Internet Engineering Task Force Johan Sjoberg, Ericsson
Audio Video Transport WG Magnus Westerlund, Ericsson
INTERNET-DRAFT Ari Lakaniemi, Nokia
July 14, 2001 Petri Koskelainen, Nokia
Expires: January 14, 2000
RTP payload format for AMR
<draft-sjoberg-avt-rtp-amr-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Abstract
This document describes a proposed real-time transport protocol (RTP)
[8] payload format for AMR speech encoded [1] signals. The AMR
payload format is designed to be able to interoperate with existing
AMR transport formats. This document also includes a MIME type
registration for AMR. The MIME type is specified for both real-time
transport and storage.
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1. Introduction
The adaptive multi-rate (AMR) speech codec was developed by the
European Telecommunications Standards institute (ETSI). The AMR codec
is standardized for GSM, and is also chosen by 3GPP as the mandatory
codec for third generation systems. It is currently under
standardization for TDMA. I.e. the AMR codec will be widely used in
cellular systems. The AMR codec is developed to preserve high speech
quality under a wide range of transmission conditions.
The AMR codec is a multi-mode codec with 8 narrow band modes with bit
rates between 4.75 and 12.2 kbps. The sampling frequency is 8000 Hz
and processing is done on 20 ms frames, i.e. 160 samples per frame.
The AMR modes are closely related to each other and uses the same
coding framework. Three of the AMR modes are already adopted and used
standards of there own, the 6.7 kbps mode as PDC-EFR [7], the 7.4
kbps mode as IS-641 codec in TDMA [6], and the 12.2 kbps mode as GSM-
EFR [5].
AMR implementations must support all 8 speech coding modes, and mode
switching can occur to any mode at any time. The mode information
must therefore be transmitted together with the speech encoded bits,
to indicate the mode.
It is possible for the decoder to signal to the encoder the mode it
prefers to receive. The reason can be e.g. transmission bandwidth or
quality.
The AMR codec is designed with a voice activity detector (VAD) and
generation of comfort noise (CN) parameters during silence periods.
Hence, the AMR codec can reduce the number of transmitted bits and
packets during silence periods to a minimum. The operation to send CN
parameters at regular intervals during silence periods is usually
called discontinuous transmission (DTX) or source controlled rate
(SCR) operation. The three codec standards that are part of AMR
[5][6][7] also have SCR/CN functionality specified. To enable
interoperability with terminals supporting these standards the AMR
can optionally be extended to support also these CN schemes, see [2].
Due to the flexibility and robustness of AMR, it is suitable also for
other purposes than circuit switched cellular systems. Other suitable
applications are real-time services over packet switched networks,
e.g. over RTP. To be optimized for transmission over networks with
high packet loss rates, the possibility to use extra redundancy is
built into the RTP payload format for AMR. The speech encoded bits
have different perceptual sensitivity to bit errors and cellular
systems exploit this by using unequal error protection and detection
(UEP and UED). This mechanism concentrates the correction and
detection of corrupted bits to the perceptually most sensitive bits.
A frame is only regarded as lost or damaged if errors are detected in
the most sensitive bits. The UED can also be employed on RTP if UDP
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lite is used as transport layer protocol (UDP lite [10] is work in
progress). To enable this, the bits in the payload have to be ordered
in sensitivity order. The AMR encoded bits are defined in sensitivity
order in [2]. If the receiver supports option to retransmit redundant
frames, the different sensitivity could also be used for transmitting
only the most sensitive bits of a redundant frame. The special
problems with IP real-time traffic over cellular access networks are
further discussed in [9].
Other AMR scenarios are possible, e.g. one end is circuit switched
GSM, which is connected through a gateway to IP network and an IP
terminal in the other end. To improve quality, also frames damaged by
the GSM radio should be transmitted to the decoder in the IP network.
To make this possible, frame quality information has to be
transmitted over the IP network. The quality bit is also needed for
the AMR RTP payload format to interwork with for example the ATM AAL2
AMR profile.
