Computer Networking

COMPUTER NETWORKING

INTRODUCTION
Up to 70s there used to be isolated EDP centers with mainframe computers in large companies with specialist programmers & for any reports etc. one had to give requisition to the dept. and reports used to take any length of time depending on complexities and in case the report was not satisfactory another requisition and another waiting period was required. However, in early 80s, when desktop PCs started entering market, it was assumed that PCs would be use independently by their users in the way they linked. With desktop available with user, he could enter information himself, manipulate him and produced the report he really needed. However, information already available in mainframes needed to be entered again. At that time nobody imagined what would happen next. Gradually PCs became more and more powerful and powerful application like spreadsheets; database and word processors were available. PC market exploded with the fierce competition. Because of intense competition, there was increase power and dramatic fall in prices of PCs. PCs were soon outperforming older, slower mainframe applications and accomplished what was earlier thought as miracles, desktop publishing, graphics, computer-aided drafting, more powerful database & sophisticated user interfaces. Small business also started getting benefits of information management, which earlier very large companies could afford. But soon it was re-discovered that “Information is useful only when it is communicated between individuals”. Passing information through paper was cumbersome as each individual had to re-enter. Passing files after coping in floppies was inconvenient particularly when the users were geographical separated and also there was no guarantee that the floppy contains the latest version of information. And thus started networks of PCs.

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COMPUTER NETWORK

Collection of autonomous computers interconnected by a shared network medium. Networking is the concept of sharing resources and services. A network of computers is a group of interconnected systems sharing resources and interacting using a shared communications link in the figure below.

A network, therefore, is a set of interconnected systems with something to share. The shared resource can be data, a printer, a fax modem, or a service such as a database or an email system. The individual systems must be connected through a pathway (called the transmission medium) that is used to transmit the resource or service between the computers. All systems on the pathway must follow a set of common communication rules for data to arrive at its intended destination and for the sending and receiving systems to understand each other. The rules governing computer communication are called protocols.

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USE OF COMPUTER NETWORK

Resource Sharing- programs, equipment, data & information are available to any one the network without regard to his physical location. Networks help keep information reliable and up-to-date. Networks help to communicate more efficiently. Networks help businesses service their clients more effectively. Networks greatly expand a business’s marketing and customer service capability.

NETWORK TYPES

We can classify network technologies as belonging to one of two basic groups. Local area network (LAN) technologies connect many devices that are relatively close to each other, usually in the same building. The library terminals that display book information would connect over a local area network. Wide area network (WAN) technologies connect a smaller number of devices that can be many kilometers apart. For example, if two libraries at the opposite ends of a city wanted to share their book catalog information, they would most likely make use of a wide area network technology, which could be a dedicated line leased from the local telephone company, intended solely to carry their data.

There are two basic types of networking: 1. Local Area Network (LAN) 2. Wide Area Network (WAN)

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▫ LAN (Local Area Network)
They are privately – owned networks within a single building or campus of up up-to a few kilometers in size. LANs can be small, linking as a few as three computers, but often link hundreds of computers used by thousands of people. The development of standard networking protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organization. A LAN connects network devices over a relatively short distance. A networked office building, distance. school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs, and occasionally a LAN will span a group of nearby buildings. In IP networking, one can conceive of a LAN as a single IP subnet (though this is not necessarily true single in practice). Besides operating in a limited space, LANs include several other distinctive features. LANs are typically owned, controlled, and managed by a single person or organization. They also use certain specific connectivity technologies, primarily Ethernet and Token Ring. tain

They are characterized by the following: High data transfer speeds Generally less expensive technologies Limited geographic area

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▫ WAN (Wide Area Network)
A network that covers a larger area such as a company. This is accomplished by connecting the different LANs using services such as dedicated leased phone lines, dial up phone lines (both dial-up synchronous & asynchronous), satellite links and data packet carrier services. Wide Area packet networking can be as simple as a modem and remote access server for employees to dial into, or it can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the expense of sending data sent expense over vast distances. As the term implies, a wide-area network spans a large physical distance. A WAN like the area Internet spans most of the world! A WAN is a geographically dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains outer both a LAN address and a WAN address. WANs differ from LANs in several important ways. Like the Internet, most WANs are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs use technology like ATM, Frame Relay and X.25 for connectivity.

