Beacon Process
The beacon process is the last chance of ring recovering in case of hardware errors. Beacon frames are sent by an adapter if it doesn't receive any more frames (due to possible input line errors). Beacon frames are sent in intervals of 20 milliseconds until the ring is healed.
There are four types of beacon frames identified by different subvector IDs:
Because beacon frames contain the originator's NAUN address, the ring error monitor (REM) or any protocol analyzer can immediately isolate the error's location.
1st. Possibility
Beacon frames sent by station S are received by its NAUN station S-1 that recognizes its own MAC address as the NAUN address mentioned in the beacon frame. The station S-1 assumes that its transmit line is out of order. After receipt of eight beacon frames station S-1 gets off the ring and performs a lobe media test.
If this test confirms that the lobe cable is defect, the NAUN station S-1 off the ring. The station S that send the beacon frames now receives its own beacon frames; this fact indicates that the ring is closed again. Station S stops beaconing and starts monitor contention process..
Monitor Contention Process: Claim Token
2nd. Possibility
The NAUN station S-1 passed the lobe media test successfully. The NAUN station S-1 inserts again into the ring. Now the station S that sends the beacon frames gets off the ring and performs a lobe media test. In case that the lobe cable is defect, station S stays off the ring. Its downstream neighbor S+1 starts the monitor contention process.
Monitor Contention Process: Claim Token
3rd. Possibility
The lobe media test of station S was successful, too, and station S inserts into the ring again. Now the downstream station S+1 begins to send beacon frames - endless. This process can only be stopped by human intervention.
If the beacon process didn't heal the ring within 26 seconds, the beacon process will last forever.