GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3267

Network Working Group J. Sjoberg Request for Comments: 3267 M. Westerlund Category: Standards Track Ericsson

                                                          A. Lakaniemi
                                                                 Nokia
                                                                Q. Xie
                                                              Motorola
                                                             June 2002
 Real-Time Transport Protocol (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 Internet Society (2002).  All Rights Reserved.

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 MIME type registrations are
 included, one for AMR and one for AMR-WB, specifying use of both the
 RTP payload format and the storage format.

Sjoberg, et. al. Standards Track [Page 1] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

Table of Contents

 1. Introduction.................................................... 3
 2. Conventions and Acronyms........................................ 3
 3. Background on AMR/AMR-WB and Design Principles.................. 4
   3.1. The Adaptive Multi-Rate (AMR) Speech Codec.................. 4
   3.2. The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec...... 5
   3.3. Multi-rate Encoding and Mode Adaptation..................... 5
   3.4. Voice Activity Detection and Discontinuous Transmission..... 6
   3.5. Support for Multi-Channel Session........................... 6
   3.6. Unequal Bit-error Detection and Protection.................. 7
     3.6.1. Applying UEP and UED in an IP Network................... 7
   3.7. Robustness against Packet Loss.............................. 9
     3.7.1. Use of Forward Error Correction (FEC)................... 9
     3.7.2. Use of Frame Interleaving...............................11
   3.8. Bandwidth Efficient or Octet-aligned Mode...................11
   3.9. AMR or AMR-WB Speech over IP scenarios......................12
 4. AMR and AMR-WB RTP Payload Formats..............................14
   4.1. RTP Header Usage............................................14
   4.2. Payload Structure...........................................16
   4.3. Bandwidth-Efficient Mode....................................16
     4.3.1. The Payload Header......................................16
     4.3.2. The Payload Table of Contents...........................17
     4.3.3. Speech Data.............................................19
     4.3.4. Algorithm for Forming the Payload.......................20
     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.........................30
     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
 5. AMR and AMR-WB Storage Format...................................34
   5.1. Single Channel Header.......................................34
   5.2. Multi-channel Header........................................35
   5.3. Speech Frames...............................................36
 6. Congestion Control..............................................37
 7. Security Considerations.........................................37
   7.1. Confidentiality.............................................37

Sjoberg, et. al. Standards Track [Page 2] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

   7.2. Authentication..............................................38
   7.3. Decoding Validation.........................................38
 8. Payload Format Parameters.......................................38
   8.1. AMR MIME Registration.......................................39
   8.2. AMR-WB MIME Registration....................................41
   8.3. Mapping MIME Parameters into SDP............................44
 9. IANA Considerations.............................................45
 10. Acknowledgements...............................................45
 11. References.....................................................45
   11.1 Informative References......................................46
 12. Authors' Addresses.............................................48
 13. Full Copyright Statement.......................................49

1. Introduction

 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 MIME 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 RFC2119 [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

Sjoberg, et. al. Standards Track [Page 3] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

    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
 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.

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 MIME 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 codecs 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 8 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, each encoded AMR speech
 frame represents 160 samples of the original speech.

Sjoberg, et. al. Standards Track [Page 4] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 Among the 8 AMR encoding modes, three are already separately adopted
 as standards of their own.  Particularly, the 6.7 kbps mode is
 adopted as PDC-EFR [14], the 7.4 kbps mode as IS-641 codec in TDMA
 [13], and the 12.2 kbps mode as GSM-EFR [12].

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 9 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.  E.g., 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 to enable 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 5] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 limitations or other constraints.  For this reason, the decoder is
 allowed to indicate its acceptance of a particular mode or a subset
 of the defined modes for the session using out-of-band means.
 For example, the GSM radio link can only use a subset of at most four
 different modes in a given session.  This subset can be any
 combination of the 8 AMR modes for an AMR session or any combination
 of the 9 AMR-WB modes for an AMR-WB session.
 Moreover, for better interoperability with GSM through a gateway, the
 decoder is allowed to use out-of-band means to set the minimum number
 of frames between two mode changes and to limit the mode change among
 neighboring modes only.
 Section 8 specifies a set of MIME parameters that may be used to
 signal these mode adaptation controls at session setup.

3.4. Voice Activity Detection and Discontinuous Transmission

 Both codecs support voice activity detection (VAD) and generation of
 comfort noise (CN) parameters during silence periods.  Hence, the
 codecs have the option to reduce the number of transmitted bits and
 packets during silence periods to a minimum.  The operation of
 sending CN parameters at regular intervals during silence periods is
 usually called discontinuous transmission (DTX) or source controlled
 rate (SCR) operation.  The AMR or AMR-WB frames containing CN
 parameters are called Silence Indicator (SID) frames.  See more
 details about VAD and DTX functionality in [9] and [10].

3.5. Support for Multi-Channel Session

 Both the RTP payload format and the storage format defined in this
 document support multi-channel audio content (e.g., a stereophonic
 speech session).
 Although AMR and AMR-WB codecs themselves do not support encoding of
 multi-channel audio content into a single bit stream, they can be
 used to separately encode and decode each of the individual channels.
 To transport (or store) the separately encoded multi-channel content,
 the speech frames for all channels that are framed and encoded for
 the same 20 ms periods are logically collected in a frame-block.
 At the session setup, out-of-band signaling must be used to indicate
 the number of channels in the session and the order of the speech
 frames from different channels in each frame-block.  When using SDP
 for signaling, the number of channels is specified in the rtpmap

Sjoberg, et. al. Standards Track [Page 6] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 attribute and the order of channels carried in each frame-block is
 implied by the number of channels as specified in Section 4.1 in
 [24].

3.6. Unequal Bit-error Detection and Protection

 The speech bits encoded in each AMR or AMR-WB frame have different
 perceptual sensitivity to bit errors.  This property has been
 exploited in cellular systems to achieve better voice quality by
 using unequal error protection and detection (UEP and UED)
 mechanisms.
 The UEP/UED mechanisms focus the protection and detection of
 corrupted bits to the perceptually most sensitive bits in an AMR or
 AMR-WB frame.  In particular, speech bits in an AMR or AMR-WB frame
 are divided into class A, B, and C, where bits in class A are most
 sensitive and bits in class C least sensitive (see Table 1 below for
 AMR and [4] for AMR-WB).  A frame is only declared damaged if there
 are bit errors found in the most sensitive bits, i.e., the class A
 bits.  On the other hand, it is acceptable to have some bit errors in
 the other bits, i.e., class B and C bits.
                                  Class A   total speech
                Index   Mode       bits       bits
                ----------------------------------------
                  0     AMR 4.75   42         95
                  1     AMR 5.15   49        103
                  2     AMR 5.9    55        118
                  3     AMR 6.7    58        134
                  4     AMR 7.4    61        148
                  5     AMR 7.95   75        159
                  6     AMR 10.2   65        204
                  7     AMR 12.2   81        244
                  8     AMR SID    39         39
        Table 1.  The number of class A bits for the AMR codec.
 Moreover, a damaged frame is still useful for error concealment at
 the decoder since some of the less sensitive bits can still be used.
 This approach can improve the speech quality compared to discarding
 the damaged frame.

3.6.1. Applying UEP and UED in an IP Network

 To take full advantage of the bit-error robustness of the AMR and
 AMR-WB codec, the RTP payload format is designed to facilitate
 UEP/UED in an IP network.  It should be noted however that the
 utilization of UEP and UED discussed below is OPTIONAL.

Sjoberg, et. al. Standards Track [Page 7] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 UEP/UED in an IP network can be achieved by detecting bit errors in
 class A bits and tolerating bit errors in class B/C bits of the AMR
 or AMR-WB frame(s) in each RTP payload.
 Today there exist some link layers that do not discard packets with
 bit errors, e.g., SLIP and some wireless links.  With the Internet
 traffic pattern shifting towards a more multimedia-centric one, more
 link layers of such nature may emerge in the future.  With transport
 layer support for partial checksums, for example those supported by
 UDP-Lite [15], bit error tolerant AMR and AMR-WB traffic could
 achieve better performance over these types of links.
 There are at least two basic approaches for carrying AMR and AMR-WB
 traffic over bit error tolerant IP networks:
 1) Utilizing a partial checksum to cover headers and the most
    important speech bits of the payload.  It is recommended that at
    least all class A bits are covered by the checksum.
 2) Utilizing a partial checksum to only cover headers, but a frame
    CRC to cover the class A bits of each speech frame in the RTP
    payload.
 In either approach, at least part of the class B/C bits are left
 without error-check and thus bit error tolerance is achieved.
    Note, it is still important that the network designer pay
    attention to the class B and C residual bit error rate.  Though
    less sensitive to errors than class A bits, class B and C bits are
    not insignificant and undetected errors in these bits cause
    degradation in speech quality.  An example of residual error rates
    considered acceptable for AMR in UMTS can be found in [20] and for
    AMR-WB in [21].
 The application interface to the UEP/UED transport protocol (e.g.,
 UDP-Lite) may not provide any control over the link error rate,
 especially in a gateway scenario.  Therefore, it is incumbent upon
 the designer of a node with a link interface of this type to choose a
 residual bit error rate that is low enough to support applications
 such as AMR encoding when transmitting packets of a UEP/UED transport
 protocol.
 Approach 1 is a bit efficient, flexible and simple way, but comes
 with two disadvantages, namely, a) bit errors in protected speech
 bits will cause the payload to be discarded, and b) when transporting
 multiple frames in a payload there is the possibility that a single
 bit error in protected bits will cause all the frames to be
 discarded.

