26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
Typically, "DTMF events" are triggered by an end-user (on either side of the speech call) by pressing some key of the user’s terminal, with the goal to control services in the network or in an application server. This may happen throughout the duration of the speech call on both ends of the call.
It might happen that DTMF events are forwarded as telephone events to the (remote) MTSI client in terminal. In such a case, the MTSI client in terminal may convert the telephone events into audible DTMF tone-pairs. There is, however, no requirement in 3GPP for handling downlink telephone events.
This annex describes a method for sending DTMF events as telephone events within the same RTP media stream as the speech, i.e. "inband", using the same IP address and UDP port as the RTP for speech, but using a different RTP Payload Type number.
– MTSI clients in terminal offering speech communication shall support the below described method in the transmitting direction (uplink) and should support it in the receiving direction (downlink).
– MTSI media gateways offering speech communication shall support the below described method in both directions with respect to the MTSI client in terminal, the transmitting direction (downlink) and in the receiving direction (uplink). For MTSI media gateways, the described method applies only to the PS session between the MTSI gateway and the MTSI client in terminal.
– The use of the telephone-event codec between MTSI media gateways is recommended. Typically, DTMF events are also transported between MTSI media gateways as telephone events in both directions.