They are characterized by the following: Multiple interconnected LANs Generally more expensive technology More sophisticated to implement than LANs Exist in an unlimited geographic area Less error resistance due to transmission travel distances

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▫ INTERNET
It is a single network but a network of networks. The internet is a system of linked networks that are worldwide in scope and facilitate data communication services such as remote login, file transfer, electronic mail, the World Wide Web and newsgroups. With the meteoric rise in demand of connectivity, the internet has become a communication highway for millions of users. The internet was initially restricted to military and academic institutions, but now it is a full fledged conduct for all and any form of information and commerce. Internet websites now provide personal, educational, public and economic resources to every corner of the planet.

▫ INTRANET
With the advancements made in browser-based software for the internet, many private organizations are implementing intranets. As intranet is a private network utilizing Internet-type tools & protocols, but available only within that organization. For large organizations, an intranet provides an easy access mode to corporate information for employees.

▫ CLIENT/SERVER
This type of network is designed to support a large number of users and uses dedicated server/s to accomplish this. Clients log on to the server/s in order to run applications or obtain files. Security and permissions can be managed by 1 or more administrators which cut down on confusion. This type of network also allows for convenient backup services, reduces network traffic and provides a host of other services that come with the network operating system (NOS)

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NETWORKING DEVICES
▫ BRIDGE
A bridge is a L2 device that divides a network into separate collision domains while retaining the broadcast domain. It divides a network into segments with each segment having a reduced number of nodes that are competing for access to the transmission medium. Bridges operate at the MAC sub layer of the OSI Data Link layer (see Chapter 2). A repeater passes on all signals that it receives. A bridge, on the other hand, is more selective and passes only those signals targeted for a computer on the other side. A bridge can make this determination because each device on the network is identified by a unique physical address. Each packet that is transmitted bears the address of the device to which it should be delivered.

The process works as follows: 1. The bridge receives every packet from either side of it on LAN A. 2. The bridge references an internal table of addresses. This table is either learned by the bridge, from previous packet deliveries on the network, or manually programmed into the bridge.

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3. Packets on LAN A – Side 1 that are addressed to devices on LAN A – Side 1 and packets on LAN A – Side 2 that are addressed to devices on LAN A – Side 2, are not passed along to the other side by the bridge. These packets can be delivered without the help of the bridge.

3. Packets on LAN A – Side 1 addressed to devices on LAN A – Side 2 are retransmitted, by the bridge to LAN A – Side 2 for delivery. Similarly, the appropriate packets on LAN A – Side 2 are retransmitted to LAN A – Side 1.

▫ SWITCH
A network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Technically, network switches operate at layer two (Data Link Layer) of the OSI model. Network switches appear nearly identical to network hubs, but a switch generally contains more intelligence (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting data packets as they are received, determining the source and destination device of each packet, and forwarding them appropriately. By delivering messages only to the connected device intended, a network switch conservesnetwork bandwidth and offers generally better performance than a hub. A network hub, or repeater, is a fairly unsophisticated network device. Hubs do not manage any of the traffic that comes through them. Any packet entering a port is broadcast out or "repeated"

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on every other port, except for the port of entry. Since every packet is repeated on every other port, packet collisions result, which slows down the network. There are specialized applications where a hub can be useful, such as copying traffic to multiple network sensors. High end switches have a feature which does the same thing called port mirroring. There is no longer any significant price difference between a hub and a low-end switch.

When a packet arrives, the header is checked to determine which segment the packet is destined for, and then it is forwarded to that segment. If the packet is destined for the same segment that it arrives on, the packet is dropped and not retransmitted. This prevents the packet being “broadcasted” onto unnecessary segments, reducing the traffic. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 13

▫ ROUTER
Routers are a step up from bridges. They are able to route and filter information to different networks. Some routers can automatically detect problems and redirect information around the problem area. These are called "intelligent routers." Routers organize the large network in terms of logical network segments. Each network segment is assigned an address so that every packet has both a destination network address and a destination device address. Routers are more “intelligent” than bridges. Not only do routers build tables of network locations, but they also use algorithms to determine the most efficient path for sending a packet to any given network. Even if a particular network segment isn’t directly attached to the router, the router knows the best way to send a packet to a device on that network. In Figure below, for example, Router A knows that the most efficient step is to send the packet to Router C, not Router B.

Router B presents a redundant path to the path Router A provides. Routers can cope with this situation because they exchange routing information to ensure that packet loops don’t occur. In Figure below, if Router A fails, Router B provides a backup message path, thus making this network more robust. One consequence of all the processing a router performs on a packet is that Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 14

routers generally are slower than bridges. You can use routers to divide large, busy LANs into smaller segments, much as you can use bridges. But that’s not the only reason to select a router. Routers also can connect different network types. An example of this would be a router that connected a token-ring Segment with the Ethernet segments. On such networks, a router is the device of choice, as a bridge cannot perform this function. The protocols used to send data through a router must be specifically designed to support routing functions. IP, IPX, and DDP (the AppleTalk Network-layer protocol) are routable transport protocols. NetBEUI is a nonroutable transport protocol.. Because routers can determine route efficiencies, they usually are employed to connect a LAN to a wide area network (WAN). WANs frequently are designed with multiple paths, and routers can ensure that the various paths are used most efficiently.

