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SYNCHRONIZATION OF DIGITAL FACILITIES
5-29
CONCLUSIONS ON SYNCHRONIZATION
Make no assumptions regarding synchronization. Reverify items such as the availability of a
synchronization source, the clock stratum, and compatibility of every T1 span. The best guarantee is
written confirmation that the local exchange carrier, AT&T Communications, or other vendor will
either synchronize to a System 75, System 85, Generic 1, Generic 2, or provide an appropriate
synchronization reference.
Develop a diagram showing the network synchronization plan. Make multiple copies of this diagram
and keep a copy at each switch site. This diagram is essential for installing, administering, and
tuning up a DS1 network. Such a diagram can also be used by maintenance personnel to troubleshoot
network problems associated with synchronization.
USE OF GENERIC 2 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 DS1 as a clock
reference are provided next.
ISDN-PRI Trunk Facilities (ANN35 or TN767 with TN755)
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.
Line-Only Mode DS1/DMI-BOS (ANN11_ or TN767)
When using a line-only mode interface for synchronization purposes, the following facts should be
known:
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Since the D4-channel bank (or equivalent) at the far end will be timed to the signal received from
a DS1, no slips should occur on a line-only mode DS1 facility. Because of this, slip counts from
line-only mode DS1s should not be used in the process of determining 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 the far end D4-channel bank will not be locked to the AT&T reference
frequency.