Figure 29: Bipolar violations
Cyclic Redundancy Check (CRC-4). When the 2.0 Mb DTI card runs in CRC-4 mode, the
CRC-4 word contains a checksum of all data in the multiframe.
The receiving side uses the
checksum to verify the data.
The primary difference between BPV and CRC is that bipolar violation tracking indicates errors
in the local span, while CRC indicates errors on an end-to-end span. For example, on a satellite
link, BPV detects errors only in the span between the system and the satellite connection.
Since CRC traverses the entire span, it indicates an end-to-end bit error rate.
Slip (SLP). Digital signals must have accurate clock synchronization for data to interleaf into,
or extract from, the appropriate timeslot during multiplexing and demultiplexing operations. A
frame slip is defined as the repetition or deletion of 193 bits (one frame) due to a discrepancy
in the read and write rates at the buffer (clocks aren't operating at EXACTLY the same
speed).
When data bits write to (add to) a buffer at a slightly higher rate than they read (empty), sooner
or later the buffer overflows. This is called slip-frame deletion.
In the opposite situation, when data bits write (add) to a buffer at a slightly lower rate than they
read (empty), eventually the buffer runs dry or underflows. This is called slip-frame
repetition.
All degradations shown in
Table 63: Performance impact of one slip on page
133 can be
controlled or avoided by proper clock (network) synchronization.
Table 63: Performance impact of one slip
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 4-8 scan lines. Drop Call.
Monitor system DTI operation
ISDN Primary Rate Interface Maintenance November 2011 133
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