11 Codec Mismatch Resolution and Codec Optimization

08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

It is not mandatory for a BSS to support the resolution of Codec Mismatch or the Codec Optimization. In that case the Local_Codec_List shall include only the Local_Used_Codec. However, in the optional case, if a BSS sends a Local_Codec_List that includes more than the Local_Used_Codec, then it is mandatory for that BSS to support the resolution of Codec Mismatch or the Codec Optimisation, considering the reported Codec_Types.

The determination of the Local_Codec_List (i.e. the list of all Codecs supported by the local radio leg, consisting of the local MS, the local BSS and the local radio resources) and the communication of the TRAU with the local BSS, is a BSS specific matter and is outside the scope of this specification. However, only Codec_Types that are real alternatives, considering all resources, shall be reported within the Local_Codec_List. The Local_Codec_List shall be updated and resent as soon as these local resource conditions have changed, if the BSS wants a have these new conditions considered within the Codec Mismatch Resolution or Codec Optimisation.

Whenever a new Distant_Codec_List or a new Local_Codec_List becomes available, then the BSS shall attempt to resolve the Codec_Mismatch or optimize the Codec_Type as soon as possible by following the rules outlined in Table 19 and shall perform a subsequent intra cell handover to the new Local_Used_Codec.

Table 19: Rules for Resolving Codec Mismatch

Radio Leg 2 (T2)

Radio Leg 1
(T1)

EFR
FR HR

EFR
FR

EFR
HR

EFR

FR
EFR HR

FR
EFR

FR
HR

FR

HR
EFR FR

HR
FR

HR
EFR

HR

EFR
FR HR

=

=

=

=

T2=EFR

T2=EFR

T1=FR

T1=FR

T2=EFR

T1=FR T2=FR

T2=EFR

T1=HR

EFR
FR

=

=

=

T2=EFR

T2=EFR

T1=FR

T1=FR

T2=EFR

T1=FR
T2=FR

T2=EFR

MIS

EFR
HR

=

=

T2=EFR

T2=EFR

T1=HR
T2=HR

MIS

T2=EFR

T1=HR

T2=EFR

T1=HR

EFR

=

T2=EFR

T2=EFR

MIS

MIS

T2=EFR

MIS

T2=EFR

MIS

FR
EFR HR

T1=EFR
T2=EFR

T1=EFR
T2=EFR

=

=

T1=EFR
T2=EFR

T2=FR

T1=EFR
T2=EFR

T1=HR

FR
EFR

T1=EFR
T2=EFR

=

=

T1=EFR
T2=EFR

T2=FR

T1=EFR
T2=EFR

MIS

FR
HR

=

=

T2=FR

T2=FR

T1=HR

T1=HR

FR

=

T2=FR

T2=FR

MIS

MIS

HR
EFR FR

T1=EFR
T2=EFR

T1=FR
T2=FR

T1=EFR
T2=EFR

=

HR
FR

T1=FR
T2=FR

=

=

HR
EFR

T1=EFR
T2=EFR

=

HR

=

The first column of Table 19 contains in each cell a definition of the Used_Codec in Radio Leg 1 followed on the next line by the list of supported Codecs. The first row contains similar information for the other, Radio Leg 2. The matrix elements indicate the change to be made in the Used_Codec. For example T2=HR means that Radio Leg 2 shall use the Half Rate Codec. The grey shaded area is intentionally left blank, since it would contain redundant information. The ´=´ sign indicates that no mismatch is present. The ´MIS´ indicates that mismatch can not be resolved. The light (green) shaded areas represent no Codec mismatch, but in several cases double sided handover is recommended to gain speech quality.

Annex A (Normative):
Inband Signalling Protocol: Generic Structure

Scope

Inband Signalling Messages (IS Messages) can be used to construct a specific IS Protocol for the communication between telecommunication entities for various purposes. The original purpose is to establish tandem free operation of mobile-to-mobile calls in GSM networks. The IS Messages provide communication channels inside the speech signal paths between the speech transcoders.

In addition IS Messages allow the control of equipment within the speech signal paths between these telecommunication entities (e.g. speech transcoders). These equipments are termed „In Path Equipments“ (IPEs).

Annex A defines the generic structure of these IS Messages and rules for the IS_Sender.

Annex B defines the generic rules with respect to these IS Messages for the IPEs.

Annex A is mandatory for TFO_TRAU Equipment and informative for IPEs.

Annex B is informative for TFO_TRAU Equipment.

Annex B shall be followed by IPEs, which want to be compatible to IS Messages.