Short note: Cambridge Ring LAN

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

        [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

+-+-+-+-------+-------+----------+--+-+
|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.