On alocal area network,token passing is achannel access method where a packet called atoken is passed between nodes to authorize that node to communicate.[1][2][3] In contrast topolling access methods, there is no pre-defined "master" node.[4] The most well-known examples areIBM Token Ring andARCNET, but there were a range of others, includingFDDI (Fiber Distributed Data Interface), which was popular in the early to mid 1990s.
Token passing schemes degrade deterministically under load, which is a key reason why they were popular for industrial control LANs such asMAP, (Manufacturing Automation Protocol).[5] The advantage overcontention based channel access (such as theCSMA/CD of early Ethernet), is that collisions are eliminated, and that the channelbandwidth can be fully utilized without idle time when demand is heavy.[6] The disadvantage is that even when demand is light, a station wishing to transmit must wait for the token, increasinglatency.
Some types of token passing schemes do not need to explicitly send a token between systems because the process of "passing the token" is implicit. An example is the channel access method used during "Contention Free Time Slots" in theITU-TG.hn standard for high-speedlocal area networking using existing home wires (power lines, phone lines and coaxial cable).[citation needed]
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