IEEE 802.5, Token Ring

Token ring network is different from both CSMA/CD and token bus in that it does NOT use broadcast channel. It's a collection of point to point links forming a physical ring . Therefore, the physical layer of Token Ring is also completely different from CSMA/CD. Only one token can be in the ring at any time. Furthermore, only token holder has the right to transmit.

• Runs at 1 or 4 , therefore, it's used mostly for small scale LANs (imagine 200 workstations fighting for 1!).
• Covers up to 10 in cable length;
• There are two modes: listen and transmit. In listen mode, one bit delay is inserted for passing data streams. In transmit mode, the station
  • removes token,
  • transmits data frame,
  • removes the frame when it's wrapped back,
  • regenerates and releases a new token.

  The delay introduced by all the stations is used to store the token in the ring so that, together with propagation delay, the token can fly around the ring without overlapping with itself.

• Acknowledgement of successful reception is done by letting the receiving station set the ack bit in the frame, sender then see the acknowledgement when the frame wraps back. Note that if the frame is corrupted somehow, MAC layer does nothing to correct it, upper layer has to retransmit or discard the corrupted frame.
• One station is elected as monitor station. If no one is the monitor, any station can issue CLAIM_TOKEN message to become one. Monitor station's main responsibilities:
  • Make sure that the delay in the ring is enough to store the token (24bits), if not, introduce more delays;
  • Make sure that the token is not lost,
  • Cleans up the ring of garbled frames and damaged token;
  • Delete orphan (unclaimed) frames, etc.