If the subscription succeeds, the digits are sent in the NOTIFY messages. The conference server then sends a SUBSCRIBE to the endpoint with the event package kpml. Similarly, the conference server may indicate the same in the response. An endpoint can advertise that it supports KPML by including the string kpml in the Allow-Events header in the initial offer message.
A KPML flow is established using SUBSCRIBE/NOTIFY. KPML is an out-of-band mechanism to collect DTMF digits. When the user presses a digit, the endpoint sends an encoded RTP packet with the payload type of 116 (as negotiated in the preceding example). The conference server then can include the same in the SDP answer. The endpoint is indicating that it is capable of supporting RFC 2833 through the MIME type telephone-event (dynamic payload type 116). Consider the following SDP offer, which is sent from a SIP endpoint to the conference server: This capability is specified as MIME type telephone-event in the SDP offer. RFC 2833 defines RTP payload types for carrying DTMF digits in-band in the media stream.
RFC 2833 is a special way of sending DTMF in-band, and Key Press Markup Language (KPML) provides a way for the endpoints to send DTMF out-of-band. Endpoints connecting via an IP network send DTMF information in-band or out-of-band. Endpoints that dial into a PSTN gateway must play DTMF tones in the media stream so that the PSTN gateway can "hear" the tones. Voice-band DTMF tones are modulated as actual tones in the media. Endpoints can use three methods to send DTMF digits:
Endpoints that connect to a conference server via a PSTN gateway often must navigate through an IVR using DTMF tones, and therefore DTMF support in the endpoints and the conference server is important to the conferencing support.
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