Guidelines - Nortel Communication Server 1000S Installation And Configuration Manual

When data bits are written into (added to) a buffer at a slightly higher rate
than that at which they are being read (emptied), sooner or later the buffer
overflows This is a slip-frame deletion.
In the opposite situation, when data bits are written (added) into a buffer at
slightly lower rate than that at which they are being read (emptied), eventually
the buffer runs dry or underflows. This is also a slip-frame repetition.
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.
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
Encrypted Text
Video
Digital Data
Facsimile
Voice Band Data
Voice

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.
Clock Controller description and installation
Potential Impact
Encryption key must be resent.
Freeze frame for several seconds. Loud pop on
audio.
Deletion or repetition of data. Possible misframe.
Deletion of 4 to 8 scan lines. Dropped call.
Transmission Errors for 0.01 to 2 s. Dropped
call.
Possible click.
ISDN Primary Rate Interface
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Installation and Configuration