▫ MAIN COMPONENTS OF ROUTER

ROM contains microcode for basic functions to start and maintain the router. RAM contains the software and data structures that allow the router to function. The principle software running in RAM is the Cisco IOS image and the running configuration. Flash is primarily used to contain the IOS software image. Non-volatile RAM is mainly used to store the configuration. NVRAM uses a battery to maintain the data when power is removed from the router. Configuration Register is used to control how the router boots up.

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Packets are only passed to the network segment they are destined for. They work similar to bridges and switches in that they filter out unnecessary network traffic and remove it from network segments. Routers generally work at the protocol level (L3). Routers were devised in order to separate networks logically. For instance, a TCP/IP router can segment the network based on IP subnets. Filtering at this level (on IP addresses) will take longer than that of a bridge or switch which only looks at the MAC layer. Most routers can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving the networking.

Typically, an organization which connects to the Internet will install a router as the main gateway link between their network and the outside world. By configuring the router with access lists (which define what protocols and what hosts have access) this enforces security by restricted (or allowing) access to either internal or external hosts. For example, an internal WWW server can be allowed IP access from external networks, but other company servers which contain sensitive data can be protected, so that external hosts outside the company are prevented access (you could even deny internal workstations access if required). Routers are devised to separate the network logically. Routers filter the network traffic based on the protocol level. Therefore it also acts as a firewall. This is a basically a barrier which restricts the un-wanted packets to come in and go out of the network.

▫ FEATURES OF ROUTER
Use dynamic routing. Operate at the protocol level. Remote administration and configuration via SNMP. Support complex networks. The more filtering done, the lower the performance. Provides security. Segment networks logically. Broadcast storms can be isolated. Often provides bridge function also. More complex routing protocols used [RIP, IGRP, and OSPF]

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▫ HUB
A hub is repeater with multiple port Each port is RJ-45 connector. ports. Hubs, also called wiring concentrators, provide a central attachment point for network cabling.

Hubs come in three types: 1. Passive 2. Active 3. Switching

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▫ Passive Hub
Passive hubs do not contain any electronic components and do not process the data signal in any way. The only purpose of a passive hub is to combine the signals from several network cable segments. All devices attached to a passive hub receive all the packets that pass through the hub. Hubs follow the 5-4-3 rules, where no more than 5 network segments can be connected to 4 hubs with only 3 of them being populated.

▫ Active HUB
Active hubs incorporate electronic components that can amplify and clean up the electronic signals that flow between devices on the network. This process of cleaning up the signals is called signal regeneration. Signal regeneration has the following benefits: . The network is more robust (less sensitive to errors). . Distances between devices can be increased.

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▫ Intelligent HUB
Intelligent hubs are enhanced active hubs. Several functions can add intelligence to a hub: . Hub management. Hubs now support network management protocols that enable the hub to send packets to a central network console. . Switching. The latest development in hubs is the switching hub, which includes circuitry that very quickly routes signals between ports on the hub. Instead of repeating a packet to all ports on the hub, a switching hub repeats a packet only to the port that connects to the destination computer for the packet.

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▫ REPEATER
A physical layer device that extends the distance of a network by amplifying and retiming signals. A repeater corrects for attenuation problems. Each media type, therefore, has a maximum range that it can reliably carry data. The purpose of a repeater is to extend the maximum range for the network cabling. A repeater is a network device that repeats a signal from one port onto the other ports to which it is connected . Repeaters operate at the OSI Physical layer. (Refer to “The OSI Reference Model” section in Chapter 2.) A repeater does not filter or interpret—it merely repeats (regenerates) a signal, passing all network traffic in all directions. A repeater doesn’t require any addressing information from the data frame because a repeater merely repeats bits of data. This means that if data is corrupt, a repeater will regenerate the signal anyway. A repeater will even repeat a broadcast storm caused by a malfunctioning adapter. The advantages of repeaters are that they are inexpensive and simple. Also, although they cannot connect networks with dissimilar data

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OS I MOD E L
The International Standards Organization, defines a 7 layer model for describing interconnected systems. It is referred to as the Open Systems interconnection (OSI) model. Here is an easy way to memorize the order of the 7 layer model and it is as follows: “All People Seem To Need Data Processing”. The first letter of each word corresponds to the first letter of one of the layers. The functions of the seven layers of the OSI model are:

▫ Layer Seven of the OSI Model
The Application Layer of the OSI model is responsible for providing end-user services, such as file transfers, electronic messaging, e-mail, virtual terminal access, and network management. This is the layer with which the user interacts.

