An interconnected collection of autonomous computers is called Computer Network. It means two computers are said to be interconnected if they are capable of exchanging the information.

Need for Networking Sharing of Hardware , Software

Resources and the database. Reliability To enable different computers to

communicate To reduce cost of transfer of

data

Disadvantages of Network

Protection from Viruses Central dependency on

software and files System becomes more

complex and sophisticated

Evolution of Networking 1. ARPANET : This was the first network which was planted in 1969. It means Advanced Research Projects Agency NETwork. Its aim was to connect different computers of different universities and U.S. defense to exchange messages and data. This network was expanded very soon by the students , scientists , engineers etc. for their work. The users were able to share their interests.

2. NSFnet : This was the network which was started in 1980 by National Science Foundation. It was a high capacity network than ARPANET which was more capable than ARPANET . It was used to do research work on the network and some kind of private business.

3. INTERNET : It is a worldwide network of Computers network. It was developed in 1990 by different companies to allow the users to connect one network to the other to communicate. It is a super network. It uses a set of protocols called TCP/IP. 4. INTERSPACE : It refers to a vision of what the internet will become in future. It is a client/server software program that allows multiple users to communicate online with real time audio , video and text in dynamic 3D environment.

Basic TerminologyNodes or Workstations

The different terminals or computers which are attached to a network and are able to share the resources are called nodes or workstations.

Server A Computer which facilitates the sharing of data , software and hardware resources like Printer , Modem etc. on the network is called a Server.

Network Interface Unit

It is an interpreter which is attached to server and all the workstations to maintain the connection between them. It is also called Terminal Access Point (TAP).

TYPES OF NETWORK

LOCAL AREA NETWORK (LAN )

METROPOLITAN AREA NETWORK (MAN)

WIDE AREA NETWORK (WAN )

LOCAL AREA NETWORK ( LAN ) It is a small computer network that are

confined to a localized area like an office , a building etc. Only a definite number of computers can be connected in this network to share the resources . Its range can be up to a few Kilometers.Its main features are

• Total diameter not more than 5 km• Data communication at the rate of 100 Mbps• Owned by single organization• Low Error Rates• Provides user access to several resources like printers, Modem etc.

Metropolitan Area Network ( MAN )

This network is larger in size than LAN and it spreads over a city. It means a very large number of computers can be connected in this in a city or from one city to the other.

Example : The Cable TV networks

which are spread over a city.

WIDE AREA NETWORK ( WAN ) This network spreads across the countries and

it is a group of computers that are separated by very large distances. The computers of even one continent can be connected to the other continent.

Its main features are :• Spans large geographical area, often a country

or continent • Helps in E-Mailing• Data can be received from or sent to remote

areas with the help of satellite with less cost

LAN WAN .

• Diameter of not more than a few kms.

• Total data rate of at least several Mbps

• Complete ownership be a single organization

• Very low error rates

• Span entire countries

• Data rate less than 1 Mbps

• Owned by multiple organizations

• Comparatively higher error rates.

MODEM( MOdulation/DEModulation )

The device which is used to connect and communicate with computers with the help of telephonic lines . It converts digital data into analog form and vice versa.

Working :A modem is a computer peripheral that allows to connect and communicate with the other computers through telephone lines. In MoDem , Mo stands Modulation and Dem stands for Demodulation . Modulation means to convert digital signals to analog signals and Demodulation is reverse of that. The data is transferred form source to the destination through telephone wires and they can only understand analog signals so there is need to convert digital signals to analog . And computer can only understand digital signals . This conversion is done by MoDem.

MODEM MODEM

Digital Signals

Analog signals Analog signals

Digital Signals

Source Computer Destination (Remote Computer)

WORKING OF MODEM

Types of MoDem ( 1 ) Internal MoDem : The MoDem which is connected inside the computer is called Internal MoDem.( 2 ) External MoDem : The MoDem which is connected externally to a computer is called External MoDem.

Network TopologiesThe way of interconnection of the nodes in a network is called the Topology.The different factors on which selection of a topology depends are : ( a ) Cost : A Network of computers can reduce the cost to a large extent.( b ) Reliability : It is more reliable to work in a Computer Network . Hence if a node fails to work the data can be achieved from some other nodes.( c ) Flexibility : The topology should allow for easy reconfiguration of the network.

