AT&T DS1/DMi/ISDN-PRI manual Criteria for Switching to the Secondary Facility

	ADMINISTRATION OPTIONS AND REQUIREMENTS — SYSTEM 85	7-11
Field 10	V1-V4
	This options enables (1) or disables (0) switching between the primary, secondary,

or internal high-accuracy clock. The decision to switch from one source to the other is normally based on an internal slip count calculation (software record). However, hardware events (such as primary link failures) may take precedence over any software controls.

Slips are caused by differences in clock frequencies. A slip results in the deletion or repetition of a single frame. Slips are not caused by noise on the line.

DS1/T1 spans that are used to provide the primary and secondary synchronization reference should be administered for slip enable (1). Since the switch software does this automatically, this task is not mandatory but is a good procedure to follow. Typically, other DS1/T1 spans that are used for data applications and deemed important should also be administered for slip enable. This excludes all T1-spans connecting channel banks, unless the channel bank is externally timed. Normally, DS1/T1 spans that are used exclusively for voice and not assigned as the primary or secondary synchronization source should be administered for slip disable (0). The goal is to keep that reference on-line, which minimizes slips for all those DS1/DMI Interfaces for which slips cannot tolerated.

The digital switch always maintains a slip-count record for each DS1. Slip counts are calculated on a 24-hour continuous interval. As a historical record, the slip counts for each DS1 are maintained for the last 24 consecutive intervals. The slip count is used to determine if a DS1 span is experiencing errors and, if so, the severity of the errors (type alarm).

If the primary facility uses 24th-channel signaling and if the secondary facility uses RBS, then the primary will always be on-line unless a hardware event forces a switch to the secondary. A software algorithm is used to select the facility (primary or secondary) that is on-line for the cases where:

a . Both primary and secondary facilities use the same type of signaling (either 24th-channel or RBS)

b . The primary uses robbed-bit, and the secondary uses 24th-channel signaling

The slip count can be viewed in procedure 625 test 1.

Criteria for Switching to the Secondary Facility

If 50% of the spans administered for slip enable are experiencing slips (with respect to the primary), then a decision is made to switch to the secondary. When a System 85 switches to its secondary, a software bit is set making the primary appear as though it exceeded its maximum slip limit. The primary is not reevaluated for one hour.

Criteria for Switching Back to the Primary Facility

At the end of the one-hour interval, the slip count is analyzed. If the primary slip count is less than 2, then a switch back to the primary is made. If the primary has a