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Computer Networks

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COMPUTER NETWORKS
INTRODUCTION
A computer network or data network is a telecommunications network that allows
computers to exchange data. In computer networks, networked computing devices pass
data to each other along data connections. The connections (network links) between
nodes are established using either cable media or wireless media. The best-known
computer network is the Internet.

Network computer devices that originate, route and terminate the data are called network
nodes. Nodes can include hosts such as personal computers, phones, servers as well as
networking hardware. Two such devices are said to be networked together when one
device is able to exchange information with the other device, whether or not they have a
direct connection to each other. Computer networks support applications such as access
to the World Wide Web, shared use of application and storage servers, printers, and fax
machines, and use of email and instant messaging applications. Computer networks differ
in the physical media used to transmit their signals, the communications protocols to
organize network traffic, the network's size, topology and organizational intent.
Properties
Computer networking may be considered a branch of electrical engineering,
telecommunications, computer science, information technology or computer engineering,
since it relies upon the theoretical and practical application of the related disciplines.

A computer network has the following properties:
Facilitates interpersonal communications : People can communicate efficiently and
easily via email, instant messaging, chat rooms, telephone, video telephone calls, and
video conferencing.

Allows sharing of files, data, and other types of information: Authorized users may
access information stored on other computers on the network. Providing access to
information on shared storage devices is an important feature of many networks.

Allows sharing of network and computing resources: Users may access and use
resources provided by devices on the network, such as printing a document on a shared
network printer. Distributed computing uses computing resources across a network to
accomplish tasks.

May be insecure : A computer network may be used by computer Crackers to deploy
computer viruses or computer worms on devices connected to the network, or to prevent
these devices from accessing the network (denial of service).

May interfere with other technologies : Power line communication strongly disturbs
certain forms of radio communication, e.g., amateur radio. It may also interfere with last
mile access technologies such as ADSL and VDSL.

May be difficult to set up : A complex computer network may be difficult to set up. It
may be costly to set up an effective computer network in a large organization.
TYPES OF NETWORKS (ACCORDING TO AREA COVERED)
Different types of (private) networks are distinguished based on their size (in terms of the
number of machines), their data transfer speed, and their reach. Private networks are
networks that belong to a single organization. There are usually said to be three
categories of networks:

 LAN (local area network)
 MAN (metropolitan area network)
 WAN (wide area network)

There are two other types of networks:
TANs (Tiny Area Network), which are the same as LANs but smaller (2 to 3 machines),
and CANs (Campus Area Networks), which are the same as MANs (with bandwidth
limited between each of the network's LANs).

LAN
LAN stands for Local Area
Network.: It's a group of computers
which all belong to the same
organization, and which are linked
within a small geographic area using
a network, and often the same
technology (the most widespread
being Ethernet).

A local area network is a network in
its simplest form. Data transfer
speeds over a local area network can reach up to 10 Mbps (such as for an Ethernet
network) and 1 Gbps (as with FDDI or Gigabit Ethernet). A local area network can reach
as many as 100, or even 1000, users. By expanding the definition of a LAN to the
services that it provides, two different operating modes can be defined:

In a "peer-to-peer" network, in which communication is carried out from one computer to
another, without a central computer, and where each computer has the same role. in a
"client/server" environment, in which a central computer provides network services to
users.

MAN
MANs (Metropolitan Area Networks)
connect multiple geographically nearby LANs
to one another (over an area of up to a few
dozen kilometres) at high speeds. Thus, a
MAN lets two remote nodes communicate as
if they were part of the same local area
network. A MAN is made from switches or
routers connected to one another with high-
speed links (usually fibre optic cables).

WANs
A WAN (Wide Area Network or extended
network) connects multiple LANs to one
another over great geographic distances. The
speed available on a WAN varies depending
on the cost of the connections (which
increases with distance) and may be low.
WANs operate using routers, which can
"choose" the most appropriate path for data to
take to reach a network node. The most well-known WAN is the Internet.

ACCORDING TO NETWORK TOPOLOGY

The physical topology of a network refers to the configuration of cables, computers, and
other peripherals. Physical topology should not be confused with logical topology which
is the method used to pass information between workstations. Logical topology was
discussed in the Protocol chapter. Main Types of Network Topologies In networking, the
term "topology" refers to the layout of connected devices on a network. This article
introduces the standard topologies of computer networking.
One can think of a topology as a network's virtual shape or structure. 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 in a family room,
but it would be highly unlikely to find an actual ring topology there.
Network topologies are categorized into the following basic types:

STAR TOPOLOGY
Star Topology Many home
networks use the star topology. A
star network features a central
connection point called a "hub" that
may be a hub, switch or router.
Devices typically connect to the
hub with Unshielded Twisted Pair
(UTP) Ethernet.

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.

