rfc4867
Network Working Group J. Sjoberg
Request for Comments: 4867 M. Westerlund
Obsoletes: 3267 Ericsson
Category: Standards Track A. Lakaniemi
Nokia
Q. Xie
Motorola
April 2007
RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB)
Audio Codecs
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies a Real-time Transport Protocol (RTP) payload
format to be used for Adaptive Multi-Rate (AMR) and Adaptive Multi-
Rate Wideband (AMR-WB) encoded speech signals. The payload format is
designed to be able to interoperate with existing AMR and AMR-WB
transport formats on non-IP networks. In addition, a file format is
specified for transport of AMR and AMR-WB speech data in storage mode
applications such as email. Two separate media type registrations
are included, one for AMR and one for AMR-WB, specifying use of both
the RTP payload format and the storage format. This document
obsoletes RFC 3267.
Sjoberg, et al. Standards Track [Page 1]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Table of Contents
1. Introduction ....................................................4
2. Conventions and Acronyms ........................................4
3. Background on AMR/AMR-WB and Design Principles ..................5
3.1. The Adaptive Multi-Rate (AMR) Speech Codec .................5
3.2. The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec .....6
3.3. Multi-Rate Encoding and Mode Adaptation ....................6
3.4. Voice Activity Detection and Discontinuous Transmission ....7
3.5. Support for Multi-Channel Session ..........................7
3.6. Unequal Bit-Error Detection and Protection .................8
3.6.1. Applying UEP and UED in an IP Network ...............8
3.7. Robustness against Packet Loss ............................10
3.7.1. Use of Forward Error Correction (FEC) ..............10
3.7.2. Use of Frame Interleaving ..........................12
3.8. Bandwidth-Efficient or Octet-Aligned Mode .................12
3.9. AMR or AMR-WB Speech over IP Scenarios ....................13
4. AMR and AMR-WB RTP Payload Formats .............................15
4.1. RTP Header Usage ..........................................15
4.2. Payload Structure .........................................17
4.3. Bandwidth-Efficient Mode ..................................17
4.3.1. The Payload Header .................................17
4.3.2. The Payload Table of Contents ......................18
4.3.3. Speech Data ........................................20
4.3.4. Algorithm for Forming the Payload ..................21
4.3.5. Payload Examples ...................................21
4.3.5.1. Single-Channel Payload Carrying a
Single Frame ..............................21
4.3.5.2. Single-Channel Payload Carrying
Multiple Frames ...........................22
4.3.5.3. Multi-Channel Payload Carrying
Multiple Frames ...........................23
4.4. Octet-Aligned Mode ........................................25
4.4.1. The Payload Header .................................25
4.4.2. The Payload Table of Contents and Frame CRCs .......26
4.4.2.1. Use of Frame CRC for UED over IP ..........28
4.4.3. Speech Data ........................................30
4.4.4. Methods for Forming the Payload ....................31
4.4.5. Payload Examples ...................................32
4.4.5.1. Basic Single-Channel Payload
Carrying Multiple Frames ..................32
4.4.5.2. Two-Channel Payload with CRC,
Interleaving, and Robust Sorting ..........32
4.5. Implementation Considerations .............................33
4.5.1. Decoding Validation ................................34
5. AMR and AMR-WB Storage Format ..................................35
5.1. Single-Channel Header .....................................35
5.2. Multi-Channel Header ......................................36
Sjoberg, et al. Standards Track [Page 2]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
5.3. Speech Frames .............................................37
6. Congestion Control .............................................38
7. Security Considerations ........................................38
7.1. Confidentiality ...........................................39
7.2. Authentication and Integrity ..............................39
8. Payload Format Parameters ......................................39
8.1. AMR Media Type Registration ...............................40
8.2. AMR-WB Media Type Registration ............................44
8.3. Mapping Media Type Parameters into SDP ....................47
8.3.1. Offer-Answer Model Considerations ..................48
8.3.2. Usage of Declarative SDP ...........................50
8.3.3. Examples ...........................................51
9. IANA Considerations ............................................53
10. Changes from RFC 3267 .........................................53
11. Acknowledgements ..............................................55
12. References ....................................................55
12.1. Normative References .....................................55
12.2. Informative References ...................................56
Sjoberg, et al. Standards Track [Page 3]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
1. Introduction
This document obsoletes RFC 3267 and extends that specification with
offer/answer rules. See Section 10 for the changes made to this
format in relation to RFC 3267.
This document specifies the payload format for packetization of AMR
and AMR-WB encoded speech signals into the Real-time Transport
Protocol (RTP) [8]. The payload format supports transmission of
multiple channels, multiple frames per payload, the use of fast codec
mode adaptation, robustness against packet loss and bit errors, and
interoperation with existing AMR and AMR-WB transport formats on
non-IP networks, as described in Section 3.
The payload format itself is specified in Section 4. A related file
format is specified in Section 5 for transport of AMR and AMR-WB
speech data in storage mode applications such as email. In Section
8, two separate media type registrations are provided, one for AMR
and one for AMR-WB.
Even though this RTP payload format definition supports the transport
of both AMR and AMR-WB speech, it is important to remember that AMR
and AMR-WB are two different codecs and they are always handled as
different payload types in RTP.
2. Conventions and Acronyms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
The following acronyms are used in this document:
3GPP - the Third Generation Partnership Project
AMR - Adaptive Multi-Rate (Codec)
AMR-WB - Adaptive Multi-Rate Wideband (Codec)
CMR - Codec Mode Request
CN - Comfort Noise
DTX - Discontinuous Transmission
ETSI - European Telecommunications Standards Institute
FEC - Forward Error Correction
SCR - Source Controlled Rate Operation
SID - Silence Indicator (the frames containing only CN
parameters)
VAD - Voice Activity Detection
UED - Unequal Error Detection
UEP - Unequal Error Protection
Sjoberg, et al. Standards Track [Page 4]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
The term "frame-block" is used in this document to describe the
time-synchronized set of speech frames in a multi-channel AMR or
AMR-WB session. In particular, in an N-channel session, a frame-
block will contain N speech frames, one from each of the channels,
and all N speech frames represents exactly the same time period.
