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Ring Monitors

Active Monitors and Standby Monitors

Every station on a token ring network is either an Active Monitor (AM) or a Standby Monitor (SM). The Active Monitor is chosen in an election process called monitor contention and has the general responsibility of making sure that everything is running properly on the ring. Any station that is not acting as the Active Monitor is acting as a Standby Monitor because it is standing by to become the Active Monitor, should the Active Monitor fail to perform its duties.

At any given time on a properly operating token ring network, there will be only one Active Monitor. Should the circumstance arise that there are two Active Monitors on one ring (for example, two separate rings being joined into one while all stations are up and running), both Active Monitors will detect the presence of the other and instantly become Standby Monitors. The ring is then left without an Active Monitor until the Standby Monitors realize that an AM is lacking and initiate monitor contention.

Responsibilities of the Active Monitor

The specific responsibilities of the Active Monitor are:

• Provide master clocking to the ring.
  The Active Monitor is responsible for putting onto the wire a master clock signal, which all other stations use to "synch up" their internal clocks so that every station knows when a bit time begins and ends.
• Provide a minimum 24 bit "latency buffer".
  The purpose of the latency buffer is to simulate a ring length that assures that the token can be circulated properly. Because most token ring networks have a small (in terms of the speed of propagation of electrical signals) ring length, the latency buffer assures that no station other than the AM itself is ever stripping the initial bits of the token off of the wire before it has finished sending the last bits. Initiate Ring Polling every seven seconds.
  The AM will send an Active Monitor Present frame every seven seconds.
• Monitor Ring Polling.
  If the AM does not receive an AMP or SMP frame from its nearest upstream neighbor within seven seconds of initiating ring polling, it reports a Ring Poll Failure to the ring's Error Monitor.
• Ensure proper token passing.
  The Active Monitor watches for problems in token passing such as circulating tokens due to a station raising the ring's priority and then never lowering it, or lost tokens (specifically, the AM must see a good token every 10 milliseconds).
• Compensate for frequency jitter.
  As stations bit repeat data around the ring, they each introduce a small amount of phase shift in the signal. This phase shift is referred to as "jitter". The AM compensates for this by generating a master clock signal based on its own internal clock rather than the clock signal being received from its upstream neighbor.

Responsibilities of a Standby Monitor

The general responsibility of a Standby Monitor is to make sure that the Active Monitor is performing its duties properly. Specifically, its responsibilities are:

• Monitor token passing on the ring.
  Similar to the AM, Standby Monitors watch token passing on the ring, but much more leniently. A SM must see either a data frame or a good token at least once every 2.6 seconds. If it does not, it initiates monitor contention.
• Monitor Ring Polling.
  If a Standby Monitor does not see an Active Monitor Present frame at least once every 15 seconds, it initiates monitor contention.
• Monitor ring frequency.
  If a Standby Monitor detects that the clock signal being produced by the AM differs significantly from its own internal clock, it will initiate monitor contention.