The different topologies are :

Star Topology It is consisted of a central node to which all other nodes are

connected by a single path. Its different advantages and disadvantages are :

Advantages : ( 1 ) One Device per connection : In this topology failure of a

single connection typically involves disconnecting one node from an otherwise fully functional network.

( 2 ) Simple Access protocols : In this access protocols are very simple.

Disadvantages :( 1 ) Long Cable Length : This topology uses a very long cable as

each node id directly connected to the centre.( 2 ) Central Dependency : If the central node fails , the entire

network stops working.

WS1 WS3

WS4

Server

WS2

Bus or Linear Topology It is consisted of a single length of the transmission medium

onto which the various nodes are attached . Its different advantages and disadvantages are :

Advantages : ( 1 ) Short Cable Length : There is a single common data path

connecting all nodes , so this topology uses a very short cable .( 2 ) Easy to Expand : In this topology additional nodes can be

connected at any point along its length.Disadvantages :( 1 ) Fault detection : In this topology it is very difficult to detect

a faulty node .( 2 ) Nodes must be intelligent : All the nodes working in this

topology must be intelligent.

Server WS2

WS4

WS1

WS3 Printer

Ring or Circular Topology In this each node is connected to two and only two

neighbouring nodes. Data is transmitted from one to another node and is transmitted onwards to another. In this topology the data travels only in one direction.

Its different advantages and disadvantages are :Advantages : ( 1 ) Short Cable Length : There is very short cable which is

used to develop a Computer Network ( 2 ) Suitable for optical fibers : In this topology for very fast

speed transmission optical fibers can be used.Disadvantages :( 1 ) Node Failure causes network failure : In this topology if one

node fails to pass the data through itself , the entire network fails.

( 2 ) Network reconfiguration is difficult : It is not possible to shut down a small section of the ring .

C1 C3

C4

C2

C6 C5

Tree Topology A variation of bus topology is the tree topology. The shape of

the network is just like an inverted tree with the central root branching and sub-branching to the extremities of the network.

Advantages : ( 1 ) Easy to Expand : It is very easy expand this topology . It

means nodes can be connected easily.( 2 ) Fault isolation is easy : It is easy to find the faulty node in

this topology.Disadvantages :( 1 ) Dependency on root computer : The total dependence is on the root

computer . If it fails to work , the entire network will stop working.( 2 ) Complex Access Protocols : The protocols designed for this topology are

generally very complex.

WS1 WS3

WS6

Server

WS2 WS5WS4

Backbone Network• A backbone is a central interconnecting

structure which is used to connect two or more networks e.g. LANs together to form a WAN

e.g. FDDI – Fiber Distributed Data Interface

REPEATER

A repeater is a device that amplifies a signal being transmitted on a network for long distances.

It is used in long network lines, which exceed the maximum rated distance for a single run.

Bridge

A bridge is used to connect two LANs which are physically separated but logically same i.e. following the same topology.

Router

It is a device that links two networks which can handle different protocols. The routers use physical addresses. It improves the performance of the network.

Gateway

A gateway is a device that connects dissimilar networks. It gives an intelligent connection between a local network and an external network with different structures .

It is a special device which allows different networks to talk to internet that uses TCP/IP.

The 80-20 Rule80 percent of the traffic on a given network segment is local and 20 percent of the network traffic should need to move across a backbone networks (sub-network.)

How to Improve Performance of a LAN ?

1.Moving resources ( files etc. ) from one server to another server to contain traffic locally within a workgroup.2.Moving users logically within a workgroup.3.Adding servers to access them locally without crossing the backbone networks.

Network SecurityThe different Prevention methods are used while working in Network environment for security.

( 1 ) Authorization : It determines whether the ISP has granted the access to the web service or not. It is done by asking the user legal login-Id. ( 2 ) Authentication : It is a way to check the password protection of an authorised user. ( 3 ) Firewall : It is a system designed to prevent the unauthorised users to access to or from a private network. In this we can also encrypt the data.( 4 ) Encrypted Smart cards : It is smart hand-held card that can generate a token that a computer can recognise. Every time a new token is generated.( 5 ) Biometric System : It includes some unique aspect of a person’s body such as finger prints etc. To establish his identity.

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