A token ring always includes an active monitor station. Periodically this station initiates an active monitor poll by sending an Active Monitor Present MAC frame.

Every frame on a token ring contains an Address Recognised bit and a Frame Copied bit. These are collectively known as the AC bits. When a station processes a frame it must set either or both of these bits. A station that simply forwards a frame to the next station sets neither bit. Valid combinations of the bits are:

AC Meaning 00 Frame repeated to next station. 10 The station recognised the address. That station did not however copy the frame. Generally this indicates a lack of resource in the station that recognised the address. 11 The station recognised and managed to copy the frame.

These bits are used in an active monitor poll sequence. Each station obeys two rules. 1) When a standby monitor station receives an active or standby monitor present frame with the AC field set to 00 it sets the bits to 11 and starts a timer. When the timer expires it sends a standby monitor present frame. 2) When the active monitor receives any monitor present frame the active poll is considered complete.

The Monitor Error counter is increased by a station if it receives either and active or standby monitor present frame with AC=00, then receives a standby monitor present frame also with AC=00. If such a situation occurs there is some error on the ring. This may only be a reconfiguration because the active monitor station has been removed from the ring.

GENERAL: Soft Errors

The token ring architecture includes an inherent error monitoring protocol. Each station records local errors. These errors are generically referred to as 'soft errors'. Soft errors represent intermittent faults that temporarily disrupt normal operation of the ring. Stations periodically transmit their soft error counters onto the ring where they can be received by any stations which have the ring error monitor functional address active.

There are two categories of soft error. Isolating errors and Non-isolating errors. An isolating error will be reported by exactly one station on the ring and the error is known to have occurred between that station and it's upstream neighbour. The second category are non-isolating. That is, errors of this nature may be seen and reported by more than one ring station. With these errors it is not possible to resolve the source of the error.