5.9 DTMF Handling For TrFO Connections

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

DTMF from the UE is sent via DTAP procedures out-band. For a TrFO call the Originating MSC shall use an out-band DTMF procedure, all CN nodes shall support this procedure in their call control protocol. The out-band DTMF procedure shall also be used when TrFO is not achieved in order that TFO is possible. Insertion of DTMF in the PCM payload can result in the break of the TFO connection.

For terminating calls DTMF may need to be received by the core network (for voice-prompted services, voicemail control procedures etc). If the DTMF is received out-band then out-band procedures shall be maintained in core network.

If the DTMF is received for a TrFO call from an external network inband, in I.366.2 profile or RTP payload type, then the gateway MGW which interworks between Iu Framing and the external framing protocol shall report the DTMF tones via H.248 procedures to its server. The server shall then use out-band procedures to pass the DTMF through the CN. See Figure 5.9/1.

The same shall apply if a DTMF tone is received for a TrFO call from an external network inband in a PCM coded stream. The DTMF tone shall be detected by the MGW and reported via H.248 procedures to its server. In order to prevent duplication of DTMF tones due to subsequent PCM legs in the call, when encoding to compressed codecs the detected tones shall not be allowed to continue in the compressed stream; the DTMF Digits shall be deleted by the MGW before entering the speech encoding stage.

The MGW may also optionally pass DTMF inband where such an option exists for the Nb interface, and is supported by the proceeding MGW.

Transcoding to default PCM to send DTMF tones shall be avoided for TrFO connections.

Figure 5.9/1:DTMF received inband from external network

5.10 Framing Protocol for GERAN AoIP mode

AoIP user plane does not use IuFP framing protocol or associated 3GUP procedures. Rate control procedures are performed within RTP.