Nortel Networks OPTera Metro 3500 - Manual (Page 192)

2-154 Operation, administration, and maintenance (OAM) features
OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
When the network element timing mode is set to Internal, the synchronization
block in the STX and VTX-series circuit pack produces network element
timing without any external timing sources. In Internal timing mode, the STX
and VTX-series circuit packs provide a network element timing quality
defined in
Table 2-32 on page 2-151 .
External timing
External timing is shown in Figure 2-49 on page 2-155 , example (b).
An external timing signal can be obtained from a BITS clock of stratum 3
quality or better.
External timing uses a timing source independent of any internal clock or
received transport signal. The external timing source is a highly accurate
stratum clock. If the external source is lost, the STX and VTX-series circuit
packs provide network element timing internally, for short periods, based on
the last received reference (a function called holdover).
Primary and secondary timing references can also be provisioned; for
example, the primary timing reference is set to BITS-A and the secondary
reference is set to BITS-B. The system selects the active timing reference
based on the stability of the transport signal, the synchronization message, and
any incoming synchronization status provisioned by the user.
An external timing signal can be obtained from a BITS clock of stratum 3 or
better.
Line timing
Line timing is a signal derived from an incoming SONET frame (EC1, OC-3,
OC-12, OC-48 or OC-192) or an incoming DS1 signal.
Transport line timing is shown in Figure 2-49 on page 2-155 , example (c).
When using transport line timing, a network element derives timing from a
received transport signal. The network element selects one of the two timing
sources (primary and secondary timing references) as the active timing
reference. The selection is made based on the stability of the transport signal,
the synchronization message, and any incoming synchronization status
provisioned by the user. (For information on synchronization messaging, see
Synchronization-status messages on page 2-156 .) The derived signal is used in
the network element to synchronize outgoing transport signals in both
directions, and all synchronous tributary signals terminated by the network
element. The quality of synchronization depends on the stability of the
transport signal received from the remote end.