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How to minimize the impact of high and inconsistent delay across a satellite connection

Core Issue

A customer's network setup includes a satellite link from ship to land. The adaptive jitter buffer on the Cisco IOS  gateway is not able to adjust properly to the inconsistency and magnitude of the delay and causes choppy voice.

If the voice card is configured for high codec complexity, the highest value that can be configured for nominally compressed codecs is 250 ms. For medium-complexity codec configurations, the highest nominal value is 150 ms.

Resolution

In order to minimize the impact of the delay and jitter in this customer's network, perform these steps:

  1. Set the codec complexity to high using the codec complexity command.
  2. Turn off the adaptive de-jitter buffer operation using the playout-delay mode command.
  3. Adjust the nominal playout-delay to 250 ms.

Note: Nominal playout-delay is the amount of delay applied at the beginning of a call by the jitter buffer in the gateway, in milliseconds. In fixed mode, this is also the maximum size of the jitter buffer throughout the call.

This is an example configuration:

voice-card 1

codec complexity high

voice-port 1/0/0

playout-delay mode fixed

playout-delay nominal 250

This configuration results in an initial lag on the call, but after that there is enough in the buffer to playout a continuous audio stream. In this instance, the voice clarity is satisfactory.

Note: Depending on the severity of the delay and jitter characteristics of the network, these actions may or may not resolve the problem.

Note: Before Cisco IOS Software Release 12.1(5)T, the playout-delay command was configured only in voice-port configuration mode. For Cisco IOS Software Release 12.1(5)T and later, in most cases, playout-delay should be configured in dial-peer configuration mode on the VoIP dial peer that is on the receiving end of the voice traffic that is to be buffered. This dial peer senses network conditions and relays them to the Digital Signal Processors (DSPs), which adjust the jitter buffer as necessary. When multiple applications are configured on the gateway, playout-delay should be configured in dial-peer configuration mode. However, when there are numerous dial peers to configure, it might be simpler to configure playout-delay on a voice port. If conflicting playout-delay values have been configured on a voice port and on a dial peer, the dial-peer configuration takes precedence.

For more information on playout-delay and the de-jitter buffer operation, refer to the playout-delay (dial-peer) section of  the Cisco IOS Voice Commands documentation.

Version history
Revision #:
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Last update:
‎06-22-2009 06:10 PM
Updated by:
 
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