A Communications channel, models the medium through which information is transmitted from a sender (or transmitter) to a receiver. For example, in wireless communications, the channel is often modelled by a random attennuation of the transmitted signal, followed by additive noise. The attennuation captures the loss in signal power over the course of the transmission, and the noise in the model captures external interference and/or electronic noise in the receiver. Hence, depending on the application, the mathematical model for the communication system includes a model for the distortion introduced by the tranmission medium, and termed the communication channel, or channel for short.

Types of Communications channels

• Simplex Communication
  A simplex communication system is one where all signals flow in one direction. These systems are often employed in broadcast networks, where the receivers do not need to send any data back to the transmitter/broadcaster.
• Duplex Communication
  A duplex communication system is one where signal can flow in both directions between connected parties. These systems are employed in nearly all communications networks, either to allow for a "two-way street" between connected parties or to provide a "reverse path" for the monitoring and remote adjustment of equipment in the field.

Communications channel

• a connection between initiating and terminating nodes of a circuit.
• a single path provided by a transmission medium via either
• physical separation, such as by multipair cable or
• electrical separation, such as by frequency- or time-division multiplexing.
• a path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
• the portion of a storage medium, such as a track or a band, that is accessible to a given reading or writing station or head.
• in a communications system, the part that connects a data source to a data sink.
• a specific radio frequency or band of frequencies, usually in conjunction with a predetermined letter, number, or codeword, and allocated by international agreement. For example 802.11b consists of unlicensed channels 1-13 from 2412MHz to 2484MHz in 5MHz steps.
• in particular, a television channel. Examples include North American TV Channel 2 = 55.25MHz, Channel 13 = 211.25MHz