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Acommunication channel refers either to a physicaltransmission medium such as a wire, or to alogical connection over amultiplexed medium such as a radio channel intelecommunications andcomputer networking. A channel is used forinformation transfer of, for example, a digitalbit stream, from one or severalsenders to one or severalreceivers. A channel has a certaincapacity for transmitting information, often measured by itsbandwidth inHz or itsdata rate inbits per second.
Communicating an informationsignal across distance requires some form of pathway or medium. These pathways, called communication channels, use two types of media:Transmission line-basedtelecommunications cable (e.g.twisted-pair,coaxial, andfiber-optic cable) andbroadcast (e.g.microwave,satellite,radio, andinfrared).
Ininformation theory, a channel refers to a theoreticalchannel model with certain error characteristics. In this more general view, astorage device is also a communication channel, which can be sent to (written) and received from (reading) and allows communication of an information signal across time.
Examples of communications channels include:
All of these communication channels share the property that they transfer information. The information is carried through the channel by asignal.
Mathematical models of the channel can be made to describe how the input (the transmitted signal) is mapped to the output (the received signal). There exist many types and uses of channel models specific to the field of communication. In particular, separate models are formulated to describe each layer of a communication system.
A channel can be modeled physically by trying to calculate the physical processes which modify the transmitted signal. For example, in wireless communications, the channel can be modeled by calculating the reflection from every object in the environment. A sequence of random numbers might also be added to simulate external interference or electronic noise in the receiver.
Statistically, a communication channel is usually modeled as atuple consisting of an input alphabet, an output alphabet, and for each pair(i, o) of input and output elements, a transition probabilityp(i, o). Semantically, the transition probability is the probability that thesymbolo is received given thati was transmitted over the channel.
Statistical and physical modeling can be combined. For example, in wireless communications the channel is often modeled by a random attenuation (known asfading) 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 or electronic noise in the receiver. If the attenuation term iscomplex it also describes the relative time a signal takes to get through the channel. The statistical properties of the attenuation in the model are determined by previous measurements or physical simulations.
Communication channels are also studied in discrete-alphabetmodulation schemes. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. Ininformation 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 or analog.
In a digital channel model, the transmitted message is modeled as adigital signal at a certainprotocol layer. Underlying protocol layers are replaced by a simplified model. The model may reflect channel performance measures such asbit rate,bit errors,delay,delay variation, etc. Examples of digital channel models include:
In an analog channel model, the transmitted message is modeled as ananalog signal. The model can be alinear ornon-linear,time-continuous or time-discrete (sampled),memoryless or dynamic (resulting inburst errors),time-invariant ortime-variant (also resulting in burst errors),baseband,passband (RF signal model),real-valued orcomplex-valued signal model. The model may reflect the following channel impairments:
These are examples of commonly usedchannel capacity and performance measures:
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In networks, as opposed topoint-to-point communication, the communication media can be shared between multiple communication endpoints (terminals). Depending on the type of communication, different terminals can cooperate or interfere with each other. In general, any complex multi-terminal network can be considered as a combination of simplified multi-terminal channels. The following channels are the principal multi-terminal channels first introduced in the field of information theory[citation needed]:
