Channel

Old telephone wires are a challenging communications channel for modern digital communications.

Channel, in communications (sometimes called communications channel), refers to the medium used to convey information from a sender (or transmitter) to a receiver.

Overview

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:

1. A connection between initiating and terminating nodes of a circuit.
2. A buffer from which messages can be put and gotten. See Actor model and process calculi for discussion on the use of channels.
3. 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.
4. A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
5. 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.
6. In a communications system, the part that connects a data source to a data sink.
7. A specific radio frequency, pair or band of frequencies, usually named with a letter, number, or codeword, and often allocated by international agreement.
   Examples:
   • Marine VHF radio uses some 88 channels in the VHF band for two-way FM voice communication. Channel 16, for example, is 156.800MHz. In the US, seven additional channels, WX1 - WX7, are allocated for weather broadcasts.
   • Television channels such as North American TV Channel 2 = 55.25MHz, Channel 13 = 211.25MHz. Each channel is 6MHz wide. Besides these "physical channels", television also has "virtual channels".
   • Wi-Fi consists of unlicensed channels 1-13 from 2412MHz to 2484MHz in 5MHz steps.
8. A room in the Internet Relay Chat (IRC) network, in which participants can communicate with each other.

All of these communications channels share the property that they transfer information. The information is carried through the channel by a signal.

Channel models

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 discrete-alphabet 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 include:

• Noise model, for example Additive white Gaussian noise (AWGN)
• Distortion model, for example non-linear channel causing intermodulation distortion (IMD)
• Interference model, for example cross-talk (co-channel interference) and intersymbol interference (ISI)
• Modelling of underlying physical layer transmission techniques, for example an equivalent baseband model of modulation and frequency response
• Radio frequency propagation model
• Fading model, for example Rayleigh fading, Ricean fading, log-normal shadow fading and frequency selective (dispersive) fading
• Doppler shift model, which combined with fading results in a time-invariant system
• Mobility models, which also causes a time-invariant system

Types of communications channels

• Digital or analog channel
• Baseband and passband channel
• Transmission medium, for example a fibre channel
• Multiplexed channel
• Computer network Virtual channel
• Simplex communication, duplex communication or half duplex communication channel
• Return channel
• Uplink or downlink (upstream or downstream channel)
• Broadcast channel, unicast channel or multicast channel

See also

• Traffic generation model

References

• C. E. Shannon, A mathematical theory of communication, Bell System Technical Journal, vol. 27, pp. 379–423 and 623–656, (July and October, 1948)
• Amin Shokrollahi, LDPC Codes: An Introduction