AT&T DS1/DMi/ISDN-PRI manual Facilities, for details, 6ADMINISTRATION Requirements

A-6ADMINISTRATION REQUIREMENTS

Primary	System 75 must obtain synchronization timing from a 4ESS. Typically, the
	carrier slot location for the DS1 connecting to a 4ESS is used as the primary
	source. The switch must also be equipped with a TN741.
Secondary	If multiple DS1s are present in a System 75 and are connected to this or other
	4ESSs, then it may be desirable to enter their slot locations as a secondary
	synchronization source. Refer to chapter 5, Synchronization of Digital Network
	Facilities, for details.
Location	Circuit pack slot location (as required)
Name	Enter the facility name (if applicable)
Slip	Generally, DS1 facilities are very stable. Therefore, in most applications, a y
	should be entered.