---
title: "Short note: IBM Ring LAN"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:short-note-ibm-ring-lan"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-ibm-ring-lan
site: BCT Engineering
---

# Short note: IBM Ring LAN

Telecommunications · Chapter 8: Data Communication

- Asked in: 2068 Bhadra (old course), 10 marks
- Video: [Token ring network and how it works](https://www.youtube.com/watch?v=p72R2uGglnU) — Sunny Classroom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-ibm-ring-lan
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on IBM Ring LAN.

## Short answer

IBM Token Ring (IEEE 802.5) is a token-passing LAN operating at 4 or 16 Mbps. Stations are star-wired to a MAU, forming a logical ring. A circulating token grants transmission; the sender captures it, sends a frame, and strips it after a round trip, ensuring collision-free access.

## Answer

**IBM Token Ring** is a LAN developed by IBM in the 1980s and standardised as **IEEE 802.5**. Stations form a logical ring and get the right to transmit by capturing a circulating **token**, so there are no collisions. It runs at **4 Mbps or 16 Mbps** with differential Manchester coding.

#### Physical structure – star-wired ring
```text
          +---------------+
   A -----|               |----- B
          |   MAU (hub)   |
   D -----|  ring inside  |----- C
          +---------------+
 Logical ring: A -> B -> C -> D -> A
 Each lobe = 2 pairs (send + receive)
```
- Stations connect to a **MAU (Multistation Access Unit)**, a wiring concentrator, by shielded twisted pair (STP, IBM Type 1) or UTP.
- Inside the MAU the lobes are joined into a ring. A relay in the MAU **bypasses** a station that is switched off or faulty, so the ring is not broken. MAUs are joined by Ring-In / Ring-Out ports to make larger rings.
- Up to 260 stations on STP (72 on UTP).

#### Frame and token format
```text
Token (3 bytes):
+----+----+----+
| SD | AC | ED |
+----+----+----+
Data frame:
+--+--+--+--+--+------+---+--+--+
|SD|AC|FC|DA|SA| Data |FCS|ED|FS|
|1 |1 |1 |6 |6 |  0-n | 4 |1 |1 |
+--+--+--+--+--+------+---+--+--+
AC byte = P P P T M R R R
```
- **SD / ED:** start and end delimiters (use code violations J, K of differential Manchester).
- **AC (access control):** PPP = priority, T = token bit (0 token, 1 frame), M = monitor bit, RRR = reservation.
- **FC:** frame control (data or MAC control frame).
- **DA, SA:** 6-byte addresses; **FCS:** 32-bit CRC.
- **FS (frame status):** A (address recognised) and C (frame copied) bits set by the receiver.

#### Working (token passing)
1. A free token circulates round the ring; each station repeats it bit by bit (1-bit delay).
2. A station with data waits for the token, changes the T bit to 1 (token becomes a start-of-frame), and sends its frame. It may hold the token for at most the token holding time (10 ms).
3. Each station repeats the frame. The destination copies it and sets the A and C bits in FS.
4. The frame returns to the sender, which **removes it** from the ring (source stripping), checks A and C for delivery, and releases a new free token.
5. At 16 Mbps, **early token release** lets the sender release the token right after its frame, so two frames may be on the ring at once.

**Priority:** a station with a high-priority frame writes its priority in the RRR bits of a passing frame; the next token is issued at that priority, so only stations with equal or higher priority may use it.

#### Ring maintenance – active monitor
One station is elected **active monitor**; others are standby monitors.
- Detects a **lost token** (no token within a timeout) and issues a new one.
- Removes **orphan/circulating frames**: it sets the M bit on each frame; a frame seen again with M = 1 is purged.
- Supplies the master clock and a **latency buffer** so the ring is long enough to hold a 24-bit token.
- Sends Active Monitor Present frames; **beacon** frames locate cable breaks.

#### Features

| Point | IBM Token Ring (802.5) |
|---|---|
| Access method | Token passing, collision-free |
| Data rate | 4 or 16 Mbps |
| Topology | Logical ring, physical star (MAU) |
| Coding | Differential Manchester |
| Max frame | ~4500 bytes (4 Mbps), ~18 kB (16 Mbps) |
| Priority | 8 levels |

**Advantages:** deterministic access (bounded waiting time), good performance under heavy load, priority support, fault bypass in MAU. **Disadvantages:** more complex and costly than Ethernet, monitor and token management needed, now replaced by switched Ethernet.

## Check yourself

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

1. Why are there no collisions in IBM Token Ring?
   A. Each station has a separate cable to every other
   B. A station may transmit only after capturing the circulating token
   C. Stations listen while sending and back off
   D. Frames are sent in fixed time slots for each station
   Answer: B. Only the station holding the token transmits, so access is collision-free. (Key point: Token passing, no collisions)

2. What does the MAU do in the physical structure?
   A. Joins lobes into a ring inside and bypasses a faulty or switched-off station
   B. Converts 4 Mbps to 16 Mbps
   C. Elects the active monitor
   D. Removes frames from the ring
   Answer: A. The MAU is a wiring concentrator whose relays bypass dead stations so the logical ring stays intact. (Key point: Star-wired ring and MAU)

3. In the AC byte (PPP T M RRR), what does T = 1 mean?
   A. The frame was copied by the destination
   B. It is a free token
   C. It is a data frame (token captured)
   D. The frame has been seen by the monitor
   Answer: C. T = 0 is a free token; the sender changes it to 1, turning the token into a start-of-frame. (Key point: Token bit in AC byte)

4. Who removes a data frame from the ring and what follows?
   A. The destination removes it and releases the token
   B. The active monitor removes every frame
   C. The next station removes it
   D. The sender removes it after checking A and C bits, then releases a new free token
   Answer: D. The frame returns to the sender, which strips it, checks the A and C bits for delivery and issues a free token. (Key point: Source stripping)

5. How does the active monitor purge an orphan circulating frame?
   A. It sets the M bit on frames and purges any frame seen again with M = 1
   B. It checks the FCS of every frame
   C. It lowers the priority bits
   D. It waits for the token holding time of 10 ms
   Answer: A. The monitor sets M on each passing frame; a frame returning with M = 1 has circled more than once and is removed. (Key point: Active monitor orphan frames)

## Related in Telecommunications

- [Short note: Network topologies and IBM Ring LAN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-topologies-ibm-ring) — Telecommunications, chapter 8
- [Short note: Cambridge Ring LAN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-cambridge-ring) — Telecommunications, chapter 8