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Communications Channel Channel, in communications (sometimes called communications channel), refers to the medium used to convey information from a sender (or transmitter) to a receiver.
A Channel can take many forms. Examples of communications channels include:
All of these communications channels share the property that they transfer information. The information is carried though the channel by a signal.
Types of communications channelsSimplex communicationSimplex 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.
One way at a timeAccording to the ITU-T definition, a simplex circuit is one where all signals can flow in only one direction at a time. This was also the way Western Union used the term when describing the duplex and simplex capacity of their new transatlantic telegraph cable completed between Newfoundland and the Azores in 1928[1]. The same definition for a simplex radio channel was used by the National Fire Protection Association in 2002[2].
One way onlyAccording to the ANSI definition, a simplex circuit is one where all signals can flow in only one direction. These systems are often employed in broadcast networks, where the receivers do not need to send any data back to the transmitter/broadcaster.
Examples according to ANSI definition
Duplex communicationA duplex communication system is one where signal can flow in both directions between connected parties.
These systems are employed in nearly all communications networks, either to allow for a "two-way street" between connected parties or to provide a "reverse path" for the monitoring and remote adjustment of equipment in the field.
Half-duplexA half-duplex system allows communications in both directions, but only one direction at a time (not simultaneously). Any radio system where you must use "Over" to indicate the end of transmission, or any other procedure to ensure that only one party broadcasts at a time would be a half-duplex system.
A good analogy for a half-duplex system would be a one lane road with traffic controllers at each end. Traffic can flow in both directions, but only one direction at a time with this being regulated by the controllers. Another analogy is a walkie-talkie format as mentioned above to be "Over".
Full-duplexA full-duplex system allows communication in both directions, and unlike half-duplex allows this to happen simultaneously. All wire telephone networks are full duplex as they allow both callers to speak and be heard at the same time. A good analogy for a full-duplex system would be a two lane road with one lane for each direction.
Example: Telephone, Mobile Phone, etc.
These terms also apply to early PC sound cards, however almost all are now full-duplex. Two way radios are either simplex or duplex. A simplex service is one where you send and receive (ie listen and talk) on the same frequency. A duplex service is where you send (talk) on one frequency but receive (listen) on a different frequency.
A duplex service has two capabilities that a simplex service does not. Firstly, a duplex service can be used so that you can simultaneously talk and listen at the same time - just like on a telephone or cell phone, which are duplex services.
Secondly, a duplex service can be used with a repeater station. A repeater station is usually situated somewhere with good coverage, and has an excellent aerial system, sensitive receiver and high power transmitter. It rebroadcasts signals that it receives, but on a different frequency. The repeater gives much longer range coverage.
Usage in point-to-multipoint networksWhere channel access methods are used in point to multipoint networks such as cellular networks for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as:
Time division duplex (TDD) is the application of time-division multiple access to separate outward and return signals. Time division duplex has a strong advantage in the case where the asymmetry of the uplink and downlink data speed is variable. As the amount of uplink data increases, more bandwidth can be allocated to that and as it shrinks it can be taken away. Another advantage is that the uplink and downlink radio paths are likely to be very similar in the case of a slow moving system. This means that techniques such as beamforming work well with TDD systems.
Frequency division duplex (FDD) is the application of frequency-division multiple access to separate outward and return signals. The uplink and downlink sub-bands are said to be separated by the "frequency offset". Frequency division duplex is much more efficient in the case of symmetric traffic. In this case TDD tends to waste bandwidth during switchover from transmit to receive, has greater inherent latency, and may require more complex, more power-hungry circuitry.
Another advantage of FDD is that it makes radio planning easier and more efficient since base stations do not "hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere each other. With TDD systems, care must be taken to keep guard bands between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations so they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio)
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Last modified at : Thursday, December 11st 2008 14:03:02.
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