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.

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.

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: 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: 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. 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. 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. 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].

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 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.

The following new codec format parameters are introduced: 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. 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. 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.

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 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: All codec data present. Codec type is obvious, so not explicitly stated. 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. 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.

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.

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.

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.

Andras Stefan (Nokia Siemens Networks) provided great support in writing this document by explaining his viewes on issues of AMR codecs.

This memo includes no request to IANA. Two media types (audio/AMR and audio/AMR-WB) have already been registered.

There are no specific security issues regarding AMR codec syntax extension.