Sjoberg, et. al. Standards Track [Page 8] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 These disadvantages can be avoided, if needed, with some overhead in
 the form of a frame-wise CRC (Approach 2).  In problem a), the CRC
 makes it possible to detect bit errors in class A bits and use the
 frame for error concealment, which gives a small improvement in
 speech quality.  For b), when transporting multiple frames in a
 payload, the CRCs remove the possibility that a single bit error in a
 class A bit will cause all the frames to be discarded.  Avoiding that
 gives an improvement in speech quality when transporting multiple
 frames over links subject to bit errors.
 The choice between the above two approaches must be made based on the
 available bandwidth, and desired tolerance to bit errors.  Neither
 solution is appropriate to all cases.  Section 8 defines parameters
 that may be used at session setup to select between these approaches.

3.7. Robustness against Packet Loss

 The payload format supports several means, including forward error
 correction (FEC) and frame interleaving, to increase robustness
 against packet loss.

3.7.1. Use of Forward Error Correction (FEC)

 The simple scheme of repetition of previously sent data is one way of
 achieving FEC.  Another possible scheme which is more bandwidth
 efficient is to use payload external FEC, e.g., RFC2733 [19], which
 generates extra packets containing repair data.  The whole payload
 can also be sorted in sensitivity order to support external FEC
 schemes using UEP.  There is also a work in progress on a generic
 version of such a scheme [18] that can be applied to AMR or AMR-WB
 payload transport.
 With AMR or AMR-WB, it is possible to use the multi-rate capability
 of the codec to send redundant copies of the same mode or of another
 mode, e.g., one with lower-bandwidth.  We describe such a scheme
 next.

Sjoberg, et. al. Standards Track [Page 9] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 This involves the simple retransmission of previously transmitted
 frame-blocks together with the current frame-block(s).  This is done
 by using a sliding window to group the speech frame-blocks to send in
 each payload.  Figure 1 below shows us an example.
  1. -+——–+——–+——–+——–+——–+——–+——–+–

| f(n-2) | f(n-1) | f(n) | f(n+1) | f(n+2) | f(n+3) | f(n+4) |

  1. -+——–+——–+——–+——–+——–+——–+——–+–
   <---- p(n-1) ---->
            <----- p(n) ----->
                     <---- p(n+1) ---->
                              <---- p(n+2) ---->
                                       <---- p(n+3) ---->
                                                <---- p(n+4) ---->
            Figure 1: An example of redundant transmission.
 In this example each frame-block is retransmitted one time in the
 following RTP payload packet.  Here, f(n-2)..f(n+4) denotes a
 sequence of speech frame-blocks and p(n-1)..p(n+4) a sequence of
 payload packets.
 The use of this approach does not require signaling at the session
 setup.  In other words, the speech sender can choose to use this
 scheme without consulting the receiver.  This is because a packet
 containing redundant frames will not look different from a packet
 with only new frames.  The receiver may receive multiple copies or
 versions (encoded with different modes) of a frame for a certain
 timestamp if no packet is lost.  If multiple versions of the same
 speech frame are received, it is recommended that the mode with the
 highest rate be used by the speech decoder.
 This redundancy scheme provides the same functionality as the one
 described in RFC 2198 "RTP Payload for Redundant Audio Data" [24].
 In most cases the mechanism in this payload format is more efficient
 and simpler than requiring both endpoints to support RFC 2198 in
 addition.  There are two situations in which use of RFC 2198 is
 indicated: if the spread in time required between the primary and
 redundant encodings is larger than 5 frame times, the bandwidth
 overhead of RFC 2198 will be lower; or, if a non-AMR codec is desired
 for the redundant encoding, the AMR payload format won't be able to
 carry it.
 The sender is responsible for selecting an appropriate amount of
 redundancy based on feedback about the channel, e.g., in RTCP
 receiver reports.  A sender should not base selection of FEC on the
 CMR, as this parameter most probably was set based on none-IP

Sjoberg, et. al. Standards Track [Page 10] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 information, e.g., radio link performance measures.  The sender is
 also responsible for avoiding congestion, which may be exacerbated by
 redundancy (see Section 6 for more details).

3.7.2. Use of Frame Interleaving

 To decrease protocol overhead, the payload design allows several
 speech frame-blocks be encapsulated into a single RTP packet.  One of
 the drawbacks of such an approach is that in case of packet loss this
 means loss of several consecutive speech frame-blocks, which usually
 causes clearly audible distortion in the reconstructed speech.
 Interleaving of frame-blocks can improve the speech quality in such
 cases by distributing the consecutive losses into a series of single
 frame-block losses.  However, interleaving and bundling several
 frame-blocks per payload will also increase end-to-end delay and is
 therefore not appropriate for all types of applications.  Streaming
 applications will most likely be able to exploit interleaving to
 improve speech quality in lossy transmission conditions.
 This payload design supports the use of frame interleaving as an
 option.  For the encoder (speech sender) to use frame interleaving in
 its outbound RTP packets for a given session, the decoder (speech
 receiver) needs to indicate its support via out-of-band means (see
 Section 8).

3.8. Bandwidth Efficient or Octet-aligned Mode

 For a given session, the payload format can be either bandwidth
 efficient or octet aligned, depending on the mode of operation that
 is established for the session via out-of-band means.
 In the octet-aligned format, all the fields in a payload, including
 payload header, table of contents entries, and speech frames
 themselves, are individually aligned to octet boundaries to make
 implementations efficient.  In the bandwidth efficient format only
 the full payload is octet aligned, so fewer padding bits are added.
    Note, octet alignment of a field or payload means that the last
    octet is padded with zeroes in the least significant bits to fill
    the octet.  Also note that this padding is separate from padding
    indicated by the P bit in the RTP header.
 Between the two operation modes, only the octet-aligned mode has the
 capability to use the robust sorting, interleaving, and frame CRC to
 make the speech transport robust to packet loss and bit errors.

Sjoberg, et. al. Standards Track [Page 11] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

3.9. AMR or AMR-WB Speech over IP scenarios

 The primary scenario for this payload format is IP end-to-end between
 two terminals, as shown in Figure 2.  This payload format is expected
 to be useful for both conversational and streaming services.
              +----------+                         +----------+
              |          |    IP/UDP/RTP/AMR or    |          |
              | TERMINAL |<----------------------->| TERMINAL |
              |          |    IP/UDP/RTP/AMR-WB    |          |
              +----------+                         +----------+
                 Figure 2: IP terminal to IP terminal scenario
 A conversational service puts requirements on the payload format.
 Low delay is one very important factor, i.e., few speech frame-blocks
 per payload packet.  Low overhead is also required when the payload
 format traverses low bandwidth links, especially as the frequency of
 packets will be high.  For low bandwidth links it also an advantage
 to support UED which allows a link provider to reduce delay and
 packet loss or to reduce the utilization of link resources.
 Streaming service has less strict real-time requirements and
 therefore can use a larger number of frame-blocks per packet than
 conversational service.  This reduces the overhead from IP, UDP, and
 RTP headers.  However, including several frame-blocks per packet
 makes the transmission more vulnerable to packet loss, so
 interleaving may be used to reduce the effect packet loss will have
 on speech quality.  A streaming server handling a large number of
 clients also needs a payload format that requires as few resources as
 possible when doing packetization.  The octet-aligned and
 interleaving modes require the least amount of resources, while CRC,
 robust sorting, and bandwidth efficient modes have higher demands.
 Another scenario occurs when AMR or AMR-WB encoded speech will be
 transmitted from a non-IP system (e.g., a GSM or 3GPP network) to an
 IP/UDP/RTP VoIP terminal, and/or vice versa, as depicted in Figure 3.

Sjoberg, et. al. Standards Track [Page 12] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

        AMR or AMR-WB
        over
        I.366.{2,3} or +------+                        +----------+
        3G Iu or       |      |   IP/UDP/RTP/AMR or    |          |
        <------------->|  GW  |<---------------------->| TERMINAL |
        GSM Abis       |      |   IP/UDP/RTP/AMR-WB    |          |
        etc.           +------+                        +----------+
                           |
         GSM/3GPP network  |           IP network
                           |
                   Figure 3: GW to VoIP terminal scenario
 In such a case, it is likely that the AMR or AMR-WB frame is
 packetized in a different way in the non-IP network and will need to
 be re-packetized into RTP at the gateway.  Also, speech frames from
 the non-IP network may come with some UEP/UED information (e.g., a
 frame quality indicator) that will need to be preserved and forwarded
 on to the decoder along with the speech bits.  This is specified in
 Section 4.3.2.
 AMR's capability to do fast mode switching is exploited in some non-
 IP networks to optimize speech quality.  To preserve this
 functionality in scenarios including a gateway to an IP network, a
 codec mode request (CMR) field is needed.  The gateway will be
 responsible for forwarding the CMR between the non-IP and IP parts in
 both directions.  The IP terminal should follow the CMR forwarded by
 the gateway to optimize speech quality going to the non-IP decoder.
 The mode control algorithm in the gateway must accommodate the delay
 imposed by the IP network on the response to CMR by the IP terminal.
 The IP terminal should not set the CMR (see Section 4.3.1), but the
 gateway can set the CMR value on frames going toward the encoder in
 the non-IP part to optimize speech quality from that encoder to the
 gateway.  The gateway can alternatively set a lower CMR value, if
 desired, as one means to control congestion on the IP network.

