Nortel Networks NTRN10AN, 3500 manual Stratum clocks, Synchronization hierarchy, Smc

2-150Operation, administration, and maintenance (OAM) features

the module automatically enters acquire mode. The maximum time a module can remain in holdover mode is 24 hours. After 24 hours, the module enters the free run mode.

Stratum clocks

Stratum clocks are stable timing reference signals that are graded according to their accuracy. American National Standards Institute (ANSI) standards have been developed to define four levels of stratum clocks. The accuracy requirements of these stratum levels are shown in Table 2-31 on page 2-150.

Table 2-31

ANSI-required standard clock strata

Clock quality	Minimum accuracy	Minimum holdover stability
Stratum 1	+1.0 x 10-11	not applicable
Stratum 2	+1.6 x 10-8	1 x 10-10per day
Stratum 3	+4.6 ppm	+0.37 ppm during first 24 hours
SMC	+20 ppm	+4.6 ppm during first 24 hours
Stratum 4	+32 ppm	not required

Synchronization hierarchy

A synchronization hierarchy is a network of stratum clocks that contains one stratum 1 clock and several lower stratum clocks, as shown in Figure 2-48 on page 2-152. The stratum 1 clock sends a reference signal to several stratum 2 clocks. These stratum 2 clocks, in turn, transmit synchronization signals to other stratum 2 and stratum 3 clocks. Similarly, stratum 3 clocks synchronize other stratum 3 and stratum 4 clocks.

For reliable operation, the synchronization network includes primary and secondary synchronization facilities to each stratum 2 and 3 node, and to many stratum 4 nodes. In addition, each stratum 2 and 3 node is equipped with an internal clock that can bridge short disruptions to the synchronization reference.

Each network element transmits a synchronization-status message (SSM) from all SONET interfaces (DS1, EC-1x3, OC-3, OC-12, OC-48 and OC-192). When the timing reference to a network element is disrupted, the network element enters holdover mode.

A network element in holdover mode transmits timing signals with the quality level of its internal clock, depending on the quality of the alternate timing reference. If the alternate timing reference is of a higher quality than the internal clock of a network element, then the network element uses the alternate timing reference. Table 2-32 on page 2-151describes the transmitted

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