When we communicate we are sharing information. This sharing can be local or global. Between individuals, local communication usually occurs face to face, while global communication take place over distance. The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance (tele is Greek for "far").

Data commucation implies the transmission of information between two devices through a transmission medium. Data communication is considered local if the communicating devices are in the same building or a similarly restricted geographical area, and is considered global if the devices are farther apart.

A data communication system may be represented by way of a simple conceptual model as shown in fig .......

Simplified Data Communication Model

The effectiveness of a data communication system depends on the three fundamental characteristics:

Delivery. The system must deliver data to the correct destination Data must be received by the intended device.

Accuracy. The system must deliver the data accurately. Data that has been altered during transmission and left uncorrected is unusable.

Timeliness. The system must deliver data in a timely manner. Data delivered late is useless. In the case of video and audio, timely delivery means delivering data as it is produced, in the same order, that it is produced, and without significant delay. This kind of delivery is known as real time transmission.