Sjoberg, et. al. Standards Track [Page 13] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 A third likely scenario is that IP/UDP/RTP is used as transport
 between two non-IP systems, i.e., IP is originated and terminated in
 gateways on both sides of the IP transport, as illustrated in Figure
 4 below.
 AMR or AMR-WB                                        AMR or AMR-WB
 over                                                 over
 I.366.{2,3} or +------+                     +------+ I.366.{2,3} or
 3G Iu or       |      |  IP/UDP/RTP/AMR or  |      | 3G Iu or
 <------------->|  GW  |<------------------->|  GW  |<------------->
 GSM Abis       |      |  IP/UDP/RTP/AMR-WB  |      | GSM Abis
 etc.           +------+                     +------+ etc.
                    |                           |
  GSM/3GPP network  |          IP network       |  GSM/3GPP network
                    |                           |
                      Figure 4: GW to GW scenario
 This scenario requires the same mechanisms for preserving UED/UEP and
 CMR information as in the single gateway scenario.  In addition, the
 CMR value may be set in packets received by the gateways on the IP
 network side.  The gateway should forward to the non-IP side a CMR
 value that is the minimum of three values:
  1. the CMR value it receives on the IP side;
  1. the CMR value it calculates based on its reception quality on

the non-IP side; and

  1. a CMR value it may choose for congestion control of transmission

on the IP side.

 The details of the control algorithm are left to the implementation.

4. AMR and AMR-WB RTP Payload Formats

 The AMR and AMR-WB payload formats have identical structure, so they
 are specified together.  The only differences are in the types of
 codec frames contained in the payload.  The payload format consists
 of the RTP header, payload header and payload data.

4.1. RTP Header Usage

 The format of the RTP header is specified in [8].  This payload
 format uses the fields of the header in a manner consistent with that
 specification.

Sjoberg, et. al. Standards Track [Page 14] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The RTP timestamp corresponds to the sampling instant of the first
 sample encoded for the first frame-block in the packet.  The
 timestamp clock frequency is the same as the sampling frequency, so
 the timestamp unit is in samples.
 The duration of one speech frame-block is 20 ms for both AMR and
 AMR-WB.  For AMR, the sampling frequency is 8 kHz, corresponding to
 160 encoded speech samples per frame from each channel.  For AMR-WB,
 the sampling frequency is 16 kHz, corresponding to 320 samples per
 frame from each channel.  Thus, the timestamp is increased by 160 for
 AMR and 320 for AMR-WB for each consecutive frame-block.
 A packet may contain multiple frame-blocks of encoded speech or
 comfort noise parameters.  If interleaving is employed, the frame-
 blocks encapsulated into a payload are picked according to the
 interleaving rules as defined in Section 4.4.1.  Otherwise, each
 packet covers a period of one or more contiguous 20 ms frame-block
 intervals.  In case the data from all the channels for a particular
 frame-block in the period is missing, for example at a gateway from
 some other transport format, it is possible to indicate that no data
 is present for that frame-block rather than breaking a multi-frame-
 block packet into two, as explained in Section 4.3.2.
 To allow for error resiliency through redundant transmission, the
 periods covered by multiple packets MAY overlap in time.  A receiver
 MUST be prepared to receive any speech frame multiple times, either
 in exact duplicates, or in different AMR rate modes, or with data
 present in one packet and not present in another.  If multiple
 versions of the same speech frame are received, it is RECOMMENDED
 that the mode with the highest rate be used by the speech decoder.  A
 given frame MUST NOT be encoded as speech in one packet and comfort
 noise parameters in another.
 The payload is always made an integral number of octets long by
 padding with zero bits if necessary.  If additional padding is
 required to bring the payload length to a larger multiple of octets
 or for some other purpose, then the P bit in the RTP in the header
 may be set and padding appended as specified in [8].
 The RTP header marker bit (M) SHALL be set to 1 if the first frame-
 block carried in the packet contains a speech frame which is the
 first in a talkspurt.  For all other packets the marker bit SHALL be
 set to zero (M=0).

Sjoberg, et. al. Standards Track [Page 15] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The assignment of an RTP payload type for this new packet format is
 outside the scope of this document, and will not be specified here.
 It is expected that the RTP profile under which this payload format
 is being used will assign a payload type for this encoding or specify
 that the payload type is to be bound dynamically.

4.2. Payload Structure

 The complete payload consists of a payload header, a payload table of
 contents, and speech data representing one or more speech frame-
 blocks.  The following diagram shows the general payload format
 layout:
 +----------------+-------------------+----------------
 | payload header | table of contents | speech data ...
 +----------------+-------------------+----------------
 Payloads containing more than one speech frame-block are called
 compound payloads.
 The following sections describe the variations taken by the payload
 format depending on whether the AMR session is set up to use the
 bandwidth-efficient mode or octet-aligned mode and any of the
 OPTIONAL functions for robust sorting, interleaving, and frame CRCs.
 Implementations SHOULD support both bandwidth-efficient and octet-
 aligned operation to increase interoperability.

4.3. Bandwidth-Efficient Mode

4.3.1. The Payload Header

 In bandwidth-efficient mode, the payload header simply consists of a
 4 bit codec mode request:
  0 1 2 3
 +-+-+-+-+
 |  CMR  |
 +-+-+-+-+
 CMR (4 bits): Indicates a codec mode request sent to the speech
    encoder at the site of the receiver of this payload.  The value of
    the CMR field is set to the frame type index of the corresponding
    speech mode being requested.  The frame type index may be 0-7 for
    AMR, as defined in Table 1a in [2], or 0-8 for AMR-WB, as defined
    in Table 1a in [4].  CMR value 15 indicates that no mode request
    is present, and other values are for future use.

Sjoberg, et. al. Standards Track [Page 16] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The mode request received in the CMR field is valid until the next
 CMR is received, i.e., a newly received CMR value overrides the
 previous one.  Therefore, if a terminal continuously wishes to
 receive frames in the same mode X, it needs to set CMR=X for all its
 outbound payloads, and if a terminal has no preference in which mode
 to receive, it SHOULD set CMR=15 in all its outbound payloads.
 If receiving a payload with a CMR value which is not a speech mode or
 NO_DATA, the CMR MUST be ignored by the receiver.
 In a multi-channel session, CMR SHOULD be interpreted by the receiver
 of the payload as the desired encoding mode for all the channels in
 the session.
 An IP end-point SHOULD NOT set the CMR based on packet losses or
 other congestion indications, for several reasons:
  1. The other end of the IP path may be a gateway to a non-IP

network (such as a radio link) that needs to set the CMR field

       to optimize performance on that network.
  1. Congestion on the IP network is managed by the IP sender, in

this case at the other end of the IP path. Feedback about

       congestion SHOULD be provided to that IP sender through RTCP or
       other means, and then the sender can choose to avoid congestion
       using the most appropriate mechanism.  That may include
       adjusting the codec mode, but also includes adjusting the level
       of redundancy or number of frames per packet.
 The encoder SHOULD follow a received mode request, but MAY change to
 a lower-numbered mode if it so chooses, for example to control
 congestion.
 The CMR field MUST be set to 15 for packets sent to a multicast
 group.  The encoder in the speech sender SHOULD ignore mode requests
 when sending speech to a multicast session but MAY use RTCP feedback
 information as a hint that a mode change is needed.
 The codec mode selection MAY be restricted by a session parameter to
 a subset of the available modes.  If so, the requested mode MUST be
 among the signalled subset (see Section 8).

4.3.2. The Payload Table of Contents

 The table of contents (ToC) consists of a list of ToC entries, each
 representing a speech frame.

Sjoberg, et. al. Standards Track [Page 17] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 In bandwidth-efficient mode, a ToC entry takes the following format:
  0 1 2 3 4 5
 +-+-+-+-+-+-+
 |F|  FT   |Q|
 +-+-+-+-+-+-+
 F (1 bit): If set to 1, indicates that this frame is followed by
    another speech frame in this payload; if set to 0, indicates that
    this frame is the last frame in this payload.
 FT (4 bits): Frame type index, indicating either the AMR or AMR-WB
    speech coding mode or comfort noise (SID) mode of the
    corresponding frame carried in this payload.
 The value of FT is defined in Table 1a in [2] for AMR and in Table 1a
 in [4] for AMR-WB.  FT=14 (SPEECH_LOST, only available for AMR-WB)
 and FT=15 (NO_DATA) are used to indicate frames that are either lost
 or not being transmitted in this payload, respectively.
 NO_DATA (FT=15) frame could mean either that there is no data
 produced by the speech encoder for that frame or that no data for
 that frame is transmitted in the current payload (i.e., valid data
 for that frame could be sent in either an earlier or later packet).
 If receiving a ToC entry with a FT value in the range 9-14 for AMR or
 10-13 for AMR-WB the whole packet SHOULD be discarded.  This is to
 avoid the loss of data synchronization in the depacketization
 process, which can result in a huge degradation in speech quality.
 Note that packets containing only NO_DATA frames SHOULD NOT be
 transmitted.  Also, frame-blocks containing only NO_DATA frames at
 the end of a packet SHOULD NOT be transmitted, except in the case of
 interleaving.  The AMR SCR/DTX is described in [6] and AMR-WB SCR/DTX
 in [7].
 The extra comfort noise frame types specified in table 1a in [2]
 (i.e., GSM-EFR CN, IS-641 CN, and PDC-EFR CN) MUST NOT be used in
 this payload format because the standardized AMR codec is only
 required to implement the general AMR SID frame type and not those
 that are native to the incorporated encodings.
 Q (1 bit): Frame quality indicator.  If set to 0, indicates the
    corresponding frame is severely damaged and the receiver should
    set the RX_TYPE (see [6]) to either SPEECH_BAD or SID_BAD
    depending on the frame type (FT).

