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.

If a station receives a packet in which the AC bits are set to '11' and which has that station as the destination of the frame then there are probably two stations on the ring with same address. When a station detects this condition it increases the Frame Copy error counter.

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.