Nortel Networks OPTera Metro 3500 - Manual (Page 189)

Operation, administration, and maintenance (OAM) features 2-151
Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
timing signal quality from a network element in holdover mode in the event a
timing reference is disrupted. See
Synchronization-status messages on page
2-156 .
Hierarchy violations
A hierarchy violation occurs when a clock of one stratum level is used to
synchronize a clock of a higher stratum level. A stratum 3 clock synchronizing
a stratum 2 clock is one such example. The synchronization network must be
carefully planned so that no hierarchy violations occur.
Timing loops
A timing loop is created when a clock is synchronizing itself, either directly or
through intermediate equipment. A timing loop causes excessive jitter and can
result in traffic loss.
Timing loops can be caused by a hierarchy violation, or by having clocks of
the same stratum level synchronize each other. In a digital network, timing
loops can be caused during the failure of a primary reference source, if the
secondary reference source is configured to receive timing from a derived
transport signal within the network.
A timing loop can also be caused by incorrectly provisioned synchronization
status message (SSM) for some of the facilities in a linear or ring system.
Under normal conditions, if there is a problem in the system (for example,
pulled fiber), the SSM functionality will heal the timing in the system.
However, if the SSM is incorrectly provisioned, the system might not be able
to heal itself and might segment part of itself in a timing loop.
Table 2-32
Transmitted timing signal quality in holdover mode
Module Quality of disrupted
timing reference Quality of transmitted
timing signal in holdover
mode
VTX-48 ≥ ST3 ST3
< ST3
(NE ignores signal) ST3
VTX-48e
≥ ST3
ST3
< ST3
(NE ignores signal) ST3
STX-192 ≥ ST3 ST3
< ST3
(NE ignores signal) ST3