Sjoberg, et. al. Standards Track [Page 18] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The frame quality indicator is included for interoperability with the
 ATM payload format described in ITU-T I.366.2, the UMTS Iu interface
 [16], as well as other transport formats.  The frame quality
 indicator enables damaged frames to be forwarded to the speech
 decoder for error concealment.  This can improve the speech quality
 comparing to dropping the damaged frames.  See Section 4.4.2.1 for
 more details.
 For multi-channel sessions, the ToC entries of all frames from a
 frame-block are placed in the ToC in consecutive order as defined in
 Section 4.1 in [24].  When multiple frame-blocks are present in a
 packet in bandwidth-efficient mode, they will be placed in the packet
 in order of their creation time.
 Therefore, with N channels and K speech frame-blocks in a packet,
 there MUST be N*K entries in the ToC, and the first N entries will be
 from the first frame-block, the second N entries will be from the
 second frame-block, and so on.
 The following figure shows an example of a ToC of three entries in a
 single channel session using bandwidth efficient mode.
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1|  FT   |Q|1|  FT   |Q|0|  FT   |Q|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Below is an example of how the ToC entries will appear in the ToC of
 a packet carrying 3 consecutive frame-blocks in a session with two
 channels (L and R).
 +----+----+----+----+----+----+
 | 1L | 1R | 2L | 2R | 3L | 3R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
   Frame-    Frame-    Frame-
   Block 1   Block 2   Block 3

4.3.3. Speech Data

 Speech data of a payload contains one or more speech frames or
 comfort noise frames, as described in the ToC of the payload.
    Note, for ToC entries with FT=14 or 15, there will be no
    corresponding speech frame present in the speech data.

Sjoberg, et. al. Standards Track [Page 19] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 Each speech frame represents 20 ms of speech encoded with the mode
 indicated in the FT field of the corresponding ToC entry.  The length
 of the speech frame is implicitly defined by the mode indicated in
 the FT field.  The order and numbering notation of the bits are as
 specified for Interface Format 1 (IF1) in [2] for AMR and [4] for
 AMR-WB.  As specified there, the bits of speech frames have been
 rearranged in order of decreasing sensitivity, while the bits of
 comfort noise frames are in the order produced by the encoder.  The
 resulting bit sequence for a frame of length K bits is denoted d(0),
 d(1), ..., d(K-1).

4.3.4. Algorithm for Forming the Payload

 The complete RTP payload in bandwidth-efficient mode is formed by
 packing bits from the payload header, table of contents, and speech
 frames, in order as defined by their corresponding ToC entries in the
 ToC list, contiguously into octets beginning with the most
 significant bits of the fields and the octets.
 To be precise, the four-bit payload header is packed into the first
 octet of the payload with bit 0 of the payload header in the most
 significant bit of the octet.  The four most significant bits
 (numbered 0-3) of the first ToC entry are packed into the least
 significant bits of the octet, ending with bit 3 in the least
 significant bit.  Packing continues in the second octet with bit 4 of
 the first ToC entry in the most significant bit of the octet.  If
 more than one frame is contained in the payload, then packing
 continues with the second and successive ToC entries.  Bit 0 of the
 first data frame follows immediately after the last ToC bit,
 proceeding through all the bits of the frame in numerical order.
 Bits from any successive frames follow contiguously in numerical
 order for each frame and in consecutive order of the frames.
 If speech data is missing for one or more speech frame within the
 sequence, because of, for example, DTX, a ToC entry with FT set to
 NO_DATA SHALL be included in the ToC for each of the missing frames,
 but no data bits are included in the payload for the missing frame
 (see Section 4.3.5.2 for an example).

Sjoberg, et. al. Standards Track [Page 20] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.3.5 Payload Examples

4.3.5.1. Single Channel Payload Carrying a Single Frame

 The following diagram shows a bandwidth-efficient AMR payload from a
 single channel session carrying a single speech frame-block.
 In the payload, no specific mode is requested (CMR=15), the speech
 frame is not damaged at the IP origin (Q=1), and the coding mode is
 AMR 7.4 kbps (FT=4).  The encoded speech bits, d(0) to d(147), are
 arranged in descending sensitivity order according to [2].  Finally,
 two zero bits are added to the end as padding to make the payload
 octet aligned.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=15|0| FT=4  |1|d(0)                                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                     d(147)|P|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sjoberg, et. al. Standards Track [Page 21] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.3.5.2. Single Channel Payload Carrying Multiple Frames

 The following diagram shows a single channel, bandwidth efficient
 compound AMR-WB payload that contains four frames, of which one has
 no speech data.  The first frame is a speech frame at 6.6 kbps mode
 (FT=0) that is composed of speech bits d(0) to d(131).  The second
 frame is an AMR-WB SID frame (FT=9), consisting of bits g(0) to
 g(39).  The third frame is NO_DATA frame and does not carry any
 speech information, it is represented in the payload by its ToC
 entry.  The fourth frame in the payload is a speech frame at 8.85
 kpbs mode (FT=1), it consists of speech bits h(0) to h(176).
 As shown below, the payload carries a mode request for the encoder on
 the receiver's side to change its future coding mode to AMR-WB 8.85
 kbps (CMR=1).  None of the frames is damaged at IP origin (Q=1).  The
 encoded speech and SID bits, d(0) to d(131), g(0) to g(39) and h(0)
 to h(176), are arranged in the payload in descending sensitivity
 order according to [4]. (Note, no speech bits are present for the
 third frame).  Finally, seven 0s are padded to the end to make the
 payload octet aligned.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=1 |1| FT=0  |1|1| FT=9  |1|1| FT=15 |1|0| FT=1  |1|d(0)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                         d(131)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |g(0)                                                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          g(39)|h(0)                                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                           h(176)|P|P|P|P|P|P|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sjoberg, et. al. Standards Track [Page 22] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.3.5.3. Multi-Channel Payload Carrying Multiple Frames

 The following diagram shows a two channel payload carrying 3 frame-
 blocks, i.e., the payload will contain 6 speech frames.
 In the payload all speech frames contain the same mode 7.4 kbit/s
 (FT=4) and are not damaged at IP origin.  The CMR is set to 15, i.e.,
 no specific mode is requested.  The two channels are defined as left
 (L) and right (R) in that order.  The encoded speech bits is
 designated dXY(0).. dXY(K-1), where X = block number, Y = channel,
 and K is the number of speech bits for that mode.  Exemplifying this,
 for frame-block 1 of the left channel the encoded bits are designated
 as d1L(0) to d1L(147).

Sjoberg, et. al. Standards Track [Page 23] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=15|1|1L FT=4|1|1|1R FT=4|1|1|2L FT=4|1|1|2R FT=4|1|1|3L FT|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |4|1|0|3R FT=4|1|d1L(0)                                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                               d1L(147)|d1R(0) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       d1R(147)|d2L(0)                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |d2L(147|d2R(0)                                                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                       d2R(147)|d3L(0)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               d3L(147)|d3R(0)                                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                       d3R(147)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sjoberg, et. al. Standards Track [Page 24] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.4. Octet-aligned Mode

4.4.1. The Payload Header

 In octet-aligned mode, the payload header consists of a 4 bit CMR, 4
 reserved bits, and optionally, an 8 bit interleaving header, as shown
 below:
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+- - - - - - - -
 |  CMR  |R|R|R|R|  ILL  |  ILP  |
 +-+-+-+-+-+-+-+-+- - - - - - - -
 CMR (4 bits): same as defined in section 4.3.1.
 R: is a reserved bit that MUST be set to zero.  All R bits MUST be
    ignored by the receiver.
 ILL (4 bits, unsigned integer): This is an OPTIONAL field that is
    present only if interleaving is signalled out-of-band for the
    session.  ILL=L indicates to the receiver that the interleaving
    length is L+1, in number of frame-blocks.
 ILP (4 bits, unsigned integer): This is an OPTIONAL field that is
    present only if interleaving is signalled.  ILP MUST take a value
    between 0 and ILL, inclusive, indicating the interleaving index
    for frame-blocks in this payload in the interleave group.  If the
    value of ILP is found greater than ILL, the payload SHOULD be
    discarded.
 ILL and ILP fields MUST be present in each packet in a session if
 interleaving is signalled for the session.  Interleaving MUST be
 performed on a frame-block basis (i.e., NOT on a frame basis) in a
 multi-channel session.
 The following example illustrates the arrangement of speech frame-
 blocks in an interleave group during an interleave session.  Here we
 assume ILL=L for the interleave group that starts at speech frame-
 block n.  We also assume that the first payload packet of the
 interleave group is s and the number of speech frame-blocks carried
 in each payload is N. Then we will have:
 Payload s (the first packet of this interleave group):
   ILL=L, ILP=0,
   Carry frame-blocks: n, n+(L+1), n+2*(L+1), ..., n+(N-1)*(L+1)
    Payload s+1 (the second packet of this interleave group):

Sjoberg, et. al. Standards Track [Page 25] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

   ILL=L, ILP=1,
   frame-blocks: n+1, n+1+(L+1), n+1+2*(L+1), ..., n+1+(N-1)*(L+1)
     ...
 Payload s+L (the last packet of this interleave group):
   ILL=L, ILP=L,
   frame-blocks: n+L, n+L+(L+1), n+L+2*(L+1), ..., n+L+(N-1)*(L+1)
 The next interleave group will start at frame-block n+N*(L+1).
 There will be no interleaving effect unless the number of frame-
 blocks per packet (N) is at least 2.  Moreover, the number of frame-
 blocks per payload (N) and the value of ILL MUST NOT be changed
 inside an interleave group.  In other words, all payloads in an
 interleave group MUST have the same ILL and MUST contain the same
 number of speech frame-blocks.
 The sender of the payload MUST only apply interleaving if the
 receiver has signalled its use through out-of-band means.  Since
 interleaving will increase buffering requirements at the receiver,
 the receiver uses MIME parameter "interleaving=I" to set the maximum
 number of frame-blocks allowed in an interleaving group to I.
 When performing interleaving the sender MUST use a proper number of
 frame-blocks per payload (N) and ILL so that the resulting size of an
 interleave group is less or equal to I, i.e., N*(L+1)<=I.

