Nortel Networks OPTera Metro 3500 - Manual (Page 188)

2-150 Operation, administration, and maintenance (OAM) features
OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
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 .
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 2-151 describes the transmitted
Table 2-31
ANSI-required standard clock strata
Clock quality Minimum accuracy Minimum holdover stability
Stratum 1
Stratum 2
Stratum 3
SMC
Stratum 4
+1.0 x 10
-11
+1.6 x 10
-8
+4.6 ppm
+20 ppm
+32 ppm
not applicable
1 x 10
-10
per day
+0.37 ppm during first 24 hours
+4.6 ppm during first 24 hours
not required