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Modem and fax calls fail to train when connected through 2600 series gateways using an AIM-VOICE-30 module

Core Issue

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

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 digital errors cause the fax machines to misinterpret the digital signal, resulting in unsuccessful fax session negotiation or training.

The Digital Signal Processor (DSP) resources of the Advanced Integration Module (AIM) module 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. Issue the clock source line command under the appropriate controller. The Voice and WAN Interface Card (VWIC) 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 and T1 VWIC and pass it across the backplane to the AIM, issue the network-clock-participate command in global configuration mode. If multiple VWICS are installed, the command is appropriately repeated.
  3. 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-select 1 t1 0/0    
    Router(config)#^Z
    
  4. To confirm the system clocking, issue the show network-clocks command, 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 
    

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

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

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