The following are the basic requirements for working of a communication system.
1. A sender (source), which creates the message to be transmitted.
2. A medium that carries the message.
3. A receiver (sink), which receives the message.
In data communication four basic terms are frequently used. They are

Data: A collection of facts in raw form that become information after processing.
Signals: Electric or electromagnetic encoding of data.
Signaling: Propagation of signals across a communication medium.
Transmission: Communication of data achieved by the processingof signals.