A Computer Network is a network of geographically distributed multiple computers connected in a manner to enable meaningful transmission and exchange of information among them.Network is a set of devices (nodes) connected through links.
Objectives of Computer Networks

–Sharing of information, sharing of resources ( hardware and software), and sharing of processing load are some of the major objectives of a computer network. 

Applications of Networks
Resource Sharing
Hardware (computing resources, disks, printers) 
Software (application software) 

Information Sharing 
Easy accessibility from anywhere (files, databases) 
Search Capability (WWW) 

Message broadcast 

Remote computing 
Distributed processing (GRID Computing)

Basic Elements of a Communication System
Communication is the process of transferring messages from one point to another.
Three basic elements of any communication system are
–A sender, which creates the message to be transmitted.
–A medium, which creates a message.
–A receiver, which receives a message.

Data Transmission Modes

There are three ways, or modes, of transmitting data from one point to another.

–Half duplex
–Full duplex


–In this Transmission mode, communication takes place in only one direction.
–Simplex circuits are seldom used because a return path is generally needed to send acknowledgement, error signals.
•Printer (receive only mode)

Half Duplex 
–In this Transmission mode, system can transmit data in both
directions, but only in one direction at a time. Hence, a half duplex line can alternately send and receive data. 
 Full Duplex 
–Full Duplex system allows information to flow simultaneously in both directions on a transmission path.


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