In the token ring architecture a station that transmits a frame will eventually receive that frame after it has gone all the way around the ring. The station that sent the frame then removes that frame from the ring. Whenever a station places a frame onto the ring it starts a timer. It is expected that the frame will return to the station before that timer expires. If this does not happen then this error counter is incremented.

Note that this is a non-isolating error and so it is not possible to determine where on the network the fault occurred.

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.