▫ Layer Six of the OSI Model
The Presentation Layer of the OSI model is responsible for defining the syntax which two network hosts use to communicate. Encryption and compression should be Presentation Layer functions.

▫ Layer Five of the OSI Model
The Session Layer of the OSI model is responsible for establishing process-to-process communications between networked hosts.

▫ Layer Four of the OSI Model
The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.

▫ Layer Three of the OSI Model
The Network Layer of the OSI model is responsible for establishing paths for data transfer through the network. Routers operate at the Network Layer.

▫ Layer Two of the OSI Model
The Data Link Layer of the OSI model is responsible for communications between adjacent network nodes. Hubs and switches operate at the Data Link Layer.

▫ Layer One of the OSI Model
The Physical Layer of the OSI model is responsible for bit-level transmission between network nodes. The Physical Layer defines items such as: connector types, cable types, voltages, and pinouts. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 21

▫ The 7 – layers

1. 2. 3. 4. 5. 6. 7.

Physical layer Data link layer Network layer Transport layer Session layer Presentation layer Application layer

Figure showing the OSI Reference Model Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 22

▫ PHYSICAL Layer
The physical layer describes the medium used to connect the systems. Examples are coax cable and fiber optic cable. Example of protocol is ETHERNET.

The Physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are: Establishment and termination of a connection to a communications medium. Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 23

Protocols defined at the Physical Layer standardize physical connections. Specifications include voltage levels, maximum transmission distances, data rates, and physical connectors. Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4. Thus, the major functions of this layer are: Transmits the unstructured raw bit stream over a physical medium. Relates the electrical, optical mechanical and functional interfaces to the cable. Defines how the cable is attached to the network adapter card. Defines data encoding and bit synchronization.

▫ DATA LINK Layer
The data link layer describes representation of bits on the physical medium and the format of messages on the medium. The data link layer may do some checking of messages on the medium. The data link layer may do some checking of messages and error recovery, but does not guarantee delivery of a message. Many descriptions of link layer systems break the link layer into a number of sub layers. The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The best known example of this is Ethernet. This layer manages the interaction of devices with a shared medium. Other examples of data link protocols are HDLC and ADCCP for point-to-point Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 24

or packet-switched networks and Aloha for local area networks. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from the physical layer into logical chunks of data, known as frames. This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain. The data link layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-toPoint Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as TCP.

Thus, the major functions of this layer are: Sends data frames from the Network layer to the Physical layer. Packages raw bits into frames for the Network layer at the receiving end. Responsible for providing error free transmission of frames through the Physical layer.

▫ Data Link Sublayers

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Logical Link Control (LLC) responsible for identifying Network layer protocols and encapsulating them. Media Access Control (MAC) defines how packets are placed on media.

▫ ETHERNET
Ethernet is a shared local area networking technology. The basic design consists of a shared transmission medium in the form of a cable or hub. Since the communication medium is shared, nodes listen to make sure the cable is not in use before transmitting. Since it is possible that two computers might transmit at the same time, causing a “collision”, a collision detection scheme ght was built into the system. When collision occurs any packets being transmitted are corrupted and thus collisions can be detected. When collision occurs, both stations back o and try off retransmitting later. Each computer waits for a few thousandths of a second, this interval is random & different from other computers wait period, and then tries again. Hence this type of method is called CSMA/CD (Carrier Sense Multiple access/Collision Detection). access/Collision Ethernet was developed by Xerox in 1970. It was implemented through thicknet cable running at through 10Mbps. Ethernet is a connection media access method that allows all hosts on a network to share the same bandwidth of a link. Ethernet actually just refers to the LAN implementations that include three principal categories. ually Ethernet / IEEE 802.3 operates at 10 Mbps on coaxial cable and twisted pair cable. 802.3---operates 100-Mbps Ethernet---(also known as Fast Ethernet) operates at 100 Mbps over ---(also twisted-pair cable. 1000-Mbps Ethernet ( also known as Gigabit Ethernet) operates at 1000 Mbps (1 Mbps Ethernet---( Gbps) over fiber and twisted twisted-pair cables. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 26