2. Requirements
The AMR payload format for RTP was designed to meet the following
requirements:
o Different levels of robustness must be supported, from no
redundant data to extreme robustness capable of handling very
high packet loss rates with no or small speech quality
degradation.
o Fast, frame-wise AMR mode adaptation must be supported. This
means that it must be possible to send Codec Mode Requests back
from the receiving side to the transmitting side with information
on the preferred mode. Slower AMR mode adaptation may also be
accomplished with external signaling.
o Source controlled rate operation (SCR) and comfort noise
parameter (CN) transmission defined in AMR must be supported.
3. Payload format
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [3].
The AMR payload format is designed to be flexible, ranging from very
low overhead to an extended format with the possibility to send
redundancy information and several speech frames in one packet.
The payload format consists of payload header and zero or more
payload frames. Neither the payload header nor the payload frames are
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octet aligned on their own but the full payload is. If the option to
transmit redundant information is enabled and employed, the full
payload SHALL finally be ordered in descending bit error sensitivity
order to be prepared for unequal error protection or unequal error
detection schemes, e.g. UDP lite. The AMR encoded bit streams are
defined in sensitivity order in Annex B of [2], the original order as
delivered from the speech encoder is defined in [1].
The last octet of an AMR payload packet is padded with zeroes at the
end if not all bits are used.
The AMR frame types, or modes, are defined in [2]. Frame type 15, no
transmission, is needed to indicate not transmitted frames or lost
frames. Not transmitted could mean both no data produced by the
speech encoder for this frame or no data transmitted in this payload,
i.e. valid data for this frame could be sent in another payload. For
example, when multiple frames are sent in each payload and comfort
noise starts. A frame type sequence in a payload with 8 frames,
speech frames with AMR mode 7 are interrupted by CN in the
fifth frame, could look like: {7,7,7,7,8,15,15,8}. The AMR SCR is
described in [4].
The AMR payload format supports robust transmission, multiple frames
in one payload packet, and the use of fast codec mode adaptation.
The robust behavior is accomplished by using the optional possibility
to retransmit previously transmitted frames together with the current
frame or frames. The redundant frames could be transmitted in their
entirety or only partly. If only a part of the redundant frame is
transmitted, the least sensitive bits are omitted. A partially
transmitted redundant frame SHALL fill the number of used octets for
that frame. The bits in the payload are sorted in descending
sensitivity order to support UED, like in UDP lite [10], if partial
redundancy is used.
When bits in redundant frames are not transmitted, the not
transmitted/received bits MUST be reconstructed on the receiver side.
It is RECOMMENDED to produce the non received bits with state of the
art ECU actions. Nothing giving worse quality than using a random
generated bits SHOULD be used. To use a fixed pattern SHOULD be
avoided for speech quality reasons.
Note that the possibility to transmit partial redundant frames can be
employed only if the receiver has signaled support for this in
capability description.
A frame quality indicator is included for interoperability with the
ATM payload format described in ITU-T I.366.2 and the UTRAN Iu
interface [14]. The speech quality is significantly increased if
damaged frames are forwarded to the speech decoder error concealment
unit and not dropped.
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3.1. The payload header
The payload header has dynamic length, 3 or 7 bits. The bits in the
header are specified as follows:
Q (1 bit): The payload quality bit indicates, if not set, that the
payload is severely damaged and the receiver should set the RX_TYPE,
see [4], to SPEECH_BAD or SID_BAD depending on the frame type (FT).
L (1 bit): Indicates the existence of LEN fields in the payload
frames and that sensitivity sorting is used. Note that this bit can
be set only if the receiver has signaled support for option to
transmit redundant data.
R (1 bit): Indicates, if set, that the Codec Mode Request (CMR) is
sent.
CMR (4 bits): OPTIONAL field, depending on the R bit. Requested codec
mode for the other communication direction. The mapping of existing
AMR modes are given in Table 1a in [2].
0
0 1 2
+-+-+-+
|Q|L|R|
+-+-+-+
Figure 1: AMR payload header, R=0
0
0 1 2 3 4 5 6
+-+-+-+-+-+-+-+
|Q|L|R| CMR |
+-+-+-+-+-+-+-+
Figure 2: AMR payload header, R=1
3.2. AMR payload frame
An AMR payload frame represent one encoded speech frame. Each payload
frame includes several specified fields as follows:
F (1 bit): Indicates if this frame is followed by further frames. F=1
further frames follow, F=0 last frame.
