1
NETWORKING
9.1.1
Course Objectives
9.1.1.1
This section of the self-study package is intended to provide trainees with: i) the principles of data communications including the requirements, rationale, technology, and the intent of Open Systems Interconnection ii) a broad appreciation of the principles of different types of Data Communications Networks iii) an understanding of some of the basic concepts of how Data Communications Networks can be integrated by Open Systems Interconnection iv) an introduction to the range of applications which can be supported. v) a foundation from which trainees should progress to other courses on data communications applications and to making intelligent choices with regard to different data networks, OSI services and applications.
9.1.2
Networks In Use Today
9.1.2.1 A wide range of different types of data networks are in use today. Many have been installed to support special applications [e.g. the Aeronautical Fixed Telecommunications Network (AFTN) )]. Other, more general purpose data networks, which can be used for a wide range of applications have been established over the last 20 years on the basis of standardised data communications network services and standard protocols. The standardisation activity which led to these began is the early 1970s. Some of these latter networks are used to support 'public' data network services. 9.1.2.2 During that time, the problems of data communications have been well understood, and the technology basis supporting communications has changed dramatically with the ever greater sophistication of communications and transmission technologies and the wide availability of relatively cheap computing power. These influences have lead to an ever greater diversity of data communications network types and data communications requirements. 9.1.2.3 So, this course starts with a broad overview of the range of the network types in common use today, including some of those that are liable to be in use tomorrow. 9.1.2.4
Data Communications provide the following benefits: Allow fast transfer of information between locations; Allow access to databases, services and computing facilities; Allow duplication and mirroring of important information resources; Allow application load sharing. Many data communications requirements can be satisfied by collections of point to point communications links, and this type of interconnection has been used many times in the past.
9.1.2.5
However, use of network technology offers a number of distinct advantages by: Increasing resilience by providing alternative communications paths; Allowing resource sharing of communications system components; Allowing rationalisation of the use of communications resources; Simultaneously supporting a range of different data communications applications.
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9.1.2.6 This leads to the concept of a general purpose backbone data communications network. As more and more Information Technology (IT) and communications applications use such a backbone network, the cost/benefit ratio improves.
9.2 Topological are collections of data switching nodes connected to each other by fixed communications links. They come in many different shapes and sizes. Topological Networks can provide connections between a community of end systems to exchange data (called stations). Topologies consisting of stations and end systems can take a wide variety of forms. Each topology has its own benefits, and its precise shape will be determined by many factors including the geographic distribution of the end systems, the traffic loads expected, resilience requirements, quality of service requirements etc. 9.2.1 Star Networks: This type of network is easy to manage and administer but a single fault at the centre can cause the entire network to fail because there are no alternative routing capabilities. Since all traffic must traverse the central node, it can become congested. Only the central node needs a switching capability. 9.2.3 Ring Networks: May be economical in terms of communications link costs, but a single node or connection failure can cause the entire network to fail with some types of protocols (e.g. where the communication links carry data in one direction only). 9.2.4 Complete Networks: Have the highest resilience, lowest transit delays and throughput, but depending on the geographical topology, they may be expensive. Each node must be able to handle connections to each other node, requiring a degree of intelligence at each node. Single and multiple communications link failures can be overcome with appropriate re-routing intelligence at each node. 9.2.5 Tree Networks: Allows distribution of control and administration (which might suit some organisational requirements), a single failure will isolate only a part of the network - there are no alternative routing possibilities. 65
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9.2.6 Irregular Networks: Are constructed of combinations of the above topologies, and each component and connection will be subject to the advantages and disadvantages of its type of local topology. However, this type of network is commonly constructed as a compromise between all of the above topologies which can suit a particular purpose, organisation or application.
9.3 Broadcast Networks operate on a different principle: A station wishing to send data to one or more other stations 'broadcasts' the data over a common communications media so that all of the other stations can receive it. Only the destination stations for which the data is intended actually accept and use the data, the others ignore it. In effect, the communications media and the end stations operate the 'switching' themselves. 9.3.1 There are many types of broadcast networks in use, and all have different strategies for directing the data to the appropriate recipient and preventing clashes, where two or more stations try to send data simultaneously over the common channel. The figure above illustrates a number of quite different broadcast network techniques.
