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CS 635 Lecture 5, Part 1

Token Bus General Description

 Token Bus  Stations are added to an 802.4 bus by an approach called response windows: A node can drop out of the transmission sequence. Upon receiving a token, it sends a set-successor  frame to the predecessor, which orders the next node to give the token hereafter to its successor

MAC Sublayer

 Token Bus MAC sublayer

The MAC sublayer consists of four major functions:

  1. The interface machine (IFM)
  2. The access control machine (ACM)
  3. The receive machine (RxM)
  4. The transmit machine (TxM)\

ACM

TxM

RxM

 Token Bus MAC Frame

Priority Scheme

THT = token holding time; the maximum time that a station can hold the token to transmit class 6 data
TRTi = token rotation time for class i, (i=4,2,0); the maximum time that a token can take to circulate and still permit class i transmission Principal disadvantage of token bus is complexity. Second disadvantage is the overhead - under lightly loaded condition a station may have to wait a while to transmit.

Can overcome the disadvantage by using multiple priority levels; different stations maybe allowed to hold the token different amounts of time

Advantages
There is no minimum packet length. No need to listen and talk. Superior performance under heavy load.

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