LEN (5 bits): OPTIONAL field, exists if the payload header bit L is
set, L=1. LEN specifies the number of octets in the FT field and AMR
encoded bits field in this frame. If LEN indicates more bits than the
AMR mode information in the FT field, the implicit knowledge of the
number of bits for the AMR mode indicated by FT is the valid number
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of AMR encoded bits. If LEN indicates fewer bits than given by the
mode information in the FT field, LEN gives the number of encoded
bits. If a frame is transmitted only partially the least sensitive
bits at the end of the frame are omitted. This use is intended for
partial redundant data.
FT (4 bits): Frame type indicator, indicating the AMR speech coding
mode or comfort noise (CN) mode. The mapping of existing AMR modes
are given in Table 1a in [2]. If FT=15 (No transmission) no LEN or
AMR encoded bits follow.
AMR encoded bits: This is the speech codec encoded data field. The
length of this field is either defined implicitly by the AMR mode in
the FT field, or by the LEN field. The last payload frame SHALL
always contain a full AMR frame, i.e. no LEN field is needed.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| LEN | FT | |
+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
/ AMR encoded bits /
+ +-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Payload frame format, F=1 and L=1
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| FT | |
+-+-+-+-+-+ +
| |
+ +
/ AMR encoded bits /
+ +-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Payload frame format, F=0 or L=0
3.3. Payload block sorting
A bit error in a more sensitive bit is subjectively more annoying
than in a less sensitive bit. Therefore, to be able to protect the
most sensitive bits in a payload packet with a forward error
detection code, e.g. a CRC outside RTP, the bits inside a frame are
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ordered into sensitivity order. If the option to transmit redundant
data is employed, the full RTP payload MUST be further sorted into
sensitivity order. The protection MAY then cover an appropriate
number of octets from the beginning of the payload. How many octets
depend on the channel and application. This can for example be
accomplished by UDP lite [10] (work in progress). To maintain
sensitivity ordering inside the AMR payload when more than one speech
frame is transmitted in one packet reordering of the data is needed.
The reordering is only performed if partial redundancy is used, i.e.
L=1.
The reordering to maintain the sensitivity ordered AMR payload SHALL
be performed on bit level. The AMR payload header SHALL still be
placed unchanged in the beginning of the payload. Thereafter, the
payload frames are sorted with one bit alternating from each payload
frame.
+-------------+
| h(0)-h(H-1) |
+------------------------+
| f(0,0) _ f(0,F(0)) |
+----------------------------+
| f(1,0) _ f(1,F(1)) |
+----------------------------+
| f(2,0) _ f(2,F(2)) |
+----------------------+
\ \
+-------------------------------+
| f(N-1,0) _ f(N-1,F(N-1)) |
+-------------------------------+
Figure 5: The payload header and N payload frames before sorting.
The sorting algorithm can be described in C-code.
b(m) - bit m of RTP final payload
f(n,m) - bit m in payload frame n
F(n) - number of bits in payload frame n, defined by FT or by LEN
h(m) - bit m of payload header
H - number of payload header bits, 3 or 7 bits
N - number of payload frames in the payload
S - number of unused bits
Payload frames f(n,m) are ordered in consecutive order, where frame
n=1 is preceding frame n=2.
The sorting algorithm is defined in C-style as:
for (i = 0; i < H; i++){
b(i) = h(i);
}
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max = max(F(0),..,F(N-1));
k = H;
for (i = 0; i < max; i++){
for (j = 0; j < N; j++){
if (i < F(j)){
b(k++) = f(j,i);
}
}
}
S = 8 - k%8;
if (S < 8){
for (i = 0; i < S; i++){
b(k++) = 0;
}
}
Note that if multiple new frames are encapsulated into the payload
and partial redundant data is not transmitted, payload bit-sorting
SHALL NOT be performed but the payload is formed by concatenating the
payload header and the bits from each AMR frame in the payload.
However, the bits inside a frame are ordered into sensitivity order
as defined in [2]. In this case the bits are stored into payload
according to C-style algorithm below (see the definition of symbols
above).
for (i = 0; i < H; i++){
b(i) = h(i);
}
k = H;
for (j = 0; j < N; j++){
for (i = 0; i < F(j); i++){
b(k++) = f(j,i);
}
}
}
S = 8 - k%8;
if (S < 8){
for (i = 0; i < S; i++){
b(k++) = 0;
}
}
4. RTP header usage
The RTP header marker bit (M) is used to mark (M=1) the packages
containing the first speech frame after CN. All other packages the
marker bit is set to 0 (M=0).