▫ NETWORKING Layer
The network layer describes how to transmit messages between connected network segments. The network layer does not guarantee delivery of a message. The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it's easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks that carry sacks of mail to other post offices or airports, airplanes that carry airmail between major cities, trucks that distribute mail sacks in a city, and carriers that take a letter to its destinations. Think of fragmentation as splitting a large document into smaller envelopes for shipping, or, in the case of the network layer, splitting an application or transport record into packets. Thus, the main functions of this layer are: Responsible for addressing messages and translating logical addresses and names into physical addresses. Determines the route from the source to the destination computer. Manages traffic such as packet switching, routing and controlling the congestion of data. Example of protocol is IP. The network layer attempts to deliver packets from a node on one network segment to another node that may be on another segment. All network layer protocols use a header that includes both a source and destination address. IP is categorized in five classes. The bits in the first octet identify the address class. The router uses the first bits to identify how many bits it must match to interpret the network portion of the address. Class A addresses include the following: The first bit is 0. Range of network numbers: 1.0.0.0 to 126.0.0.0 Number of possible networks: 127 (1-126 usable, 127 is reserved) Number of possible values in the host portion: 16,777,216.

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Class B addresses include the following: The first two bits are 10. Range of network numbers: 128.0.0.0 to 191.255.0.0 Number of possible networks: 16,384 Number of possible values in the host portion: 65,536

Class C addresses include the following: The first three bits are 110. Range of network numbers: 192.0.0.0 to 223.255.255.0 Number of possible networks: 2,097,152 Number of possible values in the host portion: 256

Class D addresses include the following: Range of network numbers: 224.0.0.0 to 239.255.255.255

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IP provides connectionless, best-effort delivery routing of datagrams. It is not concerned with the content of the datagrams. Instead, it looks for a way to move the datagrams to their destination.

IP Datagram

Network layer addresses consist of two parts: A network segment prefix which identifies the network segment and a node suffix, which identifies the node on the segment. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 29

▫ ICMP
The Internet Control Message Protocol (ICMP) is implemented by all TCP/IP hosts. ICMP messages are carried in IP datagrams and are used to send error and control messages.

ICMP uses the following types of defined messages: 1. Destination Unreachable 2. Time Exceeded 3. Parameter Problem 4. Subnet Mask Request 5. Redirect 6. Echo 7. Echo Reply 8. Information Request 9. Information Reply 10.Address Request 11.Address Reply

▫ TRANSPORT Layer
The transport layer multiplexes message streams used by various applications. It may guarantee in order delivery of data. Example of protocol is TCP, but it also includes some features that OSI includes in level 5. The transport layer multiplexes data for various connections. Each connections represent a path between a pair of applications on a pair of computers. The connections multiplexed by the transport protocol may come from several computers and may be for several applications.

The Transport layer provides transparent transfer of data between end users, providing reliable data transfer while relieving the upper layers of it. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 30

have the equivalent of double envelopes, such as cryptographic Presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a network layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packets. The basic roles of the Transport Layer are to establish end-to-end connections from one computer to another on the network and provide reliable "transport" of data between devices.

TCP Segment

UDP Segment

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▫ Basic Transport Layer Services
Resource Utilization (multiplexing) Connection Management (establishing) Flow Control (Buffering / Windowing) Reliable Transport (positive acknowledgment / error checking)

▫ Flow Control
Once the connection has occurred and transfer is in progress, congestion of the data flow can occur at a destination for a variety of reasons. Possible options include: The destination can become overwhelmed if multiple devices are trying to send it data at the same time. It may become overwhelmed if the source is sending faster than it can physically receive.

▫ CONGESTION PREVENTION
The Transport Layer is responsible for providing flow control to alleviate the issue of congestion and provide reliability in the data transfer. Two main methods for flow control include: Buffering Windowing

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▫ Buffering
Buffering is a form of data flow control regulated by the Transport Layer. It is responsible for ensuring that sufficient buffers are available in the destination for the processing of data and that is data transmitted at a rate that does not exceed what the buffer can handle.

▫ Windowing
Windowing is a flow control scheme in which the source computer will monitor and make adjustments to the amount of information sent based on successful, reliable receipt of data segments by the destination computer. The size of the data transmission, called the "window e size", is negotiated at the time of connection establishment. It is determined by the amount of memory or buffer that is available.

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Given a window size of 3, the source (in this case a router) sends 3 data segments to the ize destination. The destination sends an acknowledgement asking for the next set of data segments. If the destination does not receive all three of the negotiated data segments, for e example, due to a buffer overflow, it sends no acknowledgment. Since the source does not receive an acknowledgment, it knows the data segments should be retransmitted retransmitted.

