 - Aloha, also called the Aloha method, refers toa simple communications scheme in which each source (transmitter) in a network sends data wheneverthere is a frame to send. If the framesuccessfully reaches the destination (receiver), the next frame is sent. If theframe fails to be received at the destination, it is sent again. This protocol wasoriginally developed at the University of Hawaii for use with satellite communication systems in the Pacific.In a wireless broadcast systemor a half-duplex two-way link, Aloha works perfectly. But as networks become morecomplex, for example in an Ethernet systeminvolving multiple sources and destinations that share a common data path,trouble occurs because data frames collide (conflict). The heavier thecommunications volume, the worse the collision problems become. The result isdegradation of system efficiency, because when two frames collide, the data contained inboth frames is lost. To minimize the number of collisions, thereby optimizing networkefficiency and increasing the number of subscribers that can use a given network, a schemecalled slotted Aloha was developed. This system employs signals called beacons that are sentat precise intervals and tell each source when the channel is clear to send aframe. Further improvement can be realized by a more sophisticated protocol called Carrier Sense Multiple Access withCollision Detection (CSMA/CD).
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06 Sep 2005
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