Network Working Group G. Somogyi
Internet-Draft Nokia Siemens Networks
Updates: 4867 (if approved) November 21, 2008
Intended status: Standards Track
Expires: May 25, 2009
draft-somogyi-sdp-amr-bicc-like-00
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Abstract
This document describes and update to File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB)
Audio Codecs. The new format allows better interworking with widely
deployed mobile telecommunication switching systems. This document
updates RFC 4867 [RFC4867].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Shortcomings of the original format . . . . . . . . . . . . . . 3
2.1. Information loss . . . . . . . . . . . . . . . . . . . . . 3
2.2. Significant signaling overhead . . . . . . . . . . . . . . 4
3. SDP format extension . . . . . . . . . . . . . . . . . . . . . 5
3.1. New parameters . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Backward compatibility . . . . . . . . . . . . . . . . . . . . 6
4.1. Full backward compatibility . . . . . . . . . . . . . . . . 6
4.2. Partial backward compatibility . . . . . . . . . . . . . . 7
4.3. Deciding backward compatibility mode . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
The SDP [RFC4566] format description of AMR and AMR-WB codecs are
described in RFC 4867 [RFC4867]. That format causes trouble for SIP
[RFC3261] based mobile switching systems, as the described SDP format
did not contain all the necessary information for proper codec
negotiation. And that format caused significant signaling overhead.
The aim of this document to address these issues, while remaining
backward compatible. This document introduces new parameters,
allowing describing codecs of mobile world in full detail. The new
parameters make the SDP even larger. But it makes several payload
types of the same codec redundant, thus those could be omitted.
Omitting those payload types limits backward compatibility, and
therefore optional.
The new syntax elements were designed to fully support 3GPP
Transcoder Free Operation (TrFO; 3GPP TS 23.153 [3GPP23153]) and
Tandem Free Operation (TFO; 3GPP TS 28.062 [3GPP28062]). Compliance
with 3GPP IP Multimedia Subsystem (IMS; TS 26.114[3GPP26114]) was
taken care of. Although 3GPP Release 8 standards were used as
reference, the descriptions apply to earlier and possibly later
releases, as well. Besides mobile switching systems this document
was created with the intention to be able to be used within any kind
of SIP systems.
1.1. Requirements Language
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 inRFC 2119 [RFC2119].
2. Shortcomings of the original format
2.1. Information loss
The old syntax did not provide a way to represent all the AMR codec
information defined for mobile switches at 3GPP TS 23.153
[3GPP23153]. The information loss could result extra transcoding and
speech quality degradation.
Using the syntax defined in RFC 4867 only the following 3GPP single
codec format parameter is presented in SDP in full detail:
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ACS: Active Codec mode Set is presented by "mode-set"
parameter.
Using the syntax defined in RFC 4867 the following 3GPP single codec
format parameters are NOT presented in full detail:
SCS: Supported Codec mode Set is the list of all supported
modes. It is a superset of ACS. Using codec
negotiation methods ACS can be modified to use any of
the supported modes.
As the syntax defined in RFC 4867 does not have
parameter representing SCS, several distinct payload
types present the most preferred mode sets, in
accordance with 3GPP TS 29.163 [3GPP29163].
Therefore only a fraction of the mode combinations
are presented, compared to what is intended.
MACS: Maximal number of codec modes in the ACS shows the
maximum number of codec modes that may be selected
for the ACS at a time during speech codec
negotiation.
OM: Optimization Mode flag shows whether optimization
(modification) of Active Codec mode Set is supported
or not. If optimization is not supported, SCS and
MACS have no meaning for the relevant codec.
Codec type: There are numerous AMR codec types, described at
3GPP TS 26.103 [3GPP26103]. Those form two groups,
narrow-band (NB) and wide-band (WB) AMR codecs. In
SDP representation there is one NB and one WB codec
type. As codecs within one group have different
properties, those properties are described by
additional codec parameters in SDP. According to
3GPP TS 29.163 [3GPP29163] when only one mode is
defined in "mode-set" parameter, "mode-change-period"
and "mode-change-neighbor" parameters should not be
present. That causes trouble, as in single mode
configuration "UMTS AMR" and "FR AMR" codecs become
indistinguishable, even though they are not
compatible, according to 3GPP TS 23.153 [3GPP23153].
2.2. Significant signaling overhead
As described beforehand, RFC 4867 [RFC4867] does not provide a
mechanism to signal the SCS, MACS or OM parameters in SDP. Therefore
codecs with OM allowed should have been translated into a list of SDP
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payload formats, where each includes a "mode-set" parameter with a
unique value derived from the ACS, SCS and MACS. That resulted in
quite huge SDPs. Regarding signaling that significantly increased
the traffic, required significantly mode processing resources to
parse and significant extra memory to store the information elements
of SDP.
