DTMF Interworking
Multimedia devices and applications can exchange user-input DTMF information end-to-end over IP networks. The Oracle® Enterprise Session Border Controller provides the capabilities required to interconnect networks and devices that use different DTMF indication signaling protocols.
DTMF Indication
There are three ways to convey DTMF information for packet-based communications:
- DTMF audio tones: DTMF digit waveforms are encoded inline with voice packets. This method only works with uncompressed audio codecs like G.711. Compressed audio codecs like G.729 and G.723 are incompatible with DTMF audio. DTMF audio is also referred to as in-band tones.
- Out-of-band signaling events:
SIP INFO messages with Content-Type: application/dtmf-relay define out-of-band signaling events for transmitting DTMF information. SIP INFO messages separate DTMF digits from the voice stream and send them in their own signaling message.
- RTP named telephony events (NTE): RFC 2833 telephone-events are a standard that describes how to transport DTMF tones in RTP packets according to section 3 of RFC 2833. Of the three RTP payload formats available, the Oracle® Enterprise Session Border Controller supports RTP NTE.
RFC 2833 telephone-event
RFC 2833 specifies a way of encoding DTMF-indications in RTP media streams. It does not encode the audio of the tone itself, instead data represents the sent tone. RFC 2833 can be used with SIP.
RFC 2833 defines the format of NTE RTP packets used to transport DTMF digits and other telephony events between two peer endpoints. With the NTE method, the endpoints perform per-call negotiation of the DTMF transfer method. They also negotiate to determine the payload type value for the NTE RTP packets.
The NTE payload takes the place of codec data in a standard RTP packet. The payload type number field of the RTP packet header identifies the contents as 2833 NTE. The payload type number is negotiated per call. The local device sends the payload type number to use for RFC 2833 packets using SDP, which tells the other side what payload type number to use when sending the named event packets to the local device. Most devices use payload type number 101 for RFC 2833 packets, although no default is specified in the standard.
The RFC 2833 packet’s RTP header also makes use of the timestamp field. Because events often last longer than the RFC 2833 packets sending interval, the timestamp of the first 2833 packet for an event represents the beginning reference time for subsequent RFC 2833 packets for that same event. For events that span multiple RTP packets, the RTP timestamp identifies the beginning of the event. As a result, several RTP packets might carry the same timestamp.