---
title: "Token bus also called token ring"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:token-bus-token-ring"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/token-bus-token-ring
site: BCT Engineering
---

# Token bus also called token ring

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2073 Shrawan, 4 marks
- Video: [Lec-40: Token Ring (IEEE 802.5) in Computer Networks](https://www.youtube.com/watch?v=-u4Dzu63eZc) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/token-bus-token-ring
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Explain why token bus is also called the 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.

## Answer

**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.

```text
 Physical bus:  [A]--[D]--[B]--[C]
 Logical ring:  A -> B -> C -> D -> A
```

5. **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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is token bus called a token ring?
   A. It is physically a ring
   B. It is physically a bus but stations form a logical ring passing a token
   C. It uses ring cable
   D. It uses IEEE 802.5
   Answer: B. Physical bus, logical ring in order of station addresses. (Key point: Logical ring)

2. In token bus, to whom does a station pass the token?
   A. The physically nearest station
   B. Its successor in the logical ring order
   C. A random station
   D. The station with the longest queue
   Answer: B. Each station knows its successor; the last passes it back to the first. (Key point: Token passing order)

3. Which is true about physical position?
   A. Logical neighbours must be adjacent
   B. Logical neighbours may be at opposite ends of the cable
   C. Position sets priority
   D. Position must be sorted
   Answer: B. Logical order is by address, not cable position. (Key point: Physical position)

4. What is shared with IEEE 802.5?
   A. CSMA/CD
   B. Only the token holder transmits for a limited holding time
   C. Random backoff
   D. Frequency division
   Answer: B. Both give collision-free, bounded-delay access via token. (Key point: Access rule)

## Related in other subjects

- [Token ring mutual exclusion](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Atoken-ring-mutex) — Distributed System, chapter 6, asked 3×
- [Ring bus rearrangement and zones](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aring-bus-comparison) — Switchgear and Protection, chapter 1
- [Reactor function and ring bus-bar scheme](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aring-bus-reactor-scheme) — Power Plant Equipment, chapter 4