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▫ SESSION LAYER
The session layer describes how a session is terminated and whether both ends can transmit at the same time or they must one at a time. The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite. The Session Layer establishes, manages, and terminates sessions (different from connections) between applications as they interact on different hosts on a network. Its main job is to coordinate the service requests and responses between different hosts for applications. The main functions of this layer are: Allows two applications running on different computers to establish use and end a connection called a Session. Performs name recognition and security. Provides synchronization by placing checkpoints in the data stream. Implements dialog control between communicating processes.

Three different communication modes exists for data transfer within a session connection: Single-duplex Half-duplex Full-duplex

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▫ PRESENTATION Layer
The presentation layer describes the format of the data. In the TCP/IP protocol family, the part of level 5 not in TCP, level 6, and level 7 are combined into an application layer. SMTP and Telnet are examples.

The Presentation layer transforms the data to provide a standard interface for the Application layer. MIME encoding, data encryption and similar manipulation of the presentation are done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

The Presentation Layer is responsible for the following services: Determines the format used to exchange data among the networked computers. Translates data from a format from the Application layer into an intermediate format. Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion. Redirector operates at this level.

▫ APPLICATION Layer
The application layer describes messages for an application. The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces. The common application services sublayer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 36

Control, and Transaction Processing (according to the ACID requirements). The common application service sublayer are functions meaningful to user application programs such as programs, messaging (X.400), directory (X.500), file transfer (FTAM), virtual terminal (VTAM), and batch job manipulation (JTAM).

OSI Model Data unit Layer Function

Application Network process to application Host layers Data Presentation Data representation and encryption Session Segments Transport Media layers Packets Frames Network Data link Interhost communication End-to-end connections and reliability (TCP) Path determination and logical addressing (IP) Physical addressing (MAC & LLC) Page 37

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In the TCP/IP family of protocols, there are 2 transport layer protocols: TCP which guarantees in order delivery of all data sent on a connection and UDP which does not guarantee delivery of all data sent on a connection and does not guarantee in order de equipment types.

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▫ TOPOLOGY
One can think of a topology as a network's "shape." This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle, but it would be highly unlikely to find an actual ring topology there.

▫ Types of Topology
Network topologies are categorized into the following basic types: 1. 2. 3. 4. 5. 6. Bus Topology Ring Topology Star Topology Tree Topology Mesh Topology Hybrid Topology

▫ BUS Topology
In a bus topology a single cable connects each workstation in a linear, daisy-chained fashion. signals are broadcasted to all stations, but stations only act on the frames addressed to them. Bus networks use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium, that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.

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▫ RING Topology
In a ring topology: Unidirectional links connect the transmit side of one device to the receive side of another device. Devices transmit frames to the next device (downstream member) in the ring.

In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (effectively either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

▫ STAR Topology
In a star topology, each station is connected to a central hub or concentrator that functions as a multi-port repeater. Each station broadcasts to all of the devices connected to the hub. Physical LAN topologies are usually characterized as either bus or ring. Many home networks use the star topology. A star network features a central connection point called a "hub" that may be an actual hub or a switch. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.

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Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

▫ TREE Topology
Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub ports) alone.

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▫ MESH Topology
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, like the Internet, employ mesh routing.

▫ HYBRID Topology

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▫ CO MMUNICATING ME D IU M
The Communication medium means what kind of medium we are going to use for networking between different computing devices. As above said that every network means communication between computers so we need a medium through which we can connect the computers.

There are two types of communication medium: I. Wired Copper cables Optical fiber II. Wireless Micro waves Radio waves Infrared waves

▫ CABLES
The following are the types of cables used in networks and other related topics.

Unshielded Twisted Pair (UTP) Cable Shielded Twisted Pair (STP) Cable Coaxial Cable Fiber Optic Cable Wireless LANs Cable Installation Guides

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▫ UNSHIELDED TWISTED PAIR (UTP) CABLE IS
Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair unshielded. (UTP) is the most popular and is generally the best option for school networks.

Unshielded Twisted Pair

UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. UTP cables are found in many ethernet networks and telephone systems.

The quality of UTP may vary from t telephone-grade wire to extremely high-speed cable. The speed cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per hter foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire.

