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

# Short note: Cambridge Ring LAN

Telecommunications · Chapter 8: Data Communication

- Asked in: 2065 Baisakh (old course), 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-cambridge-ring
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on Cambridge Ring LAN.

## Short answer

The Cambridge Ring is a slotted ring LAN at 10 Mbps using fixed 38-bit mini-packets with 16 data bits. A station fills an empty slot and the source empties it after the destination sets response bits. Useful capacity is 16/38 times 10 Mbps, about 4.21 Mbps.

## Answer

The **Cambridge Ring** is a **slotted ring** LAN developed at the University of Cambridge (UK) in the late 1970s. A fixed number of small, fixed-size **mini-packets (slots)** circulate continuously round the ring; a station sends data by filling an empty slot. The raw ring speed is **10 Mbps**.

#### Structure
```text
        [Monitor]
       /         \
   [R]-S1       S4-[R]
    |             |
   [R]-S2 ---- S3-[R]
 R = repeater, S = station unit + access box
```
- **Repeaters** (about 100 m apart) regenerate the signal; they are powered from the ring, so a station can be switched off without breaking the ring. Two twisted pairs are used.
- **Station unit / access box:** connects a host to its repeater and handles slot filling and response checking.
- **Monitor station:** sets up the slot structure at start-up, marks and clears lost or damaged slots, and checks parity; there is also an error-logging station.

#### Mini-packet (slot) format – 38 bits
```text
+-+-+-+-------+-------+----------+--+-+
|S|F|M| Dest  |Source |   Data   |R |P|
|1|1|1|   8   |   8   |    16    |2 |1|
+-+-+-+-------+-------+----------+--+-+
```
- **S:** start bit (always 1); **F:** full/empty bit.
- **M:** monitor bit, used to remove orphan slots.
- 8-bit destination and source addresses (up to 255 stations).
- **Data:** 2 bytes per slot.
- **R:** 2 response bits; **P:** parity bit.

#### Working
1. A station that wants to send waits for a slot with **F = empty**.
2. It sets F = full, writes the addresses and 2 bytes of data.
3. The destination copies the data and sets the **response bits** to show: accepted, busy, rejected, or ignored (station absent).
4. The slot goes all round the ring back to the **source**, which reads the response and marks the slot **empty**.
5. The source may **not reuse the same slot** at once; it passes it on, so every station gets a fair share.
6. The monitor sets M as a full slot passes it; if a full slot returns with M already set (source failed to empty it), the monitor empties it.

#### Efficiency
Only 16 of 38 bits are user data:
$$
\frac{16}{38} = 0.421, \qquad 0.421 \times 10\ \text{Mbps} = 4.21\ \text{Mbps}
$$
so the useful capacity is at most about 4.2 Mbps; inter-slot gaps and the slot-return rule lower one station's rate further (around 1 Mbps).

#### Advantages and disadvantages
- Simple, fixed-size slots; fair and bounded access; no collisions.
- Immediate acknowledgement through response bits; powered repeaters improve reliability.
- Low data efficiency (large overhead per 2 bytes), small slots unsuitable for big transfers, single ring break stops the network.

The Cambridge Ring was widely used in UK universities and research labs and influenced later slotted-ring and ATM-style cell designs.

## Check yourself

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

1. How does a station send data on the Cambridge Ring?
   A. It captures a single token
   B. It senses the carrier and transmits if idle
   C. It fills an empty fixed-size slot circulating on the ring
   D. It is polled by the monitor
   Answer: C. The Cambridge Ring is a slotted ring: a few mini-packet slots circulate and a station fills an empty one. (Key point: Slotted ring principle)

2. A slot has 38 bits of which 16 are data on a 10 Mbps ring. What is the maximum useful capacity?
   A. About 4.2 Mbps
   B. 10 Mbps
   C. About 2.4 Mbps
   D. About 6.3 Mbps
   Answer: A. 16/38 = 0.421, and 0.421 x 10 Mbps = 4.21 Mbps. (Key point: Efficiency calculation)

3. What do the response bits in the returned slot tell the source?
   A. The ring speed
   B. Whether the destination accepted, was busy, rejected or ignored (absent)
   C. The parity of the slot
   D. Which station is the monitor
   Answer: B. The destination sets the response bits so the source gets immediate acknowledgement. (Key point: Response bits)

4. Why may the source not reuse the same slot at once?
   A. The slot is damaged after use
   B. The monitor needs it
   C. To give every station a fair share of slots
   D. Parity must be recalculated
   Answer: C. The source marks it empty and passes it on so other stations can use it, giving fair access. (Key point: Slot reuse rule and fairness)

5. What does the monitor do when a full slot returns with M already set?
   A. Empties the slot, since the source failed to do so
   B. Doubles its priority
   C. Delivers it again
   D. Shuts down the ring
   Answer: A. A full slot with M already set has circled twice, so the monitor empties it to remove the orphan. (Key point: Monitor bit)

## Related in Telecommunications

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