Nortel Networks OPTera Metro 3500 - Manual (Page 194)

2-156 Operation, administration, and maintenance (OAM) features
OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
For an OPTera Metro 3500 system to be synchronized with high-quality timing
signals from an external source such as a BITS. If the system contains network
elements not connected to external timing sources, then high-quality timing
signals must be distributed from the network elements that are connected to
external timing sources. The timing signals are distributed in the SONET
signal. A network element that receives its timing signals in the SONET signal
is line timed.
External timing reference input signals to STX and VTX-series circuit
packs
An OPTera Metro 3500 system can receive timing signals from an external
timing source such as a stratum clock or a BITS. The BITS is connected to the
network element by wire-wrap connectors on the Left OAM (LOAM). The
VTX-series or STX-192 circuit packs are connected to the BITS through the
backplane of the network element. The timing signals from an external timing
source are called BITSIN-A and BITSIN-B.
The VTX-series or STX-192 circuit packs provide a stable reference frequency
of 38.88 MHz from an external timing source to the transport and tributary
circuit packs in the network element. Each STX and VTX-series circuit packs
contains a synchronization block. Each synchronization block uses the stable
reference frequency as the basis for the two system clocks (38.88-MHz and
2-kHz clock).
If the signal from the external timing source is interrupted, the STX and
VTX-series circuit packs enters holdover mode, and continues to provide a
stable reference frequency to the transport and tributary circuit packs.
BITSIN-A and BITSIN-B can be DS1 signals or composite clock signals. The
ability to switch between BITSIN-A and BITSIN-B provides non-revertive
1+1 reference protection. If the two external references fail, then the node
switches to holdover mode.
Synchronization-status messages
Synchronization-status messages (SSM) indicate the quality of the timing
signals currently available to a network element. The timing sources that can
be provisioned in a network element include external timing from a BITS,
timing derived from SONET interfaces, and the internal clock of the network
element.
A network element can select the better of the two timing signals provided by
the primary and secondary timing sources provisioned by the user. The
selection is based on the quality values carried in the SSMs.
See Figure 2-49 on page 2-155 for an example of a network showing the
synchronization flow, head-end network element, synchronization boundary,
and synchronization status messaging.