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IEEE 802.4 Token Ring

• CSMA/CD – probabilities
  • MAC model – bad link – station wait for infinitely a long time! – no priorities
    • not useful for real time system.
      • Use a ring:

IEEE 802.4 Token Ring

• stations take time sending frames.

n

frame ,

nT

sec to wait

• physical ring broken
  • use logical ring

13

11

7

20

17

Token Bus Ring Organisation

• Token passing from higher to lower order

station address

• Token acquired station transmits for certain

amount of time

Hand over token either at end of time or no frame to transmit

prioritise tokens

Token Bus

• each maintains a queue of frames • each has timers • handover token from higher priority to

lower priority.

• fraction of token holding time allocated to

each priority.

• useful for implementing real – time traffic.

Token Bus Frame Format

• Preamble – clock synchronisation • Starting and ending delimiter • frame boundaries
  • analog encoding symbols (other than 0 or 1 – does not occur in analog dat
    • no need of length field

Token Bus: Issues

• Frame Control
  • Successors, – predecessors – Entry of new station – Clain token
    • Token lost, station with token dead
      • Protocols to handle all issues – Useful for real time traffic