Advantages of a Star Topology
 Easy to install and wire.
 No disruptions to the network then connecting or removing devices.
 Easy to detect faults and to remove parts.

Disadvantages of a Star Topology
 Requires more cable length than a linear topology.
 If the hub or concentrator fails, nodes attached are disabled.
 More expensive than linear bus topologies because of the cost of the
concentrators.
 The protocols used with star configurations are usually Ethernet or LocalTalk.
Token Ring uses a similar topology, called the star-wired ring.

RING TOPOLOGY
Ring Topology In a ring network, every
device has exactly two neighbors for
communication purposes. All messages
travel through a ring in the same direction
(either "clockwise" or "counterclockwise").
A failure in any cable or device breaks the
loop and can take down the entire network.
To implement a ring network, one typically
uses FDDI, SONET, or Token Ring
technology. Ring topologies are found in
some office buildings or school campuses.



BUS TOPOLOGY
Bus networks (not to be confused with the system bus of a computer) 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.
Ethernet bus topologies are relatively easy to install and don't require much cabling
compared to the alternatives. 10Base-2
("ThinNet") and 10Base-5 ("ThickNet")
both were popular Ethernet cabling
options many years ago for bus
topologies. However, bus networks work
best with a limited number of devices. If
more than a few dozen computers are
added to a network bus, performance
problems will likely result. In addition, if
the backbone cable fails, the entire
network effectively becomes unusable.
Advantages of a Linear Bus Topology
 Easy to connect a computer or peripheral to a linear bus.
 Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology
 Entire network shuts down if there is a break in the main cable.
 Terminators are required at both ends of the backbone cable.
 Difficult to identify the problem if the entire network shuts down.
 Not meant to be used as a stand-alone solution in a large building.
TREE TOPOLOGY
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 connection points) alone.
Advantages of a Tree Topology
 Point-to-point wiring for individual segments.
 upported by several hardware and software venders.
Disadvantages of a Tree Topology
 Overall length of each segment is limited by the type of cabling used.
 If the backbone line breaks, the entire segment goes down.
 More difficult to configure and wire than other topologies.

HYBRID TOPOLOGY
A combination of any two or more network topologies. Note 1: Instances can occur
where two basic network topologies, when connected together, can still retain the basic
network character, and therefore not be a hybrid network. For example, a tree network
connected to a tree network is still a tree network. Therefore, a hybrid network accrues
only when two basic networks are connected and the resulting network topology fails to
meet one of the basic topology definitions. For example, two star networks connected
together exhibit hybrid network topologies. Note 2: A hybrid topology always accrues
when two different basic network topologies are connected.

BASIC INTERNET TERMS
Below are basic internet terms or concepts useful for understanding what happens when
searching for information in the internet. Terms are in yellow and definitions are in black.

Domain abbreviations (last three characters in URL or address)

 edu - Educational institutions
 com - Commercial businesses
 gov - government
 mil - military
 org - organizations
 net - Network/Internet Service Providers

In a URL address, means an individual is responsible for the contents

Favorites
A menu option at the top of the screen that shows any websites that have been previously
saved for you to return to on a different day, much like a traditional bookmark is used for
books.

Go
A command in Explorer that sits on the top of the screen with other commands. In Go,
Explorer keeps track of most sites that a user has visited during a session of using
Explorer. This is not saved once you quit Explorer.

Homepage
Default setting, or the page that opens up when you start your Browser. A homepage can
also be the entry point for people viewing information that is provided.
Internet
Network of millions of computers used to send information back and forth to one
another. No one person or group decides what can and cannot be published on the
internet. This means that virtually anyone can publish, and means that there are large
number of sites that contain false and misleading information. It is important to check
who takes credit for the site you are viewing.

Links
Hypertext which, when clicked, connects you to another site or another page within the
same site. In most cases, these links are usually highlighted in blue and are always
underlined.

Online Databases
These are sites that provide access to mostly newspaper and magazine articles which are
not available through Google or any other search engine. The only way to see these
articles online is to pay a fee. Online databases are usually specialized and focus on
subjects like social studies or science.

Page
What is on your screen at any given time. The page includes all the information available
to you by scrolling up or down; however, when you click on hypertext your computer
will 'jump' to a new page.

Search Engines
Software that allows you to search the Web by typing in a topic of interest. Examples of
search engines we use are Google, HotBot, and Northern Light. These search engines
find exact matches from what has been typed in the search screen to either documents
(files) or subjects of files on the WWW.

URL or Uniform Resource Locator –
The address for a specific file on the web, as well as a method for directing users to a
specific file, at a specific site. To type in a different URL, press command and L at the
same time on a Macintosh computer, control and L on a PC, or locate the address bar at
the top of the screen, type in the URL, and then press return. The URL for the Blaine
School District Home page is: http://www.blaine.k12.wa.us.