Ring, the stations all connect to their immediate neighbours in a ring. This might take the form of a number of communication lines which link adjacent stations or a continuous cable. Data sent from one station is received by the adjacent node and passed onward so that it eventually reaches all the others. In this case each end station acts as a node.;
Satellite, the stations can all send and receive radio signals to/from a satellite over a common radio channel. The satellite simply re-broadcasts the data so that all of the stations can receive it; 66
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Bus, all of the stations connect to a single 'multidrop' communications line. Senders wait till the line is not busy and simply transmit data addressed to a recipient station.
9.4 These technologies, use a variety of different protocols to overcome or prevent clashes where several stations try to send simultaneously.
9.4.1 LANs are usually networks of the 'broadcast' type which are found in situations as diverse as large corporate networks, and home computing. There are many different ways to use a LAN, and a single LAN may, for example, be simultaneously used to support terminal access to a mainframe, and to share data between PCs. The distinction between the two is not the LAN technology itself, rather the way it is used. A LAN is simply the medium to exchange data, and the terms "Peer-to-Peer" and "Client/Server" refer to the way the software that uses the LAN works, rather than the LAN itself. 9.4.2 Peer-to-Peer operation is typical of networking software used in small isolated workgroups, and is the operational mode of products such as Microsoft's Windows for Workgroups or Novel's Netware Lite. In Peer-to-Peer networking all systems are equals. Resources on any PC (e.g. Disks, Printers, Comms Ports) may be made available to users on other PCs. There is no need for one PC to be dedicated to a task, such as being a file server; potentially each PC is every other PC's file server. With the right software, a user on one PC can "mount" the disk on another PC as if it was locally attached to their PC, and, simultaneously the reverse may also happen. 9.4.3 Peer-to-Peer networking is ideal for sharing data in small workgroups, where each user is responsible for their own machine. However, it is difficult to administer and really does require awareness amongst its users and their co-operation to work properly. For example, who is responsible for data backup or file deletion? If one user saves their file on another's PC, can they be sure it will still be there when they go back to find it, and who is responsible for ensuring that the file is properly backed up?
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9.5 On Client/Server networks, one or more systems are configured as dedicated servers, providing access to shared data (as File Servers), shared resources [e.g. Print Servers) and shared service access (i.e. to external communications services via Wide Area Networks (WAN)]. Users on Client PCs can, for example, mount the disk on the File Server to appear as a local disk, but they cannot mount disks on another user's PC. 9.5.1 It is also common for servers to have a different operating system from their client PCs. While a client PC will typically be running DOS or DOS/Windows, the server is more likely to be running a more powerful operating system, such as Unix, Netware, OS/2 or Windows NT. Server operating systems usually need to support pre-emptive multi-tasking in order to serve their users efficiently, and to provide better security than a single user system does. 9.5.2 The advantage of a server approach is that by concentrating common resources at a single location, it is possible to manage and protect those resources better. A server may be out of the office environment and in a computer room with a protected power supply and protected from intruders. Responsibility for data backup can be made clear, with all user data being backed up on a controlled and regular basis. 9.5.3 The disadvantage of the server approach is that the server is a single point of failure, and organisations that implement a server approach must either be prepared for occasions when the network is unavailable (and typically most client PCs will be unusable), or invest in multiple servers.
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9.6 LANs can only link communities in a relatively confined locality. A typical LAN is limited by its technology to a maximum distance of 2 Kilometres. 9.6.1 There are a number of different types Wide Area Network (WAN) technologies, each have different communications characteristics. The first type of WAN which was developed to offer a global service was the X.25 Packet Switch Technology. A functional diagram of an X.25 Packet Switched Network is illustrated. 9.6.2 Users connect their systems over a standardised interface and protocol (the X.25 Protocol) to a packet switch which is a node of the packet switched network. Packet switches serving different user communities and localities interconnect so that they can forward data to intended recipient end stations. The service is able to switch data communications channels between different end user stations on demand, and each user can establish a large number of different channels simultaneously over a single access link to the PSN. 9.6.3
The standards are: X.25 - which specifies the interface between an X.25 Host and the packet switching service; X.3 - which specifies the operation of a Packet Assembler/Disassembler (PAD) which is used to interface comparatively simple terminals using Asynchronous modems to the Packet switching service; X.29 - which specifies how an X.3 PAD communicates with a distant host using X.25. 69
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X.25 PSN technology is ageing and has a number of disadvantages. More recent technologies such as Frame Relay and Asynchronous Transfer Mode do not have these disadvantages and will probably replace X.25 in the future.
