23.1533GPPOut of band transcoder controlRelease 17Stage 2TS
For the purposes of the present document, the following definitions apply:
Codec: device to encode information from its original representation into an encoded form and to decode encoded information into its original representation
Codec Lists, Selected Codecs: The OoBTC procedures pass a number of codec lists created by comparing the capabilities of the different nodes or equipment involved. For the different interfaces involved during call setup, handover, and relocation, the following codec lists and selected codecs need to be distinguished – where codec lists are ordered, "ordered" is included in the description:
1) Supported Codecs List (DTAP) – this is the list of codecs supported by the UE. It is subdivided into codecs supported for the currently used radio access and codecs that can be used for other radio accesses supported by the UE. The list contains only the codec types, but not the individual configuration, as the UE is mandated to support all configurations for a given codec type.
2) Supported Codecs List (BSSMAP) – "BSC-SCL" – this is the list of codecs supported by the BSS (BSS-SCL). The list contains the codec types as well as the individual codec configurations supported by the radio access at the very moment of call setup.
3) Supported Codecs List (BICC) – this ordered list is used on NNI (BICC) OoBTC signalling. At call setup it is sent forward by the node originating the OoBTC signalling and contains the default PCM codec and a set of codecs that is common to the nodes and the equipment involved in setting up the call. For a mobile originating call, these are the UE and the MGWs involved in the connection and, for UTRAN, GERAN Iu-mode and GERAN AoIP mode, also the originating radio access. At inter-MSC relocation and inter-MSC handover, the Supported Codecs List (BICC) is sent forward by the anchor MSC towards the target MSC and contains the default PCM codec and a set of codecs that is common to the anchor MSC and the nodes involved in setting up the new call leg towards the target MSC. For UDI/RDI multimedia calls with fallback and service change according to 3GPP TS 23.172 , the multimedia dummy codec will be included (see 3GPP TS 26.103 ).
4) Available Codecs List (BICC) – this is the list of codecs available for the NNI connection. It is returned in the backward signalling to the node that originated the OoBTC and is a subset of the Supported Codecs List (BICC) sent forward. At call setup the Available Codecs List (BICC) contains the default PCM codec and a common set of codecs that can be supported by all nodes and, if Transcoder Free Operation has been achieved end-to-end, also by the UEs and the radio access networks that are involved in the call. At inter-MSC relocation and inter-MSC handover to UMTS, the Available Codecs List (BICC) contains the default PCM codec and a set of codecs that can be supported by all nodes involved in setting up the new call leg towards the target MSC and, if Transcoder Free Operation can be maintained end-to-end after the handover or relocation, also by the UE and the target radio access network.
5) Selected Codec (BICC) – this is the codec selected to be used on the NNI connection. It is one of the codecs contained in the Available Codecs List (BICC) and may be different from the codec that is used on the radio interface, but if end-to-end Transcoder Free Operation has been achieved, this will be the common codec for all nodes, the UEs, and the radio accesses.
6) Iu-Supported Codecs List (MAP) – this ordered list is used for MAP signalling from the anchor MSC to the target MSC. It is subdivided into lists for UTRAN and GERAN Iu-mode and contains the codecs common to the UE and to the anchor MGW for each radio access supported by the UE. The codec capabilities of the serving radio access, i.e. the radio access used prior to the inter-MSC handover or relocation, are not taken into account. Codecs that are only applicable to the NNI, e.g. the default PCM codec or the multimedia dummy codec (see 3GPP TS 26.103 ), are not included.
7) Iu-Available Codecs List (MAP) – this is the list of codecs available for the target Iu interface. When returned by the target MSC to the anchor MSC in response to an initial Prepare Handover message it is the Iu-Supported Codecs List (MAP) reduced according to the capabilites of the target MGW and the target radio access. After a subsequent intra-MSC handover or relocation, the target MSC may update the Iu-Available Codecs List (MAP) according to the capabilites of its associated MGW and the new target radio access, if necessary.
8) Iu-Selected Codec (MAP) – this is the codec selected for the target Iu interface. It is one of the codecs contained in the Iu-Available Codecs List (MAP). In response to a Prepare Handover request message this is the codec selected by the target MSC and indicated back to the anchor MSC. When sent from the anchor MSC in a Forward Access Signalling request message during a codec modification, it contains the codec type and configuration chosen by the anchor MSC.
9) Iu-Currently Used Codec (MAP) – this is the codec in use on the serving Iu interface prior to an inter-MSC handover.
10) TFO Codec List (H.248) – this is the list of codecs for use by the MGW during TFO in-band negotiations with a distant node. The list is passed via the Mc interface from the server to the MGW. The first entry of the TFO Codec List (H.248) is to be used by the MGW as the ‘Local Used Codec’ (see ).
11) Distant Codec List (H.248) – this is the list of codecs received by the MGW from a distant node during TFO in-band negotiations. The list is passed via the Mc interface from the MGW to the server. The first entry of the Distant Codec List (H.248) is the ‘Distant Used Codec’ received by the MGW (see ).