The byte order used in this document is network byte order, i.e., the
most significant byte first. The bit order is also the most
significant bit first. This is presented in all figures as having
the most significant bit leftmost on a line and with the lowest
number. Some bit fields may wrap over multiple lines in which cases
the bits on the first line are more significant than the bits on the
next line.
3. Background on AMR/AMR-WB and Design Principles
AMR and AMR-WB were originally designed for circuit-switched mobile
radio systems. Due to their flexibility and robustness, they are
also suitable for other real-time speech communication services over
packet-switched networks such as the Internet.
Because of the flexibility of these codecs, the behavior in a
particular application is controlled by several parameters that
select options or specify the acceptable values for a variable.
These options and variables are described in general terms at
appropriate points in the text of this specification as parameters to
be established through out-of-band means. In Section 8, all of the
parameters are specified in the form of media subtype registrations
for the AMR and AMR-WB encodings. The method used to signal these
parameters at session setup or to arrange prior agreement of the
participants is beyond the scope of this document; however, Section
8.3 provides a mapping of the parameters into the Session Description
Protocol (SDP) [11] for those applications that use SDP.
3.1. The Adaptive Multi-Rate (AMR) Speech Codec
The AMR codec was originally developed and standardized by the
European Telecommunications Standards Institute (ETSI) for GSM
cellular systems. It is now chosen by the Third Generation
Partnership Project (3GPP) as the mandatory codec for third
generation (3G) cellular systems [1].
The AMR codec is a multi-mode codec that supports eight narrow band
speech encoding modes with bit rates between 4.75 and 12.2 kbps. The
sampling frequency used in AMR is 8000 Hz and the speech encoding is
performed on 20 ms speech frames. Therefore, each encoded AMR speech
frame represents 160 samples of the original speech.
Sjoberg, et al. Standards Track [Page 5]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Among the eight AMR encoding modes, three are already separately
adopted as standards of their own. Particularly, the 6.7 kbps mode
is adopted as PDC-EFR [18], the 7.4 kbps mode as IS-641 codec in TDMA
[17], and the 12.2 kbps mode as GSM-EFR [16].
3.2. The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec
The Adaptive Multi-Rate Wideband (AMR-WB) speech codec [3] was
originally developed by 3GPP to be used in GSM and 3G cellular
systems.
Similar to AMR, the AMR-WB codec is also a multi-mode speech codec.
AMR-WB supports nine wide band speech coding modes with respective
bit rates ranging from 6.6 to 23.85 kbps. The sampling frequency
used in AMR-WB is 16000 Hz and the speech processing is performed on
20 ms frames. This means that each AMR-WB encoded frame represents
320 speech samples.
3.3. Multi-Rate Encoding and Mode Adaptation
The multi-rate encoding (i.e., multi-mode) capability of AMR and
AMR-WB is designed for preserving high speech quality under a wide
range of transmission conditions.
With AMR or AMR-WB, mobile radio systems are able to use available
bandwidth as effectively as possible. For example, in GSM it is
possible to dynamically adjust the speech encoding rate during a
session so as to continuously adapt to the varying transmission
conditions by dividing the fixed overall bandwidth between speech
data and error protective coding. This enables the best possible
trade-off between speech compression rate and error tolerance. To
perform mode adaptation, the decoder (speech receiver) needs to
signal the encoder (speech sender) the new mode it prefers. This
mode change signal is called Codec Mode Request or CMR.
Since in most sessions speech is sent in both directions between the
two ends, the mode requests from the decoder at one end to the
encoder at the other end are piggy-backed over the speech frames in
the reverse direction. In other words, there is no out-of-band
signaling needed for sending CMRs.
Every AMR or AMR-WB codec implementation is required to support all
the respective speech coding modes defined by the codec and must be
able to handle mode switching to any of the modes at any time.
However, some transport systems may impose limitations in the number
of modes supported and how often the mode can change due to bandwidth
Sjoberg, et al. Standards Track [Page 6]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 eight AMR modes for an AMR session or any
combination of the nine 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 media type 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
attribute and the order of channels carried in each frame-block is
Sjoberg, et al. Standards Track [Page 7]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
implied by the number of channels as specified in Section 4.1 in
[12].
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 the
most sensitive and bits in class C the least sensitive (see Table 1
below for AMR and [4] for AMR-WB). An AMR or AMR-WB 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 8]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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.
Link-layer protocols exist that do not discard packets containing 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 [19]), bit error tolerant AMR and AMR-WB traffic could
achieve better performance over these types of links. The
relationship between UDP-Lite's partial checksum at the transport
layer and the checksum coverage provided by the link-layer frame is
described in UDP-Lite specification [19].
There are at least two basic approaches for carrying AMR and AMR-WB
traffic over bit error tolerant IP networks:
a) Utilizing a partial checksum to cover the IP, transport protocol
(e.g., UDP-Lite), RTP and payload headers, and the most important
speech bits of the payload. The IP, UDP and RTP headers need to
be protected, and it is recommended that at least all class A bits
are covered by the checksum.
b) Utilizing a partial checksum to only cover the IP, transport
protocol, RTP and payload headers, but an AMR or AMR-WB 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 pays
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 the Universal Mobile
Telecommunications System (UMTS) can be found in [24] and for
AMR-WB in [25].
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.
Sjoberg, et al. Standards Track [Page 9]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Approach 1 is bit efficient, flexible and simple, 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
AMR or AMR-WB frames in a RTP payload, there is the possibility that
a single bit error in protected bits will cause all the frames to be
discarded.
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
improves the speech quality when transporting multiple AMR or AMR-WB
frames over links subject to bit errors.
The choice between the above two approaches must be made based on the
available bandwidth, and the desired tolerance to bit errors.
Neither solution is appropriate for all cases. Section 8 defines
parameters that may be used at session setup to choose 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., RFC 2733 [23], 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 [22] 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 a frame using either the
same mode or another mode, e.g., one with lower bandwidth. We
describe such a scheme next.
Sjoberg, et al. Standards Track [Page 10]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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.
