26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
DTMF events shall be encoded and transmitted in MTSI sessions as "telephone events" in RTP packets, using the selected "telephone-event" codec. DTMF events in this Annex refer to the DTMF Named Events described in Section 3.2, Table 3 in IETF RFC 4733 , i.e. events (0-9, *, # and A-D) which are encoded with event code values 0-9, 10, 11 and 12-15 respectively.
DTMF events are carried as part of the audio stream which can either be narrowband, wideband, super-wideband or fullband audio, i.e. use 8 kHz or 16 kHz RTP clock rate respectively. MTSI clients that in addition to narrowband also support wideband, super-wideband or fullband speech shall support telephone-event RTP clock rates matching all supported codecs. When switching between speech and DTMF, telephone events shall use the same RTP clock rate as the currently used codec for the speech signal in the same RTP stream.
The encoding of DTMF events includes specifying the duration time for the events, see IETF RFC 4733 . Supporting long-lasting DTMF events, where the duration time exceeds the maximum duration time expressible by the duration field, is optional. If supported, long-lasting DTMF events shall be divided into segments, see IETF RFC 4733 . To harmonize with legacy DTMF signalling, , , the tone duration of a DTMF event shall be at least 65 ms and the pause duration in-between two DTMF events shall be at least 65 ms. The duration of the DTMF event and the pause time to the next DTMF event, where applicable, should be selected such that it enables incrementing the RTP Time Stamp with an integer multiple of the number of timestamp units corresponding to the frame length of the speech codec used for the speech media before the DTMF event.