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B-14 SYNCHRONIZATION OF DIGITAL FACILITIES
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With automatic reference switching enabled, most Generic 1 error counters are decremented by 1 every 15
minutes and initialized to zero on reaching the threshold value. The following conditions cause an offline
reference to be restored to online (items 2 through 7 apply only when the stratum-3 option is not used).
1. The system configuration maintenance detects DS1 circuit pack sanity (for example, a DS1 has been
reinitialized, reinserted into the carrier, and/or replaced).
2. The DS1 reference determines that the LOS error no longer exists.
3. The remote loop-around test is completed.
4. The red alarm has been cleared.
5. The blue alarm has been cleared.
6. The slip error counter has cleared.
7. The misframe error counter has cleared.
The External Synchronization Clock
It is recommended that all customer premises switches (except for System 75 and Generic 1) that connect
to the AT&T public network and transfer timing use a stratum-4E clock or better (see Technical Publication
62411). Such a clock already exists in System 75. For AT&T customer-premises switches (except
Generic 1), the method chosen to meet this recommendation is the external synchronization clock
(Generic 1 uses an internal-clock upgrade). For System 85, the external clock, new cabling, and TN2131
external clock interface circuit pack are used instead of the SCS circuit pack and its associated cabling and
administration.
One advantage of the stratum-3 clock is that it offers 24-hour hold over accuracy. Using an external
stratum-3 clock, therefore, can avoid many slips when a network is severed.
The external clock may be retrofit into existing System 85s (R2V3 and R2V4). Furthermore, most
Generic 2s and Generic 1s (R1V5) will use the external clock. This clock is physically mounted external
to the switch cabinets.
The external clock requires duplicated -48VDC power. This power source may be an existing -48VDC
power plant (which is located on customer premises), or a duplicated two cabinet-mounted power supply
with battery backup. The input voltage tolerance is -45V to -52V with a worst-case current drain of 3A.
The cabinet mounted power supply provides up to 10 minutes of holdover.
In terms of application, the external clock connects between the NCTEs and the switch. Cabling for the I/O
and alarm connections is via 25-pair cables with 50-pin telephone connectors. The NCTE-to-external clock
connection has the same distance limitations as the NCTE-to-DS1 connection (655 feet maximum). For
System 85 and Generic 2, the cable run from the external clock to the TN2131 circuit pack cannot exceed
3000 feet. Cable connections to and from the external clock will route via the yellow cross-connect field.
Figure B-7, Public-Network External Clock, shows a public-network configuration of the external clock.
The external clock provides two timing reference inputs and may connect to two NCTEs. These timing
reference inputs are called Reference 1 and Reference 2. They are functionally equivalent to the primary
and secondary references used with the stratum-4E or stratum-4 clock; however, the references must be
from a stratum 3 or stratum 2 source. The same basic considerations that apply to the selection of primary