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Regular clicking noise or echo heard in voice calls across a 7200VXR series router, acting as a digital voice gateway, and clock slips are incrementing on the E1 or T1 controller

Core Issue

The show controllers command shows a repeating clock slip on the E1 or T1 interface, as shown in this command output:

Router#show controllers e1 0/0
 E1 0/0 is up.
  Applique type is Channelized E1 - balanced
  No alarms detected.
  alarm-trigger is not set
  Version info Firmware: 20020812, FPGA: 11
  Framing is CRC4, Line Code is HDB3, Clock Source is Line.
  Data in current interval (97 seconds elapsed):
     0 Line Code Violations, 0 Path Code Violations
     4 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
     4 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
  Total Data (last 14 15 minute intervals):
     1 Line Code Violations, 4 Path Code Violations,
     425 Slip Secs, 0 Fr Loss Secs, 1 Line Err Secs, 0 Degraded Mins,
     427 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

The occurrence of clock slips on the digital trunks can cause an audible clicking noise (or echo) during the conversation.

The Digital Signal Processor (DSP) resources on the 7200VXR series router reside on the PA-VXC-2TE1+ or PA-VXB-2TE1+ card, but DSP can be shared on the card E1 and T1 interfaces. DSP can also be shared with other E1 and T1 interfaces installed in other port adapter slots across the Time-Division Multiplexing (TDM) backplane architecture.

To allow all the interfaces and DSP resources to be correctly shared across the TDM backplane of the router chassis, the system clocking needs to be enabled. This allows the DSP resources to be synchronized to the clock signal coming in from the appropriate digital line.

Note: The default clocking configuration is for the port adapter to source clock from the router backplane. On the other hand, the digital interface defaults to clocking from the line. In situations such as these, where digital interfaces need to source clock across the TDM backplane, its default clocking mode needs to be changed.

Resolution

To resolve the issues on 7200VXR series routers, issue the frame-clock-select command.

To confirm the 7200VXR system clocking, issue the show network-clocks command, as shown in this output:

Router#show network-clocks 
  Network Clock Configuration 
  --------------------------- 
  Priority      Clock Source    Clock State     Clock Type 
    0             T1 0/0             Good               T1  
    5          Backplane             Good               PLL 
  Current Primary Clock Source 
  --------------------------- 
  Priority      Clock Source    Clock State     Clock Type 
     0             T1 0/0            Good               T1   

For more information on clocking configurations, refer to Clocking Configurations On Voice-Capable IOS-Based Platforms.

Voice Gateway Hardware Components

PA-VXB-2TE1+

PA-VXC-2TE1+

PA-VXA-1TE1-24+

PA-VXA-1TE1-30+

Voice Gateways

7200 Series Multiservice Platform

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