IBM 3745 Series, Hardware Maintenance Reference

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Access Protocol
A token is a short message. It travels around a communication ring that allows attaching different systems
to the communications facilities provided by that ring. When a token is used for transmission by an
attachment, it is known as a frame. Actually, a token is a very short frame (3 bytes) that has no
addressing, message, or error-checking capabilities. Those are added when the token becomes a true
frame.
General Frame Format
┌──────────────┬────────────────┬────────────────┐
│
Physical │ │ Physical │
│ Transmission │ Data │ Transmission │
│ Header │ │ Trailer │
└──────────────┴────────────────┴────────────────┘
/ \
/ \
/ \
/ \
/ \
/ \
┌──────────┬─────────────────┐
│
Data │ │
│ Link │ Information │
│ Control │ │
└──────────┴─────────────────┘
Figure 6-5. Frame Format
Once a station is physically attached to a ring, it first synchronizes itself to the data patterns passing
through it over the ring.
Token-Ring Access Control Protocol
Figure 6-6. Token-Ring Access Protocol Example
Token-Ring Encoding
The differential Manchester protocol is used as means to transmit information
around the ring, to all devices connected to that ring and power-ON.
Major System Components
Chapter 6. The Token-Ring Subsystem
6-7