Token passing

In telecommunication, token passing is a channel access method where a signal called a token is passed around between nodes that authorizes the node to communicate.

Token passing schemes are a technique in which only the system which has the token can communicate. The token is a control mechanism which gives authority to the system to communicate or use the resources of that network. Once the communication is over, the token is passed to the next candidate in a sequential manner. The most well-known examples are token ring and ARCNET.

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 of this implicit method is the channel access method used during "Contention Free Time Slots" in the ITU-T G.hn standard for high-speed Local area networking using existing home wires (power lines, phone lines and coaxial cable).

Token passing schemes provide round-robin scheduling. If the packets are equally sized, the scheduling is max-min fair.

The advantage over contention based channel access is that collisions are eliminated, and that the channel bandwidth can be fully utilized without idle time when demand is heavy.

The disadvantage is that even when demand is light, a station wishing to transmit must wait for the token, increasing latency.

See also

• Token ring
• ARCNET
• Cambridge Ring

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