Specifications

Cisco ONS 15454 Product Overview

Timing

Cisco ONS 15454 Product Overview, R3.3

May 2002

Timing Parameters

You must set the SONET timing parameters for each ONS 15454. ONS 15454

timing is set to one of three modes: external, line, or mixed. An externally-timed

node derives its timing from a BITS source wired to the BITS inputs on the

backplane. The BITS source, in turn, derives its timing from a Primary Reference

Source (PRS) such as a Stratum 1 clock or GPS signal. A line-timed node derives

its timing for an incoming optical signal on one of the OC-N cards.

Figure 31 shows an example of an ONS 15454 network-timing setup. Node 1 is set to

external timing. Two references are set to BITS, and the third reference is set to

internal. The BITS output pin on the backplane of Node 3 provides timing to outside

equipment, such as a digital access line access multiplexer.

Synchronization Status Messaging

Synchronization status messaging is a mechanism for managing synchronization

(or network timing) in SONET networks. It allows BITS timing sources, nodes,

and combinations of the two to exchange information about the quality of timing

sources. SSM enables SONET devices such as the ONS 15454 to automatically

select the highest quality timing reference and to avoid timing loops (particularly

in ring architecture). Synchronization status messages are carried in the S1 byte

of the SONET line overhead (Line layer) and as a bit-patterned message in the

Extended Super Frame (ESF) datalink of the BITS DS-1. The ONS 15454

supports BITS inputs with or without SSM.