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SAMPLE INSTALLATION AND MAINTENANCE PROBLEMS
B-11
common equipment shelf of the D4. External timing may only be optioned from the OIU-2. Loop
timing should be selected (digital switch as the master reference) in every instance but one: when a
lower stratum clock is available to time the D4-channel bank externally. Figures B-12, N
O
Synchronization Reference Assigned at Location A, and B-13, No, Primary, or Secondary Sync
Reference Assigned at Location A, show these arrangements.
DS1 OR DMI
D4
INTERFACE
CHANNEL
LOCATION A
BANK
INTERNAL TIMING
LOOP TIMING AT THE OIU
Figure B-12. No Synchronization Reference Assigned at Location A
DS1 OR DMI
INTERFACE
D4
CHANNEL
LOCATION A
BANK
INTERNAL TIMING
LOOP TIMING AT THE OIU
OR
OR
PRIMARY REFERENCE
EXTERNAL TIMING AT THE OIU
OR
SECONDARY REFERENCE
Figure B-13. No, Primary, or Secondary Sync Reference Assigned at Location A
Synchronization remains an issue when interfacing with COs. An analog CO (such as a 1A ESS)
does not have an internal clock compatible with DS1, nor does it have a direct DS1. Normally, the
CO is interfaced to a DS1/DMI via a D4-channel bank. The local exchange company (LEC) may or
may not have a DS1 reference available for connection to customer facilities. The options are
expanded, but the issues are the same as when the D4 is located on customer premises; it is either
timed from the customer premises switch A or it is externally timed.