CyberData Products and RFC 2833.

CyberData Products and RFC 2833.

CyberData devices only support RFC 2833 Dual-Tone Multi-Frequency (DTMF) digits. Our devices require Real-time Transport Protocol (RTP) event notifications to process DTMF; which is supported by the vast majority of the telecom industry. Our devices cannot process in-band style DTMF, meaning old-school frequency based dual tone frequency DTMF. We must receive RTP Events in order to process any DTMF sent to a device.

RFC 2833 is considered by some to be both In-Band and Out-of-Band DTMF. This comes down to the interpretation of the original patent for DTMF filed by AT&T and how DTMF is required to be in-band. This dealt with the old implementation of DTMF where the characters sent as a combination of tones In-Band with the audio itself. This meant that the audio for the tones was in the same audio stream as the conversation in the call.

As we move into the greater VoIP world, audio and DTMF is no longer passed in the same style. We now send audio as RTP packets and DTMF as RTP events in an RTP stream. This becomes a part of contention between the writers of the RFC. Adhering to the original DTMF patent, DTMF must be sent in the audio stream for the call, so some consider RFC 2833 DTMF to be out of band, because it is not in the audio stream. However, if you look at DTMF patent through a modern lens where audio is sent as RTP and DTMF as events it could be considered In-Band. Both the RTP audio stream and DTMF events use a similar packet structure and network port, so it can be argued that it is indeed in-band.

In the context of CyberData products we expect and can only process RFC 2833 RTP event style DTMF. As an example, say you wanted to automate a message through a SIP Paging Server. You would need to call the paging server, select a paging group, and then play the message. If this was done in a single audio file recording where the DTMF is embedded in the audio file itself the paging server will not be able to process the DTMF. The DTMF would need to be sent before the audio file plays as RFC 2833 RTP Event style DTMF.
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