RFC2833 [Schulzrinne and Petrack (2000)] describes how to carry DTMF signaling, other tone signals, and telephony events in Reat Time Protocol (RTP) packets. The RTP payload format for a DTMF event is designated as a “telephone-event,” with the media type as an “audio/telephone-event” in negotiation. The RFC2833 payload format for telephone events does not have a static payload type number. The payload type number is established dynamically. The default clock frequency is 8000 Hz, but the clock frequency can be redefined when assigning the dynamic payload type. The named telephone events are carried as part of the voice stream and use the same sequence number and time-stamp base as the regular voice channel. RFC2833-based out-of-band telephone events are used for five different types of events/signals—namely DTMF tones, fax-related tones, standard subscriber line tones, country-specific subscriber line tones, and trunk events.
The RTP header contains 12 bytes, and the details are given in Chapter 10. The use of the RTP header fields time stamp, sequence number, and marker bit are updated in the RFC2833 packet. Chapter 10 has some additional details on RTP header fields, which are common for both RFC2833 packets and voice packets. The RTP marker bit indicates the beginning of a new event. The marker bit should be set to “1” for the beginning of the DTMF packet and to “0” for all succeeding packets relating to the same event.