Random access systems: ALOHA´s progeny

Random access algorithms allow a large number of bursty data sources to share a common transmission medium. Such algorithms are essentially refinements of the distributed algorithm first introduced in the ALOHA system. Unlike techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and polling, the performance of random access algorithms is insensitive to the number of stations in the system. The weakness of early random access systems is extreme sensitivity to traffic, resulting in instability. Later implementations of the technique have improved throughput, eliminating instability. This paper discusses the three forms of random access techniques: stable backoff algorithms, tree search algorithms and carrier sensing techniques. Forms of the last of these have been used in an optical-fibre local area network. The particular application of random access techniques to satellite communication systems is also considered.