Token passing

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On a local area network, token passing is a channel access method where a packet called a token is passed between nodes to authorize that node to communicate. [1] [2] [3] In contrast to polling access methods, there is no pre-defined "master" node. [4] The most well-known examples are IBM Token Ring and ARCNET, but there were a range of others, including FDDI (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 as MAP, (Manufacturing Automation Protocol). [5] The advantage over contention based channel access (such as the CSMA/CD of early Ethernet), is that collisions are eliminated, and that the channel bandwidth 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, increasing latency.

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 the ITU-T standard for high-speed local area networking using existing home wires (power lines, phone lines and coaxial cable).[ citation needed ]

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  1. Sheldon, Tom. "Token and Token-Passing Access Methods". Linktionary. Retrieved 8 July 2017.
  2. Littlejohn Shinder, Debra (2001). Computer Networking Essentials. Cisco Press. p. 123. ISBN   978-1587130380 . Retrieved 8 July 2017.
  3. Savage, Stefan. "Lecture 6: Media Access Control" (PDF). Computer Science and Engineering. UC San Diego. Retrieved 8 July 2017.
  4. DiNicolo, Dan. "Essential Network Concepts Part 3". Security Pro News. Retrieved 9 July 2017.
  5. "Token Bus (IEEE 802.4)". Retrieved 8 July 2017.
  6. Hura, Gurdeep (2001). Data and Computer Communications: Networking and Internetworking. CRC Press. p. 327. ISBN   978-0849309281.