Active Monitor - AM

Functional MAC Address = C0 00 00 00 00 01

Only one ring adapter acts as active monitor. It has to...

- generate ring speed (4 Mbit/s or 16 Mbit/s);

- provide 24-bits latency buffer;

- initiate ring poll process (NAUN process) in intervals of 7 seconds;

- strip frames off the ring that pass more than once;

- release new tokens;

- initiate the ring again if errors had happened.

Ring Master Clock

The active monitor always transmits a stream of idle signals (all binary zeros). This enables all other adapters to synchronize to the given ring speed. If token-ring adapters receive frames they do not need to resynchronize againn because they are always synchronized. (Ethernet adapters need to get synchronized to each new frame.) Frames have a starting delimiter in order to reset the differential manchester code (the starting delimiter is not a synchronization pattern as Ethernet's preamble is).

Latency Buffer

Because most rings are too short even for token frames (frames don't cover the whole ring) transmitting adapters already receive the first bits of their frame before the frame's last bit is sent. In order to ensure that the token passes the ring properly, the active monitor has to provide a 24-bits latency buffer that is large enough for the token frame.

Ring Poll Process / NAUN Process

Token-ring adapters are able to isolate (locate) certain ring errors. This can only be done because each adapter knows its next addressable upstream neighbor (NAUN); the NAUN's MAC address is referred to as SUA or UNA. Token-ring error messages always contain the NAUN address in order to enable ring recovery.

Learning the NAUN addresses depends on the ring poll process or NAUN process initiated by the active monitor.

The active monitor (AM) sends active monitor present (AMP) notification MAC frames in intervals of 7 seconds (AC bits set to zero). The following station detects that it is the first station behind the active monitor because the AC bits are still set to zero; it learns its NAUN address and sets AC=11. All other stations will ignore this AMP frame because of AC bits set to all binary one. After the AMP frame has passed, all other stations subsequently send standby monitor present (SMP) notification MAC frames.

Ring Poll Process: NAUN - AMP - SMP

Token & Frame Monitoring

Each frame may only pass the ring once. The source adapter has to strip its own frame off the ring as soon as it is detected at the receive port.

Because the source station may be unable to strip the frame, all frames are monitored by the active monitor by means of the monitor bit within the access control field. Any new frame has its monitor bit set to zero (M=0). As soon as the frame passes the active monitor, the active monitor will switch the monitor bit to one (M=1). In case that the source station doesn't strip the frame off the ring, the frame will pass the monitor once again; the active monitor will recognize that the frame had already passed once and will strip the frame off the ring.

This can only be done because the active monitor provides a 24-bit latency buffer.

If the active monitor detects a loss of the token frame (timer no token [TNT] exceeded) or a corrupted token frame, it purges the ring (ring purge MAC frame) and releases a new token.