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User experiences no audio or one-way audio on IP phones when calls are placed across 2691 or 3700 gateways

Core Issue

No audio or one-way audio is experienced, even though the show call active voice and show voice dsp commands issued at the gateway indicate that packets are transmitted and received.

The most obvious symptom is the repeating clock slips on the E1 or T1 interface. The show controller command shows the repeating clock slips, as shown in the following example:

Router#show controller 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 digital errors cause the fax machines to misinterpret the digital signal, and training is unsuccessful.

The Digital Signal Processor (DSP) resources of the Advanced Integration Module (AIM) on a Cisco 2600 and 3700 are accessed through Time-Division Multiplexing (TDM) based architecture. Clocking errors occur as a result of the AIM module not being clocked from the appropriate digital line.

Note: The default clocking configuration is for the AIM module 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 issue, perform these steps:

  1. Ensure that the clock source line command is issued under the appropriate controller. The Voice and WAN Interface Card (VWIC) controller configuration defaults to clock source line. However, the recovered clock is not used to drive the AIM module. Therefore, this configuration is not sufficient to resolve the clock slip problem.
  2. To allow the router to recover the clock from the E1/T1 VWIC and pass it across the backplane to the AIM, the network-clock-participate command is added in global configuration mode. If multiple VWICS are installed, the command is appropriately repeated.
  3. To allow the AIM modules in a 3700 or 2691 to participate in the system clocking, the network-clock-participate command is added for each AIM module.
  4. Specify the network clock source and priority. Issue the network-clock-select command in global configuration mode.

This is an example of the required configuration:

Router#conf t            
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#network-clock-participate wic 0
Router(config)#network-clock-participate aim 0    
Router(config)#network-clock-select 1 t1 0/0    
Router(config)#^Z

Issue the show network-clocks command to confirm the system clocking, as shown in this example:

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 synchronized clocking, refer to the NAS Clocking Commands section of Clock Synchronization for AS5xxx Network Access Servers.

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


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