--+--------+--------+--------+--------+--------+--------+--------+--
| f(n-2) | f(n-1) | f(n) | f(n+1) | f(n+2) | f(n+3) | f(n+4) |
--+--------+--------+--------+--------+--------+--------+--------+--
<---- 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. However, a parameter for providing a maximum delay in
transmitting any redundant frame is defined in Section 8. 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" [27].
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 the duration of 5 frames, 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
Sjoberg, et al. Standards Track [Page 11]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
receiver reports. A sender should not base selection of FEC on the
CMR, as this parameter most probably was set based on non-IP
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 to be encapsulated into a single RTP packet. One
of the drawbacks of such an approach is that packet loss can cause
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 more robust to packet loss and bit errors.
Sjoberg, et al. Standards Track [Page 12]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 is 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.
A streaming service has less strict real-time requirements and
therefore can use a larger number of frame-blocks per packet than a
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 that 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 is when AMR or AMR-WB encoded speech is transmitted
from a non-IP system (e.g., a GSM or 3GPP UMTS network) to an
IP/UDP/RTP VoIP terminal, and/or vice versa, as depicted in Figure 3.
Sjoberg, et al. Standards Track [Page 13]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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/ | IP network
3GPP UMTS 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 IP terminal's response to CMR.
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.
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.
Sjoberg, et al. Standards Track [Page 14]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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/ | IP network | GSM/
3GPP UMTS network | | 3GPP UMTS 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:
- the CMR value it receives on the IP side;
- the CMR value it calculates based on its reception quality on
the non-IP side; and
- 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.
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.
Sjoberg, et al. Standards Track [Page 15]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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, in exact
duplicates, 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 length is always made an integral number of octets 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).
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.
Sjoberg, et al. Standards Track [Page 16]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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.
The codec mode request received in the CMR field is valid until the
next codec mode request is received, i.e., a newly received CMR value
corresponding to a speech mode, or NO_DATA overrides the previously
received CMR value corresponding to a speech mode or NO_DATA.
Therefore, if a terminal continuously wishes to receive frames in the
Sjoberg, et al. Standards Track [Page 17]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 that is not a speech mode or
NO_DATA, the CMR MUST be ignored by the receiver.
In a multi-channel session, the codec mode request 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 codec mode request based on packet
losses or other congestion indications, for several reasons:
- 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.
- 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 codec 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 codec mode
requests when sending speech to a multicast session but MAY use RTCP
feedback information as a hint that a codec 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). If the received CMR
value is outside the signalled subset of modes, it MUST be ignored.
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 18]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 no data for that frame
has been produced by the speech encoder 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 in any payload format configuration, except in the case
of interleaving. Also, frame-blocks containing only NO_DATA frames
at the end of a packet SHOULD NOT be transmitted in any payload
format configuration, 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 19]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
The frame quality indicator is included for interoperability with the
ATM payload format described in ITU-T I.366.2, the UMTS Iu interface
[20], 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
more than 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 [12]. 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 three 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 zero 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 20]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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), and to bring the payload to octet alignment, 0 to 7
padding bits. Padding bits MUST be set to zero and MUST be ignored
on reception. They are packed 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).
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.
Sjoberg, et al. Standards Track [Page 21]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 padding bits (P) 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 a 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
kbps 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 are 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 zero bits are padded to the end to
make the payload octet aligned.
Sjoberg, et al. Standards Track [Page 22]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 kbps
(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 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 interleaving 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 interleaving group during an interleaving session. Here
we assume ILL=L for the interleaving group that starts at speech
frame-block n. We also assume that the first payload packet of the
interleaving group is s, and the number of speech frame-blocks
carried in each payload is N. Then we will have:
Sjoberg, et al. Standards Track [Page 25]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Payload s (the first packet of this interleaving 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 interleaving group):
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 interleaving 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 interleaving 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 interleaving group. In other words, all payloads in an
interleaving 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 media type 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
interleaving group is less or equal to I, that is, 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. That
is, the ToC is as follows:
+---------------------+
| 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.
Sjoberg, et al. Standards Track [Page 26]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 consecutively in time. Instead, the presence and
order of the frame-blocks in a packet will follow the pattern
described in 4.4.1.
The following example shows the ToC of three consecutive packets,
each carrying three 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 results in the interleaving group size of 9 frame-blocks.
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
Sjoberg, et al. Standards Track [Page 27]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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.
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, and MUST be ignored on
reception.
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 bits 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 [19]) to protect the IP,
transport protocol (e.g., UDP-Lite), and RTP headers, as well as the
payload header and the table of contents in the payload. The frame
CRC, when used, MUST be calculated only over all class A bits in the
AMR or AMR-WB frame. Class B and C bits in the AMR or AMR-WB 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.
Sjoberg, et al. Standards Track [Page 28]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
If the transport layer checksum or link layer checksum detects any
errors within the protected (sensitive) part, it is assumed that the
complete packet will be discarded as defined by UDP-Lite [19].
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.
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 CRCs takes 8 bits
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| c0| c1| c2| c3| c4| c5| c6| c7|
+---+---+---+---+---+---+---+---+
(MSB) (LSB)
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 appears as follows: 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 (LSB) bit of the CRC register and the bit. The CRC
Sjoberg, et al. Standards Track [Page 29]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
register is then right-shifted one step (each bit's significance is
reduced by one), inputting a "0" as the leftmost bit (MSB). If the
result of the XOR operation mentioned above is a "1", then "10111000"
is 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) has been used in this CRC calculation, the contents in
CRC register should simply be copied to the corresponding field in
the list of CRCs.
Fast calculation of the CRC on a general-purpose CPU is possible
using a table-driven algorithm.
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:
- The last octet of each speech frame MUST be padded with zero
bits at the end if all bits in the octet are not used. The
padding bits MUST be ignored on reception. In other words,
each speech frame MUST be octet-aligned.
- When multiple speech frames are present in the speech data
(i.e., compound payload), the speech frames are arranged either
one whole frame after another as usual, or with the octets of
all frames interleaved together at the octet level, depending
on the media type parameters negotiated for the payload type.