3. SDP format extension
3.1. New parameters
The following new codec format parameters are introduced:
extra-mode-set: Lists supported modes that are not listed in mode-set
(ACS). Value syntax is the same as for parameter
"mode-set", defined at RFC 4867 [RFC4867]. If
optimization of ACS is supported and SCS differs from
ACS, then the parameter MUST be present, otherwise it
SHOULD NOT be present.
macs: Defines MACS. Valid values between 1 and 8. (Note,
that 3GPP TS 26.103 [3GPP26103] defines range between
0 and 7, value zero meaning eight. Here "8"
represents value eight.) If optimization of ACS is
supported, then the parameter MUST be present,
otherwise it MUST NOT be present. Consequently, the
presence of "macs" parameter indicates the status of
OM.
codec-type: Codec type. Valid values are capitalized names of
AMR codecs defined in 3GPP TS 26.103 [3GPP26103],
spaces replaced by underscores. Namely FR_AMR,
HR_AMR, UMTS_AMR, UMTS_AMR_2, OHR_AMR, FR_AMR-WB,
UMTS_AMR-WB, OFR_AMR-WB and OHR_AMR-WB. This
parameter MAY be omitted if the codec type is obvious
from mode change parameters defined at RFC 4867
[RFC4867]. In such case omitting codec type is
RECOMMENDED. Codec type MUST NOT conflict with mode
change parameters.
3.2. Example
The example shows a possible SDP representation of FR AMR codec with
ACS=4,5,6,7; SCS=2,3,4,5,6,7; OM=supported; MACS=4.
Note: There is an extra line-break in the SDP example at extra-mode-
set attribute. That was added so that to match traditional RFC
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formatting. Real SDP does not contain such line break.
v=0
o=- 0 1 IN IP4 1.2.3.4
s=-
c=IN IP4 5.6.7.8
t=0 0
m=audio 6000 RTP/AVP 96 97 98 99 100
b=AS:16
a=rtpmap:96 AMR/8000
a=fmtp:96 mode-set=4,5,6,7; mode-change-period=2;
extra-mode-set=2,3; macs=4
a=rtpmap:97 AMR/8000
a=fmtp:97 mode-set=2,4,5,6; mode-change-period=2
a=rtpmap:98 AMR/8000
a=fmtp:98 mode-set=2,4,6; mode-change-period=2
a=rtpmap:99 AMR/8000
a=fmtp:99 mode-set=3,6; mode-change-period=2
a=rtpmap:100 AMR/8000
a=fmtp:100 mode-set=3; codec-type=FR_AMR
Explanation on RTP payload type descriptions:
96: All codec data present. Codec type is obvious, so not
explicitly stated.
97-99: These lines are not necessary to be present, if full backward
compatibility is not needed. Codec type is obvious, so not
explicitly stated. These lines do not contain any new
parameter defined in this document, as the extra information
is available at the definition of payload 96.
100: This line is not necessary to be present, if full backward
compatibility is not needed. As codec type is not obvious,
it is explicitly stated here.
4. Backward compatibility
4.1. Full backward compatibility
Full backward compatibility with RFC 4867 [RFC4867] can be achieved
by enumerating possible mode sets, as before. Extra parameters must
not cause any problem for older implementations.
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4.2. Partial backward compatibility
The aim of providing only partial compatibility is to reduce
signaling bandwidth, CPU and RAM usage. Partial compatibility is
achieved by not presenting redundant information (payload type
description 97-100 in the example). Older implementations interpret
the received payload type description as usual, but they are provided
with less rich set of choices.
4.3. Deciding backward compatibility mode
It is up to the implementation which approach to take. For example
the compatibility mode can be fixed, configurable, or even route
specific decisions could be made.
5. Acknowledgements
Andras Stefan (Nokia Siemens Networks) provided great support in
writing this document by explaining his viewes on issues of AMR
codecs.
6. IANA Considerations
This memo includes no request to IANA. Two media types (audio/AMR
and audio/AMR-WB) have already been registered.
7. Security Considerations
There are no specific security issues regarding AMR codec syntax
extension.
8. References
8.1. Normative References
[3GPP23153]
3rd Generation Partnership Project, 3GPP., "3GPP TS 23.153
Out of band transcoder control; Stage 2, version 8.0.0",
2007-12.
[3GPP26103]
3rd Generation Partnership Project, 3GPP., "3GPP TS 26.103
Speech codec list for GSM and UMTS, version 8.0.0",
2008-09.
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[3GPP29163]
3rd Generation Partnership Project, 3GPP., "3GPP TS 29.163
Interworking between the IP Multimedia (IM) Core Network
(CN) subsystem and Circuit Switched (CS) networks, version
8.1.0", 2007-12.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007.
8.2. Informative References
[3GPP26114]
3rd Generation Partnership Project, 3GPP., "3GPP TS 26.114
IP Multimedia Subsystem (IMS); Multimedia Telephony; Media
handling and interaction, version 8.0.0", 2008-09.
[3GPP28062]
3rd Generation Partnership Project, 3GPP., "3GPP TS 28.062
Inband Tandem Free Operation (TFO) of speech codecs;
Service description; Stage 3, version 7.0.0", 2007-06.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
Author's Address
Gabor Somogyi
Nokia Siemens Networks
Koztelek u. 6.
Budapest 1092
Hungary
Email: gabor.somogyi@nsn.com
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