Web Browser
To access the WWW, you need a Browser. A Web Browser is client software that gets
information from a server. It interprets the information, formats it, and displays it on your
computer screen. The most popular browser, and the one we use most often in Blaine, is
Internet Explorer.

WWW, World Wide Web or Web
Interactive collection of hypertext pages linked to one another. They may include text,
graphics and/or links to other spots in the Web. The Web is interactive because a user can
click on text or graphics to navigate (move) to more information or other graphics,
sounds and video that is of interest to them.

SOME MORE INTERNET TERMS

USENET
the name given to the computer network which carries newsgroups -
newsgroups are arranged in heirarchies based loosely on subject matter - the
USENET is often confused with the INTERNET in the news media - started by
two students at Duke University.
HTTPD - Hyper-text Transfer Protocol Daemon
a computer program which manages the transfer of hyper-text and multimedia
documents over the INTERNET.
HTML - Hyper Text Markup Language
the text markup language used to insert tags which allow a Web browser to
correctly display a hyper-text document. HTML1, HTML +, HTML 2 and
HTML 3 are versions of HTML in use at this time. HTML is a subset of the
Standard Generalized Markup Language (SGML) first invented to display legal
texts and now the world standard for large documentation projects.
VRML - Virtual Reality Modeling Language
a tagging language for conveying three dimensional information over the
Internet using a VRML browser.
Hypertext
documents which contain links to other documents within them - footnotes are
a form of hypertext link.
Multimedia
documents which contain text, sound, graphics and video elements that are all
capable of being displayed to the user.
Hypermedia
documents which combine hypertext links and multimedia elements.
NNTPD
the network news transfer protocol daemon which serves USENET newsgroups
across the Internet.
NNTP Server
the computer which you connect to to receive USENET newsgroups and post
USENET news articles.
DNS
the Domain Name System which identifies each computer as a network node on the
Internet using an internet protocol address system to translate from domain names to
IP numbers and vice-versa.
DNS Server
the computer you use to access the DNS to allow you to contact other
computers on the Internet.
FAQ
Frequently Asked Questions document which answers FAQs about various
subjects.
RFC - Request for Comments
a document which defines Internet operating protocols - despite the name it is
more a statement of agreed standards than a request.
Internet Access Provider (IAP)/Internet Service Provider (ISP)
an organization or commercial enterprise which provides access to the Internet.

ACCESS METHODS:
Dial-up
a computer connection that is brought up and brought down as needed - a
simple version is dialing in to a servive provider over a modem as a dumb
terminal - SLIP and PPP can also be dial-up connections - also known as a
switched connection.
Dumb Terminal
when a computer is running a terminal emulation program while connected to
another computer.
SLIP - Serial Line Internet Protocol
a non-standard method of electronically connecting a remote computer to an IP
network as a node on the network most popular method at present to use
graphical browsers on the Internet - first used to connect ham radios to the
Internet.


PPP - Point to Point Protocol
an Internet standard for electronically connectiong a remote computer to an IP
network - the method slowly replacing SLIP.
Ethernet
a method for directly connecting a computer to a network in the same physical
location - much faster connection than SLIP or PPP.
Wireless Network
a method using infra-red, ultra-violet or radio waves, of connecting computers
into a network.
ISDN - Integrated Services Digital Network
a digital telephone network that allows personal home computers to connect to
remote networks.
Modem
a device that modulates and demodulates telephone toned to allow for the
multiplexing of information on the telephone network.
Cable Modem
a device that allows a computer to connect to a cable television system and
connect to a computer network - cable modems work at speeds approaching
Ethernet connections - probable wide-spread future way to connect to the
Internet.

INTERNET TOOLS:
FTP - File Transfer Protocol
a method of serving and obtaining files over the Internet.
telnet
a method of logging into another computer as a terminal on that computer.


NFS - Network File System
a set of protocols that allow transparent access to a remote computers file
system - another type is the Andrew File System (AFS).
gopher
a method of serving or retrieving files over the Internet - it has largely been
replaced by the Web.
Browser
a client program that retrieves documents and other materials from an HTTPD
server and displays them in accord with the HTML specification - MOSAIC
was the first widely available browser - CELLO was the second browser -
NETSCAPE is the most used browser at this time.
WinSock
a specification for a program that implements a "socket" connection under the
MS Windows environment - a winsock is needed for SLIP or PPP connections
under WINDOWS.
MACTCP
a program used on Macintosh computers to provide socket connections for
SLIP and PPP connections.
EMail
a software application that allows one to exchange messages with someone
else.
IRC - Internet Relay Chat
a method for serving and retrieving connections for real-time conferencing over
computer networks.
WAIS - Wide Area Information Service
a method of finding information on the Internet.
archie
a system to find files available for retrieval by FTP.

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