4.4.2. The Payload Table of Contents and Frame CRCs

 The table of contents (ToC) in octet-aligned mode consists of a list
 of ToC entries where each entry corresponds to a speech frame carried
 in the payload and, optionally, a list of speech frame CRCs, i.e.,
 +---------------------+
 | list of ToC entries |
 +---------------------+
 | list of frame CRCs  | (optional)
  - - - - - - - - - - -
    Note, for ToC entries with FT=14 or 15, there will be no
    corresponding speech frame or frame CRC present in the payload.
 The list of ToC entries is organized in the same way as described for
 bandwidth-efficient mode in 4.3.2, with the following exception; when
 interleaving is used the frame-blocks in the ToC will almost never be
 placed consecutive in time.  Instead, the presence and order of the
 frame-blocks in a packet will follow the pattern described in 4.4.1.

Sjoberg, et. al. Standards Track [Page 26] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The following example shows the ToC of three consecutive packets,
 each carrying 3 frame-blocks, in an interleaved two-channel session.
 Here, the two channels are left (L) and right (R) with L coming
 before R, and the interleaving length is 3 (i.e., ILL=2).  This makes
 the interleave group 9 frame-blocks large.
 Packet #1
 ---------
 ILL=2, ILP=0:
 +----+----+----+----+----+----+
 | 1L | 1R | 4L | 4R | 7L | 7R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
   Frame-    Frame-    Frame-
   Block 1   Block 4   Block 7
 Packet #2
 ---------
 ILL=2, ILP=1:
 +----+----+----+----+----+----+
 | 2L | 2R | 5L | 5R | 8L | 8R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
   Frame-    Frame-    Frame-
   Block 2   Block 5   Block 8
 Packet #3
 ---------
 ILL=2, ILP=2:
 +----+----+----+----+----+----+
 | 3L | 3R | 6L | 6R | 9L | 9R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
   Frame-    Frame-    Frame-
   Block 3   Block 6   Block 9
 A ToC entry takes the following format in octet-aligned mode:
  0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |F|  FT   |Q|P|P|
 +-+-+-+-+-+-+-+-+
 F (1 bit): see definition in Section 4.3.2.

Sjoberg, et. al. Standards Track [Page 27] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 FT (4 bits unsigned integer): see definition in Section 4.3.2.
 Q (1 bit): see definition in Section 4.3.2.
 P bits: padding bits, MUST be set to zero.
 The list of CRCs is OPTIONAL.  It only exists if the use of CRC is
 signalled out-of-band for the session.  When present, each CRC in the
 list is 8 bit long and corresponds to a speech frame (NOT a frame-
 block) carried in the payload.  Calculation and use of the CRC is
 specified in the next section.

4.4.2.1. Use of Frame CRC for UED over IP

 The general concept of UED/UEP over IP is discussed in Section 3.6.
 This section provides more details on how to use the frame CRC in the
 octet-aligned payload header together with a partial transport layer
 checksum to achieve UED.
 To achieve UED, one SHOULD use a transport layer checksum, for
 example, the one defined in UDP-Lite [15], to protect the RTP header,
 payload header, and table of contents bits in a payload.  The frame
 CRC, when used, MUST be calculated only over all class A bits in the
 frame.  Class B and C bits in the frame MUST NOT be included in the
 CRC calculation and SHOULD NOT be covered by the transport checksum.
    Note, the number of class A bits for various coding modes in AMR
    codec is specified as informative in [2] and is therefore copied
    into Table 1 in Section 3.6 to make it normative for this payload
    format.  The number of class A bits for various coding modes in
    AMR-WB codec is specified as normative in table 2 in [4], and the
    SID frame (FT=9) has 40 class A bits.  These definitions of class
    A bits MUST be used for this payload format.
 Packets SHOULD be discarded if the transport layer checksum detects
 errors.
 The receiver of the payload SHOULD examine the data integrity of the
 received class A bits by re-calculating the CRC over the received
 class A bits and comparing the result to the value found in the
 received payload header.  If the two values mismatch, the receiver
 SHALL consider the class A bits in the receiver frame damaged and
 MUST clear the Q flag of the frame (i.e., set it to 0).  This will
 subsequently cause the frame to be marked as SPEECH_BAD, if the FT of
 the frame is 0..7 for AMR or 0..8 for AMR-WB, or SID_BAD if the FT of
 the frame is 8 for AMR or 9 for AMR-WB, before it is passed to the
 speech decoder.  See [6] and [7] more details.

Sjoberg, et. al. Standards Track [Page 28] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The following example shows an octet-aligned ToC with a CRC list for
 a payload containing 3 speech frames from a single channel session
 (assuming none of the FTs is equal to 14 or 15):
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1|  FT#1 |Q|P|P|1|  FT#2 |Q|P|P|0|  FT#3 |Q|P|P|     CRC#1     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     CRC#2     |     CRC#3     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Each of the CRC's takes 8 bits
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | c0| c1| c2| c3| c4| c5| c6| c7|
 +---+---+---+---+---+---+---+---+
 and is calculated by the cyclic generator polynomial,
   C(x) = 1 + x^2 + x^3 + x^4 + x^8
 where ^ is the exponentiation operator.
 In binary form the polynomial has the following form: 101110001
 (MSB..LSB).
 The actual calculation of the CRC is made as follows:  First, an 8-
 bit CRC register is reset to zero: 00000000.  For each bit over which
 the CRC shall be calculated, an XOR operation is made between the
 rightmost bit of the CRC register and the bit.  The CRC register is
 then right shifted one step (inputting a "0" as the leftmost bit).
 If the result of the XOR operation mentioned above is a "1"
 "10111000" is then bit-wise XOR-ed into the CRC register.  This
 operation is repeated for each bit that the CRC should cover.  In
 this case, the first bit would be d(0) for the speech frame for which
 the CRC should cover.  When the last bit (e.g., d(54) for AMR 5.9
 according to Table 1 in Section 3.6) have been used in this CRC
 calculation, the contents in CRC register should simply be copied to
 the corresponding field in the list of CRC's.
 Fast calculation of the CRC on a general-purpose CPU is possible
 using a table-driven algorithm.

Sjoberg, et. al. Standards Track [Page 29] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.4.3. Speech Data

 In octet-aligned mode, speech data is carried in a similar way to
 that in the bandwidth-efficient mode as discussed in Section 4.3.3,
 with the following exceptions:
  1. The last octet of each speech frame MUST be padded with zeroes

at the end if not all bits in the octet are used. In other

       words, each speech frame MUST be octet-aligned.
  1. When multiple speech frames are present in the speech data

(i.e., compound payload), the speech frames can be arranged

       either one whole frame after another as usual, or with the
       octets of all frames interleaved together at the octet level.
       Since the bits within each frame are ordered with the most
       error-sensitive bits first, interleaving the octets collects
       those sensitive bits from all frames to be nearer the beginning
       of the packet.  This is called "robust sorting order" which
       allows the application of UED (such as UDP-Lite [15]) or UEP
       (such as the ULP [18]) mechanisms to the payload data.  The
       details of assembling the payload are given in the next
       section.
 The use of robust sorting order for a session MUST be agreed via
 out-of-band means.  Section 8 specifies a MIME parameter for this
 purpose.
 Note, robust sorting order MUST only be performed on the frame level
 and thus is independent of interleaving which is at the frame-block
 level, as described in Section 4.4.1. In other words, robust sorting
 can be applied to either non-interleaved or interleaved sessions.

4.4.4. Methods for Forming the Payload

 Two different packetization methods, namely normal order and robust
 sorting order, exist for forming a payload in octet-aligned mode.  In
 both cases, the payload header and table of contents are packed into
 the payload the same way; the difference is in the packing of the
 speech frames.
 The payload begins with the payload header of one octet or two if
 frame interleaving is selected.  The payload header is followed by
 the table of contents consisting of a list of one-octet ToC entries.
 If frame CRCs are to be included, they follow the table of contents
 with one 8-bit CRC filling each octet.  Note that if a given frame
 has a ToC entry with FT=14 or 15, there will be no CRC present.

Sjoberg, et. al. Standards Track [Page 30] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 The speech data follows the table of contents, or the CRCs if
 present.  For packetization in the normal order, all of the octets
 comprising a speech frame are appended to the payload as a unit. The
 speech frames are packed in the same order as their corresponding ToC
 entries are arranged in the ToC list, with the exception that if a
 given frame has a ToC entry with FT=14 or 15, there will be no data
 octets present for that frame.
 For packetization in robust sorting order, the octets of all speech
 frames are interleaved together at the octet level.  That is, the
 data portion of the payload begins with the first octet of the first
 frame, followed by the first octet of the second frame, then the
 first octet of the third frame, and so on.  After the first octet of
 the last frame has been appended, the cycle repeats with the second
 octet of each frame.  The process continues for as many octets as are
 present in the longest frame.  If the frames are not all the same
 octet length, a shorter frame is skipped once all octets in it have
 been appended.  The order of the frames in the cycle will be
 sequential if frame interleaving is not in use, or according to the
 interleave pattern specified in the payload header if frame
 interleaving is in use.  Note that if a given frame has a ToC entry
 with FT=14 or 15, there will be no data octets present for that frame
 so that frame is skipped in the robust sorting cycle.
 The UED and/or UEP is RECOMMENDED to cover at least the RTP header,
 payload header, table of contents, and class A bits of a sorted
 payload.  Exactly how many octets need to be covered depends on the
 network and application.  If CRCs are used together with robust
 sorting, only the RTP header, the payload header, and the ToC SHOULD
 be covered by UED/UEP.  The means to communicate to other layers
 performing UED/UEP the number of octets to be covered is beyond the
 scope of this specification.

