Jan 18, 2013

DATA COMMUNICATION or DATA TRANSMISSION


DATA COMMUNICATION
Many organizations have multiple users of computers,some of these users are geographically remote from the organizations headquarter offices.Even within an office building,there may be hundreds or thousands of employees who use a particular computer.
Users have several options to choose from in communicating data
to and in receiving data from the computer.Data transmission may take place from a data terminal in one location to a data terminal in another location.In this case both data terminals can be off-line to the computer or data transmission may be transmitted on-line to a computer from a remote data terminal or from another computer.
Data  Communication

COMMUNICATION SERVICES
Normally an organization that wishes to transmit data users one of the common carrier services to carry the messages from station to station. Following is a brief description of these services.

1.Narrow Bands Services

Usually this service is used where data volume is relatively low,the transmission rates usually range from 45 to 300 bits per second.

2.Voice Band Services

This service use ordinary telephone lines to send data messages. Transmission rates vary from 300 to 4800 bits per second and higher.

3.Wide Band Services

Wide band services provide data transmission rates several thousands to several million bits per second.These services are limited to high volume users. Such services generally use coaxial cable or microwave communication.Space satellites, a more exotic development have been employed to rapidly transmit data from any part of the world to another part to the world. Communication services may be either leased or dialed up. A leased communication channel, which gives the user exclusive use of the channel is used where there are continuing data transmission needs. The dial up variety requires the person to dial the computer (and of course the line may be busy), thus this alternative is appropriate for periodic data to transmit.

Transmission Modes 

Data transmission may occur in one of three modes ......

A> Simplex

Data is transmitted in one direction only the sending station cannot receive data.

B>Half Duplex

Data is transmitted back and forth between two stations.But data can only go in one of the directions at one point in time.

C>Duplex

Data can be transmitted simultaneously between two stations.Both stations can send and receive data at any point in time.

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