Specifications

2-19
TOKEN RING OVERVIEW
Each station on the ring, as it receives the PRG frame, cancels all timers
and resets to normal repeat mode, if the frame cannot be copied. If the
frame can be copied, action is taken according to the information field of
the MAC frame. The information field contains the address of the Nearest
Active Upstream Neighbor (NAUN).
If the PRG frame successfully makes it around the ring, the monitor has
been properly reset on the ring. A new token would then be issued by the
monitor.
It is possible that the PRG frame may not make it around the ring, in
which case the monitor’s ring purge timer will expire and the monitor will
enter Monitor Contention Mode (MCM).
Beaconing
Beaconing is initiated when a station on the ring has detected a ring
failure. A beacon frame is transmitted by the station to notify the other
stations on the ring of the failure.
From the NAUN process each station knows where it is in ring and the
beaconing process helps to identify the failure domain, which consists of
the station that first spots the failure (beaconing station) and its NAUN
and the ring cable between them.
Beacon Receive (Auto Removal)
The automatic recovery process starts when a station loses signal. Either
its receiver side is faulty, or the transmitter of its NAUN is faulty. The
station enters the beaconing transmit state and sends a Beacon MAC
(BCN) frame around the ring. The BCN frame is read by all stations and
contains the address of the sending station’s NAUN. When the NAUN
sees its address it performs a self test. If the test fails, the station remains
off the ring. Otherwise, it reinserts onto the ring.
After transmitting the BCN the beaconing station runs a Timer Beaconing
Transmit (TBT) which has a default of 16 seconds. If the beaconing station
receives its BCN within this timer period, then the ring is good and the
station moves to Claim Token state. If the BCN is not received before the
expiration of the TBT, the Beaconing station removes itself from the ring
and performs a self test.
The Beaconing sequence provides a method by which the NAUN checks
itself and its associated lobe wiring, and if this passes satisfactorily, the
expired TBT of the Beaconing Station causes it to check itself and its
associated lobe wiring. At the end of the sequence both stations and the
cabling have been checked and either station, if detecting a failure,
remains off-line.