Nortel Networks NN43001-301 - Manual (Page 161)

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
.