Short note: Network topologies and IBM Ring LAN

Network topologies

The topology of a network is the geometric arrangement of the nodes and the links that join them. It decides cost, reliability and the access method.

 Bus:   H---H---H---H        Star:     H   H
        ===============               \ /
                                  H--[Hub]--H
 Ring:  H---H                         / \
        |   |                        H   H
        H---H
 Mesh:  every node linked to every other
        (n(n-1)/2 links)

IBM Ring LAN

IBM Token Ring (IEEE 802.5) is a LAN in which stations form a logical ring and gain the right to send by capturing a circulating token; there are no collisions. Rates are 4 or 16 Mbps, coding is differential Manchester. Physically it is a star-wired ring – a hybrid topology:

          +---------------+
   A -----|               |----- B
          |   MAU (hub)   |
   D -----|  ring inside  |----- C
          +---------------+
 Logical ring: A -> B -> C -> D -> A
 Each lobe = 2 pairs (send + receive)

• Stations connect to an MAU by STP/UTP lobes; relays in the MAU bypass a dead station.
• Up to 260 stations per ring (STP).

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.

Advantages: deterministic, fair access, good at high load, priorities. Disadvantages: costly and complex compared with Ethernet.