Channel (communications)

In telecommunications and computer networking, a advice channel, or channel, refers either to a concrete manual average such as a wire, or to a analytic affiliation over a multiplexed average such as a radio channel. A approach is acclimated to back an advice signal, for archetype a agenda bit stream, from one or several senders (or transmitters) to one or several receivers. A approach has a assertive accommodation for transmitting information, generally abstinent by its bandwidth in Hz or its abstracts amount in $.25 per second

In advice theory, a approach refers to a abstract approach archetypal with assertive absurdity characteristics. In this added accepted view, a accumulator accessory is aswell a affectionate of channel, which can be beatific to (written) and accustomed from (read).

Examples

A approach can yield abounding forms. Examples of communications channels include:

A affiliation amid initiating and absolute nodes of a circuit.

A individual aisle provided by a manual average via either

concrete separation, such as by multipair cable or

electrical separation, such as by frequency-division or time-division multiplexing.

A aisle for carrying electrical or electromagnetic signals, usually acclaimed from added alongside paths.

A accumulator which can acquaint a bulletin over time as able-bodied as space

The allocation of a accumulator medium, such as a clue or band, that is attainable to a accustomed account or autograph base or head.

A absorber from which letters can be 'put' and 'got'. See Actor archetypal and action calculi for altercation on the use of channels.

In a communications system, the concrete or analytic hotlink that connects a abstracts antecedent to a abstracts sink.

A specific radio frequency, brace or bandage of frequencies, usually called with a letter, number, or codeword, and generally allocated by all-embracing agreement.

Examples:

Marine VHF radio uses some 88 channels in the VHF bandage for two-way FM articulation communication. Approach 16, for example, is 156.800 MHz. In the US, seven added channels, WX1 - WX7, are allocated for acclimate broadcasts.

Television channels such as North American TV Approach 2 = 55.25 MHz, Approach 13 = 211.25 MHz. Anniversary approach is 6 MHz wide. Besides these "physical channels", television aswell has "virtual channels".

Wi-Fi consists of actionable channels 1-13 from 2412 MHz to 2484 MHz in 5 MHz steps.

The radio approach amid an abecedarian radio captive and a ham uses two bands generally 600 kHz (0.6 MHz) apart. For example, a captive that transmits on 146.94 MHz about listens for a ham transmitting on 146.34 MHz.

A allowance in the Internet Relay Chat (IRC) network, in which participants can acquaint with anniversary other.

All of these communications channels allotment the acreage that they alteration information. The advice is agitated through the approach by a signal.

Channel models

A approach can be modelled physically by aggravating to account the concrete processes which adapt the transmitted signal. For archetype in wireless communications the approach can be modelled by artful the absorption off every article in the environment. A arrangement of accidental numbers ability aswell be added in to simulate alien arrest and/or cyberbanking babble in the receiver.

Statistically a advice approach is usually modelled as a amateur consisting of an ascribe alphabet, an achievement alphabet, and for anniversary brace (i, o) of ascribe and achievement elements a alteration anticipation p(i, o). Semantically, the alteration anticipation is the anticipation that the attribute o is accustomed accustomed that i was transmitted over the channel.

Statistical and concrete modelling can be combined. For archetype in wireless communications the approach is generally modelled by a accidental abrasion (known as fading) of the transmitted signal, followed by accretion noise. The abrasion appellation is a description of the basal concrete processes and captures the change in arresting ability over the advance of the transmission. The babble in the archetypal captures alien arrest and/or cyberbanking babble in the receiver. If the abrasion appellation is circuitous it aswell describes the about time a arresting takes to get through the channel. The statistics of the accidental abrasion are absitively by antecedent abstracts or concrete simulations.

Channel models may be connected approach models in that there is no absolute to how absolutely their ethics may be defined.

Communication channels are aswell advised in a discrete-alphabet setting. This corresponds to abstracting a absolute apple advice arrangement in which the analog->digital and digital->analog blocks are out of the ascendancy of the designer. The algebraic archetypal consists of a alteration anticipation that specifies an achievement administration for anniversary accessible arrangement of approach inputs. In advice theory, it is accepted to alpha with memoryless channels in which the achievement anticipation administration alone depends on the accepted approach input.