9.7 Internetworking is about joining dissimilar types of network, whether they be LAN or WAN or wireless network, to form an 'internet'. The individual networks which are joined to form an internet are referred to as 'subnetworks'. An internet provides a common communications service to its networked applications users regardless of whether the system on which they run is mainframe or PC, or whether they are connected to LAN or WAN. The design of these applications is therefore independent of the system that runs them or the subnetwork to which they are attached. The graphic above illustrates a simple internet consisting of a LAN and two X.25 Packet Switched subnetworks. The LAN is coupled to the WANs using a 'Router' which acts as a gateway. It maps between the different network protocols and manages the internetwork's addressing and routing. 9.7.1 A larger scale internetwork has been created by 'The Internet'. This interconnects a vast number of computer subnetworks into a single global system, allowing the users of each subnetwork to exchange data with any other user, and access services provided by any service providers attached to the Internet. It also supports a comprehensive range of data communications applications, such as e-mail, file transfer, and remote access to computing resources. 9.7.2 The Internet is the realisation of the potential of internetworking. It is not an organisation or service, but the result of the application of internetworking technology. The Internet is the simple consequence of many organisations adopting common communications protocols, applications that use them, a common addressing scheme, and then interconnecting their respective subnetworks. 70
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9.7.3
The Internet illustrates that: Internetworking can interconnect subnetworks and data communications users on a global scale; An internetwork can support a wide range of applications using a common data communications technology and infrastructure
9.7.4 Internetworking is about joining dissimilar types of network, whether they be LAN or WAN or wireless network, to form an 'internet'. The individual networks which are joined to form an internet are referred to as 'subnetworks'. An internet provides a common communications service to its networked applications users regardless of whether the system on which they run is mainframe or PC, or whether they are connected to LAN or WAN. The design of these applications is therefore independent of the system that runs them or the subnetwork to which they are attached. The LAN is coupled to the WANs using a 'Router' which acts as a gateway. It maps between the different network protocols and manages the internetwork's addressing and routing. 9.7.5 Following on from the requirements for internetworks outlined previously, this section introduces the trainee to the Open System Interconnection Reference Model. The OSI Reference Model is a complete model of data communications supporting distributed applications and it provides terminology which can be used to define data communications services and protocols in a standardised way.
9.8
Open System Interconnect (OSI)
9.8.1
This section introduces OSI: the requirement for standards; background and introduction to the OSI Reference Model; protocol and service layering; connection oriented and Connectionless communications; general concepts and structure of OSI protocols. The trainee will be made aware of the needs for standardisation in this area.
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9.8.2 There are a number of organisations which have formally developed the OSI Standards. The International Standards Organisation (ISO) and the International Telecommunications
Union (ITU) have co-operated to develop the OSI standards over the period since the mid 1970s. In the field of OSI, ISO has formed a joint technical committee (JTC1) with the International Electrical Committee, so this combination is often referenced as ISO/IEC JTC1. 9.8.3 The OSI Reference Model was constructed to provide a framework of concepts within which it was possible to discuss networks and data communications in an abstract sense. It was designed to be unrelated to any particular network or application's terminology. This was necessary because OSI is about all data networks and all data communications applications. In this sense, the OSI reference model provides a meta-language for describing data communications systems. It provides: A common terminology for all aspects of data communications interworking; A common specification of the data communications services provided by the different layers; The concepts of protocol layers and layer services as a means of allocating the different data communications functions into different protocol layers. This has additionally made OSI highly modular and it explicitly outlines all of the different communications functions which must be defined in a complete data communications system; In doing this it does not constrain a supplier to implement the OSI protocols in any particular way, but merely to obey the protocols which are defined in the context of OSI. 9.8.4 Without an OSI Reference model, open communications would be difficult, there would be many islands of 'data communications' which could only exchange information with difficulty.
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9.9
The OSI reference model is a layered model, and identifies two types of Systems:
End Systems which are 'Host Computers', the users of network services, and which comprise seven protocol layers, providing communications services to applications. Intermediate Systems which are either Routers or Packet Switches, and comprise only the three layers appropriate to network communications.
