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Choppy voice heard on VoIP or VoFR calls across a public Frame Relay network

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Wed, 11/18/2009 - 18:27
Jun 18th, 2009
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Core Issue

The issue is that the public Frame Relay network is inserting a delay and dropping packets.

This is an example network:

  • A simple site-to-site connection across a public Frame Relay circuit with a Committed Information Rate (CIR) of 64Kbps
  • Two calls supported with a low bit rate codec and appropriate queueing and shaping

These are the symptoms:


  • Issuing a show frame-relay pvc command reveals that no Backward Explicit Congestion Notification (BECN) or Discard Eligible (DE) bit packets are being received at either end. Therefore, Frame Relay CIR is not being exceeded.
  • An extended ping with 2000 byte packets yields dropped packets and a return trip time of over 1 second, even when no data is sharing the Permanent Virtual Circuit (PVC). Expected delay on a 64Kbps circuit should be 250ms each way at the worst.
  • Issuing a show interfaces command shows that neither router is dropping packets or experiencing errors.
  • The sound of the voice can be described as a broken or choppy voice.


Resolution

The problem could be any one of these issues:



  • Queuing:

    This issue can also be removed from consideration with tests that only include voice. If choppy voice is not eliminated, continue resolving the issue elsewhere. If the choppy voice symptom is eliminated when the data is removed or when such a test is not viable, check the queuing configuration. For more information, refer to the Strict Priority for Voice Traffic (LLQ or IP RTP Priority) section of VoIP over Frame Relay with Quality of Service (Fragmentation, Traffic Shaping, LLQ / IP RTP Priority).


  • Frame Relay Traffic Shaping (FRTS):

    In this example, no BECNs or DE bit packets are being received at either end. Therefore, traffic shaping appears to be operating correctly. Problems experienced with 0K CIR services and services with inappropriate shaping to CIR ratios would manifest themselves differently. For more information on FRTS for voice, refer to these documents:


  • Lack of bandwidth and Connection Admission Control (CAC):

    Ensure that the required number of calls are supported for the codec being used. Also check assumptions regarding the use of compressed RTP (cRTP) and Voice Activity Detection (VAD). For more information on bandwidth consumption, refer to Voice over IP - Per Call Bandwidth Consumption.


  • Delay or packet drop in the path of the call:

    In this example, the round trip ping time is excessive. In addition, there are no drops on router interfaces. These facts lead to the conclusion that packets are either being delayed or dropped in the Frame Relay network.

    Check with the carrier and ask them to check the round trip delay and drop characteristics of their network with reference to Loopback Tests for T1/56K Lines.

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