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Categories of Unshielded Twisted Pair tegories

Type Category 1 Category 2 Category 3 Category 4 Category 5

Use Voice Only (Telephone Wire) Data to 4 Mbps (Local Talk) Data to 10 Mbps (Ethernet) Data to 20 Mbps (16 Mbps Token Ring) Data to 100 Mbps (Fast Ethernet)

▫ UNSHIELDED TWISTED PAIR CONNECTOR IS
The standard connector for unshielded twisted pair cabling is an RJ 45 connector. This is a RJ-45 plastic connector that looks like a large telephone style connector (See fig. 2). A slot allows the telephone-style RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector 45 Jack, follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.

RJ-45 connector Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 45

▫ USES

Twisted pair cabling headed with a registered jack

▫ UTP Cables
Twisted pair cables were first used in telephone systems by Bell in 1881 and by 1900 the entire American network was twisted pair, or else open wire with similar arrangements to guard against interference. Most of the billions of conductor feet (millions of Kilometers) of twisted pairs in the world are outdoors, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers. The majority of data or Internet connections use those wires. For indoor telephone applications, UTP is often grouped into sets of 25 pairs according to a standard 25-pair color code originally developed by AT&T. A typical subset of these AD1L colors (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to fiber and coaxial cabling. Cable Shielding

Twisted pair cables are often shielded to prevent electromagnetic interference. Because the shielding is made of metal, it also serves as a ground. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work.

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▫ SHIELDED TWISTED PAIR (STP) CABLE
A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology.

STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications and used with token ring networks.

▫ SCREENED SHIELDED TWISTED PAIR (S/STP) CABLE

S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP), is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 47

▫ SCREENED U SHIELDED TWISTED PAIR (S/UTP) CABLE

Screened Unshielded Twisted Pair, Foiled Twisted Pair, Screened Foiled Twisted Pair

▫ COAXIAL CABLE

Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other co computers.

Coaxial cable is the kind of copper cable used by cable TV companies between the community antenna and user homes and businesses. Coaxial cable is sometimes used by telephone

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companies from their central office to the telephone poles near users. It is also widely installed users. for use in business and corporation Ethernet and other types of local area network. network Coaxial cable is called "coaxial" because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. The outer channel serves as a ground. Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance. Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twis twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.

▫ COAXIAL CABLE C ONNECTORS
The most common type of connector used with coaxial cables is the Bayone Neill-Concelman Bayone-Neill (BNC) connector (See fig. 4). Different types of adapters are available for BNC connectors, adapters including a T-connector, barrel connector, and terminator. Connectors on the cable are the connector, weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable. n

BNC connector

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▫ FIBER OPTIC CABLE
Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

Fiber optic cable

Cladding and Core Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 50

Data Transfer in Optical Fiber

An optical fiber (or fiber) is a glass or plastic fiber designed to guide light along its length by total internal reflection. Fiber optics is the branch of applied science and engineering concerned with such optical fibers. Optical fibers are widely used in fiber-optic communication, which permits digital data transmission over longer distances and at higher data rates than electronic communication. They are also used to form sensors, and in a variety of other applications. The operating principle of optical fibers applies to a number of variants including multi-mode optical fibers, single-mode optical fibers, graded-index optical fibers, and step-index optical fibers. Because of the physics of the optical fiber, special methods of splicing fibers and of connecting them to other equipment are needed. A variety of methods are used to manufacture optical fibers, and the fibers are also built into different kinds of cables depending on how they will be used. The light-guiding principle behind optical fibers was first demonstrated in Victorian times, but modern optical fibers were only developed beginning in the 1950s. Optical fibers became practical for use in communications in the late 1970s, and since then several technical advances have been made to extend the reach and speed capability of optical fibers, and lower the cost of fiber communications systems.

Facts about fiber optic cables: Outer insulating jacket is made of Teflon or PVC. Kevlar fiber helps to strengthen the cable and prevent breakage. A plastic coating is used to cushion the fiber center. Center (core) is made of glass or plastic fibers.

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▫ FIBER OPTIC CONNECTOR
The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space.

Ethernet Cable Summary

Specification 10BaseT 10Base2 10Base5 10BaseF 100BaseT 100BaseTX

Cable Type Unshielded Twisted Pair Thin Coaxial Thick Coaxial Fiber Optic Unshielded Twisted Pair Unshielded Twisted Pair

Maximum length 100 meters 185 meters 500 meters 2000 meters 100 meters 220 meters

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▫ ROUTING
Routing is the process by which an item gets from one location to another. Many items get routed: for example, mail, telephone calls, and trains. In networking, a router is the device used to route traffic.

