Nortel Networks 3500 manual Internal timing, External timing, Line timing, Transport line timing

Operation, administration, and maintenance (OAM) features 2-147

These configurations require synchronization among the network elements to avoid the effects of the SONET synchronous transport signal pointer repositioning within the frame. When a network element is synchronized, all synchronous tributaries and high-speed signals generated by that network element are synchronized to its timing source.

Normally, one network element in a ring (UPSR or BLSR) is externally timed. To protect the network timing against complete nodal failure, two network elements in a ring can be externally timed.

See Figure 2-49 on page 2-155for an example of a network showing the synchronization flow, head-end network element, synchronization boundary, and synchronization status messaging.

Each network element is synchronized by one of the following methods:

•internal timing

•external timing

•line timing

Internal timing

A SONET-compliant free-running clock produced within the network element provides internal timing. Network elements with VTX-48, VTX-48e and STX-192 modules provide timing signals of Stratum 3 (ST3) quality.

External timing

An external timing signal is obtained from a building-integrated timing supply (BITS) clock of ST3 or better.

Line timing

Line timing is when a timing signal is derived from an incoming SONET frame (OC-3, OC-12, OC-48, OC-192), DS1 facility or EC-1 facility.

Note 1: Line timing is derived from DS1 circuit pack (NTN430AA, BA) in OPTera Metro 3500 shelf equipped with VTX-series circuit packs.

Note 2: Line timing is derived from EC-1x3 (NTN436AA) circuit pack.

There are two types of line timing: transport and tributary.

Transport line timing

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. Possible sources of transport line timing are OC-3, OC-12, OC-48 and OC-192 facilities.

Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004