How many voice channels can a VoIP system support?

As with a PSTN Blueworx Voice Response telephony configuration, the number of channels available on each pSeries computer or BladeCenter computer depends on the number of adapters installed in the system unit.

A single DTNA adapter supports 96 channels of VoIP streaming when configured in T1 mode, and 120 channels when configured to work in E1 mode.

In Blueworx Voice Response, the internal coding of voice is always µ-law for T1 trunks and A-law for E1 trunks. If one or more DTTA PSTN adapters, are present in the same system as one or more DTNA adapters, the DTNAs must operate in the same mode (either T1 or E1) as the PSTN adapters (whose operating mode is determined by the network attachment). For VoIP-only configurations, with no PSTN adapters, it is possible to use all 120 channels of the DTNA in a T1 country⁄region by configuring the system to be E1 with a default µ-law VoIP encoding (however, in this case, voice segments must be stored as A-law).

Note: When you configure Blueworx Voice Response for VoIP, you still need to select either T1 or E1 mode even though the DTNAs are not actually connected to PSTN trunks.

Blueworx Voice Response supports up to a total of four DTEAs and DTTAs, so for a VoIP-only configuration, the maximum number of TDM or mixed VoIP channels available on each pSeries computer (or LPAR on such a computer) is 384 (4×96) for T1 and 480 (4×120) for E1.

Capacity planning for DTNA

The use of DTNA imposes some additional load on your system processor over and above that of the DTTA and the DTEA adapters (both of which are now unavailable). This is because work which was previously done on hardware adapter cards is now being done in the main system processor.

Individual core processing speed is no longer relevant for Blueworx Voice Response. The overall system relative performance, or rPerf, is the defining factor in how many channels can be supported. As a guideline, an rPerf of 20 can support a 480-channel Blueworx Voice Response LPAR running speech-enabled VoiceXML applications. See http://www.ibm.com/systems/power/hardware/notices/rperf.html for further information on rPerf.

The use of secure SIP and secure RTP, increases the system performance requirements of Blueworx Voice Response by approximately 30%. As a guideline, a system performance rPerf of 26 can support a 480-channel secure SIP and secure RTP Blueworx Voice Response LPAR running speech-enabled VoiceXML applications.

Up to four Blueworx Voice Response LPARs are supported per pSeries computer or BladeCenter blade.

To minimize CPU impact, consider the following:

Ensure that RFC2833 RTP Payloads are used for DTMF transmission rather than DTMF tones. Most modern SIP devices support RFC2833, usually by default. The VOIP_MONITOR utility, provided with Blueworx Voice Response to support SIP, can be used to ensure that Telephone Event is being offered in the SDP (body) part of the SIP Invite message, and then accepted as the DTMF method to be used, as indicated in the SDP (body) part of the 200 OK response to the SIP Invite. If this is not found to be the case, either Blueworx Voice Response (DTEA and DTNA parameters) or the remote SIP endpoint (or both) need to be reconfigured to support RFC2833.
Ensure that your application uses uncompressed play and recording. This is because the DTNA processing overhead of converting between the Blueworx Voice Response internal 5:1 algorithm voice format and uncompressed G711 A and µ law as sent/received over the line, is significant.
- For Java and VoiceXML applications, uncompressed play and recording is always used, and no further action need be taken.
- Most State Table applications will already have been written to use uncompressed voice (on account of the higher quality) and therefore no further actions are required.
- However, if your State Table application uses compressed prompts (or messages, audio names and/or greetings) but does no recording in the compressed format, you need to take this into account in capacity sizings. If your application also does compressed recording, this factor also needs to be taken into account when planning channel capacity. Alternatively, consider changing your application to use uncompressed voice throughout.
- Unified Messaging for Blueworx Voice Response is a voice messaging application. The Blueworx Voice Response and Unified Messaging supports the use of compressed or uncompressed voice in messages, audio names and greetings through setting options in Blueworx Voice Response System Configuration. If you are planning to operate Unified Messaging for Blueworx Voice Response in uncompressed mode for all voice objects from the outset, or to migrate your existing Unified Messaging for Blueworx Voice Response system to uncompressed voice, you need take no further action.

For the most recent information on suitable processors and capacity planning for DTNA, contact your IBM representative.