Sjoberg, et. al. Standards Track [Page 31] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

4.4.5. Payload Examples

4.4.5.1. Basic Single Channel Payload Carrying Multiple Frames

 The following diagram shows an octet aligned payload from a single
 channel session that carries two AMR frames of 7.95 kbps coding mode
 (FT=5).  In the payload, a codec mode request is sent (CMR=6),
 requesting the encoder at the receiver's side to use AMR 10.2 kbps
 coding mode.  No frame CRC, interleaving, or robust-sorting is in
 use.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=6 |R|R|R|R|1|FT#1=5 |Q|P|P|0|FT#2=5 |Q|P|P|   f1(0..7)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   f1(8..15)   |  f1(16..23)   |  ....                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         ...   |f1(152..158) |P|   f2(0..7)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   f2(8..15)   |  f2(16..23)   |  ....                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         ...   |f2(152..158) |P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note, in above example the last octet in both speech frames is padded
 with one 0 to make it octet-aligned.

4.4.5.2. Two Channel Payload with CRC, Interleaving, and Robust-sorting

 This example shows an octet aligned payload from a two channel
 session.  Two frame-blocks, each containing 2 speech frames of 7.95
 kbps coding mode (FT=5), are carried in this payload,
 The two channels are left (L) and right (R) with L coming before R.
 In the payload, a codec mode request is also sent (CMR=6), requesting
 the encoder at the receiver's side to use AMR 10.2 kbps coding mode.
 Moreover, frame CRC and frame-block interleaving are both enabled for
 the session.  The interleaving length is 2 (ILL=1) and this payload
 is the first one in an interleave group (ILP=0).
 The first two frames in the payload are the L and R channel speech
 frames of frame-block #1, consisting of bits f1L(0..158) and

Sjoberg, et. al. Standards Track [Page 32] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 f1R(0..158), respectively.  The next two frames are the L and R
 channel frames of frame-block #3, consisting of bits f3L(0..158) and
 f3R(0..158), respectively, due to interleaving.  For each of the four
 speech frames a CRC is calculated as CRC1L(0..7), CRC1R(0..7),
 CRC3L(0..7), and CRC3R(0..7), respectively.  Finally, the payload is
 robust sorted.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=6 |R|R|R|R| ILL=1 | ILP=0 |1|FT#1L=5|Q|P|P|1|FT#1R=5|Q|P|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1|FT#3L=5|Q|P|P|0|FT#3R=5|Q|P|P|      CRC1L    |      CRC1R    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      CRC3L    |      CRC3R    |   f1L(0..7)   |   f1R(0..7)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   f3L(0..7)   |   f3R(0..7)   |  f1L(8..15)   |  f1R(8..15)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  f3L(8..15)   |  f3R(8..15)   |  f1L(16..23)  |  f1R(16..23)  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | f3L(144..151) | f3R(144..151) |f1L(152..158)|P|f1R(152..158)|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |f3L(152..158)|P|f3R(152..158)|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note, in above example the last octet in all the four speech frames
 is padded with one zero bit to make it octet-aligned.

4.5. Implementation Considerations

 An application implementing this payload format MUST understand all
 the payload parameters in the out-of-band signaling used.  For
 example, if an application uses SDP, all the SDP and MIME parameters
 in this document MUST be understood.  This requirement ensures that
 an implementation always can decide if it is capable or not of
 communicating.
 No operation mode of the payload format is mandatory to implement.
 The requirements of the application using the payload format should
 be used to determine what to implement.  To achieve basic
 interoperability an implementation SHOULD at least implement both
 bandwidth-efficient and octet-aligned mode for single channel.  The
 other operations mode: interleaving, robust sorting, frame-wise CRC
 in both single and multi-channel is OPTIONAL to implement.

Sjoberg, et. al. Standards Track [Page 33] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

5. AMR and AMR-WB Storage Format

 The storage format is used for storing AMR or AMR-WB speech frames in
 a file or as an e-mail attachment.  Multiple channel content is
 supported.
 In general, an AMR or AMR-WB file has the following structure:
 +------------------+
 | Header           |
 +------------------+
 | Speech frame 1   |
 +------------------+
 : ...              :
 +------------------+
 | Speech frame n   |
 +------------------+
 Note, to preserve interoperability with already deployed
 implementations, single channel content uses a file header format
 different from that of multi-channel content.

5.1. Single channel Header

 A single channel AMR or AMR-WB file header contains only a magic
 number and different magic numbers are defined to distinguish AMR
 from AMR-WB.
 The magic number for single channel AMR files MUST consist of ASCII
 character string:
    "#!AMR\n"
    (or 0x2321414d520a in hexadecimal).
 The magic number for single channel AMR-WB files MUST consist of
 ASCII character string:
    "#!AMR-WB\n"
    (or 0x2321414d522d57420a in hexadecimal).
 Note, the "\n" is an important part of the magic numbers and MUST be
 included in the comparison, since, otherwise, the single channel
 magic numbers above will become indistinguishable from those of the
 multi-channel files defined in the next section.

Sjoberg, et. al. Standards Track [Page 34] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

5.2. Multi-channel Header

 The multi-channel header consists of a magic number followed by a 32
 bit channel description field, giving the multi-channel header the
 following structure:
 +------------------+
 | magic number     |
 +------------------+
 | chan-desc field  |
 +------------------+
 The magic number for multi-channel AMR files MUST consist of the
 ASCII character string:
    "#!AMR_MC1.0\n"
    (or 0x2321414d525F4D43312E300a in hexadecimal).
 The magic number for multi-channel AMR-WB files MUST consist of the
 ASCII character string:
    "#!AMR-WB_MC1.0\n"
    (or 0x2321414d522d57425F4D43312E300a in hexadecimal).
 The version number in the magic numbers refers to the version of the
 file format.
 The 32 bit channel description field is defined as:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Reserved bits                                    | CHAN  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Reserved bits: MUST be set to 0 when written, and a reader MUST
                ignore them.
 CHAN (4 bit unsigned integer): Indicates the number of audio channels
    contained in this storage file.  The valid values and the order of
    the channels within a frame block are specified in Section 4.1 in
    [24].

Sjoberg, et. al. Standards Track [Page 35] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

5.3. Speech Frames

 After the file header, speech frame-blocks consecutive in time are
 stored in the file.  Each frame-block contains a number of octet-
 aligned speech frames equal to the number of channels, and stored in
 increasing order, starting with channel 1.
 Each stored speech frame starts with a one octet frame header with
 the following format:
  0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |P|  FT   |Q|P|P|
 +-+-+-+-+-+-+-+-+
 The FT field and the Q bit are defined in the same way as in Section
 4.1.2. The P bits are padding and MUST be set to 0.
 Following this one octet header come the speech bits as defined in
 4.3.3.  The last octet of each frame is padded with zeroes, if
 needed, to achieve octet alignment.
 The following example shows an AMR frame in 5.9 kbit coding mode
 (with 118 speech bits) in the storage format.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |P| FT=2  |Q|P|P|                                               |
 +-+-+-+-+-+-+-+-+                                               +
 |                                                               |
 +          Speech bits for frame-block n, channel k             +
 |                                                               |
 +                                                           +-+-+
 |                                                           |P|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Frame-blocks or speech frames lost in transmission and non-received
 frame-blocks between SID updates during non-speech periods MUST be
 stored as NO_DATA frames (frame type 15, as defined in [2] and [4])
 or SPEECH_LOST (frame type 14, only available for AMR-WB) in complete
 frame-blocks to keep synchronization with the original media.

Sjoberg, et. al. Standards Track [Page 36] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

6. Congestion Control

 The general congestion control considerations for transporting RTP
 data apply to AMR or AMR-WB speech over RTP as well.  However, the
 multi-rate capability of AMR and AMR-WB speech coding may provide an
 advantage over other payload formats for controlling congestion since
 the bandwidth demand can be adjusted by selecting a different coding
 mode.
 Another parameter that may impact the bandwidth demand for AMR and
 AMR-WB is the number of frame-blocks that are encapsulated in each
 RTP payload.  Packing more frame-blocks in each RTP payload can
 reduce the number of packets sent and hence the overhead from
 IP/UDP/RTP headers, at the expense of increased delay.
 If forward error correction (FEC) is used to combat packet loss, the
 amount of redundancy added by FEC will need to be regulated so that
 the use of FEC itself does not cause a congestion problem.
 It is RECOMMENDED that AMR or AMR-WB applications using this payload
 format employ congestion control.  The actual mechanism for
 congestion control is not specified but should be suitable for real-
 time flows, e.g., "Equation-Based Congestion Control for Unicast
 Applications" [17].

7. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the general security considerations discussed in [8].
 As this format transports encoded speech, the main security issues
 include confidentiality and authentication of the speech itself.  The
 payload format itself does not have any built-in security mechanisms.
 External mechanisms, such as SRTP [22], MAY be used.
 This payload format does not exhibit any significant non-uniformity
 in the receiver side computational complexity for packet processing
 and thus is unlikely to pose a denial-of-service threat due to the
 receipt of pathological data.

7.1. Confidentiality

 To achieve confidentiality of the encoded AMR or AMR-WB speech, all
 speech data bits will need to be encrypted.  There is less a need to
 encrypt the payload header or the table of contents due to 1) that
 they only carry information about the requested speech mode, frame
 type, and frame quality, and 2) that this information could be useful
 to some third party, e.g., quality monitoring.