9.9.1 End systems may either communicate directly, using the services of a physical communications medium, or communicate via one or more Intermediate Systems. The definition of each protocol layer is:
Application Layer: contains all the information (or semantics) that is exchanged between End Systems. In particular, it contains all user information that is exchanged. It also provides the means to allow the End Systems to agree to the semantics of the information exchanged; Presentation Layer: provides the means to represent the information exchanged (i.e. the Syntax) between the End Systems without changing the semantics of the information; Session Layer: provides the means to mark significant part of the information exchanged between systems: for example, a Unit or Word, a page, a chapter, a file or a message. It also provides applications with a set of tools to structure communications dialogues; 73
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Transport Layer: provides end-to-end control and information interchange with the level of reliability that is needed for the application. The services provided to the upper layers are independent of the underlying network implementation. The Transport Layer is therefore the "user's liaison, acting as the go-between for the user and the network, enhancing the network's service to that required by the application; Network Layer: provides the means to establish, maintain and terminate the switched connections between End Systems, or to transfer datagrams between two End Systems. Addressing and routing functions are included in the Network Layer; Data Link Layer: provides the synchronisation and error control for the information transmitted over each individual physical link accessing the network or connecting network components; Physical Layer: provides the functional and procedural characteristics to activate, maintain and deactivate the physical connection. It also includes the electrical and mechanical characteristics of the physical interface to the external transmission media.
9.9.2 The seven layer model has proved remarkably resilient to change since it was first introduced, although it has been considerably enhanced over the years. The original reference model considered only connection mode communications. It was later amended to include connectionless communications, and now also includes a Security Model, a Naming and Addressing Model, and a Systems Management Model. A Quality of Service (QoS) Management Framework is a likely future addition. 9.9.3 Each Layer is defined by a 'layer service' which is provided to the next layer up. Each layer service is supported by a 'layer protocol'. Each layer of the model uses the layer services provided by its next lower layer and enhances it by using its layer protocol to provide the layer's service to the next higher layer. In this way, the OSI Reference model defines a 'stack' of seven layers of protocols.
9.9.4 A further important characteristic of OSI is that it specifies both connection oriented and connectionless communications services. Examples of connection oriented services include the Telephone network, X.25 Packet Switched Data Network Services, Telex, Integrated Service Data Network (ISDN). In all of these services, a connection is established between the two communicating parties before data or information is transmitted. The data usually arrives in the same order as it was sent, and there are mechanisms which prevent information arriving too fast for the recipient to deal with (i.e. Flow Control). When the information flow has been completed, the connection between the two parties is disconnected. 9.9.5 Examples of connectionless mode communications include the postal service, messaging systems, and Local Area Networks. In such systems, the information is prepared with an address and other 74
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envelope information and passed through at least one intermediary system on its way to the destination. The originating end system establishes a connection only with the first intermediary system and passes the information over that connection. As soon as the information has been transferred to the intermediary system and stored by it, the connection is broken. There is no concept of a connection between the originating system and the recipient system. 9.9.6
The connection-mode data communications characteristics are:
9.9.7
The connection can reserve resources to guarantee a particular quality of service Information arrives and the recipient's system in the same order as it was sent. There is no need to sort and re-order the information; The flow of information can be easily regulated so that the receiving system does not become overloaded; The network can control congestion more easily by regulating the flow of information through circuits and by regulating the number of connections accepted; Because of the reliability of true connection mode, higher level protocols need not implement rigorous checks and recovery procedures to overcome data loss or unsequenced data; However, the connection mode of working involves a lot of protocol overhead, resulting in inefficiency and a limited throughput of information.
The connectionless mode characteristics are:
It is efficient in terms of network provision; Such a network can be more resilient when nodes fail - information simply takes another path; It is of use in certain applications (e.g. where repetitive data is sent as a continuous stream, and the loss of individual items is tolerable, or where an application can operate in 'send and forget mode).
9.9.8 Connectionless versus connection mode networks was one of the great debates in OSI during the 1980s. On one side, the common carriers argued for the X.25 model because of their perceived need to provide a contractual Quality of Service to their users. On the other hand LAN vendors did not want the overhead of X.25 slowing LANs down. 9.9.9 A number of different issues must be resolved by the OSI network service if it is to span a number of different network types:
Congestion management: The different networks all have different approaches to controlling traffic congestion. Some networks react to congestion by discarding data in transit. Others react by inhibiting traffic entering the network. This has the consequence that either the OSI network service must recover from loss of data or the transport layer must do it;
Addressing: Each different subnetwork type has a different addressing scheme to identify the user systems connected to it. However, the network layer must offer a consistent and well organised addressing scheme which applies to all ISO systems;
Internetwork connections: In order to establish a data transfer capability between all OSI applications, it will be necessary to interconnect different networks;
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Connection oriented vs. connectionless networks: these are fundamentally different techniques for which there are two possibilities for interconnection: ·Either the applications requires a connection oriented service, in which case the individual connectionless networks must be enhanced to provide a connection oriented service, or the application must upgrade the interconnection itself; or ·The application can tolerate a connectionless service, and the individual connection oriented networks can be used in a connectionless fashion.
