Token Ring
Logically a Ring – Physically a Star
Most token ring networks are physically wired as stars, because a star is more reliable than a ring. A special hub makes them work like a ring.
Tokens and packets are passed from computer to computer, but through the hub.
The Algorithm
1. The first computer switched on puts an access token onto the ring.
2. The token is passed around the ring from computer to computer.
3. When a computer wishes to send it must wait until it receives the token.
4. When it gets the token it eats it and instead sends out an addressed data packet.
5. The data packet is passed from computer to computer until it gets to its destination.
6. The receiving computer reads the data then sends out a confirmation of receipt packet.
7. The confirmation goes around the ring until it gets back to the sender.
8. When the sender receives the confirmation, it eats the packet and sends out a new token.
9. Any computer on the ring can now grab the token and send a packet.
Cables
The most common token ring system is sold by the IBM company. It uses UTP or STP cables and transmits at either 4 Mbps or 16 Mbps.
The Special Hubs
IBM call their token ring hubs all of the following:
MAU – Multi-station Access Unit
Each hub can link 10 computers and hubs can be joined together to build bigger rings.
The Standard
The Token Ring system used by IBM is defined in IEEE 802.5 specification.
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