Token Ring (IEEE 802.5)

A LAN technology in which stations are connected in a ring, and access to the medium is controlled by a special frame called the token.

Working

1. A small free token circulates around the ring in one direction.
2. A station with data captures the token (changes it to a busy token), attaches its data and the destination address, and sends the frame.
3. The frame passes each station; the destination copies the data and sets the acknowledge bits.
4. The frame returns to the sender, which removes it and releases a new free token.

Features

• Collision-free, deterministic access (no CSMA contention)
• Works well under heavy load; each station gets a bounded waiting time (priority levels possible)
• A monitor station handles lost or duplicate tokens
• Speeds: 4 and 16 Mbps

Disadvantages

• Token loss or a station failure can break the ring (dual rings or wiring concentrators help)
• Delay at light load while waiting for the token
• Costlier than Ethernet, which is why Ethernet replaced it