Token Ring Networks

• Token Ring Networksprovide a n alternative for accomplishing low delays and high utilization of a broadcast channel for bursty traffic.
• As the name suggests, the stations are connected in a ring, i.e., each station has a predecessor and a successor.
• In contrast to the ethernet, collisions are avoided entirely by the token ring.
• It uses the following rules to control access to the channel:
  • Only the station that has the ``token'' can transmit.
  • Upon completion of transmission, the token holder sends a short message to its successor and passes the token.
  • If a station does not have a message to transmit it immediately passes the token to its neighbore.
  • The maximum time any station can hold the token is limited.
• Since there is only one token, only one station transmits at a time, and collisions never occur.
• When all stations have messages to transmit, then the token ring protocol becomes identical to TDMA with the slot time equal to the maximum token-holding time.
• If only a single station has a message to transmit, the worst case delay is equal to the time required for passing the token around the ring.
• In comparison to the ethernet, the token ring's delay at low traffic levels is somewhat higher.
• However, under heavy traffic conditions the token ring outperforms the ethernet.