12) Codec (H.248) – this is the codec for use on a certain MGW termination. It is passed via the Mc interface from the server to the MGW.
13) MSC Preferred Codec List (BSSMAP) – "MSC-PCL" – this is the list of codecs supported by both the MSC and the MS as allowed by the MSC for this assignment or handover, ordered by the MSC with the most preferred Codec Types first (e.g. the ones that may enable TrFO or TFO).
14) AoIP-Supported Codecs List (Anchor) (MAP) – this ordered list is used for MAP signalling from the anchor MSC to the target MSC if anchor MSC supports GERAN AoIP mode. It contains codecs supported by the UE and by the anchor MSC for GSM access. Codecs that are only applicable to the NNI, e.g. the default PCM codec or the CSData Dummy Codec (see 3GPP TS 26.103 ), are not included.
15) AoIP-Selected Codec (Target) (MAP) – the codec selected by the target BSS, to be used by the UE/MS in GERAN A/Gb mode after the handover to the BSS using A interface over IP. This is the Speech Codec (BSS chosen) as returned by the Target BSS in BSSAP.
16) AoIP-Available Codecs List (MAP) – a list of codecs for GERAN A/Gb mode available for the target AoIP interface signalled via MAP. This is the Codec List (BSS Supported) returned by the Target BSS in BSSAP reduced according to the capabilities of the Target MSC.
Within the ordered codec lists, the codecs are ordered in decreasing order of priority, the first entry in the list being the highest priority codec (preferred codec).
Tandem Free Operation: configuration of a connection with two transcoders that support TFO protocol and whose external coding schemes are compatible, thus enabling compressed speech to pass between them
NOTE 1: When the TFO protocol is not supported by both transcoders or the coding schemes are not compatible then normal "Tandem" operation occurs and PCM encoded speech is passed between them.
Transcoder: device to change the encoding of information from one particular encoding scheme to a different one, most commonly to/from a compressed speech algorithm from/to PCM.
Transcoder Free Operation: configuration of a speech or multimedia call for which no transcoder device is physically present in the communication path and hence no control or conversion or other functions can be associated with it
Out of Band Transcoder Control: capability of a system to negotiate the types of codecs and codec modes on a call per call basis through out-of-band signalling, required to establish Transcoder Free Operation.
Default PCM Codec: network default 64kb/s codec for speech in PCM domain
NOTE 2: For example ITU G.711 A-law.
Transcoding free link (TrFL): bearer link, where compressed voice is being carried between bearer endpoints
NOTE 3: Within the UMTS network, the compressed voice is transmitted in Iu/ Nb User Plane format, depending on the related interface.
Tandem free link (TFOL): bearer link between transcoders that are operating in Tandem Free Operation mode, i.e. bypassing the transcoding functions
NOTE 4: The involved transcoders can be a UMTS transcoder or a GSM TRAU with TFO functionality.
Transcoder free operation (TrFO): calls that have no transcoders involved in the connection between the source codecs
NOTE 5: For mobile to mobile calls this is UE to UE, although the connection could be UE to another type of terminal. TrFO operation is considered a concatenation of TrFLs between RNCs.
NOTE 6: In case of mobile to fixed network calls the term "Transcoder free operation" is applicable for the TrFLs carrying compressed speech. The TrFO usually ends at the Gateway to the PSTN where the speech is transcoded e.g. to G.711.
Tandem free and Transcoding free operation (TaTrFO): concatenation of "transcoding free links" and "tandem free links"
Iu Framing: framing protocol used for the speech packets on both the Iu User Plane interface and the Nb User Plane interface
NOTE 7: The Iu framing protocol is specified by .
In addition, the definitions of ACS, SCS, OM, and MACS provided in  apply.
Direct Codec: is a codec that can be used without any additional transcoding stage inserted at the MGW that is offering the codec list. E.g., a direct codec can be AMR or another mobile codec when the end terminal is a mobile station, or G.711 when interworking with the PSTN.
Indirect Codec: is a codec that requires transcoding at the MGW providing the codec list.
Auxiliary RTP payload type: is a payload type used in combination with a speech codec to transmit some non-spech audio via RTP. The Telephony Event RTP Payload Type and the , Comfort Noise Codec are the only "Auxiliary" RTP payload type defined in the present Release.
For the purposes of the present document, the abbreviations defined in GSM 01.04 and the following apply:
ACS Active Codec mode Set
APM Application Transport Mechanism
BC Bearer Control
BICC Bearer Independent Call Control
CC Call Control
CCD Conference Call Device
CFNRc Call Forward Not Reachable
CFNRy Call Forward on No Reply
EVS Enhanced Voice Services
IN Intelligent Network
IuFP Iu Framing Protocol
MACS Maximal number of codec modes in the ACS
OM Optimization Mode
OoBTC Out-of-Band Transcoder Control
QoS Quality of Service
RAB Radio Access Bearer
SCS Supported Codec mode Set
TFO Tandem Free Operation
TICC Transport Independent Call Control
TrFO Transcoder Free Operation
UP User Plane