A communication system transfers information from a source to a destination through a physical or logical channel.

A basic model is

source → transmitter → channel → receiver → destination

The transmitter converts the source information into a signal suited to the channel. The receiver observes the channel output and tries to recover the intended information.

A channel can alter the transmitted signal through several distinct mechanisms:

• attenuation changes amplitude or power;
• distortion changes different signal components unequally;
• delay shifts information in time;
• noise adds unwanted random variation;
• interference adds structured signals from other sources.

The same information can be represented by many different physical signals. Communication design therefore separates the message being conveyed from the waveform chosen to convey it.

Reliability is limited by both the channel and the representation: a receiver can only distinguish messages that remain sufficiently distinguishable after channel impairments.