Token Ring
IBM's Token Ring was the main competitor to Ethernet in the 1980s and 1990s. Using a token-passing protocol rather than CSMA/CD, Token Ring guaranteed predictable network performance—but lost the standards war.
The Token Passing Protocol
Unlike Ethernet's CSMA/CD where any device can transmit when the line is idle, Token Ring uses a special packet called a "token." Only the station holding the token can transmit. After transmitting, it releases the token, which travels to the next station in the ring.
Token Ring Advantages
- Predictable performance: No collisions, deterministic latency
- Priority access: 8 priority levels with token holding
- No cable distance limits: Ring spans up to 10 km with concentrators
- Built-in diagnostics: Token monitoring, beaconing for fault isolation
Token Ring Specifications
- 4 Mbps: Original IBM standard
- 16 Mbps: Common deployed speed
- 100 Mbps: High Speed Token Ring (later)
- MAU: Multi-Station Access Unit (hub/switch)
Why Token Ring Lost to Ethernet
Despite technical elegance, Token Ring had fundamental disadvantages:
- Cost: Token Ring adapters and hubs were expensive
- Complexity: Token recovery mechanisms were complex
- Speed: Ethernet jumped to 10 Mbps, then 100 Mbps faster
- IBM ecosystem: When IBM de-emphasized Token Ring, adoption declined
The End of Token Ring
By 2000, Ethernet's 100Base-TX dominated LANs. IBM exited the Token Ring business in 2000. By 2010, Token Ring was essentially extinct, though some legacy systems still operated in industrial environments.