28 August 2011

Data communication


           Data communication refers to the transmission of the digital signals over a communication channel between the transmitter and receiver.

            The maximum permissible transmission rate of a message is directly proportional to signal power and inversely proportional to channel noise. The aim of any communications system is to provide the highest possible transmission rate at the lowest possible power and with the least possible noise.

            Efficiency in data communication valued through three properties. They are i) Delivery ii) Accuracy iii) Timeliness.

Any communications channel has a direction associated with it:

The message source is the transmitter, and the destination is the receiver. 

Simplex channel :

            A channel whose direction of transmission is unchanging is referred to as a simplex channel.

            For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back.

Half-duplex channel :

            A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood.

Full-duplex channel :

            A full-duplex channel allows simultaneous message exchange in both directions. It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points. The transmission rate of the reverse channel may be slower if it is used only for flow control of the forward channel.