The timestamp corresponds to the sampling time of the first sample
encoded for the first encoded speech frame in the packet. The
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timestamp unit is in samples. The duration of one AMR speech frame is
20 ms and the sampling frequency is 8 kHz, corresponding to 160
encoded speech samples per frame. Thus, the timestamp is increased by
160 for each consecutive frame. All frames in a packet MUST be
successive 20 ms frames.
5. Examples
5.1. Simple example
In the simple example we just send one full (L=0) frame in each RTP
packet, no Codec Mode Request CMR is sent (R=0), the payload was not
damaged at IP origin (Q=1). In this example we transmit one frame
encoded with the 5.9 kbps mode (FT=2). The speech encoded bits are
put into f(0) to f(117) in descending sensitivity order according to
[2].
| Bit no. |
Oct| 0 1 2 3 4 5 6 7 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
0 | Q=1 | L=0 | R=0 | F=0 | 0 | 0 | 1 | 0 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
1 | f(0) | f(1) | f(2) | ... | ... | ... | ... | ... |
---+-------+-------+-------+-------+-------+-------+-------+-------+
15 | ... | ... | ... | f(115)| f(116)| f(117)| 0 | 0 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 6: One frame per packet example.
5.2. Example with partial redundancy
In this example the 6.7 kbps mode (FT=3) is sent with one redundant
frame, also FT=3. Only a part of the redundant frame is sent, in this
example 12 octets, (L=1, LEN=12). A mode request is sent(R=1),
requesting the 10.2 kbps mode for the other link(CMR=6). The
redundant frame (12 octets) is r(0) to r(95) and the current frame
(134 bits) is f(0) to f(133).
| Bit no. |
Oct| 0 1 2 3 4 5 6 7 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
0 | Q=1 | L=1 | R=1 | 0 | 1 | 1 | 0 | F=1 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
1 | F=0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
2 | 1 | 0 | f(0) | 0 | f(1) | 0 | f(2) | 1 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
3 | f(3) | 1 | f(4) | r(0) | f(5) | r(1) | f(6) | r(2) |
---+-------+-------+-------+-------+-------+-------+-------+-------+
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4 | f(7) | r(3) | f(8) | ... | ... | ... | ... | ... |
---+-------+-------+-------+-------+-------+-------+-------+-------+
26 | f(95) | r(91) | f(96) | f(97) | f(98) | ... | ... | ... |
---+-------+-------+-------+-------+-------+-------+-------+-------+
30 | ... | ... | ... | ... | ... | f(131)| f(132)| f(133)|
---+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 7: Example with partial redundancy.
5.3. Example with multiple frames per payload
In this example two 5.9 kbps mode (FT=2) frames are sent in one
packet. No partial redundancy is used (L=0). A mode request is
sent(R=1), requesting the 7.95 kbps mode for the other link(CMR=5).
The first frame is represented by the 118 bits f(0) to f(117) and the
subsequent frame by g(0) to g(117).
| Bit no. |
Oct| 0 1 2 3 4 5 6 7 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
0 | Q=1 | L=0 | R=1 | 0 | 1 | 0 | 1 | F=1 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
1 | 0 | 0 | 1 | 0 | f(0) | f(1) | ... | ... |
---+-------+-------+-------+-------+-------+-------+-------+-------+
15 | ... | ... | ... | ... | ... | ... | ... | f(115)|
---+-------+-------+-------+-------+-------+-------+-------+-------+
16 | f(116)| f(117)| F=0 | 0 | 0 | 1 | 0 | g(0) |
---+-------+-------+-------+-------+-------+-------+-------+-------+
17 | g(1) | g(2) | ... | ... | ... | ... | ... | ... |
---+-------+-------+-------+-------+-------+-------+-------+-------+
31 | ... | ... | ... | g(116)| g(117)| 0 | 0 | 0 |
---+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 8: Example two frames per payload.
