RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals
RFC 2833

Document Type RFC - Proposed Standard (May 2000; No errata)
Obsoleted by RFC 4734, RFC 4733
Last updated 2013-03-02
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Network Working Group                                      H. Schulzrinne
Request for Comments: 2833                            Columbia University
Category: Standards Track                                      S. Petrack
                                                                 May 2000

   RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   This memo describes how to carry dual-tone multifrequency (DTMF)
   signaling, other tone signals and telephony events in RTP packets.

1 Introduction

   This memo defines two payload formats, one for carrying dual-tone
   multifrequency (DTMF) digits, other line and trunk signals (Section
   3), and a second one for general multi-frequency tones in RTP [1]
   packets (Section 4). Separate RTP payload formats are desirable since
   low-rate voice codecs cannot be guaranteed to reproduce these tone
   signals accurately enough for automatic recognition. Defining
   separate payload formats also permits higher redundancy while
   maintaining a low bit rate.

   The payload formats described here may be useful in at least three
   applications: DTMF handling for gateways and end systems, as well as
   "RTP trunks". In the first application, the Internet telephony
   gateway detects DTMF on the incoming circuits and sends the RTP
   payload described here instead of regular audio packets. The gateway
   likely has the necessary digital signal processors and algorithms, as
   it often needs to detect DTMF, e.g., for two-stage dialing. Having
   the gateway detect tones relieves the receiving Internet end system
   from having to do this work and also avoids that low bit-rate codecs
   like G.723.1 render DTMF tones unintelligible. Secondly, an Internet

Schulzrinne & Petrack       Standards Track                     [Page 1]
RFC 2833                         Tones                          May 2000

   end system such as an "Internet phone" can emulate DTMF functionality
   without concerning itself with generating precise tone pairs and
   without imposing the burden of tone recognition on the receiver.

   In the "RTP trunk" application, RTP is used to replace a normal
   circuit-switched trunk between two nodes. This is particularly of
   interest in a telephone network that is still mostly circuit-
   switched.  In this case, each end of the RTP trunk encodes audio
   channels into the appropriate encoding, such as G.723.1 or G.729.
   However, this encoding process destroys in-band signaling information
   which is carried using the least-significant bit ("robbed bit
   signaling") and may also interfere with in-band signaling tones, such
   as the MF digit tones. In addition, tone properties such as the phase
   reversals in the ANSam tone, will not survive speech coding. Thus,
   the gateway needs to remove the in-band signaling information from
   the bit stream. It can now either carry it out-of-band in a signaling
   transport mechanism yet to be defined, or it can use the mechanism
   described in this memorandum. (If the two trunk end points are within
   reach of the same media gateway controller, the media gateway
   controller can also handle the signaling.)  Carrying it in-band may
   simplify the time synchronization between audio packets and the tone
   or signal information. This is particularly relevant where duration
   and timing matter, as in the carriage of DTMF signals.

1.1 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
   indicate requirement levels for compliant implementations.

2 Events vs. Tones

   A gateway has two options for handling DTMF digits and events. First,
   it can simply measure the frequency components of the voice band
   signals and transmit this information to the RTP receiver (Section
   4). In this mode, the gateway makes no attempt to discern the meaning
   of the tones, but simply distinguishes tones from speech signals.

   All tone signals in use in the PSTN and meant for human consumption
   are sequences of simple combinations of sine waves, either added or
   modulated. (There is at least one tone, the ANSam tone [3] used for
   indicating data transmission over voice lines, that makes use of
   periodic phase reversals.)

   As a second option, a gateway can recognize the tones and translate
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