A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions, simultaneously. An example of a duplex device is a telephone. The people at both ends of a telephone call can speak at the same time, the earphone can reproduce the speech of the other person as the microphone transmits the speech of the local person, because there is a two-way communication channel between them.
Duplex systems are employed in many 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.
Systems that do not need the duplex capability use instead simplex communication in which one device transmits and the others just "listen." Examples are broadcast radio and television, garage door openers, baby monitors, wireless microphones, radio controlled models, surveillance cameras, and missile telemetry.
A half-duplex (HDX) system provides 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 (antennas are of trans-receiver type in these devices, so as to transmit and receive the signal as well).
An example of a half-duplex system is a two-party system such as a walkie-talkie, 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 and receive 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, regulated by the traffic controllers.
In automatically run communications systems, such as two-way data-links, the time allocations for communications in a half-duplex system can be firmly controlled by the hardware. Thus, there is no waste of the channel for switching. For example, station A on one end of the data link could be allowed to transmit for exactly one second, then station B on the other end could be allowed to transmit for exactly one second, and then the cycle repeats over and over again.
A full-duplex (FDX), 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, the transition from four to two wires being achieved by a Hybrid coil. A good analogy for a full-duplex system would be a two-lane road with one lane for each direction.
Two-way radios can be designed as full-duplex systems, transmitting on one frequency and receiving on another. This is also called frequency-division duplex. Frequency-division duplex systems can be extended to farther distances using pairs of simple repeater stations, because the communications transmitted on any one frequency always travel in the same direction.
Full-duplex Ethernet connections work by making simultaneous use of two physical pairs of twisted cable (which are inside the jacket), where one pair is used for receiving packets and one pair is used for sending packets (two pairs per direction for some types of Ethernet), to a directly connected device. This effectively makes the cable itself a collision-free environment and doubles the maximum data capacity that can be supported by the connection.
There are several benefits to using full-duplex over half-duplex. Firstly, time is not wasted, since no frames need to be retransmitted, as there are no collisions. Secondly, the full data capacity is available in both directions because the send and receive functions are separated. Thirdly, stations (or nodes) do not have to wait until others complete their transmission, since there is only one transmitter for each twisted pair.
Historically, some computer-based systems of the 1960s and 1970s required full-duplex facilities even for half-duplex operation, because their poll-and-response schemes could not tolerate the slight delays in reversing the direction of transmission in a half-duplex line.
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:
Time-division duplexing 
Time-division duplexing (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 duplexing has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction.
For radio systems that aren't moving quickly, another advantage is that the uplink and downlink radio paths are likely to be very similar. This means that techniques such as beamforming work well with TDD systems.
Examples of time-division duplexing systems are:
- UMTS 3G supplementary air interfaces TD-CDMA for indoor mobile telecommunications.
- The Chinese TD-LTE 4-G, TD-SCDMA 3-G mobile communications air interface.
- DECT wireless telephony
- Half-duplex packet mode networks based on carrier sense multiple access, for example 2-wire or hubbed Ethernet, Wireless local area networks and Bluetooth, can be considered as Time Division Duplexing systems, albeit not TDMA with fixed frame-lengths.
- IEEE 802.16 WiMAX
Frequency-division duplexing 
Frequency-division duplexing (FDD) means that the transmitter and receiver operate at different carrier 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 slightly altering the frequency at which it sends and receives. 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 duplexing can be efficient in the case of symmetric traffic. In this case time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry.
Another advantage of frequency-division duplexing 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 with each other. On the converse, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations, so that 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 Frequency Division Duplexing systems are:
- ADSL and VDSL
- Most cellular systems, including the UMTS/WCDMA use Frequency Division Duplexing mode and the cdma2000 system.
- IEEE 802.16 WiMax also uses Frequency Division Duplexing mode
Echo cancellation 
Full-duplex audio systems like telephones can create echo, which needs to be removed. Echo occurs when the sound coming out of the speaker, originating from the far end, re-enters the microphone and is sent back to the far end. The sound then reappears at the original source end, but delayed. This feedback path may be acoustic, through the air, or it may be mechanically coupled, for example in a telephone handset. Echo cancellation is a signal-processing operation that subtracts the far-end signal from the microphone signal before it is sent back over the network.
Echo cancelers are available as both software and hardware implementations. 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 sometimes will not produce good full-duplex performance.
See also 
- Duplex mismatch
- Four-wire circuit
- Communications channel
- Crossband operation
- Push to talk
- Simplex communication
- Radio resource management
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