6. The AMR MIME type registration
This chapter defines the MIME type for Adaptive Multi-Rate (AMR)
speech codec [1]. The data format and parameters are specified for
both real-time transport and for storage type applications (e.g. e-
mail attachment, multimedia messaging). The former is referred as RTP
mode and the latter as storage mode.
AMR implementations according to [1] MUST support all eight coding
modes. The mode change can occur at any time during operation and
therefore the mode information is transmitted in-band together with
speech bits to allow mode change without any additional signaling.
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In addition to the speech codec, AMR specifications also include
Discontinuous Transmission / comfort noise (DTX/CN) functionality
[11]. The DTX/CN switches the transmission off during silent parts of
the speech and only CN parameter updates are sent in regular
intervals.
6.1 RTP mode
It is possible that the decoder may want to receive certain AMR mode
or a subset of AMR modes. In the end to end transmission parts of the
chain may have limitations in the number of modes in the active codec
set, e.g. the GSM radio link can only use a subset of maximum four
modes. Therefore, it is possible to request specific set of AMR modes
in capability description and it is mandatory for encoder to abide
this request. If request for mode set is not given, encoder can
freely decide which AMR mode to use.
Although in principle AMR codec can perform a mode change at any time
between any two modes, it is possible to set limitations for mode
changes. The decoder has possibility to define the minimum number of
frames between mode changes and to limit the mode change to happen
into neighboring modes only.
In addition to AMR DTX/CN scheme, the three codec standards that are
part of the AMR also have their own DTX/CN schemes ([6][7][12]). To
enable interoperability with terminals supporting these standards,
AMR can optionally be extended to support also these CN schemes. The
CN capabilities are signaled in capability description. If no CN
capabilities are reported, it is assumed that AMR CN is supported. If
CN capabilities are reported, all supported CN types (including AMR
CN) must be signaled.
It is also possible to limit the number of AMR frames encapsulated
into one RTP packet. This is an optional feature and if no parameter
is given in capability description, the transmitter can encapsulate
any number of AMR speech frames into one RTP packet.
There is also an option to retransmit one or more previously
transmitted frames to help the receiver to recover from packet losses
in difficult transmission conditions. It also possible to transmit
these frames only partially in such a way that only the most
sensitive bits are transmitted. Since the transmission of partly
redundant frames is an optional property, it can be used only if the
receiver has signaled support for this functionality in capability
description. The partial redundancy is RECOMMENDED to be implemented
and turned on at least for conversational services.
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6.2 Storage mode
For storing AMR frames e.g. as a file or e-mail attachment, the AMR
frames must be formatted according to Annex A of [9]. Because no
exchange of particular coding parameters, e.g. specific DTX/CN mode,
can be signaled before downloading or receiving stored AMR data, the
receiving entity (AMR decoder) MUST be able to decode all eight
coding modes as well as the AMR DTX/CN [6].
6.3 MIME Registration
MIME-name for the AMR codec is allocated from IETF tree since AMR is
expected to be widely used speech codec in VoIP applications. Some
parts of this chapter will distinguish between RTP and storage modes.
Media Type name: audio
Media subtype name: AMR
Required parameters: none
Optional parameters for RTP mode:
ptime: Definition as usual in RTP audio.
mode-set: Requested AMR mode set. Restricts the active codec mode
set to a subset of all modes. Possible values are:
0,...,7 (see Table 1a [2]). If not present, all speech
modes are available.
mode-change-period: Defines a number N which restricts the mode
changes in such a way that mode changes are only allowed
on multiples of N, initial state of the phase is
arbitrary. If this parameter is not present, mode change
can happen at any time.
mode-change-neighbor: If present, mode changes SHALL be made to
neighboring modes only. If not present, change between
any two modes is allowed.
amr-cn: If present, GSM AMR DTX/CN is supported. Note that if no
CN capabilities are reported, AMR DTX/CN is assumed to
be supported, i.e. this parameter is only sent together
with one of the following CN parameters.
pdc-efr-cn:If present, PDC-EFR DTX/CN is supported, otherwise not
supported.
is-641-cn: If present, IS-641 DTX/CN is supported, otherwise not
supported.
gsm-efr-cn:If present, GSM EFR DTX/CN is supported, otherwise not
supported.
maxframes: Maximum number of AMR speech frames in one RTP packet.