Reliable end-to-end communications: One of the OSI requirements is to be able to establish reliable end-to-end communications, despite the fact that some of the subnetworks supporting the network layer are inherently unreliable;
Transparency: The OSI data communications service user requires transparency of the underlying subnetworks - the network layer has to provide a consistent service interface irrespective of the type of the subnetworks used.
Routing: Each type of network has its own routing capability and strategy, however, in interconnecting a number of 'subnetworks', it is necessary to define a new routing capability and strategy which deals with routing through those subnetworks.
9.9.10 There are many requirements for data interchange between systems connected to different types of incompatible network in the way illustrated. How do you interconnect a PC on, for example, an Ethernet LAN, with another directly connected to an X.25 network? How do you join two LANs on remote sites? 9.9.11
There is a range of different compatibility problems to be addressed: Connectionless vs. Connection Oriented Services; Frame, Packet, block length differences; Reliable transfer as opposed to unreliable transfer; Throughput and flow control differences; 76
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User system identification and addressing; Quality of Service differences; Error detection and correction; Routing maintenance; Address mapping; ·Data formats and encoding; Security functions.
9.9.12 The various techniques used to meet such requirements are known collectively as "Internetworking". Internetworking supports end-to-end data transfer between systems (e.g. between two PCs) attached to different networks, including networks of different types and characteristics.
9.9.13 There are a number of different techniques which have been developed to link data networks of similar and dissimilar types to support data communications paths which traverse different 'subnetworks'. These are: Repeaters which can directly join two networks of the same type to extend its maximum length; LAN Bridges which are used to link two similar LANS by means of a Wide Area Network; Routers which can interconnect different types of subnetwork and perform an 'internet' routing function;; Gateways which are used to link similar Wide Area Networks.
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Review Questions - SECTION III - Answers Page 83 Q49
One of the distinct advantages of using network technology is as follows: a) Decreased resilience by providing alternative communications paths; b) Prohibiting resource sharing of communications system components; c) Decreased rationalisation of the use of communications resources; d) Simultaneously supporting a range of different data communications applications.
Q50
The following shape describes what type of general topology network? a) Tree b) Ring c) Irregular d) Complete e) Star
Q51 Topological are collections of data switching nodes connected to each other by fixed communications links. They come in many different shapes and sizes. Topological Networks can provide connections between a community of end systems to exchange data (called stations). Topologies consisting of stations and end systems can take a wide variety of forms. Each topology has its own benefits, and its precise shape will be determined by many factors including the geographic distribution of the end systems, the traffic loads expected, resilience requirements, quality of service requirements etc. Please indicate if you think that these statements are true or false: a) Star Networks: This type of network is difficult to manage and administer but multiple faults at the centre can cause the entire network to fail however there are alternative routing capabilities. Since all traffic must traverse the central node, it can never become congested. Only the central node needs a switching capability. True False b) Ring Networks: May be economical in terms of communications link costs, but a single node or connection failure can cause the entire network to fail with some types of protocols (e.g. where the communication links carry data in one direction only). True False c) Complete Networks: Have the highest resilience, lowest transit delays and throughput, but depending on the geographical topology, they may be expensive. Each node must be able to handle connections to each other node, requiring a degree of intelligence at each node. Single and multiple communications link failures can be overcome with appropriate re-routing intelligence at each node. True False d) Tree Networks: Allows distribution of control and administration (which might suit some organisational requirements), a single failure will isolate only a part of the network - there are no alternative routing possibilities. True False