Key Information a Router Needs: Destination Address - What is the destination (or address) of the item that needs to be routed? Identifying sources of information - From which source (other routers) can the router learn the paths to given destinations? Discovering routes - What are the initial possible routes, or paths, to the intended destinations? Selecting routes - What is the best path to the intended destination? Maintaining routing information - A way of verifying that the known paths to destinations are the most current.

▫ ROUTED PROTOCOLS
Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from host to host based on the addressing scheme. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. The Internet protocol IP is an example of a routed protocol. Here are some examples of Routed Protocols: Internet Protocol (IP) AppleTalk (AT) Novell NetWare Protocol Xerox Network Systems (XNS)

▫ ROUTING PROTOCOLS
Supports a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. examples of routing protocols are RIP,IGRP,EIGRP and OSPF. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 53

▫ TYPES OF ROUTING
The different types of routing are: Static routing Default routing Dynamic routing

▫ Static Routing
Routes learned by the router when an administrator manually establishes the route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update.

Benefits: There is no overhead on the router CPU. There is no bandwidth usage between routers It adds security Disadvantage: The administrator must really understand the internetwork and how each router is connected to configure routes correctly. If a network is added to internetwork, the administrator has to add route to it on all routers-by hand

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▫ DEFAULT ROUTING
A default route is a special type of static route. A default route is a route to use for situations when the route from a source to a destination is not known or when it is unfeasible for the routing table to store sufficient information about the route.

▫ DYNAMIC ROUTING
Routes dynamically learned by the router after an administrator configures a routing protocol that helps determine routes. Unlike static routes, once the network administrator enables dynamic routing, route knowledge is automatically updated by a routing process whenever new topology information is received from the internetwork.

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▫ ROUTER METRICS
Routing metrics are used by routing algorithms to determine the desirability of a given route to a destination network. Different routing protocols implement different routing metrics. Routing metrics represent network characteristics. Metric information is stored in routing tables. There are a number of commonly used routing metrics, including: Path length Reliability Delay Bandwidth Load Cost

▫ Hop Count
Hop count is a value that counts the number of intermediate systems (such as routers) through which a packet must pass to travel from the source to the destination. The path length is the sum of all the hops in the path.

▫ Reliability
The reliability routing metric can be based on any of a number of network characteristics. These include: Bit-error rate (the ratio of received bits that contain errors) How often each network link fails, and, once down, how quickly each network link can be repaired.

▫ Delay
The delay routing metric is based on the length of time required to move a packet from the source to a destination through the internetwork.

▫ Bandwidth
The bandwidth routing metric is based solely on the available traffic capacity of each network link. However, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links. Pawan Kumar (Electronic Instrumentation & Control Engg.) Page 56

▫ Load
The load routing metric is based on the degree to which a network resource (such as a router) is busy. Load is calculated according to such factors as: CPU utilization Packets processed per second

▫ Cost
The cost routing metric is based on the monetary cost of using each network link. For example, a slower company-owned link can be configured as preferable over faster public links that cost money for usage time.

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▫ RIP:
RIP, or Routing Information Protocol, is a routing protocol located within IP. There are two versions of RIP supported by Cisco. RIP version 1 and an enhanced version RIPv2, a classless routing protocol.

▫ Characteristics of RIP
It is a distance vector routing protocol. Hop count is used as the metric for path selection. The maximum allowable hop count is 15. Routing updates are broadcast every 30 seconds by default. RIP is capable of load balancing over up to six equal cost paths (4 paths is the default). RIPv1 requires that for each major classful network number being advertised, only one network mask is used per network number. The mask is a fixed length subnet mask. RIPv2 permits variable-length subnet masks on the internetwork. (RIPv1 does not do triggered updates but RIPv2 does do triggered updates.)

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▫ IGRP
IGRP is an advanced distance vector routing protocol developed by Cisco in the mid-1980s. IGRP has several features that differentiate it from other distance vector routing protocols, such as RIP.

▫ Characteristics of IGRP
Increased scalability - Improved for routing in larger size networks compared to networks that use RIP. Sophisticated metric - IGRP uses a composite metric that provides significant route selection flexibility. Internetwork delay and bandwidth by default, and optionally reliability, and load are all factored into the routing decision. IGRP can be used to overcome RIP's 15-hop limit. IGRP has a default maximum hop count of 100 hops, configurable to a maximum of 255 hops. Multiple paths - IGRP can maintain up to six nonequal paths between a network source and destination; the paths do not mandate equal costs like with RIP. Multiple paths can be used to increase available bandwidth or for route redundancy.

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▫ USING Telnet TO CONNECT TO REMOTE DEVICES

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▫ USING ping & trace COMMANDS

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