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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
ALOHA is a system for coordinating and arbitrating access to a shared communication channel. It was developed in the 1970s at the University of Hawaii. The original system used terrestrial radio broadcasting, but the system has been implemented in satellite communication systems.
A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time. In the ALOHA system, a node transmits whenever data is available to send. If another node transmits at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcasts on the medium, even its own, and determine whether the frames were transmitted.
This technique is simple and elegant, but another method called slotted ALOHA was devised in 1972 to double the capacity. In the slotted scheme, distinct time slots are created in which systems can transmit a single frame. One node emits a signal at the start of each slot to let all other nodes know when the slot is available. By aligning frames on slots, overlaps in the transmissions are reduced. However, nodes must wait a fraction of a second for the beginning of a time slot before they can transmit. Also, data may be lost if nodes contend for the same slot. However, tests have shown that slotted ALOHA has a performance advantage.
One of the interesting things about ALOHA is that it inspired Robert Metcalfe in his design of Ethernet in 1972-1973 as discussed under "Ethernet."
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.