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 [19]) or UEP
(such as the ULP [22]) 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 payload type MUST be agreed via
out-of-band means. Section 8 specifies a media type 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 payload
types.
Sjoberg, et al. Standards Track [Page 30]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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
octets 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.
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 it 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 for communicating the number of
octets to be covered to other layers performing UED/UEP is beyond the
scope of this specification.
Sjoberg, et al. Standards Track [Page 31]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 payload type 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 the above example, the last octet in both speech frames is
padded with one zero bit 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
payload type. Two frame-blocks, each containing two 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, robust sorting, and frame-block interleaving are
all enabled for the payload type. The interleaving length is 2
(ILL=1), and this payload is the first one in an interleaving group
(ILP=0).
Sjoberg, et al. Standards Track [Page 32]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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
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 the above example, the last octet in all 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 media type
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 operating 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 modes for a single audio
Sjoberg, et al. Standards Track [Page 33]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
channel. The other operating modes: interleaving, robust sorting,
and frame-wise CRC (in both single and multi-channel) are OPTIONAL to
implement.
The mode-change-period, mode-change-capability, and mode-change-
neighbor parameters are intended for signaling with GSM endpoints.
When interoperability with GSM is desired, encoders SHOULD only
perform codec mode changes to neighboring modes and in integer
multiples of 40 ms (two frame-blocks), but decoders SHOULD accept
codec mode changes at any time, i.e., for every frame-block. The
encoder may arbitrarily select the initial phase (odd or even frame-
block) where codec mode changes are performed, but then SHOULD stick
to that phase as far as possible. However, in rare cases, handovers
or other events (e.g., call forwarding) may change this phase and may
also cause mode changes to non-neighboring modes. The decoder SHALL
therefore be prepared to accept changes also in the other phase and
to other modes. Section 8 specifies the usage of the parameters
mode-change-period and mode-change-capability to indicate the desired
behavior in applications.
See 3GPP TS 26.103 [28] for preferred AMR and AMR-WB configurations
for operation in GSM and 3GPP UMTS networks. In gateway scenarios,
encoders can be requested through the "mode-set" parameter to use a
limited mode-set that is supported by the link beyond the gateway.
Further, to avoid congestion on that link, the encoder SHOULD limit
the initial codec mode for a session to a lower mode, until at least
one frame-block is received with rate control information.
4.5.1. Decoding Validation
When processing a received payload packet, if the receiver finds that
the calculated payload length, based on the information for the
payload type 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.
Sjoberg, et al. Standards Track [Page 34]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 email 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.
There also exists another storage format for AMR and AMR-WB that is
suitable for applications with more advanced demands on the storage
format, like random access or synchronization with video. This
format is the 3GPP-specified ISO-based multimedia file format 3GP
[31]. Its media type is specified by RFC 3839 [32].
5.1. Single-Channel Header
A single-channel AMR or AMR-WB file header contains only a magic
number. 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).
Sjoberg, et al. Standards Track [Page 35]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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.
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 bits, 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 [12].
Sjoberg, et al. Standards Track [Page 36]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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
4.3.2. The P bits are padding and MUST be set to 0, and MUST be
ignored.
Following this one octet header come the speech bits as defined in
4.4.3. 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 kbps 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Non-received speech frames or 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]). Frames or frame-blocks lost in
transmission MUST be stored as NO_DATA frames or SPEECH_LOST (frame
type 14, only available for AMR-WB) in complete frame-blocks to keep
synchronization with the original media.
Comfort noise frames of other types than AMR SID (FT=8) (i.e., frame
type 9, 10, and 11 for AMR) SHALL NOT be used in the AMR file format.
Sjoberg, et al. Standards Track [Page 37]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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, possibly "TCP Friendly Rate Control" [21].
7. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the general security considerations discussed in [8]
and in any used profile, like AVP [12] or SAVP [26].
As this format transports encoded speech, the main security issues
include confidentiality, authentication, and integrity of the speech
itself. The payload format itself does not have any built-in
security mechanisms. External mechanisms, such as SRTP [26], need to
be used for this functionality. Note that the appropriate mechanism
to provide security to RTP and the payloads following this memo may
vary. It is dependent on the application, the transport, and the
signaling protocol employed. Therefore, a single mechanism is not
sufficient, although if suitable the usage of SRTP [26] is
RECOMMENDED. Other known mechanisms that may be used are IPsec [33]
and TLS [34] (RTP over TCP), but other alternatives may also exist.
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.
Sjoberg, et al. Standards Track [Page 38]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 of a need
to encrypt the payload header or the table of contents due to a) that
they only carry information about the requested speech mode, frame
type, and frame quality, and b) that this information could be useful
to some third party, e.g., quality monitoring.
The packetization and unpacketization of the AMR and AMR-WB payload
is done only at the endpoints. Therefore encryption should be
performed after packet encapsulation, and decryption should be
performed before packet decapsulation.
Encryption may affect interleaving. Specifically, a change of keys
should occur at the boundary between interleaving groups. If it is
not done at that boundary on both endpoints, the speech quality will
be degraded during the complete interleaving group for any receiver.
The encryption mechanism may impact the robustness of the error
correcting mechanism. This is discussed in Section 9.5 of SRTP [26].
From this, UED/UEP based on robust sorting may be difficult to apply
when the payload data is encrypted.
7.2. Authentication and Integrity
To authenticate the sender and to protect the integrity of the RTP
packets in transit, an external mechanism has to be used. As stated
before, it is RECOMMENDED that SRTP [26] be used for common
interoperability. Note that the use of UED/UEP may be difficult to
combine with some integrity protection mechanisms because any bit
errors will cause the integrity check to fail.
Data tampering by a man-in-the-middle attacker could result in
erroneous depacketization/decoding that could lower the speech
quality or produce unintelligible communications. Tampering with the
CMR field may result in a different speech quality than desired.
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 media type registrations
for the AMR and AMR-WB speech codecs. The registrations are done
following RFC 4855 [15] and the media registration rules [14].
Sjoberg, et al. Standards Track [Page 39]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 media types or SDP.
