Chances are, if you've used a regular landline phone, you've made a call into a system where you have to press keys on your numeric keypad in order to reach your intended party (e.g. dialing an extension, trying to get your account balance at your bank). Touch tones, or dual-tone multifrequency (DTMF) signaling, is what makes this happen.

In the VoIP world, voices are compressed and encoded so that the communication can be transmitted over the network. However, this compression is optimized for voice, which means DTMF doesn't work. This creates a problem when you want or need to do more than talk to a human being.

The folks at the Internet Engineering Task Force (IETF) have a couple of different solutions to this problem in the context of the SIP protocol. A lesser-known standard is called SIP INFO (RFC2976). The more common method for DTMF signaling is RFC2833, which encodes the DTMF keypresses and other telephony events as part of the RTP (audio) stream of the SIP call.

Recently, the IETF released RFC4733, which obsoletes RFC2833. In addition to DTMF signaling, RFC4733 will provide an open, extensible framework for other types of signaling. From the RFC4733 abstract:

This memo captures and expands upon the basic framework defined in RFC 2833, but retains only the most basic event codes. It sets up an IANA registry to which other event code assignments may be added. Companion documents add event codes to this registry relating to modem, fax, text telephony, and channel-associated signalling events. The remainder of the event codes defined in RFC 2833 are conditionally reserved in case other documents revive their use.

This document provides a number of clarifications to the original document. However, it specifically differs from RFC 2833 by removing the requirement that all compliant implementations support the DTMF events. Instead, compliant implementations taking part in out-of-band negotiations of media stream content indicate what events they support. This memo adds three new procedures to the RFC 2833 framework: subdivision of long events into segments, reporting of multiple events in a single packet, and the concept and reporting of state events.

Hopefully, the industry will adopt this RFC quickly and we can put an end to the various out-of-band signaling issues that occur with VoIP.

Via Disruptive Telephony