Network Working Group                                        J. Sjoberg Request for Comments: 4867                                M. Westerlund Obsoletes: 3267                                                Ericsson Category: Standards Track                                  A. Lakaniemi                                                                    Nokia                                                                  Q. Xie                                                                Motorola                                                              April 2007          RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB)
Audio Codecs
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 IETF Trust (2007).
Abstract
This document specifies a Real-time Transport Protocol (RTP) payload    format to be used for Adaptive Multi-Rate (AMR) and Adaptive Multi-
Rate Wideband (AMR-WB) encoded speech signals.  The payload format is    designed to be able to interoperate with existing AMR and AMR-WB
transport formats on non-IP networks.  In addition, a file format is    specified for transport of AMR and AMR-WB speech data in storage mode    applications such as email.  Two separate media type registrations
are included, one for AMR and one for AMR-WB, specifying use of both    the RTP payload format and the storage format.  This document
obsoletes RFC 3267.
Sjoberg, et al.            Standards Track                    [Page 1]
Table of Contents
1. Introduction (4)
2. Conventions and Acronyms (4)
3. Background on AMR/AMR-WB and Design Principles (5)
3.1. The Adaptive Multi-Rate (AMR) Speech Codec (5)
3.2. The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec (6)
3.3. Multi-Rate Encoding and Mode Adaptation (6)
3.4. Voice Activity Detection and Discontinuous Transmission (7)
3.5. Support for Multi-Channel Session (7)
3.6. Unequal Bit-Error Detection and Protection (8)
3.6.1. Applying UEP and UED in an IP Network (8)
3.7. Robustness against Packet Loss (10)
3.7.1. Use of Forward Error Correction (FEC) (10)
3.7.2. Use of Frame Interleaving (12)
3.8. Bandwidth-Efficient or Octet-Aligned Mode (12)
3.9. AMR or AMR-WB Speech over IP Scenarios (13)
4. AMR and AMR-WB RTP Payload Formats (15)
4.1. RTP Header Usage (15)
4.2. Payload Structure (17)
4.3. Bandwidth-Efficient Mode (17)
4.3.1. The Payload Header (17)
4.3.2. The Payload Table of Contents (18)
4.3.3. Speech Data (20)
4.3.4. Algorithm for Forming the Payload (21)
4.3.5. Payload Examples (21)
4.3.
5.1. Single-Channel Payload Carrying a
Single Frame (21)
4.3.
5.2. Single-Channel Payload Carrying
Multiple Frames (22)
4.3.
5.3. Multi-Channel Payload Carrying
Multiple Frames (23)
4.4. Octet-Aligned Mode (25)
4.4.1. The Payload Header (25)
4.4.2. The Payload Table of Contents and Frame CRCs (26)
4.4.2.1. Use of Frame CRC for UED over IP (28)
4.4.3. Speech Data (30)
4.4.4. Methods for Forming the Payload (31)
4.4.5. Payload Examples (32)
4.4.
5.1. Basic Single-Channel Payload
Carrying Multiple Frames (32)
4.4.
5.2. Two-Channel Payload with CRC,
Interleaving, and Robust Sorting (32)
4.5. Implementation Considerations (33)
4.5.1. Decoding Validation (34)
5. AMR and AMR-WB Storage Format (35)
5.1. Single-Channel Header (35)
5.2. Multi-Channel Header (36)
Sjoberg, et al.            Standards Track                    [Page 2]
adaptive5.3. Speech Frames (37)
6. Congestion Control (38)
7. Security Considerations (38)
7.1. Confidentiality (39)
7.2. Authentication and Integrity (39)
8. Payload Format Parameters (39)
8.1. AMR Media Type Registration (40)
8.2. AMR-WB Media Type Registration (44)
8.3. Mapping Media Type Parameters into SDP (47)
8.3.1. Offer-Answer Model Considerations (48)
8.3.2. Usage of Declarative SDP (50)
8.3.3. Examples (51)
9. IANA Considerations (53)
10. Changes from RFC 3267 (53)
11. Acknowledgements (55)
12. References (55)
12.1. Normative References (55)
12.2. Informative References (56)
Sjoberg, et al.            Standards Track                    [Page 3]
1.  Introduction
This document obsoletes RFC 3267 and extends that specification with    offer/answer rules.  See Section 10 for the changes made to this
format in relation to RFC 3267.
This document specifies the payload format for packetization of AMR
and AMR-WB encoded speech signals into the Real-time Transport
Protocol (RTP) [8].  The payload format supports transmission of
multiple channels, multiple frames per payload, the use of fast codec    mode adaptation, robustness against packet loss and bit errors, and
interoperation with existing AMR and AMR-WB transport formats on
non-IP networks, as described in Section 3.
The payload format itself is specified in Section 4.  A related file    format is specified in Section 5 for transport of AMR and AMR-WB
speech data in storage mode applications such as email.  In Section
8, two separate media type registrations are provided, one for AMR
and one for AMR-WB.
Even though this RTP payload format definition supports the transport    of both AMR and AMR-WB speech, it is important to remember that AMR
and AMR-WB are two different codecs and they are always handled as
different payload types in RTP.
