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

	ADMINISTRATION OPTIONS AND REQUIREMENTS — GENERIC 2	7-59
Field 10	Offers the options slip enable (1) or disable (0). This option enables/disables
	collection of slip data to support excessive slip rate based on 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.

Those 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, those other DS1/T1 spans that are used for data applications and that are 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, those DS1/T1 spans that are used exclusively for voice and that are 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 DS1s for which slips can’t be 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

Criteria for Switching to the Secondary

If as many as 50% of those spans that are administered for slip enable are experiencing slips (with respect to the primary), then a decision is made to switch to the secondary. On switching to the secondary, a software bit is set making the primary appear as though it has exceeded its maximum slip limit. At this point, the primary is not evaluated again for 1 hour.