Voice is choppy or synthetic over 64k WAN with LLQ configured

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Wed, 11/18/2009 - 18:27
Jun 22nd, 2009
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Core Issue


The site-to-site WAN connection has limited bandwidth, which is also carrying data and voice.

These are among the sound samples:


The site-to-site WAN connection is 64k. Communication to printers and other servers occurs over this link, in addition to the up to six phone users who generally do not all use the phone at the same time. Voice connections are working but are of poor quality.Low Latency Queueing (LLQ) is properly configured to protect voice from data. The current equipment is running G.729 voice codec. There is no compression for the WAN connection.

Without Compressed Real Time Protocol (cRTP), the G729 codec requires about 24Kbps per call at a full rate. In this example, once a third concurrent call was established across the 64Kbps link, the voice quality would suffer and become either choppy, synthetic, or both.


Resolution


To resolve the issue, perform these steps:


  1. Change the codec on both sides for this connection to be G.723.
  2. Implement Multilink Point-to-Point Protocol (MLPPP) and cRTP for the WAN.

As a result, up to seven calls can be carried out simultaneously. Also, existing LLQ ensures that voice is protected from large data transfers.

Implementing cRTP and G.723.1 reduces the bandwidth requirement per call to the order of 8Kbps. Therefore, this theoretically supports about seven calls across the 64Kbps link.

For information on per-call bandwidth considerations, including use of cRTP, Voice Activity Detection (VAD), and codec choice, refer to Voice Over IP - Per Call Bandwidth Consumption.

Although voice is protected from data, the data is not necessarily protected from voice. In addition, should more telephones be added to the network, some form of Connection Admission Control (CAC) is recommended.

The voice quality of a G.723 call may be lower than for a G.729 call. Conferencing may not be possible with lower bit rate codecs. Certain codecs may not be supported on certain hardware platforms.


For more information, refer to these documents:


For more information, refer to the Correcting Audio Problems from the Cisco IP Phone section of  Device Issues.

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