2.  Conventions and Acronyms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this    document are to be interpreted as described in RFC 2119 [5].
The following acronyms are used in this document:
3GPP  - the Third Generation Partnership Project
AMR    - Adaptive Multi-Rate (Codec)
AMR-WB - Adaptive Multi-Rate Wideband (Codec)
CMR    - Codec Mode Request
CN    - Comfort Noise
DTX    - Discontinuous Transmission
ETSI  - European Telecommunications Standards Institute
FEC    - Forward Error Correction
SCR    - Source Controlled Rate Operation
SID    - Silence Indicator (the frames containing only CN
parameters)
VAD    - Voice Activity Detection
UED    - Unequal Error Detection
UEP    - Unequal Error Protection
Sjoberg, et al.            Standards Track                    [Page 4]
The term "frame-block" is used in this document to describe the
time-synchronized set of speech frames in a multi-channel AMR or
AMR-WB session.  In particular, in an N-channel session, a frame-
block will contain N speech frames, one from each of the channels,
and all N speech frames represents exactly the same time period.
The byte order used in this document is network byte order, i.e., the    most significant byte first.  The bit order is also the most
significant bit first.  This is presented in all figures as having
the most significant bit leftmost on a line and with the lowest
number.  Some bit fields may wrap over multiple lines in which cases    the bits on the first line are more significant than the bits on the    next line.
3.  Background on AMR/AMR-WB and Design Principles
AMR and AMR-WB were originally designed for circuit-switched mobile
radio systems.  Due to their flexibility and robustness, they are
also suitable for other real-time speech communication services over    packet-switched networks such as the Internet.
Because of the flexibility of these codecs, the behavior in a
particular application is controlled by several parameters that
select options or specify the acceptable values for a variable.
These options and variables are described in general terms at
appropriate points in the text of this specification as parameters to    be established through out-of-band means.  In Section 8, all of the
parameters are specified in the form of media subtype registrations
for the AMR and AMR-WB encodings.  The method used to signal these
parameters at session setup or to arrange prior agreement of the
participants is beyond the scope of this document; however, Section
8.3 provides a mapping of the parameters into the Session Description    Protocol (SDP) [11] for those applications that use SDP.
3.1.  The Adaptive Multi-Rate (AMR) Speech Codec
The AMR codec was originally developed and standardized by the
European Telecommunications Standards Institute (ETSI) for GSM
cellular systems.  It is now chosen by the Third Generation
Partnership Project (3GPP) as the mandatory codec for third
generation (3G) cellular systems [1].
The AMR codec is a multi-mode codec that supports eight narrow band
speech encoding modes with bit rates between 4.75 and 12.2 kbps.  The    sampling frequency used in AMR is 8000 Hz and the speech encoding is    performed on 20 ms speech frames.  Therefore, ea
ch encoded AMR speech    frame represents 160 samples of the original speech.
Sjoberg, et al.            Standards Track                    [Page 5]
Among the eight AMR encoding modes, three are already separately
adopted as standards of their own.  Particularly, the 6.7 kbps mode
is adopted as PDC-EFR [18], the 7.4 kbps mode as IS-641 codec in TDMA    [17], and the 12.2 kbps mode as GSM-EFR [16].
3.2.  The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec
The Adaptive Multi-Rate Wideband (AMR-WB) speech codec [3] was
originally developed by 3GPP to be used in GSM and 3G cellular
systems.
Similar to AMR, the AMR-WB codec is also a multi-mode speech codec.
AMR-WB supports nine wide band speech coding modes with respective
bit rates ranging from 6.6 to 23.85 kbps.  The sampling frequency
used in AMR-WB is 16000 Hz and the speech processing is performed on    20 ms frames.  This means that each AMR-WB encoded frame represents
320 speech samples.
3.3.  Multi-Rate Encoding and Mode Adaptation
The multi-rate encoding (i.e., multi-mode) capability of AMR and
AMR-WB is designed for preserving high speech quality under a wide
range of transmission conditions.
With AMR or AMR-WB, mobile radio systems are able to use available
bandwidth as effectively as possible.  For example, in GSM it is
possible to dynamically adjust the speech encoding rate during a
session so as to continuously adapt to the varying transmission
conditions by dividing the fixed overall bandwidth between speech
data and error protective coding.  This enables the best possible
trade-off between speech compression rate and error tolerance.  To
perform mode adaptation, the decoder (speech receiver) needs to
signal the encoder (speech sender) the new mode it prefers.  This
mode change signal is called Codec Mode Request or CMR.
Since in most sessions speech is sent in both directions between the    two ends, the mode requests from the decoder at one end to the
encoder at the other end are piggy-backed over the speech frames in
the reverse direction.  In other words, there is no out-of-band
signaling needed for sending CMRs.
Every AMR or AMR-WB codec implementation is required to support all
the respective speech coding modes defined by the codec and must be
able to handle mode switching to any of the modes at any time.
However, some transport systems may impose limitations in the number    of modes supported and how often the mode can change due to bandwidth Sjoberg, et al.            Standards Track                    [Page 6]

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