Duplex (telecommunications)

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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.)

Duplex systems are employed in nearly all communications networks, either to allow for a communication "two-way street" between two connected parties or to provide a "reverse path" for the monitoring and remote adjustment of equipment in the field.

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[edit] Half-Duplex

A simple illustration of a half-duplex communication system.
A simple illustration of a half-duplex communication system.

A half-duplex system provides for communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying.

An example of a half-duplex system is a two-party system such as a "walkie-talkie" style two-way radio, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit on the same frequency.

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 traffic controllers.

Note that this is one of two contradictory definitions for half-duplex. This definition matches the ITU-T standard. For more detail, see Simplex communication.

[edit] Full-Duplex

A simple illustration of a full-duplex communication system.
A simple illustration of a full-duplex communication system.

A full-duplex, or sometimes double-duplex system allows communication in both directions, and unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex since 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.

Examples: Telephone, Mobile Phone, etc.

Two way radios can be, for instance, designed as full-duplex systems, which transmit on one frequency and receive on a different frequency. This is also called frequency-division duplex. Frequency-division-duplex systems can be extended to farther distances using pairs of simple repeater stations, owing to the fact that the communications transmitted on any one frequency always travel in the same direction.

Full-duplex Ethernet connections work by making simultaneous use of all four physical pairs of twisted cable (which are inside the insulation), where two pairs are used for receiving packets and two pairs are used for sending packets, to a directly connected device. This effectively makes the cable itself a collision-free environment and theoretically doubles the maximum bandwidth that can be supported by the connection.

There are several benefits to using full duplex over half duplex. First, time is not wasted since no frames need to be retransmitted as there are no collisions. Secondly, full bandwidth is available in both directions because send and receive functions are separated. Third, stations/node do not have to wait until other operations complete their transmission as there is only one transmitter for each twisted pair.

[edit] Emulation of full duplex in shared physical media

Where 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:

[edit] Time division duplex

Time division duplex (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full duplex communication over a half duplex communication link. 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 dynamically 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.

Examples of TDD systems are:

[edit] Frequency division duplex

Frequency duplex means that the radio transmitter and receiver operates at different frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a repeater station. The station must be able to send and receive a transmission at the same time, and does so by altering the frequency at which it sends and receives slightly. This mode of operation is referred to as duplex mode or offset mode.

Uplink and downlink sub-bands are said to be separated by the "frequency offset". Frequency division duplex or frequency duplex is much more efficient in the case of symmetric traffic. In this case TDD tends to waste bandwidth during switch over 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. Conversely 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)

Examples of FDD systems are:

[edit] Echo cancellation

Echo cancellation can also implement full duplex communications over certain types of shared media. In this configuration, both devices send and receive over the same medium at the same time. When processing the signal it receives, a transceiver removes the "echo" of the signal it sent, leaving, in theory, the other transceiver's signal only.

Echo cancellation is at the heart of the V.32, V.34, V.56 and V.90 modem standards.

Echo cancellers are available as both software and hardware solutions. They can be independent components in a communications system or integrated into the communication system's central processing unit. Devices that do not eliminate echo in the system will not produce good full duplex performance.

[edit] Examples

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