Short note: IBM Ring LAN

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

          +---------------+
   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

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

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.