Nortel Networks 3500 ユーザーズマニュアル

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 

 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

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 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 is shown in 

, 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.