Run Accuracy Holdover Stability Pull-in/Hold-in 10-11 +1.6 X 10-8 1 10-10 per day +1.6 X 10-8 +4.6 X 10-6 1 X 10 day 1 4.6 X 10-6 +4.6 X 10-6 <255 slips during first day of holdover 4.6 X 10-6 +32 X 10-6 No holdover 32 X 10-6 Source: Ref. 9, Table 3-1, p. 3-3. 6.12 DIGITAL NETWORK Switch w/stratum N Primary sync reference Secondary sync reference PRS = primary reference source 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. 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 11F 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