Channels of communications

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

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example, a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

Communicating data from one location to another requires some form of pathway or medium. These pathways, called communication channels, use two types of media: cable (twisted-pair wire, cable, and fiber-optic cable) and broadcast (microwave, satellite, radio, and infrared). Cable or wire line media use physical wires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and fiber-optic cable is made of glass.

In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is also a kind of channel, which can be sent to (written) and received from (read).

A channel can take many forms. Examples of communications channels include:

1) A connection between initiating and terminating nodes of a circuit.

2) A single path provided by a transmission medium via either physical

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

3) A path for conveying electrical or electromagnetic signals, are usually

distinguished from other parallel paths. It includes:

-A storage that can communicate a message over time as well as space.

-The portion of a storage medium, such as a track or band, that is accessible to a given reading or writing station or head.

-A buffer from which messages can be 'put' and 'got'.

In a communication system, the physical or logical link connects a data source to a data sink.

4) A specific radio frequency, pair or band of frequencies, are usually named

with a letter, number, or code word, and often allocated by international agreement.
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 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 either be digital (quantified, e.g. binary) or analog.

Do the following tasks:

1) Read and translate the text.

2) Write a summary.

3) Speak on the forms, which channels of communication can take.

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