6.12
DIGITAL NETWORK
141
Figure 6.25
North American hierarchical network synchronization. (From Ref. 9, Figure 11-2.)
2. Holdover Stability. This is the amount of frequency offset that a clock experiences
after it has lost its synchronization reference. Holdover is specified for stratum 2.
The stratum-3 holdover extends beyond one day and it breaks up the requirement
into components for initial offset, drift, and temperature.
3. Pull-in/Hold-in. This is a clock's ability to achieve or maintain synchronization with
a reference that may be off-frequency. A clock is required to have a pull-in/hold-in
range at least as wide as its free-run accuracy. This ensures that a clock of a given
stratum level can achieve and maintain synchronization with the clock of the same
or higher stratum level.
6.12.2.2.2
North American Holdover and Slip Performance
. When a network clock
loses its references, it enters holdover and drifts off frequency. The magnitude of this
frequency drift determines the average slip rate experienced by equipment that depends
on that clock timing source. Table 6.5 shows the number of slips expected after one day
and one week of holdover given limited ambient temperature variations of
±1
F in the
switching center. The table shows the difference between stratum levels for performance
during holdover. If maintenance actions are prompt when the unusual holdover occurs
and we base a network on stratum-2 or -3E clocks, a virtually slip-free network can be
expected (Ref. 9).
6.12.2.3
CCITT Synchronization Plans
. CCITT Rec. G.811 (Ref. 10) deals with
synchronization of international links. Plesiochronous operation is preferred (see
Section 6.12.2). The recommendation states the problem at the outset:
Table 6.5
Expected Slip Performance in Holdover
Stratum Level
Slips in Day 1
Slips in Week 1
2
1 or less
2
3E
1 or less
13
3
17
266
Source: Ref. 9, Table 5-1, p. 5-2.
Summary :
This ensures that a clock of a given stratum level can achieve and maintain synchronization with the clock of the same or higher stratum level. The recommendation states the problem at the outset: Table 6.5 Expected Slip Performance in Holdover Stratum Level Slips in Day 1 Slips in Week 1 2 1 or less 2 3E 1 or less 13 3 17 266 Source: Ref.
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