Two separate media type registrations are made, one for AMR and one
for AMR-WB, because they are distinct encodings that must be
distinguished by their own media type.
Data formats are specified for both real-time transport in RTP and
for storage type applications such as email attachments.
8.1. AMR Media Type Registration
The media type for the Adaptive Multi-Rate (AMR) codec is allocated
from the IETF tree since AMR is a widely used speech codec in general
VoIP and messaging applications. This media type 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: Restricts the active codec mode set to a subset of all
modes, for example, to be able to support transport
channels such as GSM networks in gateway use cases.
Possible values are a comma separated list of modes from
the set: 0,...,7 (see Table 1a [2]). The SID frame type
8 and NO_DATA (frame type 15) are never included in the
mode set, but can always be used. If mode-set is
specified, it MUST be abided, and frames encoded with
modes outside of the subset MUST NOT be sent in any RTP
payload or used in codec mode requests. If not present,
all codec modes are allowed for the payload type.
Sjoberg, et al. Standards Track [Page 40]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
mode-change-period: Specifies a number of frame-blocks, N (1 or
2), that is the frame-block period at which codec mode
changes are allowed for the sender. The initial phase of
the interval is arbitrary, but changes must be separated
by a period of N frame-blocks, i.e., a value of 2
allows the sender to change mode every second frame-
block. The value of N SHALL be either 1 or 2. If this
parameter is not present, mode changes are allowed at
any time during the session, i.e., N=1.
mode-change-capability: Specifies if the client is capable to
transmit with a restricted mode change period. The
parameter may take value of 1 or 2. A value of 1
indicates that the client is not capable of restricting
the mode change period to 2, and that the codec mode may
be changed at any point. A value of 2 indicates that the
client has the capability to restrict the mode change
period to 2, and thus that the client can correctly
interoperate with a receiver requiring a mode-change-
period=2. If this parameter is not present, the mode-
change restriction capability is not supported, i.e.
mode-change-capability=1. To be able to interoperate
fully with gateways to circuit switched networks (for
example, GSM networks), transmissions with restricted
mode changes (mode-change-capability=2) are required.
Thus, clients RECOMMENDED to have the capability to
support transmission according to
mode-change-capability=2.
mode-change-neighbor: Permissible values are 0 and 1. If 1, the
sender SHOULD only perform mode changes 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 that the
media present in the packet represents. The time SHOULD
be an integer 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 41]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
crc: Permissible values are 0 and 1. If 1, frame CRCs SHALL be
included in the payload. If 0 or not present, CRCs
SHALL NOT be used. 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 RFC 4566 [11].
channels: The number of audio channels. The possible values
(1-6) and their respective channel order is specified in
Section 4.1 in [12]. If omitted, it has the default
value of 1.
max-red: The maximum duration in milliseconds that elapses between
the primary (first) transmission of a frame and any
redundant transmission that the sender will use. This
parameter allows a receiver to have a bounded delay when
redundancy is used. Allowed values are between 0 (no
redundancy will be used) and 65535. If the parameter is
omitted, no limitation on the use of redundancy is
present.
Encoding considerations:
The Audio data is binary data, and must be encoded for non-
binary transport; the Base64 encoding is suitable for email.
When used in RTP context the data is framed as defined in [14].
Security considerations:
See Section 7 of RFC 4867.
Public specification:
RFC 4867
3GPP TS 26.090, 26.092, 26.093, 26.101
Sjoberg, et al. Standards Track [Page 42]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Applications that use this media type:
This media type is used in numerous applications needing
transport or storage of encoded voice. Some examples include;
Voice over IP, streaming media, voice messaging, and voice
recording on digital cameras.
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: "amr " (fourth character is space)
AMR speech frames may also be stored in the file format "3GP"
defined in 3GPP TS 26.244 [31], which is identified using the
media types "audio/3GPP" or "video/3GPP" as registered by RFC
3839 [32].
Person & email address to contact for further information:
Magnus Westerlund <magnus.westerlund@ericsson.com>
Ari Lakaniemi <ari.lakaniemi@nokia.com>
Intended usage: COMMON.
This media type is widely used in streaming, VoIP, and messaging
applications on many types of devices.
Restrictions on usage:
When this media type is used in the context of transfer over
RTP, the RTP payload format specified in Section 4 SHALL be
used. In all other contexts, the file format defined in Section
5 SHALL be used.
Author:
Magnus Westerlund <magnus.westerlund@ericsson.com>
Ari Lakaniemi <ari.lakaniemi@nokia.com>
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
Sjoberg, et al. Standards Track [Page 43]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
8.2. AMR-WB Media Type Registration
The media type for the Adaptive Multi-Rate Wideband (AMR-WB) codec is
allocated from the IETF tree since AMR-WB is a widely used speech
codec in general VoIP and messaging applications. This media type
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-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: Restricts the active codec mode set to a subset of all
modes, for example, to be able to support transport
channels such as GSM networks in gateway use cases.
Possible values are a comma-separated list of modes from
the set: 0,...,8 (see Table 1a [4]). The SID frame type
9, SPEECH_LOST (frame type 14), and NO_DATA (frame type
15) are never included in the mode set, but can always
be used. If mode-set is specified, it MUST be abided,
and frames encoded with modes outside of the subset MUST
NOT be sent in any RTP payload or used in codec mode
requests. If not present, all codec modes are allowed
for the payload type.
mode-change-period: Specifies a number of frame-blocks, N (1 or
2), that is the frame-block period at which codec mode
changes are allowed for the sender. The initial phase of
the interval is arbitrary, but changes must be separated
by multiples of N frame-blocks, i.e., a value of 2
allows the sender to change mode every second frame-
block. The value of N SHALL be either 1 or 2. If this
parameter is not present, mode changes are allowed at
Any time during the session, i.e., N=1.
