Nortel Networks OPTera Metro 3500 - Manual (Page 185)

Operation, administration, and maintenance (OAM) features 2-147
Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
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 2-155 for 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.