The receiver may set this parameter in order to limit
the buffering requirements or delay.
redundancy:If present, transmission of partly redundant frames is
supported, otherwise not supported.
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Optional parameters for storage mode: none
Encoding considerations for RTP mode: See section 3 in this document.
Encoding considerations for storage mode: Each audio frame must be
formatted in octet format according to AMR Interface Format 2 (AMR
IF2) specified in Annex A of [2]. The audio frames must be stored in
sequential order. This implies that the first octet after frame n
must be the first octet of frame (n+1). Furthermore, missing frames
and non-received frames between CN updates during non-speech period
must be stored as NO_TRANSMISSION frames (frame type 15, see
definition in [2]). Each receiving entity that accepts this MIME type
must be able to decode all eight AMR coding modes [1] and the AMR
DTX/CN [11].
Security considerations: none
Interoperability considerations for RTP mode: If CN capabilities are
not signaled in the capability description, only AMR CN is supported.
Public specification: please refer to chapter 7 "References".
Additional information for storage mode:
Magic number: none
File extensions: amr, AMR
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
johan.sjoberg@ericsson.com
ari.lakaniemi@nokia.com
Intended usage: COMMON. It is expected that many VoIP applications
(as well as mobile applications) will use this type.
Author/Change controller:
johan.sjoberg@ericsson.com
ari.lakaniemi@nokia.com
6.4 Mapping to SDP Parameters
Please note that this chapter applies to the RTP mode only.
Parameters are mapped to SDP [13] as usual.
Example usage in SDP:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 AMR
a=fmtp:97 mode-set=0,2,5,7; maxframes=2
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7. References
[1] GSM 06.90, "Adaptive Multi-Rate (AMR) speech transcoding".
[2] 3G TS 26.101, "AMR Speech Codec Frame Structure".
[3] IETF RFC 2119, "Key words for use in RFCs to Indicate
Requirement Levels".
[4] 3G TS 26.093, "AMR Speech Codec; Source Controlled Rate
operation".
[5] GSM 06.60, "Enhanced Full Rate (EFR) speech transcoding".
[6] TIA/EIA -136-Rev.A, part 410 - "TDMA Cellular/PCS - Radio
Interface, Enhanced Full Rate Voice Codec (ACELP). Formerly IS-
641. TIA published standard, 1998".
[7] ARIB, RCR STD-27H, "Personal Digital Cellular Telecommunication
System RCR Standard".
[8] IETF RFC1889, "RTP: A Transport Protocol for Real-Time
Applications".
[9] IETF draft-westberg-realtime-cellular-01.txt, "Realtime Traffic
over Cellular Access Networks".
[10] IETF draft-larzon-udplite-02.txt, "The UDP Lite Protocol".
[11] GSM 06.92, "Comfort noise aspects for Adaptive Multi-Rate (AMR)
speech traffic channels".
[12] GSM 06.62: Comfort noise aspect for Enhanced Full Rate (EFR)
speech traffic channels
[13] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998
[14] 3G TS 25.415 "UTRAN Iu Interface User Plane Protocols"
Sjoberg/Westerlund/Lakaniemi/Koskelainen [Page 14]
INTERNET-DRAFT RTP Payload Format for AMR July 14, 2000
8. Authors' addresses
Johan Sjoberg
Ericsson Research
Ericsson Radio Systems AB
Torshamnsgatan 23
SE-164 80 Stockholm
SWEDEN
E-mail: Johan.Sjoberg@ericsson.com
Magnus Westerlund
Ericsson Research
Ericsson Radio Systems AB
Torshamnsgatan 23
SE-164 80 Stockholm
SWEDEN
E-mail: Magnus.Westerlund@ericsson.com
Ari Lakaniemi
Nokia Research Center
P.O.Box 407
FIN-00045 Nokia Group
Finland
E-mail: ari.lakaniemi@nokia.com
Petri Koskelainen
Nokia Research Center
P.O.Box 100
FIN-33721 Tampere
Finland
E-mail: petri.koskelainen@nokia.com
This Internet-Draft expires January 14, 2001.
Sjoberg/Westerlund/Lakaniemi/Koskelainen [Page 15]