This article is from the Token Ring FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
Devices are either active monitors or standby monitors. There can
only be a single active monitor on a physical token ring. Any
station on the ring can assume the role of Active Monitor. All other
stations on the ring are standby monitors.
The Active Monitor provides many functions on a token ring network:
* The Active Monitor is responsible for master clocking on the token
ring network and the lower level management of the token ring
* The Active Monitor inserts a 24-bit propagation delay to prevent
the end of a frame from wrapping onto the beginning of the frame.
* The Active Monitor confirms that a data frame or good token is
received every 10 milliseconds. This timer sets the maximum possible
frame size on a token ring network to 4048 bytes on a 4 megabit
ring, and 17,997 bytes on a 16 megabit ring.
* The Active Monitor removes circulating frames from the ring. As a
frame passes the Active Monitor, a special bit called a monitor
count bit is set. If the monitor count bit is set, the Active
Monitor assumes the original sender of the frame was unable to
remove the frame from the ring. The Active Monitor purges this
frame, and sends a Token Error Soft Error to the Ring Error Monitor.
If the Active Monitor is removed from the ring or no longer performs
the Active Monitor functions, one of the Standby Monitors on the
ring will take over as Active Monitor.