Token passing

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The term token passing or English token passing refers to a media access method in computer networks . The term is based on the token in rail traffic , which regulates traffic on single-track sections of the route.

The basis of token passing is the token that is passed on in a computer network from one station to the neighboring station in a logical ring topology . There are two different ways to implement token passing: Token Ring and Token Bus .

With Token Ring, the neighbor is the physically closest accessible station, with Token Bus it is the logically closest accessible station (implemented by the addresses of the network card).


An example network

A free token (consisting of 3 bytes or 24 bits) is continuously sent from point to point. If a computer A wants to transmit data to computer C , it waits for the token to pass it and then, if it is free, appends the data packet to the token , addresses it to computer C and marks the token as occupied.

The entire package sending computer A to computer neighboring B . Computer B realizes that he is not the recipient of the data frame and sends it to his neighbor Computer C . Since C is registered as the recipient, he copies the data frame and modifies the token to receive . Then it sends the frame back onto the ring. Since the token is still occupied, no computer can attach data. Upon arrival of the frames at Computer A checks A whether the content matches the sent and the reception flag is set.

If this is the case, the transfer was successful. The data frame is removed and the token is set to free again. Even if a transmission has failed, the sender must always send a free token after receiving the reception mark (acknowledgment). This ensures that there is a free token in the ring after each data transfer.

Token Passing is used for networks with a high load, but also for real-time applications .


  • Dieter Conrads: data communication. Procedure - Networks - Services. 2nd edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 1993, ISBN 3-528-14589-7 .
  • R. Lauber (Ed.): Process computing systems '88 . Springer Verlag, Berlin / Heidelberg 1988, ISBN 3-540-18986-6 .
  • Dieter Wybranietz: Multicast communication in distributed systems . Springer Verlag, Berlin / Heidelberg 1990, ISBN 3-540-52551-3 .
  • Martin Zieher: Coupling of computer networks . Springer Verlag, Berlin / Heidelberg 1989, ISBN 3-540-51873-8 .
  • Christian Diedrich: Profibus PA. Instrumentation technology for process engineering. Oldenbourg Industrieverlag, Munich 2006, ISBN 3-8356-3056-3 .

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