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e) Irregular Networks: Are constructed of combinations of the above topologies, and each component and connection will be subject to the advantages and disadvantages of its type of local topology. However, this type of network is commonly constructed as a compromise between all of the above topologies which can suit a particular purpose, organisation or application. True False Q52
Broadcast Networks: There are many types of broadcast networks in use, and all have different strategies for directing the data to the appropriate recipient and preventing clashes, where two or more stations try to send data simultaneously over the common channel. Which one of these statements is true? a) Ring, the stations do not connect to their immediate neighbours in a ring. This might take the form of a number of communication lines which link stations. Data sent from one station is received and passed onward so that it eventually reaches all the others. In this case each end station acts as a node; True False b) Satellite, the stations can all send and receive radio signals to/from a satellite over a common radio channel. The satellite simply re-broadcasts the data so that all of the stations can receive it; True False c) Bus, all of the stations connect to a single 'multidrop' communications line. Senders wait till the line is not busy and simply transmit data addressed to a recipient station. True False d) These technologies, use a variety of different protocols to overcome or prevent clashes where several stations try to send simultaneously. True False
Q53
LANs are usually what type of networks? a) Broadcast b) Simple c) Complex d) None of the above
Q54
On Client/Server networks, one or more systems are configured as dedicated servers, providing access to shared data (as File Servers), shared resources [e.g. Print Servers) and shared service access [i.e. to external communications services via Wide Area Networks (WAN). A disadvantage of the Client/Server approach is that: a) The server is a single point of failure, and organisations that implement a server approach must either be prepared for occasions when the network is unavailable (and typically most client PCs will be unusable), or invest in multiple servers b) Responsibility for data backup can be made clear c) All user data can be backed up on a controlled and regular basis d) A server may be in a computer room with a protected power supply and protected from intruders
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Q55
Internetworking is about joining dissimilar types of network, whether they be LAN or WAN or wireless network, to form an 'internet'. The individual networks which are joined to form an internet are referred to as 'subnetworks'. An internet provides a common communications service to its networked applications users regardless of whether the system on which they run is mainframe or PC, or whether they are connected to LAN or WAN. A larger scale global internetwork has been created by a) The Airways Engineering LAN b) The STC WAN c) The Proxy Server d) The Internet
Q56
The Internet illustrates that: Internetworking can interconnect subnetworks and data communications users on a global scale and an internetwork can support a wide range of applications using a common data communications technology and infrastructure. Internetworking is about joining dissimilar types of network, whether they be LAN or WAN or wireless network, to form an 'internet'. The LAN is coupled to the WANs using a 'Router' which acts as a gateway. It maps between the different network protocols and manages the internetwork's addressing and routing. The individual networks which are joined to form an internet are referred to as: a) WAN b) LAN c) Routers d) Subnetworks
Q57
The OSI Reference Model is a complete model of data communications supporting distributed applications and it provides terminology which can be used to define data communications services and protocols in a standardised way. The OSI Reference Model was constructed to provide a framework of concepts within which it was possible to discuss networks and data communications in an abstract sense. It was designed to be unrelated to any particular network or application's terminology. This was necessary because OSI is about all data networks and all data communications applications. In this sense, the OSI reference model provides a meta-language for describing data communications systems. It provides: a) A common terminology for all aspects of data communications interworking; b) the data provided by the different layers; c) the different data communications sectional protocol layers. d) a constraint on the OSI protocols which are defined in the context of OSI.
