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 don't need the duplex capability include broadcast systems, where one station transmits, and everyone else just "listens", and in some missile control systems, where the launcher just needs to command the missile where to go, and the launcher doesn't 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 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.
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 (or rarely 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.
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 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 more than 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. Secondly, 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.
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:
Examples of Time Division Duplexing systems are:
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 each other. Conversely, 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 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 their systems sometimes will not produce good full-duplex performance.
WIPO ASSIGNS PATENT TO ADC TELECOMMUNICATIONS FOR "RANGE EXTENSION FOR TIME DIVISION DUPLEX SYSTEMS" (AMERICAN INVENTORS)
Sep 13, 2010; GENEVA, Sept. 14 -- Publication No. WO/2010/101969 was published on Sept. 10. Title of the invention: "RANGE EXTENSION FOR TIME...
WIPO PUBLISHES PATENT OF INTERDIGITAL PATENT HOLDINGS FOR "METHOD AND APPARATUS FOR DYNAMIC BANDWIDTH PROVISIONING IN FREQUENCY DIVISION DUPLEX SYSTEMS" (AMERICAN INVENTORS)
Mar 26, 2012; GENEVA, March 26 -- Publication No. WO/2012/037236 was published on March 22. Title of the invention: "METHOD AND APPARATUS FOR...
Wipo Publishes Patent of Research in Motion Limited and Research in Motion Limited for "Phich Transmission in Time Divison Duplex Systems" (American, Canadian Inventors)
Nov 18, 2013; GENEVA, Nov. 18 -- Publication No. WO/2013/169287 was published on Nov. 14.Title of the invention: "PHICH TRANSMISSION IN TIME...
Wipo Publishes Patent of Research in Motion Limited and Research in Motion Limited for "Phich Resource Provisioning in Time Division Duplex Systems" (American, Canadian Inventors)
Nov 15, 2013; GENEVA, Nov. 15 -- Publication No. WO/2013/169288 was published on Nov. 14.Title of the invention: "PHICH RESOURCE PROVISIONING...
US Patent Issued to Intel on Dec. 13 for "Methods and Apparatus for Signal Echo Cancellation and Transmitter Calibration in Full Duplex Systems" (Israeli Inventor)
Dec 19, 2011; ALEXANDRIA, Va., Dec. 19 -- United States Patent no. 8,077,642, issued on Dec. 13, was assigned to Intel Corp. (Santa Clara,...