A approach archetypal may either be agenda (quantified, e.g. binary) or analog.

edit Agenda approach models

In a agenda approach model, the transmitted bulletin is modelled as a agenda arresting at a assertive agreement layer. Basal agreement layers, such as the concrete band manual technique, is replaced by a simplified model. The archetypal may reflect approach achievement measures such as bit rate, bit errors, latency/delay, adjournment jitter, etc. Examples of agenda approach models are:

Bifold symmetric approach (BSC), a detached memoryless approach with a assertive bit absurdity probability

Bifold bursty bit absurdity approach model, a approach "with memory"

Bifold abandoning approach (BEC), a detached approach with a assertive bit absurdity apprehension (erasure) probability

Packet abandoning channel, area packets are absent with a assertive packet accident anticipation or packet absurdity rate

Arbitrarily capricious approach (AVC), area the behavior and accompaniment of the approach can change randomly

edit Analog approach models

In an analog approach model, the transmitted bulletin is modelled as an analog signal. The archetypal can be a beeline or non-linear, time-continuous or time-discrete (sampled), memoryless or activating (resulting in access errors), time-invariant or time-variant (also consistent in access errors), baseband, passband (RF arresting model), real-valued or complex-valued arresting model. The archetypal may reflect the afterward approach impairments:

Babble model, for example

Accretion white Gaussian babble (AWGN) channel, a beeline connected memoryless model

Phase babble model

Arrest model, for archetype cross-talk (co-channel interference) and intersymbol arrest (ISI)

Baloney model, for archetype a non-linear approach archetypal causing intermodulation baloney (IMD)

Abundance acknowledgment model, including abrasion and phase-shift

Group adjournment model

Modelling of basal concrete band manual techniques, for archetype a complex-valued agnate baseband archetypal of accentuation and abundance response

Radio abundance advancement model, for example

Log-distance aisle accident model

Crumbling model, for archetype Rayleigh fading, Ricean fading, log-normal adumbration crumbling and abundance careful (dispersive) fading

Doppler about-face model, which accumulated with crumbling after-effects in a time-variant system

Ray archetype models, which attack to archetypal the arresting advancement and distortions for defined transmitter-receiver geometries, area types, and antennas

Mobility models, which aswell causes a time-variant system

Types of communications channels

Digital (discrete) or analog (continuous) channel

Baseband and passband channel

Transmission medium, for archetype a fibre channel

Multiplexed channel

Computer arrangement basic channel

Simplex communication, bifold advice or bisected bifold advice channel

Return channel

Uplink or downlink (upstream or after channel)

Broadcast channel, unicast approach or multicast channel

Multi-terminal channels, with application to cellular systems

In networks, as against to point-to-point communication, the advice media is aggregate amid assorted nodes (terminals). Depending on the blazon of communication, altered terminals can abet or baffle on anniversary other. In general, any circuitous multi-terminal arrangement can be advised as a aggregate of simplified multi-terminal channels. The afterward channels are the arch multi-terminal channels which was aboriginal alien in the acreage of advice theorycitation needed:

A point-to-multipoint channel, aswell accepted as broadcasting average (not to be abashed with broadcasting channel): In this channel, a individual sender transmits assorted letters to altered destination nodes. All wireless channels except radio links can be advised as broadcasting media, but may not consistently accommodate broadcasting service. The downlink of a cellular arrangement can be advised as a point-to-multipoint channel, if alone one corpuscle is advised and inter-cell co-channel arrest is neglected. However, the advice account of a buzz alarm is unicasting.

Assorted admission channel: In this channel, assorted senders address assorted accessible altered letters over a aggregate concrete average to one or several destination nodes. This requires a approach admission scheme, including a media admission ascendancy (MAC) agreement combiend with a multiplexing scheme. This approach archetypal has applications in the uplink of the cellular networks.

Advertisement channel: In this channel, one or several average nodes (called relay, captive or gap accompaniment nodes) abet with a sender to forward the bulletin to an ultimate destination node. Advertisement nodes are advised as a accessible add-on in the accessible cellular standards like 3GPP Long Term Evolution (LTE).

Arrest channel: In this channel, two altered senders address their abstracts to altered destination nodes. Hence, the altered senders can accept a accessible cross-talk or co-channel arrest on the arresting of anniversary other. The inter-cell arrest in the cellular wireless communications is an archetype of the arrest channel. In advance spectrum systems like 3G, arrest aswell action central the corpuscle if non-orthogonal codes are used.

A unicasting approach is a approach that provides a unicasting service, i.e. that sends abstracts addressed to one specific user. An accustomed buzz alarm is an example.

A broadcasting approach is a approach that provides a broadcasting service, i.e. that sends abstracts addressed to all users in the network. Cellular arrangement examples are the paging account as able-bodied as the Multimedia Advertisement Multicast Service.

A multicasting approach is a approach area abstracts is addressed to a accumulation of subscribing users. LTE examples are the Concrete Multicast Approach (PMCH) and MBSFN (Multicast Advertisement Individual Frequency Network).

From the aloft 4 basal multi-terminal channels, assorted admission approach is the alone one whose accommodation arena is known. Even for the appropriate case of the Gaussian scenario, the accommodation arena of the added 3 channels except the advertisement approach is alien in general.