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Multiple access in IEEE 802.5

Short answer

IEEE 802.5 uses token passing: a free token circulates on a logical ring. A station needing to send captures the token, marks it busy, and appends its data. The frame goes around; the destination copies it. The sender removes the returning frame and releases a new free token, avoiding collisions and providing deterministic access.

Explain how multiple access is achieved in IEEE 802.5.

IEEE 802.5 (Token Ring) uses token passing, a controlled-access method, on a ring topology. No collisions occur because only the station holding the token may transmit.

Operation

  1. A special 3-byte frame called a token circulates continuously around the ring when all stations are idle. A free token has the token bit set to 0.
  2. A station with data waits for the free token, captures it by changing a bit (making it busy, 1) and adds the data to make a data frame with source and destination address.
  3. The frame circulates around the ring. Each station regenerates (repeats) it and passes it on. Only the destination station copies the data, and it sets the address recognised and frame copied bits.
  4. The sender removes the frame when it returns, checks the status bits for success, and releases a new free token.
  5. Token holding time is limited, so every station gets a turn.
     +---[A]---[B]---+
     |               |
    [D]-------------[C]    token -->

Features

  • Speeds 4 or 16 Mbps (maximum 100 Mbps in later versions).
  • A monitor station (active monitor) removes lost or duplicate tokens and keeps the ring healthy.
  • Priority and reservation bits in the token let high-priority frames go first.
  • Deterministic access time, so good under heavy load and for real-time traffic.
  • Drawback: waiting for the token even at low load, and a ring break affects everything unless a wiring concentrator (MAU) is used.

✓ Answer checked by Claude Opus 5.5 (high effort)

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