.gif)
Duplex (telecommunications)
Encyclopedia
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing
is used when describing communication between more than two parties or devices.
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
. These include broadcast
systems, where one station transmits and the others just "listen", and some missile guidance
systems, where the launcher needs only to command the missile where to go, and the launcher does not need to receive any information from the missile. Also, there are spacecraft such as satellites and space probes that have lost their capability to receive any commands, but they can continue to transmit radio signals through their antennas. Some early satellites (such as Sputnik 1) were designed as transmit-only spacecraft. Pioneer 6 has transmitted for decades without being able to receive anything.
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" 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 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, and then station B on the other end could be allowed to transmit for exactly one second. And then this 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. 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, 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), wherein 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. First, time is not wasted, since no frames need to be retransmitted, as there are no collisions. Second, the full data capacity is available in both directions because the send and receive functions are separated. Third, 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.
s are used in point-to-multipoint networks such as cellular network
s 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 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 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:
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:
Echo cancellation is at the heart of the V.32, V.34, V.56, and V.90 modem standards.
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.
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
is used when describing communication between more than two parties or devices.
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
Simplex communication
Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
. These include broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
systems, where one station transmits and the others just "listen", and some missile guidance
Missile guidance
Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness...
systems, where the launcher needs only to command the missile where to go, and the launcher does not need to receive any information from the missile. Also, there are spacecraft such as satellites and space probes that have lost their capability to receive any commands, but they can continue to transmit radio signals through their antennas. Some early satellites (such as Sputnik 1) were designed as transmit-only spacecraft. Pioneer 6 has transmitted for decades without being able to receive anything.
Half-duplex
- Note that this is one of two contradictory definitions for half-duplex. This definition matches the ANSIAnsiAnsi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
standard. The ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
standard uses the word simplex to refer to the definition presented here. For more detail, see Simplex communicationSimplex communicationSimplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
.
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 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, and then station B on the other end could be allowed to transmit for exactly one second. And then this cycle repeats over and over again.
Full-duplex
Examples: Telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
, Mobile Phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
, 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, 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), wherein 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. First, time is not wasted, since no frames need to be retransmitted, as there are no collisions. Second, the full data capacity is available in both directions because the send and receive functions are separated. Third, 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.
Emulation of full-duplex over a single communications link
Where channel access methodChannel access method
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...
s are used in point-to-multipoint networks such as cellular network
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
s 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 Duplex (TDD) is the application of time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
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 there is asymmetry
Asymmetry
Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....
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
Beamforming
Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
work well with TDD systems.
Examples of Time Division Duplexing systems are:
- UMTS 3G supplementary air interfaces TD-CDMATD-CDMATD-CDMA, an acronym for Time-division - CDMA, is a channel access method based on using spread spectrum across multiple time slots. It is shown that a mixture of TDMA and CDMA provides better quality of service for multimedia communications in terms of data throughput and voice/video...
for indoor mobile telecommunications. - The Chinese TD-LTE 4-G, TD-SCDMATD-SCDMATime Division Synchronous Code Division Multiple Access or UTRA/UMTS-TDD 1.28 Mcps Low Chip Rate , is an air interface found in UMTS mobile telecommunications networks in China as an alternative to W-CDMA. Together with TD-CDMA, it is also known as UMTS-TDD or IMT 2000 Time-Division .The term...
3-G mobile communications air interface. - DECT wireless telephony
- Half-duplex packet mode networks based on carrier sense multiple accessCarrier Sense Multiple AccessCarrier Sense Multiple Access is a probabilistic Media Access Control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a...
, for example 2-wire or hubbed EthernetEthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, Wireless local area networks and BluetoothBluetoothBluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
, can be considered as Time Division Duplex systems, albeit not TDMA with fixed frame-lengths. - IEEE 802.16IEEE 802.16IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers . The IEEE Standards Board in established a working group in 1999 to develop standards for broadband Wireless Metropolitan Area Networks...
WiMAXWiMAXWiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations... - PACTORPACTORPACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. A robust network of PACTOR stations has been established to relay data between radio stations and the Internet,...
Frequency-Division Duplexing
Frequency-division duplexing (FDD) means that the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
operate at different carrier frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. The term is frequently used in ham radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operation, where an operator is attempting to contact a repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
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
Lag
Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...
, 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
Spectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
) 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 Frequency Division Duplexing mode and the cdma2000CDMA2000CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
system. - IEEE 802.16IEEE 802.16IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers . The IEEE Standards Board in established a working group in 1999 to develop standards for broadband Wireless Metropolitan Area Networks...
WiMaxWiMAXWiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
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 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 cancellation is at the heart of the V.32, V.34, V.56, and V.90 modem standards.
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
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
. Devices that do not eliminate echo sometimes will not produce good full-duplex performance.
Examples
- Telephone networkTelephone networkA telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...
s - Mobile phoneMobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
networks - CBCitizens' band radioCitizens' Band radio is, in many countries, a system of short-distance radio communications between individuals on a selection of 40 channels within the 27-MHz band. Citizens' Band is distinct from the FRS, GMRS, MURS and amateur radio...
radio
See also
- Duplex mismatchDuplex mismatchIn Ethernet, a duplex mismatch is a condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex. The effect of a duplex mismatch is a network that works but is often much slower than its nominal speed...
- Four-wire circuitFour-wire circuitIn telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path...
- MultiplexingMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
- Duplexer
- Communications channel
- Push to talkPush to talkPush-to-talk , also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode....
- Simplex communicationSimplex communicationSimplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
- Radio resource managementRadio resource managementRadio resource management is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless networks and broadcasting systems...