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Q58
Without an OSI Reference model, open communications would be difficult, there would be many islands of 'data communications' which could only exchange information with difficulty. The OSI reference model is a layered model, and identifies two types of Systems: End Systems which comprise seven protocol layers, providing communications services to applications. Intermediate Systems which are either Routers or Packet Switches, and comprise only the three layers appropriate to network communications. Mark the number of the appropriate Layer beside the correct definition: 1. Application Layer 2. Presentation Layer
3. Session Layer
4. Transport Layer 5. Network Layer 6. Data Link Layer: 7. Physical Layer: Q59
provides the means to mark significant part of the information exchanged between systems: for example, a Unit or Word, a page, a chapter, a file or a message. It also provides applications with a set of tools to structure communications dialogues; provides end-to-end control and information interchange with the level of reliability that is needed for the application. The services provided to the upper layers are independent of the underlying network implementation. The Transport Layer is therefore the "user's liaison, acting as the go-between for the user and the network, enhancing the network's service to that required by the application; provides the means to establish, maintain and terminate the switched connections between End Systems, or to transfer datagrams between two End Systems. Addressing and routing functions are included in the Network Layer; contains all the information (or semantics) that is exchanged between End Systems. In particular, it contains all user information that is exchanged. It also provides the means to allow the End Systems to agree to the semantics of the information exchanged; provides the synchronisation and error control for the information transmitted over each individual physical link accessing the network or connecting network components; provides the functional and procedural characteristics to activate, maintain and deactivate the physical connection. It also includes the electrical and mechanical characteristics of the physical interface to the external transmission media. provides the means to represent the information exchanged (i.e. the Syntax) between the End Systems without changing the semantics of the information;
Each Layer is defined by a 'layer service' which is provided to the next layer up. Each layer service is supported by a 'layer protocol'. Each layer of the model uses the layer services provided by its next lower layer and enhances it by using its layer protocol to provide the layer's service to the next higher layer. In this way, the OSI Reference model defines a 'stack' of ? a) Seven layers of protocols b) Six layers of protocols c) Five layers of protocols d) Eight layers of protocols
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Q60
A number of different issues must be resolved by the OSI network service if it is to span a number of different network types. Match the correct issue to the definition: 1. Congestion Each different subnetwork type has a different addressing scheme to management: identify the user systems connected to it. However, the network layer must offer a consistent and well organised addressing scheme which applies to all ISO systems; 2. Addressing: In order to establish a data transfer capability between all OSI applications, it will be necessary to interconnect different networks; 3. Internetwork these are fundamentally different techniques for which there are two connections: possibilities for interconnection: Either the applications requires a connection oriented service, in which case the individual connectionless networks must be enhanced to provide a connection oriented service, or the application must upgrade the interconnection itself; or The application can tolerate a connectionless service, and the individual connection oriented networks can be used in a connectionless fashion. 4. Connection Each type of network has its own routing capability and strategy, oriented vs. however, in interconnecting a number of 'subnetworks', it is necessary connectionless to define a new routing capability and strategy which deals with networks routing through those subnetworks. 5. Reliable end-toThe different networks all have different approaches to controlling end traffic congestion. Some networks react to congestion by discarding communications: data in transit. Others react by inhibiting traffic entering the network. This has the consequence that either the OSI network service must recover from loss of data or the transport layer must do it; 6. Transparency: One of the OSI requirements is to be able to establish reliable end-toend communications, despite the fact that some of the subnetworks supporting the network layer are inherently unreliable; 7. Routing: The OSI data communications service user requires transparency of the underlying subnetworks - the network layer has to provide a consistent service interface irrespective of the type of the subnetworks used.
Q61
There are a number of different techniques which have been developed to link data networks of similar and dissimilar types to support data communications paths which traverse different 'subnetworks'. These are: a) Repeaters which can directly join two networks of the same type to extend its maximum length b) LAN Bridges which are used to link two similar LANS by means of a Wide Area Network c) Routers which can interconnect different types of subnetwork and perform an 'internet' routing function d) Gateways which are used to link similar Wide Area Networks e) All of the above
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ANSWERS TO REVIEW QUESTIONS -SECTION III A49 A50 A51
A52
A53 A54 A55 A56 A57 A58
A59
d) Simultaneously supporting a range of different data communications applications. e) Star False True True True True False True True True a) Broadcast a) The server is a single point of failure, and organisations that implement a server approach must either be prepared for occasions when the network is unavailable (and typically most client PCs will be unusable), or invest in multiple servers d) The Internet d) Subnetworks a) A common terminology for all aspects of data communications interworking; 1. Application Layer
3
2. Presentation Layer
4
3. Session Layer
5