Sjoberg, et al. Standards Track [Page 44]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
mode-change-capability: Specifies if the client is capable to
transmit with a restricted mode change period. The
parameter may take value of 1 or 2. A value of 1
indicates that the client is not capable of restricting
the mode change period to 2, and that the codec mode may
be changed at any point. A value of 2 indicates that the
client has the capability to restrict the mode change
period to 2, and thus that the client can correctly
interoperate with a receiver requiring a mode-change-
period=2. If this parameter is not present, the mode-
change restriction capability is not supported, i.e.
mode-change-capability=1. To be able to interoperate
fully with gateways to circuit switched networks (for
example, GSM networks), transmissions with restricted
mode changes (mode-change-capability=2) are required.
Thus, clients are RECOMMENDED to have the capability to
support transmission according to
mode-change-capability=2.
mode-change-neighbor: Permissible values are 0 and 1. If 1, the
sender SHOULD only perform mode changes 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 that the
media present in the packet represents. The time SHOULD
be an integer 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. If 0 or not present, CRCs
SHALL NOT be used. 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.
Sjoberg, et al. Standards Track [Page 45]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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 RFC 2327 [11].
channels: The number of audio channels. The possible values
(1-6) and their respective channel order is specified in
Section 4.1 in [12]. If omitted, it has the default
value of 1.
max-red: The maximum duration in milliseconds that elapses between
the primary (first) transmission of a frame and any
redundant transmission that the sender will use. This
parameter allows a receiver to have a bounded delay when
redundancy is used. Allowed values are between 0 (no
redundancy will be used) and 65535. If the parameter is
omitted, no limitation on the use of redundancy is
present.
Encoding considerations:
The Audio data is binary data, and must be encoded for non-
binary transport; the Base64 encoding is suitable for email.
When used in RTP context the data is framed as defined in [14].
Security considerations:
See Section 7 of RFC 4867.
Public specification:
RFC 4867
3GPP TS 26.190, 26.192, 26.193, 26.201
Applications that use this media type:
This media type is used in numerous applications needing
transport or storage of encoded voice. Some examples include;
Voice over IP, streaming media, voice messaging, and voice
recording on digital cameras.
Sjoberg, et al. Standards Track [Page 46]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
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)
File extensions: awb, AWB
Macintosh file type code: amrw
Object identifier or OID: none
AMR-WB speech frames may also be stored in the file format "3GP"
defined in 3GPP TS 26.244 [31] and identified using the media
type "audio/3GPP" or "video/3GPP" as registered by RFC 3839
[32].
Person & email address to contact for further information:
Magnus Westerlund <magnus.westerlund@ericsson.com>
Ari Lakaniemi <ari.lakaniemi@nokia.com>
Intended usage: COMMON.
This media type is widely used in streaming, VoIP, and messaging
applications on many types of devices.
Restrictions on usage:
When this media type is used in the context of transfer over
RTP, the RTP payload format specified in Section 4 SHALL be
used. In all other contexts, the file format defined in Section
5 SHALL be used.
Author:
Magnus Westerlund <magnus.westerlund@ericsson.com>
Ari Lakaniemi <ari.lakaniemi@nokia.com>
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
8.3. Mapping Media Type Parameters into SDP
The information carried in the 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:
Sjoberg, et al. Standards Track [Page 47]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
- The media type ("audio") goes in SDP "m=" as the media name.
- The media 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 are specified in Section 4.1 in [12].
- The parameters "ptime" and "maxptime" go in the SDP "a=ptime"
and "a=maxptime" attributes, respectively.
- Any remaining parameters go in the SDP "a=fmtp" attribute by
copying them directly from the media type parameter string as a
semicolon-separated list of parameter=value pairs.
8.3.1. Offer-Answer Model Considerations
The following considerations apply when using SDP Offer-Answer
procedures to negotiate the use of AMR or AMR-WB payload in RTP:
- Each combination of the RTP payload transport format
configuration parameters (octet-align, crc, robust-sorting,
interleaving, and channels) is unique in its bit-pattern and
not compatible with any other combination. When creating an
offer in an application desiring to use the more advanced
features (crc, robust-sorting, interleaving, or more than one
channel), the offerer is RECOMMENDED to also offer a payload
type containing only the octet-aligned or bandwidth-efficient
configuration with a single channel. If multiple
configurations are of interest to the application, they may all
be offered; however, care should be taken not to offer too many
payload types. An SDP answerer MUST include, in the SDP answer
for a payload type, the following parameters unmodified from
the SDP offer (unless it removes the payload type): "octet-
align"; "crc"; "robust-sorting"; "interleaving"; and
"channels". The SDP offerer and answerer MUST generate AMR or
AMR-WB packets as described by these parameters.
- The "mode-set" parameter can be used to restrict the set of
active AMR/AMR-WB modes used in a session. This functionality
is primarily intended for gateways to access networks such as
GSM or 3GPP UMTS, where the access network may be capable of
supporting only a subset of AMR/AMR-WB modes. The 3GPP
preferred codec configurations are defined in 3GPP TS 26.103
[25], and it is RECOMMENDED that other networks also needing to
restrict the mode set follow the preferred codec configurations
defined in 3GPP for greatest interoperability.
Sjoberg, et al. Standards Track [Page 48]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
The parameter is bi-directional, i.e., the restricted set
applies to media both to be received and sent by the declaring
entity. If a mode set was supplied in the offer, the answerer
SHALL return the mode-set unmodified or reject the payload
type. However, the answerer is free to choose a mode-set in
the answer only if no mode-set was supplied in the offer for a
unicast two-peer session. The mode-set in the answer is
binding both for offerer and answerer. Thus, an offerer
supporting all modes and subsets SHOULD NOT include the mode-
set parameter. For any other offerer it is RECOMMENDED to
include each mode-set it can support as a separate payload type
within the offer. For multicast sessions, the answerer SHALL
only participate in the session if it supports the offered
mode-set. Thus, it is RECOMMENDED that any offer for a
multicast session include only the mode-set it will require the
answerers to support, and that the mode-set be likely to be
supported by all participants.
- The parameters "mode-change-period" and "mode-change-
capability" are intended to be used in sessions with gateways,
for example, when interoperating with GSM networks. Both
parameters are declarative and are combined to allow a session
participant to determine if the payload type can be supported.
The mode-change-period will indicate what the offerer or
answerer requires of data it receives, while the mode-change-
capability indicates its transmission capabilities.
