AT&T DS1/DMi/ISDN-PRI manual Sample Installation and Maintenance Problems

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

INTERFACE

LOCATION A

INTERNAL TIMING

D4

CHANNEL

BANK

LOOP TIMING AT THE OIU

Figure B-12.No Synchronization Reference Assigned at Location A

DS1 OR DMI
INTERFACE	D4
LOCATION A	CHANNEL
	BANK

INTERNAL TIMING	LOOP TIMING AT THE OIU
O R	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.