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B-30 SYNCHRONIZATION OF DIGITAL FACILITIES
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only mode DS1s should not be used to determine the health of a clock reference (that is, they would
tend to make the reference appear to be in better condition than it may be).
• Line-only mode DS1s should not be used as system clock references unless it is known for certain that
the incoming DS1 is locked to the AT&T reference frequency. Normally, a DS1 facility terminating
directly on a far-end D4-channel bank is not locked to the AT&T reference frequency.
Line+Trunk Mode DS1/DMI-BOS (ANN35 or TN767 with TN555)
Regarding the use of a line+trunk mode DS1/DMI-BOS for synchronization, the following point should be
noted. Since it would be expected that a line+trunk mode DS1 would terminate at a class-5 or higher CO or
at another switch, the line+trunk interface should be suitable for use as a primary or secondary clock
reference (or to be the timing master for another switch).
The slip count provided by the interface should also be used in the process of choosing a healthy clock
reference. However, when the interface terminates on a D4-channel bank (or equivalent) that is not locked
to the AT&T reference frequency for its timing, then the interface should not be used for synchronization.
DMI-MOS (ANN35 or TN767 with TN555)
System 85 DS1/DMI-MOS (ANN35) will only terminate at a compatible computer. The computer must
always obtain its timing from the switch. The switch should never select the DMI-MOS link as a timing
reference.
Although those circuit packs that support connections to remote modules operate at the DS1’s rate, they are
not the same as the DS1/DMI/ISDN-PRI. Remote module connections cannot be used as a clock
synchronization reference.
USE OF GENERIC 1 AS A SYSTEM CLOCK REFERENCE
To provide increased reliability, it is recommended that (for all cases) the primary and secondary system
clock references be placed in different modules. Tips on how best to use DS1s as a clock reference are
provided next.
Trunk-Mode ISDN-PRI (TN767)
ISDN-PRI trunks may terminate on a 4ESS toll office, 5ESS digital CO, System 85 R2V4, Generic 1,
Generic 2, or compatible vendor’s switch. Depending on other considerations, any of these terminating
connections may be selected as a synchronization reference, either primary or secondary.