A mode-change-period=2 in the offer indicates a requirement on
the answerer to send with a mode-change period of 2, i.e.,
support mode-change-capability=2. If the answerer requires
mode-change-period=2, it SHALL only include it in the answer if
the offerer either has indicated support with mode-change-
capability=2 or has indicated mode-change-period=2; otherwise,
the payload type SHALL be rejected. An offerer that supports
mode-change-capability=2 SHALL include the parameter in all
offers to ensure the greatest possible interoperability, unless
it includes mode-change-period=2 in the offer. The mode-
change-capability SHOULD be included in answers. It is then
indicating the answerer's capability to transmit with that
mode-change-period for the provided payload format
configuration. The information is useful in future
re-negotiation of the payload formats.
- The parameter "mode-change-neighbor" is a recommendation to
restrict the switching of codec modes to its neighbor and
SHOULD be followed. It is intended to be used in gateway
scenarios (for example, to GSM networks) where the support of
Sjoberg, et al. Standards Track [Page 49]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
this parameter and the operations it implies improves
interoperability.
"mode-change-neighbor" is a declarative parameter. By
including the parameter, the offerer or answerer indicates that
it desires to receive streams with "mode-change-neighbor"
restrictions.
- In most cases, the parameters "maxptime" and "ptime" will not
affect interoperability; however, the setting of the parameters
can affect the performance of the application. The SDP offer-
answer handling of the "ptime" parameter is described in RFC
3264 [13]. The "maxptime" parameter MUST be handled in the
same way.
- The parameter "max-red" is a stream property parameter. For
send-only or send-recv unicast media streams, the parameter
declares the limitation on redundancy that the stream sender
will use. For recvonly streams, it indicates the desired value
for the stream sent to the receiver. The answerer MAY change
the value, but is RECOMMENDED to use the same limitation as the
offer declares. In the case of multicast, the offerer MAY
declare a limitation; this SHALL be answered using the same
value. A media sender using this payload format is RECOMMENDED
to always include the "max-red" parameter. This information is
likely to simplify the media stream handling in the receiver.
This is especially true if no redundancy will be used, in which
case "max-red" is set to 0. As this parameter was not defined
originally, some senders will not declare this parameter even
if it will limit or not send redundancy at all.
- Any unknown parameter in an offer SHALL be removed in the
answer.
8.3.2. Usage of Declarative SDP
In declarative usage, like SDP in RTSP [29] or SAP [30], the
parameters SHALL be interpreted as follows:
- The payload format configuration parameters (octet-align, crc,
robust-sorting, interleaving, and channels) are all declarative,
and a participant MUST use the configuration(s) that is provided
for the session. More than one configuration may be provided if
necessary by declaring multiple RTP payload types; however, the
number of types should be kept small.
Sjoberg, et al. Standards Track [Page 50]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
- Any restriction of the AMR or AMR-WB encoder mode-switching and
mode usage through the "mode-set", and "mode-change-period" MUST
be followed by all participants of the session. The restriction
indicated by "mode-change-neighbor" SHOULD be followed. Please
note that such restrictions may be necessary if gateways to other
transport systems like GSM participate in the session. Failure to
consider such restrictions may result in failure for a peer behind
such a gateway to correctly receive all or parts of the session.
Also, if different restrictions are needed by different peers in
the same session (unless a common subset of the restrictions
exists), some peer will not be able to participate. Note that the
usage of mode-change-capability is meaningless when no negotiation
exists, and can thus be excluded in any declarations.
- Any "maxptime" and "ptime" values should be selected with care to
ensure that the session's participants can achieve reasonable
performance.
- The usage of "max-red" puts a global upper limit on the usage of
redundancy that needs to be followed by all that understand the
parameter. However, due to the late addition of this parameter,
it may be ignored by some implementations.
8.3.3. Examples
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.
In an example of the usage of AMR in a possible GSM gateway-to-
gateway scenario, the offerer is capable of supporting three
different mode-sets and needs the mode-change-period to be 2 in
combination with mode-change-neighbor restrictions. The other
gateway can only support two of these mode-sets and removes the
payload type 97 in the answer. If the offering GSM gateway only
supports a single mode-set active at the same time, it should
consider doing the 1 out of N selection procedures described in
Section 10.2 of [13]:
Sjoberg, et al. Standards Track [Page 51]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Offer:
m=audio 49120 RTP/AVP 97 98 99
a=rtpmap:97 AMR/8000/1
a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; \
mode-change-capability=2; mode-change-neighbor=1
a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-set=0,2,3,6; mode-change-period=2; \
mode-change-capability=2; mode-change-neighbor=1
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-set=0,2,3,4; mode-change-period=2; \
mode-change-capability=2; mode-change-neighbor=1
a=maxptime:20
Answer:
m=audio 49120 RTP/AVP 98 99
a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-set=0,2,3,6; mode-change-period=2; \<
mode-change-capability=2; mode-change-neighbor=1
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-set=0,2,3,4; mode-change-period=2; \
mode-change-capability=2; mode-change-neighbor=1
a=maxptime:20
The following example shows the usage of AMR between a non-GSM
endpoint and a GSM gateway. The non-GSM offerer requires no
restrictions of the mode-change-period or mode-change-neighbor, but
must signal its mode-change-capability in the offer and abide by
those restrictions in the answer.
Offer:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 AMR/8000/1
a=fmtp:97 mode-change-capability=2
a=maxptime:20
Answer:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 AMR/8000/1
a=fmtp:97 mode-set=0,2,4,7; mode-change-period=2; \
mode-change-capability=2; mode-change-neighbor=1
a=maxptime:20
Sjoberg, et al. Standards Track [Page 52]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Example of usage of AMR-WB in a possible VoIP scenario where UEP may
be used (99) and a fallback declaration (98):
m=audio 49120 RTP/AVP 99 98
a=rtpmap:98 AMR-WB/16000
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR-WB/16000
a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2
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 media types (audio/AMR and audio/AMR-WB) have been updated; see
Section 8.
10. Changes from RFC 3267
The differences between RFC 3267 and this document are as follows:
- Added clarification of behavior in regards to mode change period
and mode-change neighbor that is expected from an IP client; see
Section 4.5.
