Nortel Networks NN43001-301 manual Guidelines

Description 161

A 1.5 Mbyte PRI contains a buffer large enough to hold about 2 full DS-1 frames (193 x 2 = 386). A 2 Mbyte PRI contains a buffer large enough to contain 2 full frames (256 x 2 = 512 bits). The buffer is normally kept half full (1 frame).

Slippage has impact on the data being transferred, as is shown in Table 63 "Performance impact of one slip on service type" (page 161). All of the degradations shown in the table can be controlled or avoided with proper clock synchronization.

Table 63

Performance impact of one slip on service type

Service	Potential Impact
Encrypted Text	Encryption key must be resent.
Video	Freeze frame for several seconds. Loud pop
	on audio.
Digital Data	Deletion or repetition of data. Possible
	misframe.
Facsimile	Deletion of 4 to 8 scan lines. Dropped call.
Voice Band Data	Transmission Errors for 0.01 to 2 s. Dropped
	call.
Voice	Possible click.

Guidelines

Some key points to keep in mind when designing Network Synchronization:

•Where possible, the Master Clock Source should always be from a Node Category/Stratum with higher clock accuracy, that is, a PBX connected to the Central Office (CO.) The CO is the Master and the PBX is the Slave.

•The source should not be in free-run itself (providing its own clock) unless it is operating in a fully independent network where the source acts as a Master (see "Plesiosynchronous operation" (page 158).)

•When connecting two PBXs together (no CO connections), the most reliable PBX should be the Master. Reliability here refers to Dual CPU/Dual Clock, battery back-up or stratum level of the clock controller.

•Avoid timing loops. A timing loop occurs when a clock using as its reference frequency a signal that it itself traceable to the output of that clock. The formation of such a closed timing loop leads to frequency instability and is not permitted. Timing loops are sometimes unavoidable on the secondary clock reference source.

•Ensure all CO/PBX links used as clock references have a traceable path back to the same stratum 1 clock source.

Nortel Communication Server 1000

ISDN Primary Rate Interface Installation and Commissioning

NN43001-301 02.03 Standard

Release 5.5 7 December 2007

Copyright © 2003-2007, Nortel Networks