Channel, in communications (sometimes called communications channel), refers to the medium used to convey information from a sender (or transmitter) to a receiver.
Contents 
A channel can take many forms, including ones suitable for storage which can communicate a message over time as well as space. Examples of communications channels include:
All of these communications channels share the property that they transfer information. The information is carried through the channel by a signal.
A channel can be modelled physically by trying to calculate the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.
Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p(i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel.
Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations.
Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined.
Communication channels are also studied in a discretealphabet setting. This corresponds to abstracting a real world communication system in which the analog>digital and digital>analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input.
A channel model may either be digital (quantified, e.g. binary) or analog.
In a digital channel model, the transmitted message is modelled as a digital signal at a certain protocol layer. Underlaying protocol layers, such as the physical layer transmission technique, is replaced by a simplified model. The model may reflect channel performance measures such as bit rate, bit errors, latency/delay, delay jitter, etcera. Examples of digital channel models are:
In an analog channel model, the transmitted message is modelled as an analog signal. The model can be a linear or nonlinear, timecontinuous or timediscrete (sampled), memoryless or dynamic (resulting in burst errors), timeinvariant or timevariant (also resulting in burst errors), baseband, passband (RF signal model), realvalued or complexvalued signal model. The model may reflect the following channel impairments:
