Beaconing

Automatic Recovery Attempt Through The Beacon Process

Beaconing is the process of isolating a a fault domain so that recovery actions can take place. The fault domain consists of:

1. The station reporting the failure (the beaconing station)
2. The station upstream from the beaconing station
3. The ring medium between them

When the ring is beaconing it alerts all stations on the ring that the token protocol has been suspended. Through ring poll / neighbor notification each station knows the address of its upstream neighbor. This allows the identification of the elements of the failure domain.

The beacon process consists of transmitting Beacon MAC frames every 20 ms without needing a token. The beaconing station derives its time base from its own internal crystal oscillator and not from the clock recovered from its receiver port.

Upon receipt of a Beacon MAC frame a station will enter either the Beacon Repeat mode or the Beacon Transmit mode (as will be discussed). A station which is in the Insertion Process will terminate its Open command with an error and will remove itself from the ring.

There are four types of Beacon MAC frames that might be seen:

• A Set Recovery Mode (Priority 1, the highest priority) is never originated by the adapter itself. It is reserved for a recovery process implemented in the attached product. Don’t expect to ever see a Priority 1 beacon. If you do, find out who sent it, and why.
• A Signal Loss (Priority 2) beacon is transmitted when a Monitor Contention timeout occurs and the Contention Transmit mode was entered because of a signal loss condition being detected.
• A Streaming Signal, Not Claim Token (Priority 3) beacon is transmitted when a Monitor Contention timeout occurs and no Claim Token MAC frames were received during the contention period. Notice that this implies that a differential Manchester clock (idle clock) was received. Otherwise a Priority 2 beacon would have resulted.
• A Streaming Signal, Claim Token (Priority 4) beacon is transmitted when a Monitor Contention timeout occurs and one or more Claim Token MAC frames were received during the contention period.