Sjoberg, et. al. Standards Track [Page 37] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 As long as the AMR or AMR-WB payload is only packed and unpacked at
 either end, encryption may be performed after packet encapsulation so
 that there is no conflict between the two operations.
 Interleaving may affect encryption.  Depending on the encryption
 scheme used, there may be restrictions on, for example, the time when
 keys can be changed.  Specifically, the key change may need to occur
 at the boundary between interleave groups.
 The type of encryption method used may impact the error robustness of
 the payload data.  The error robustness may be severely reduced when
 the data is encrypted unless an encryption method without error-
 propagation is used, e.g., a stream cipher.  Therefore, UED/UEP based
 on robust sorting may be difficult to apply when the payload data is
 encrypted.

7.2. Authentication

 To authenticate the sender of the speech, an external mechanism has
 to be used.  It is RECOMMENDED that such a mechanism protect all the
 speech data bits.  Note that the use of UED/UEP may be difficult to
 combine with authentication because any bit errors will cause
 authentication to fail.
 Data tampering by a man-in-the-middle attacker could result in
 erroneous depacketization/decoding that could lower the speech
 quality.  Tampering with the CMR field may result in speech in a
 different quality than desired.
 To prevent a man-in-the-middle attacker from tampering with the
 payload packets, some additional information besides the speech bits
 SHOULD be protected.  This may include the payload header, ToC, frame
 CRCs, RTP timestamp, RTP sequence number, and the RTP marker bit.

7.3. Decoding Validation

 When processing a received payload packet, if the receiver finds that
 the calculated payload length, based on the information of the
 session and the values found in the payload header fields, does not
 match the size of the received packet, the receiver SHOULD discard
 the packet.  This is because decoding a packet that has errors in its
 length field could severely degrade the speech quality.

8. Payload Format Parameters

 This section defines the parameters that may be used to select
 optional features of the AMR and AMR-WB payload formats.  The
 parameters are defined here as part of the MIME subtype registrations

Sjoberg, et. al. Standards Track [Page 38] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 for the AMR and AMR-WB speech codecs.  A mapping of the parameters
 into the Session Description Protocol (SDP) [11] is also provided for
 those applications that use SDP.  Equivalent parameters could be
 defined elsewhere for use with control protocols that do not use MIME
 or SDP.
 Two separate MIME registrations are made, one for AMR and one for
 AMR-WB, because they are distinct encodings that must be
 distinguished by the MIME subtype.
 The data format and parameters are specified for both real-time
 transport in RTP and for storage type applications such as e-mail
 attachments.

8.1. AMR MIME Registration

 The MIME subtype for the Adaptive Multi-Rate (AMR) codec is allocated
 from the IETF tree since AMR is expected to be a widely used speech
 codec in general VoIP applications.  This MIME registration covers
 both real-time transfer via RTP and non-real-time transfers via
 stored files.
 Note, any unspecified parameter MUST be ignored by the receiver.
 Media Type name:     audio
 Media subtype name:  AMR
 Required parameters: none
 Optional parameters:
    These parameters apply to RTP transfer only.
    octet-align: Permissible values are 0 and 1.  If 1, octet-aligned
             operation SHALL be used.  If 0 or if not present,
             bandwidth efficient operation is employed.
    mode-set:  Requested AMR mode set.  Restricts the active codec
             mode set to a subset of all modes.  Possible values are a
             comma separated list of modes from the set: 0,...,7 (see
             Table 1a [2]).  If such mode set is specified by the
             decoder, the encoder MUST abide by the request and MUST
             NOT use modes outside of the subset.  If not present, all
             codec modes are allowed for the session.
    mode-change-period: Specifies a number of frame-blocks, N, that is
             the interval at which codec mode changes are allowed.
             The initial phase of the interval is arbitrary, but

Sjoberg, et. al. Standards Track [Page 39] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

             changes must be separated by multiples of N frame-blocks.
             If this parameter is not present, mode changes are
             allowed at any time during the session.
    mode-change-neighbor: Permissible values are 0 and 1.  If 1, mode
             changes SHALL only be made to the neighboring modes in
             the active codec mode set.  Neighboring modes are the
             ones closest in bit rate to the current mode, either the
             next higher or next lower rate.  If 0 or if not present,
             change between any two modes in the active codec mode set
             is allowed.
    maxptime:  The maximum amount of media which can be encapsulated
             in a payload packet, expressed as time in milliseconds.
             The time is calculated as the sum of the time the media
             present in the packet represents.  The time SHOULD be a
             multiple of the frame size.  If this parameter is not
             present, the sender MAY encapsulate any number of speech
             frames into one RTP packet.
    crc:  Permissible values are 0 and 1.  If 1, frame CRCs SHALL be
             included in the payload, otherwise not.  If crc=1, this
             also implies automatically that octet-aligned operation
             SHALL be used for the session.
    robust-sorting: Permissible values are 0 and 1.  If 1, the payload
             SHALL employ robust payload sorting.  If 0 or if not
             present, simple payload sorting SHALL be used.  If
             robust-sorting=1, this also implies automatically that
             octet-aligned operation SHALL be used for the session.
    interleaving: Indicates that frame-block level interleaving SHALL
             be used for the session and its value defines the maximum
             number of frame-blocks allowed in an interleaving group
             (see Section 4.4.1).  If this parameter is not present,
             interleaving SHALL not be used.  The presence of this
             parameter also implies automatically that octet-aligned
             operation SHALL be used.
    ptime:     see RFC2327 [11].
    channels: The number of audio channels.  The possible values and
             their respective channel order is specified in section
             4.1 in [24].  If omitted it has the default value of 1.
 Encoding considerations:
             This type is defined for transfer via both RTP (RFC 1889)
             and stored-file methods as described in Sections 4 and 5,

Sjoberg, et. al. Standards Track [Page 40] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

             respectively, of RFC 3267.  Audio data is binary data,
             and must be encoded for non-binary transport; the Base64
             encoding is suitable for Email.
 Security considerations:
             See Section 7 of RFC 3267.
 Public specification:
             Please refer to Section 11 of RFC 3267.
 Additional information:
             The following applies to stored-file transfer methods:
             Magic numbers:
               single channel:
               ASCII character string "#!AMR\n"
               (or 0x2321414d520a in hexadecimal)
               multi-channel:
               ASCII character string "#!AMR_MC1.0\n"
               (or 0x2321414d525F4D43312E300a in hexadecimal)
 File extensions: amr, AMR
 Macintosh file type code: none
 Object identifier or OID: none
 Person & email address to contact for further information:
             johan.sjoberg@ericsson.com
             ari.lakaniemi@nokia.com
 Intended usage: COMMON.
             It is expected that many VoIP applications (as well as
             mobile applications) will use this type.
 Author/Change controller:
             johan.sjoberg@ericsson.com
             ari.lakaniemi@nokia.com
             IETF Audio/Video transport working group

8.2. AMR-WB MIME Registration

 The MIME subtype for the Adaptive Multi-Rate Wideband (AMR-WB) codec
 is allocated from the IETF tree since AMR-WB is expected to be a
 widely used speech codec in general VoIP applications.  This MIME
 registration covers both real-time transfer via RTP and non-real-time
 transfers via stored files.
 Note, any unspecified parameter MUST be ignored by the receiver.

Sjoberg, et. al. Standards Track [Page 41] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 Media Type name:     audio
 Media subtype name:  AMR-WB
 Required parameters: none
 Optional parameters:
    These parameters apply to RTP transfer only.
    octet-align: Permissible values are 0 and 1.  If 1, octet-aligned
             operation SHALL be used.  If 0 or if not present,
             bandwidth efficient operation is employed.
    mode-set:  Requested AMR-WB mode set.  Restricts the active codec
             mode set to a subset of all modes.  Possible values are a
             comma separated list of modes from the set: 0,...,8 (see
             Table 1a [4]).  If such mode set is specified by the
             decoder, the encoder MUST abide by the request and MUST
             NOT use modes outside of the subset.  If not present, all
             codec modes are allowed for the session.
    mode-change-period: Specifies a number of frame-blocks, N, that is
             the interval at which codec mode changes are allowed.
             The initial phase of the interval is arbitrary, but
             changes must be separated by multiples of N frame-blocks.
             If this parameter is not present, mode changes are
             allowed at any time during the session.
    mode-change-neighbor: Permissible values are 0 and 1.  If 1, mode
             changes SHALL only be made to the neighboring modes in
             the active codec mode set.  Neighboring modes are the
             ones closest in bit rate to the current mode, either the
             next higher or next lower rate.  If 0 or if not present,
             change between any two modes in the active codec mode set
             is allowed.
    maxptime:  The maximum amount of media which can be encapsulated
             in a payload packet, expressed as time in milliseconds.
             The time is calculated as the sum of the time the media
             present in the packet represents.  The time SHOULD be a
             multiple of the frame size.  If this parameter is not
             present, the sender MAY encapsulate any number of speech
             frames into one RTP packet.