- Updated the maxptime for better clarification. The sentence that
previously read: "The time SHOULD be a multiple of the frame
size." now says "The time SHOULD be an integer multiple of the
frame size." This should have no impact on interoperability.
- Updated the definition of the mode-set parameter for
clarification.
- Restricted the values for mode-change-period to 1 or 2, which are
the values used in circuit-switched AMR systems.
Sjoberg, et al. Standards Track [Page 53]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
- Added a new media type parameter Mode-Change-Capability that
defaults to 1, which is the assumed behavior of any non-updated
implementation. This enables the offer-answer procedures to work.
- Changed mode-change-neighbor to indicate a recommended behavior
rather than a required one.
- Added an Offer-Answer Section, see Section 8.3.1. This will have
implications on the interoperability to implementations that have
guessed how to perform offer/answer negotiation of the payload
parameters.
- Clarified and aligned the unequal detection usage with the
published UDP-Lite specification in Sections 3.6.1 and 4.4.2.1.
This included replacing a normative statement about packet
handling with an informative paragraph with a reference to UDP-
Lite.
- Clarified the bit order in the CRC calculation in Section 4.4.2.1.
- Corrected the reference in Section 5.3 for the Q and FT fields.
- Changed the padding bit definition in Sections 4.4.2 and 5.3 so
that it is clear that they shall be ignored.
- Added a clarification that comfort noise frames with frame type 9,
10, and 11 SHALL NOT be used in the AMR file format.
- Clarified in Section 4.3.2 that the rules about not sending
NO_DATA frames do apply for all payload format configurations with
the exception of the interleaved mode.
- The reference list has been updated to now published RFCs: RFC
3448, RFC 3550, RFC 3551, RFC 3711, RFC 3828, and RFC 4566. A
reference to 3GPP TS 26.101 has also been added.
- Added notes in storage format section and media type registration
that AMR and AMR-WB frames can also be stored in the 3GP file
format.
- Added a media type parameter "max-red" that allows the sender to
declare a bounded usage of redundancy. This parameter allows a
receiver to optimize its function as it will know if redundancy
will be used or not. If it is used, the maximum extra delay
introduced by the sender (that is needed to be considered by the
receiver to fully utilize the redundancy) will be known. The
addition of this parameter should have no negative effects on
older implementations as they are mandated to ignore unknown
Sjoberg, et al. Standards Track [Page 54]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
parameters per RFC 3267. In addition, older implementations are
required to operate as if the value of max-red is unknown and
possibly infinite.
- Updated the media type registration to comply with the new
registration rules.
- Moved section on decoding validation from Security Considerations
to Implementation Considerations, where it makes more sense.
- Clarified the application of encryption, integrity protection, and
authentication mechanism to the payload.
11. 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 RFC 3267 and this replacement. The authors would also
like to thank Richard Ejzak, Thomas Belling, and Gorry Fairhurst for
their input on this replacement of RFC 3267.
12. References
12.1. Normative References
[1] 3GPP TS 26.090, "Adaptive Multi-Rate (AMR) speech transcoding",
version 4.0.0 (2001-03), 3rd Generation Partnership Project
(3GPP).
[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).
Sjoberg, et al. Standards Track [Page 55]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
[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", STD 64,
RFC 3550, July 2003.
[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., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[12] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[13] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[14] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[15] Casner, S., "Media Type Registration of RTP Payload Formats",
RFC 4855, February 2007.
12.2. Informative References
[16] GSM 06.60, "Enhanced Full Rate (EFR) speech transcoding",
version 8.0.1 (2000-11), European Telecommunications Standards
Institute (ETSI).
[17] 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.
[18] ARIB, RCR STD-27H, "Personal Digital Cellular Telecommunication
System RCR Standard", Association of Radio Industries and
Businesses (ARIB).
[19] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and G.
Fairhurst, "The Lightweight User Datagram Protocol (UDP-Lite)",
RFC 3828, July 2004.
Sjoberg, et al. Standards Track [Page 56]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
[20] 3GPP TS 25.415 "UTRAN Iu Interface User Plane Protocols",
version 4.2.0 (2001-09), 3rd Generation Partnership Project
(3GPP).
[21] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol Specification", RFC 3448, January
2003.
[22] Li, A., et al., "An RTP Payload Format for Generic FEC with
Uneven Level Protection", Work in Progress.
[23] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[24] 3GPP TS 26.102, "AMR speech codec interface to Iu and Uu",
version 4.0.0 (2001-03), 3rd Generation Partnership Project
(3GPP).
[25] 3GPP TS 26.202, "AMR Wideband speech codec; Interface to Iu and
Uu", version 5.0.0 (2001-03), 3rd Generation Partnership Project
(3GPP).
[26] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[27] 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.
[28] 3GPP TS 26.103, "Speech codec list for GSM and UMTS", version
5.5.0 (2004-09), 3rd Generation Partnership Project (3GPP).
[29] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[30] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[31] 3GPP TS 26.244, "3GPP file format (3GP)", version 6.1.0 (2004-
09), 3rd Generation Partnership Project (3GPP).
[32] Castagno, R. and D. Singer, "MIME Type Registrations for 3rd
Generation Partnership Project (3GPP) Multimedia files", RFC
3839, July 2004.
[33] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
Sjoberg, et al. Standards Track [Page 57]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
[34] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.1", RFC 4346, April 2006.
ETSI documents are available from <http://www.etsi.org/>.
3GPP documents are available from <http://www.3gpp.org/>.
TIA documents are available from <http://www.tiaonline.org/>.
Authors' Addresses
Johan Sjoberg
Ericsson AB
SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000
EMail: Johan.Sjoberg@ericsson.com
Magnus Westerlund
Ericsson Research
Ericsson AB
SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000
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: Qiaobing.Xie@motorola.com
Sjoberg, et al. Standards Track [Page 58]
RFC 4867 RTP Payload Format for AMR and AMR-WB April 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
<%ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Sjoberg, et al. Standards Track [Page 59]
ERRATA
