Nortel Networks NTRN10AN, 3500 manual External timing, Line timing

2-154Operation, administration, and maintenance (OAM) features

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.

OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004