| Frequency Division Duplex |
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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. __NOTOC__ Half-duplex A ''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-duplex A ''full-duplex'' system allows communication in both directions, and unlike ''half-duplex'' allows this to happen simultaneously. Most telephone networks are ''full duplex'' as they allow both callers to speak 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 Card s, however almost all are now full-duplex. Usage in point-to-multipoint networks Where Channel Access Method 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 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) EXAMPLES
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