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Fax transmissions are incomplete or corrupted due to digital clock slips on a Cisco 2800 or 3800 Integrated Services Router (ISR) gateways

Core Issue

The show controller  command shows a repeating clock slip on the E1 or T1 interface, as shown in this 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 call to be dropped, or the fax transmissions to be incomplete or corrupted. The problem occurs with all fax machines.

In this example, a fax passthrough is used between a skinny-based Analog Telephone Adaptor (ATA) 186 registering with the Cisco CallManager 3.3(2). The Cisco IOS  Software gateway at the other end of the VoIP network is 2621. However, this issue could apply to other situations as well.

Note: The default clocking configuration is for the digital interface to take clocking from the line. In situations such as these, where digital interfaces need to source clock across the Time-Division Multiplexing (TDM) backplane, its default clocking mode needs to be changed.

If clocking is not correctly configured, clock slips result.

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) controller configuration defaults to clock source line. However, the recovered clock is not used to drive the Advanced Integration Module (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 an example configuration:

Router#configure terminal            
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

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

For additional useful information, refer to DSP on NM-HDV2 Functionality Verification for 2600XM/2691/2800/3700/3800 Platforms.

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