4. Transport Layer
1
5. Network Layer
6
6. Data Link Layer:
7
7. Physical Layer:
2
a)
provides the means to mark significant part of the information exchanged between systems: for example, a Unit or Word, a page, a chapter, a file or a message. It also provides applications with a set of tools to structure communications dialogues; provides end-to-end control and information interchange with the level of reliability that is needed for the application. The services provided to the upper layers are independent of the underlying network implementation. The Transport Layer is therefore the "user's liaison, acting as the go-between for the user and the network, enhancing the network's service to that required by the application; provides the means to establish, maintain and terminate the switched connections between End Systems, or to transfer datagrams between two End Systems. Addressing and routing functions are included in the Network Layer; contains all the information (or semantics) that is exchanged between End Systems. In particular, it contains all user information that is exchanged. It also provides the means to allow the End Systems to agree to the semantics of the information exchanged; provides the synchronisation and error control for the information transmitted over each individual physical link accessing the network or connecting network components; provides the functional and procedural characteristics to activate, maintain and deactivate the physical connection. It also includes the electrical and mechanical characteristics of the physical interface to the external transmission media. provides the means to represent the information exchanged (i.e. the Syntax) between the End Systems without changing the semantics of the information;
Seven Layers
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A60
A61
1. Congestion management:
2
2. Addressing:
3
3. Internetwork connections:
4
4. Connection oriented vs. connectionless networks 5. Reliable end-toend communications:
7
6. Transparency:
5
7. Routing:
6
e)
All of the above
1
Each different subnetwork type has a different addressing scheme to identify the user systems connected to it. However, the network layer must offer a consistent and well organised addressing scheme which applies to all ISO systems; In order to establish a data transfer capability between all OSI applications, it will be necessary to interconnect different networks; these are fundamentally different techniques for which there are two possibilities for interconnection: Either the applications requires a connection oriented service, in which case the individual connectionless networks must be enhanced to provide a connection oriented service, or the application must upgrade the interconnection itself; or The application can tolerate a connectionless service, and the individual connection oriented networks can be used in a connectionless fashion. Each type of network has its own routing capability and strategy, however, in interconnecting a number of 'subnetworks', it is necessary to define a new routing capability and strategy which deals with routing through those subnetworks. The different networks all have different approaches to controlling traffic congestion. Some networks react to congestion by discarding data in transit. Others react by inhibiting traffic entering the network. This has the consequence that either the OSI network service must recover from loss of data or the transport layer must do it; One of the OSI requirements is to be able to establish reliable end-toend communications, despite the fact that some of the subnetworks supporting the network layer are inherently unreliable; The OSI data communications service user requires transparency of the underlying subnetworks - the network layer has to provide a consistent service interface irrespective of the type of the subnetworks used.
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ADS ADS-B ADSL AGP ASCII ASM ATC ATS ATFM ATM ATN BIOS Bit BIOS CD-R CD-RW CNS/ATM CPU DGNSS DME DL DSL DRAM DVD-ROM EEPROM FANS FIRs FSK GBAS GNSS GPS HF ICAO IDE ILS IMC INS I/O IP ISO IRS ISDN IT LAAS MAC MLS MMR Modem PPP NDB PC PCI
GLOSSARY Automatic Dependent Surveillance Automatic Dependent Surveillance - Broadcast Asymmetric Digital Subscriber Line Accelerated Graphics Port American Standard Code for Information Interchange Airspace Management Air Traffic Control Air Traffic Services Air Traffic Flow Management Air Traffic Management Aeronautical Telecommunication Network Basic Input/Output System Short for "binary digit." Basic Input/Output System Compact Disk Recordable Compact Disk Rewritable Communications, Navigation, Surveillance / Air Traffic Management Central Processing Unit Differential Global Navigation Satellite System Distance Measuring Equipment Data Link Digital Subscriber Line Dynamic Random Access Memory Digital Versatile Disc, Read-Only Memory Electrically Erasable Programmable Read-Only Memory ICAO Future Air Navigation System (FANS) Flight Information Regions Frequency Shift Keying Ground Based Augmentation System Global Navigation Satellite System Global Positioning Satellite High Frequency International Civil Aviation Organization Integrated Drive Electronics Instrument Landing Systems Instrument Meteorological Conditions Inertial Navigation System Input/Output Internet Protocol International Standards Organization Inertial Reference System Integrated Service Data Network Information Technology Local Area Augmentation System Media Access Control Microwave Landing System Multi-Mode Receiver Modulator De-modulator Point to Point Protocol Non Directional Radio Beacon Personal Computers Peripheral Component Interconnect 85
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PROM PSK QAM RAM ROM RNAV RNP RS-232 port SBAS SCSI SAM SSR Mode S TCP/IP UART USB VDSL VHF VMC VOR WAAS
Programmable Read-Only Memory Phase-Shift Keying Quadrature Amplitude Modulation Random Access Memory Read Only Memory (also known as firmware) Area Navigation Required Navigation Performance Serial Port Space Based Augmentation System Small Computer System Interface Serial Access Memory Secondary Surveillance Radar (SSR) Mode S Transport Communications Protocol / Internet Protocol Universal Asynchronous Receiver/Transmitter Universal Serial Bus Very High bit-rate modem Very High Frequency Visual Meteorological Conditions Very High Frequency Omni-Directional Radio Range Wide Area Augmentation System