Codecs

The only codecs supported by DTNA are G.711 uncompressed voice (both A and µ law), and iLBC (Internet Low Bitrate Codec, RFC 3951), modes 20 and 30. Blueworx Voice Response offers one or more of these codecs (as defined in the VoIP DTNA and DTEA Media group) on an outgoing SIP Invite and accepts one of the offered codecs on an incoming SIP Invite.

Using iLBC

iLBC is a narrow-band speech audio compression format that is open-source and royalty-free. It provides audio quality better than G.729A with built-in error correction to provide improved robustness in networks with high packet loss.

iLBC compression is significantly more complex than G.711 and so, understandably, requires greater processing power. A system that can currently support 480 channels of G.711 can typically support up to 120 iLBC channels. Systems with a higher/lower rPerf will support more/less concurrent channels, respectively, up to the maximum 480 per system/partition. See http://www.ibm.com/systems/power/hardware/notices/rperf.html for further information on rPerf.

To enable iLBC in VoIP calls, it must be added to the list of codec preferences found in: Configuration->System Configuration->Change->VoIP DTEA and DTNA Media->nth Codec Preference.

If iLBC is not included in any of these settings, Blueworx Voice Response will not negotiate iLBC in any calls, inbound or outbound. The order of the configured codecs (where 1st is most preferable and 4th is least preferable) determines the order in which Blueworx Voice Response supplies them in the SDP part of an INVITE message. This in turn determines the order of preference for outbound calls from Blueworx Voice Response, where the highest codec on the list supported by both parties is selected.

It is also important to set the DTNA channel allocation method to ensure the call load is spread over the adapters on the system by setting the following parameter to Allocate calls balanced across trunks: Configuration->System Configuration->Change->VoIP SIP Signalling->Inbound Call Channel Allocation Method.

When configured in Blueworx Voice Response, if iLBC is offered with no mode specified, Blueworx Voice Response defaults to iLBC 20. All audio is converted to the standard Blueworx Voice Response audio format before it is presented to the application.

Trombone is possible with iLBC 20 and iLBC 30, however, tromboning is not supported between a 30ms codec and a 20ms codec. Therefore, iLBC 30 cannot be tromboned with ALAW, MULAW or iLBC 20. However, it is possible to trombone between two iLBC 30 lines. Table 1 below shows a compatibility matrix for tromboned calls: