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Token bus also called token ring

Short answer

Token bus (IEEE 802.4) is physically a linear bus but logically a ring: stations form a closed logical sequence and pass a token in that order. Only the token holder transmits. This token-passing creates a logical ring identical to token ring, despite the bus topology, hence the name.

Explain why token bus is also called the token ring.

Token bus (IEEE 802.4) is physically a linear bus (or tree) but logically a ring, which is why it is also described as a token ring.

Reasons

  1. Logical ring: the stations are arranged in a logical order, usually by descending station address. Each station knows its predecessor and successor.
  2. Token passing: a token (control frame) is passed from each station to its successor in this order, and the last station passes it back to the first, forming a closed logical loop. Only the token holder may send data.
  3. Same access rule as token ring: a station transmits for a limited token-holding time and then passes the token, just like in IEEE 802.5.
  4. Physical position does not matter: two neighbours in the logical ring may be at opposite ends of the physical cable.
 Physical bus:  [A]--[D]--[B]--[C]
 Logical ring:  A -> B -> C -> D -> A
  1. Ring maintenance: stations can join or leave the ring and lost tokens are recovered, as in a physical ring.

Therefore token bus has the bus wiring but the ring's token-based, collision-free behaviour. It supports priority classes and gives bounded delay, so it was used in factory automation.

✓ Answer checked by Claude Opus 5.5 (high effort)

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