Understanding the network.pdf - Back to Home

tolerances the frames, timing shifts slightly out of phase with the master
transmission clock (maintained by the active monitor) as it traverses the ring.
This phase shift is known as jitter. Token Ring's ring station density, cable
type, and length design specifications are provided, taking into account a
jitter rate that can be compensated for through the use buffer memory on
the Token Ring NIC. This buffer is known the elastic buffer. Every Token Ring
chipset contains an elastic buffer that corresponds to the NIC's operating
rate. However, the elastic buffer is only used on the station that is
designated as the ring's active monitor. The active monitor uses the elastic
buffer to reset the ring's phase as the frame/token passes around the ring.
When the jitter build-up on the ring exceeds the level that can be
compensated for, a frequency error results.
Token Ring hard errors represent a class of errors that directly impacts the
operation of the ring, typically, some type of hardware failure. Upon the detection of
a hard error, the station evokes the Token Ring beaconing process in order to
recover the ring. What makes hard errors different from soft errors is the element of
an actual failure in the physical transmission infrastructure (for example, the
complete failure of a NIC, lobe cable, or horizontal cable).
The beacon process is, in essence, a failure alarm that is sent by the station
detecting the error to the rest of the stations on the ring. The beacon frame consists
of three information elements: generating a host address, the host's NAUN address,
and the hard error beacon type. The hard error beacon type provides some
indication of the type hard error that has evoked the beacon process and the status
of the beacon process. The following lists the hard error beacon types:
• Signal loss error—Indicates that a cable fault is the source of the error
condition
• Streaming signal—Indicates that one of the stations in the fault domain is in
a failure state
• Ring recovery—Indicates that the error has been isolated and the ring is in
recovery
After the beaconing process is initiated, the other stations on the ring enter beacon
repeat mode. In most cases, the station generating the beacon is not the station at
fault. Rather, it is the beaconing station's NAUN. These two stations make up the
hard error fault domain. The recovery process that is the end-result of the beacon
process is two-fold, fault identification and ring reconfiguration.
Fault identification is achieved by both the error-generating station and it's NAUN by
performing phases 0 (lobe Test) and 2 (Duplicate Address Verification Test) from
the ring station insertion process. If either of these tests fail on either station, the
failed station removes itself from the ring. After the failed station is out of the ring,
reconfiguration can take place through automatic, or, if necessary, manual means.