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Crackling on a call between the PSTN and an IP phone

Core Issue

These are possible causes of the problem:

  • A mismatch in the line code type on the digital interface
  • A cable or interface hardware issue

Resolution

To resolve this issue, keep in mind this network example:

  • The Cisco CallManager connects to a Cisco 3640 voice gateway running Cisco IOS  Software version 12.2.2XB.
  • The gateway has four working Foreign Exchange Office (FXO) ports and one malfunctioning PRI.
  • The voice path from the PRI to the IP phone has a crackle, but the IP voice stream to the PRI is fine.
  • The problem occurs 99 percent of the time.
  • It can be an incoming or an outgoing call.

This is the output of the show controllers T1 command:

#show controllers t1
T1 1/0 is up.
Applique type is Channelized T1
Cablelength is long gain36 0db
Description: Voice PRINo alarms detected.
alarm-trigger is not set
Version info Firmware: 20010629, FPGA: 15
Framing is ESF, Line Code is B8ZS, Clock Source is Line.
Data in current interval (185 seconds elapsed):
4503 Line Code Violations, 
3901 Path Code Violations
0 Slip Secs, 
0 Fr Loss Secs, 
184 Line Err Secs, 
3 Degraded Mins
183 Errored Secs, 
182 Bursty Err Secs, 
1 Severely Err Secs, 
0 Unavail Secs
Total Data (last 24 hours)794073 Line Code Violations, 
710946 Path Code Violations,
0 Slip Secs, 
0 Fr Loss Secs, 4
7624 Line Err Secs, 
974 Degraded Mins,
47598 Errored Secs, 
45475 Bursty Err Secs, 
34 Severely Err Secs, 
0 UnavailSecs 

Looking at the current 15 minute period captured by issuing the show controllers command, there are a large number of Path Code Violations (PCVs) and Line Code Violations (LCVs) and corresponding errored and bursty errored seconds.

For more information on the meaning of these errors, refer to RFC 1406: Definitions of Managed Objects for the DS1 and E1 Interface Types.

To resolve this issue, perform these steps:

  1. Ensure that the line code type matches that of the carrier. To change the line code type, issue the linecode command on the controller. For more information on resolving the digital line, refer to T1 Error Events Troubleshooting.
  2. In rare cases, these problems can also occur when a short cable (50 feet or less) is used to connect to the next device. With short cables, the Line Interface Unit (LIU) receiver may be saturated and result in LCVs. This is particularly true if the LIU receiver is overly sensitive to a far-end equipment's transmitted signal. In these cases, change the line buildout with reference to the cablelength long section of T1/E1 Controller Commands.
  3. If these actions do not resolve the problem, try replacing the cable and gateway hardware.

For more information on symptom classifications, refer to Recognizing and Categorizing Symptoms of Voice Quality Problems.

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