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Information about Channel (communications)

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


This article is about channels specific to communications. For other uses, see channel.


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

Overview

A Channel can take many forms. 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
  4. * physical separation, such as by multipair cable or
  5. * electrical separation, such as by frequency- or time-division multiplexing.
  6. A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
  7. 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.
  8. In a communications system, the part that connects a data source to a data sink.
  9. A specific radio frequency, pair or band of frequencies, usually named with a letter, number, or codeword, and often allocated by international agreement.
    Examples:
  10. * Marine VHF radio uses some 88 channels in the VHF band for two-way FM voice communication. Channel 16, for example, is 156.800MHz. Seven additional channels, WX1 - WX7, are allocated for weather broadcasts.
  11. * Television channels such as North American TV Channel 2 = 55.25MHz, Channel 13 = 211.25MHz. Each channel is 6MHz wide.
  12. * Wi-Fi consists of unlicensed channels 1-13 from 2412MHz to 2484MHz in 5MHz steps.
  13. 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.

Types of communications channels

See also

References

A channel is a narrow, deep waterway connecting two bodies of water; the deepest part of a river or waterway. The difference between a Canal and a Channel is that a canal is man-made, whilst the channel is not, as it is often a divide between two body of land.
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In telecommunications, information transfer is the process of moving messages containing user information from a source to a sink.

Note: The information transfer rate may or may not be equal to the transmission modulation rate.
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Information is the result of processing, gathering, manipulating and organizing data in a way that adds to the knowledge of the receiver. In other words, it is the context in which data is taken.
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A sender was a circuit in a 20th century electromechanical telephone exchange which sent telephone numbers and other information to another exchange. In some American exchange designs, for example 1XB switch the same term was also used to refer to the circuit that received this
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The receiver in information theory is the receiving end of a communication channel. It receives decoded messages/information from the sender, who first encoded them. Sometimes the receiver is modeled so as to include the decoder.
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A telecommunication circuit is defined as follows:
  1. The complete path between two terminals over which one-way or two-way communications may be provided. See communications protocol.
  2. An electronic path between two or more points, capable of providing a number of channels.

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In computer science, the Actor model and process calculi are two closely related approaches to the modelling of concurrent digital computation. See Actor model and process calculi history.
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A path or pathway is a line representing the course or route of actual, potential or abstract movement (change of location over time).

Path, PATH or pathway may refer to:

In natural and built environments:

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A transmission medium (plural transmission media) is a material substance (solid, liquid or gas) which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media
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cable is one or more wires or optical fibers bound together, typically in a common protective jacket or sheath. The individual wires or fibers inside the jacket may be covered or insulated. Combination cables may contain both electrical wires and optical fibers.
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Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
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Time-Division Multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel.
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Storage may refer to:
  • Special buildings, or collections of buildings, designed to hold large objects, or a great many objects of a particular type:

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In telecommunication, a communications system is a collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment (DTE) usually capable of interconnection and interoperation to form an integrated whole.
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For other uses, see Data (disambiguation).


Debt, AIDS, Trade in Africa (or DATA) is a multinational non-government organization founded in January 2002 in London by U2's Bono along with Bobby Shriver and activists from the Jubilee 2000 Drop
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In Unix-like operating systems, /dev/null or the null device is a special file that discards all data written to it (but reports that the write operation succeeded), and provides no data to any process that reads from it (it returns EOF).
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Radio frequency, or RF, is a frequency or rate of oscillation within the range of about 3 Hz and 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves.
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Marine VHF radio is installed on all large ships and most motorized small craft. It is used for a wide variety of purposes, including summoning rescue services and communicating with harbours and marinas, and operates in the VHF frequency range, between 156 to 174 MHz.
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The term television channel generally refers to either a television station or its cable/satellite counterpart (both outlined below). Sometimes, it is confused with the term television network, which (when properly used) describes a group of geographically-distributed television
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Wi-Fi (pronounced wye-fye, IPA: /ˈwaɪfaɪ/), also unofficially known as Wireless Fidelity
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Internet Relay Chat (IRC) is a form of real-time Internet chat or synchronous conferencing. It is mainly designed for group (many-to-many) communication in discussion forums called channels, but also allows one-to-one communication and data transfers via private message.
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In telecommunication, signalling (UK spelling) or signaling (US spelling) has the following meanings:
  • The use of signals for controlling communications.

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Symbols are objects, characters, or other concrete representations of ideas, concepts, or other abstractions. For example, in the United States, Canada and Great Britain, a red octagon is a symbol for the traffic sign meaning "STOP".
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Fading refers to the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media. A fading channel is a communication channel that experiences fading.
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Information theory is a branch of applied mathematics and engineering involving the quantification of information to find fundamental limits on compressing and reliably communicating data.
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Simplex communication is a name for a type of communication circuit. There are two (contradictory) definitions that have been used for the term. In both cases, the other definition is referred to as half duplex.
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A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. (The term duplex is not used when describing communication between more than two parties or devices.
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In telecommunications

A back-channel (also reverse channel or return channel) is typically a low-speed, or less-than-optimal, transmission channel in the opposite direction to the main channel.
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Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0.
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A binary symmetric channel (or BSC) is a common communications channel model used in coding theory and information theory. In this model, a transmitter wishes to send a bit (a zero or a one), and the receiver receives a bit.
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