Sjoberg, et. al. Standards Track [Page 42] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

    crc:  Permissible values are 0 and 1.  If 1, frame CRCs SHALL be
             included in the payload, otherwise not.  If crc=1, this
             also implies automatically that octet-aligned operation
             SHALL be used for the session.
    robust-sorting: Permissible values are 0 and 1.  If 1, the payload
             SHALL employ robust payload sorting.  If 0 or if not
             present, simple payload sorting SHALL be used.  If
             robust-sorting=1, this also implies automatically that
             octet-aligned operation SHALL be used for the session.
    interleaving: Indicates that frame-block level interleaving SHALL
             be used for the session and its value defines the maximum
             number of frame-blocks allowed in an interleaving group
             (see Section 4.4.1).  If this parameter is not present,
             interleaving SHALL not be used.  The presence of this
             parameter also implies automatically that octet-aligned
             operation SHALL be used.
    ptime:     see RFC2327 [11].
    channels: The number of audio channels.  The possible values and
             their respective channel order is specified in section
             4.1 in [24].  If omitted it has the default value of 1.
 Encoding considerations:
             This type is defined for transfer via both RTP (RFC 1889)
             and stored-file methods as described in Sections 4 and 5,
             respectively, of RFC 3267.  Audio data is binary data,
             and must be encoded for non-binary transport; the Base64
             encoding is suitable for Email.
 Security considerations:
             See Section 7 of RFC 3267.
 Public specification:
             Please refer to Section 11 of RFC 3267.
 Additional information:
             The following applies to stored-file transfer methods:
             Magic numbers:
               single channel:
               ASCII character string "#!AMR-WB\n"
               (or 0x2321414d522d57420a in hexadecimal)
               multi-channel:
               ASCII character string "#!AMR-WB_MC1.0\n"
               (or 0x2321414d522d57425F4D43312E300a in hexadecimal)

Sjoberg, et. al. Standards Track [Page 43] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

             File extensions: awb, AWB
             Macintosh file type code: none
             Object identifier or OID: none
 Person & email address to contact for further information:
             johan.sjoberg@ericsson.com
             ari.lakaniemi@nokia.com
 Intended usage: COMMON.
             It is expected that many VoIP applications (as well as
             mobile applications) will use this type.
 Author/Change controller:
             johan.sjoberg@ericsson.com
             ari.lakaniemi@nokia.com
             IETF Audio/Video transport working group

8.3. Mapping MIME Parameters into SDP

 The information carried in the MIME media type specification has a
 specific mapping to fields in the Session Description Protocol (SDP)
 [11], which is commonly used to describe RTP sessions.  When SDP is
 used to specify sessions employing the AMR or AMR-WB codec, the
 mapping is as follows:
  1. The MIME type ("audio") goes in SDP "m=" as the media name.
  1. The MIME subtype (payload format name) goes in SDP "a=rtpmap"

as the encoding name. The RTP clock rate in "a=rtpmap" MUST be

       8000 for AMR and 16000 for AMR-WB, and the encoding parameters
       (number of channels) MUST either be explicitly set to N or
       omitted, implying a default value of 1.  The values of N that
       are allowed is specified in Section 4.1 in [24].
  1. The parameters "ptime" and "maxptime" go in the SDP "a=ptime"

and "a=maxptime" attributes, respectively.

  1. Any remaining parameters go in the SDP "a=fmtp" attribute by

copying them directly from the MIME media type string as a

       semicolon separated list of parameter=value pairs.
 Some example SDP session descriptions utilizing AMR and AMR-WB
 encodings follow.  In these examples, long a=fmtp lines are folded to
 meet the column width constraints of this document; the backslash
 ("\") at the end of a line and the carriage return that follows it
 should be ignored.

Sjoberg, et. al. Standards Track [Page 44] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 Example of usage of AMR in a possible GSM gateway scenario:
  m=audio 49120 RTP/AVP 97
  a=rtpmap:97 AMR/8000/1
  a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; \
    mode-change-neighbor=1
  a=maxptime:20
 Example of usage of AMR-WB in a possible VoIP scenario:
  m=audio 49120 RTP/AVP 98
  a=rtpmap:98 AMR-WB/16000
  a=fmtp:98 octet-align=1
 Example of usage of AMR-WB in a possible streaming scenario (two
 channel stereo):
  m=audio 49120 RTP/AVP 99
  a=rtpmap:99 AMR-WB/16000/2
  a=fmtp:99 interleaving=30
  a=maxptime:100
 Note that the payload format (encoding) names are commonly shown in
 upper case.  MIME subtypes are commonly shown in lower case.  These
 names are case-insensitive in both places.  Similarly, parameter
 names are case-insensitive both in MIME types and in the default
 mapping to the SDP a=fmtp attribute.

9. IANA Considerations

 Two new MIME subtypes have been registered, see Section 8.  A new SDP
 attribute "maxptime", defined in Section 8, has also been registered.
 The "maxptime" attribute is expected to be defined in the revision of
 RFC 2327 [11] and is added here with a consistent definition.

10. Acknowledgements

 The authors would like to thank Petri Koskelainen, Bernhard Wimmer,
 Tim Fingscheidt, Sanjay Gupta, Stephen Casner, and Colin Perkins for
 their significant contributions made throughout the writing and
 reviewing of this document.

11. References

 [1]   3GPP TS 26.090, "Adaptive Multi-Rate (AMR) speech transcoding",
       version 4.0.0 (2001-03), 3rd Generation Partnership Project
       (3GPP).

Sjoberg, et. al. Standards Track [Page 45] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 [2]   3GPP TS 26.101, "AMR Speech Codec Frame Structure", version
       4.1.0 (2001-06), 3rd Generation Partnership Project (3GPP).
 [3]   3GPP TS 26.190 "AMR Wideband speech codec; Transcoding
       functions", version 5.0.0 (2001-03), 3rd Generation Partnership
       Project (3GPP).
 [4]   3GPP TS 26.201 "AMR Wideband speech codec; Frame Structure",
       version 5.0.0 (2001-03), 3rd Generation Partnership Project
       (3GPP).
 [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [6]   3GPP TS 26.093, "AMR Speech Codec; Source Controlled Rate
       operation", version 4.0.0 (2000-12), 3rd Generation Partnership
       Project (3GPP).
 [7]   3GPP TS 26.193 "AMR Wideband Speech Codec; Source Controlled
       Rate operation", version 5.0.0 (2001-03), 3rd Generation
       Partnership Project (3GPP).
 [8]   Schulzrinne, H, Casner, S., Frederick, R. and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", RFC
       1889, January 1996.
 [9]   3GPP TS 26.092, "AMR Speech Codec; Comfort noise aspects",
       version 4.0.0 (2001-03), 3rd Generation Partnership Project
       (3GPP).
 [10]  3GPP TS 26.192 "AMR Wideband speech codec; Comfort Noise
       aspects", version 5.0.0 (2001-03), 3rd Generation Partnership
       Project (3GPP).
 [11]  Handley, M. and V. Jacobson, "SDP: Session Description
       Protocol", RFC 2327, April 1998.
 [24]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
       with Minimal Control" RFC 1890, January 1996.

11.1 Informative References

 [12]  GSM 06.60, "Enhanced Full Rate (EFR) speech transcoding",
       version 8.0.1 (2000-11), European Telecommunications Standards
       Institute (ETSI).

Sjoberg, et. al. Standards Track [Page 46] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

 [13]  ANSI/TIA/EIA-136-Rev.C, part 410 - "TDMA Cellular/PCS - Radio
       Interface, Enhanced Full Rate Voice Codec (ACELP)." Formerly
       IS-641.  TIA published standard, June 1 2001.
 [14]  ARIB, RCR STD-27H, "Personal Digital Cellular Telecommunication
       System RCR Standard", Association of Radio Industries and
       Businesses (ARIB).
 [15]  Larzon, L., Degermark, M. and S. Pink, "The UDP Lite Protocol",
       Work in Progress.
 [16]  3GPP TS 25.415 "UTRAN Iu Interface User Plane Protocols",
       version 4.2.0 (2001-09), 3rd Generation Partnership Project
       (3GPP).
 [17]  S. Floyd, M. Handley, J. Padhye, J. Widmer, "Equation-Based
       Congestion Control for Unicast Applications", ACM SIGCOMM 2000,
       Stockholm, Sweden .
 [18]  Li, A., et. al., "An RTP Payload Format for Generic FEC with
       Uneven Level Protection", Work in Progress.
 [19]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
       Generic Forward Error Correction", RFC 2733, December 1999.
 [20]  3GPP TS 26.102, "AMR speech codec interface to Iu and Uu",
       version 4.0.0 (2001-03), 3rd Generation Partnership Project
       (3GPP).
 [21]  3GPP TS 26.202 "AMR Wideband speech codec; Interface to Iu and
       Uu", version 5.0.0 (2001-03), 3rd Generation Partnership
       Project (3GPP).
 [22]  Baugher, et. al., "The Secure Real Time Transport Protocol",
       Work in Progress.
 [23]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley,
       M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP
       Payload for Redundant Audio Data", RFC 2198, September 1997.
 ETSI documents can be downloaded from the ETSI web server,
 "http://www.etsi.org/".  Any 3GPP document can be downloaded from the
 3GPP webserver, "http://www.3gpp.org/", see specifications.  TIA
 documents can be obtained from "www.tiaonline.org".

Sjoberg, et. al. Standards Track [Page 47] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

12. Authors' Addresses

 Johan Sjoberg
 Ericsson Research
 Ericsson AB
 SE-164 80 Stockholm, SWEDEN
 Phone:   +46 8 50878230
 EMail: Johan.Sjoberg@ericsson.com
 Magnus Westerlund
 Ericsson Research
 Ericsson AB
 SE-164 80 Stockholm, SWEDEN
 Phone:   +46 8 4048287
 EMail: Magnus.Westerlund@ericsson.com
 Ari Lakaniemi
 Nokia Research Center
 P.O.Box 407
 FIN-00045 Nokia Group, FINLAND
 Phone:   +358-71-8008000
 EMail: ari.lakaniemi@nokia.com
 Qiaobing Xie
 Motorola, Inc.
 1501 W. Shure Drive, 2-B8
 Arlington Heights, IL 60004, USA
 Phone:   +1-847-632-3028
 EMail: qxie1@email.mot.com

Sjoberg, et. al. Standards Track [Page 48] RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002

13. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Sjoberg, et. al. Standards Track [Page 49]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3267.txt · Last modified: 2002/07/02 19:40 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki