5.8.1 Modification of Selected Codec

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

The codec modification procedures shall support the following changes:

1) change of the codec type or codec configuration of the current Selected Codec (BICC) to another codec type or codec configuration within the Available Codecs List (BICC);

2) modification of the Available Codecs List (BICC) according to clause 5.8.2, (i) to (v), in combination with any change of the codec type or codec configuration of the current Selected Codec (BICC) to another codec type or codec configuration within the new Available Codecs List (BICC). The modification of the Available Codecs List (BICC) may include removal of the current Selected Codec (BICC) from the list.

The procedures described in Q.1902.4, clauses 10.4.1 to 10.4.3 [6] shall apply.

The new codec type and codec configuration may be selected freely from the Available Codecs List (BICC). For an AMR codec, AMR-WB or EVS codec, a codec configuration may be selected if it is considered to be included in the Available Codecs List (BICC) according to the criteria specified at the end of clause 5.6.4.

For the coding of the new Selected Codec (BICC), the new Available Codecs List (BICC), and the new Codec (H.248), the same rules apply as specified in clauses 5.6.4 and 5.6.5.

In Figure 5.8.1/1 and 5.8.1/2 the basic codec modification procedure is shown. This Figure is an example; the codec modification procedure may be initiated by any node within the call.

Upon Reception of a Modify Codec message (action 5 and 9 in Figure 5.8.1/1), a server node shall check if the Selected Codec is altered according to the criteria above. If the Selected Codec is not altered, the procedures in clause 5.8.2 (Modification of the Available Codec List) apply, otherwise the server node shall send a "Reserve Characteristics" procedure to the attached MGW for the corresponding termination (action 6 and 10 in Figure 5.8.1/1

To perform a modification of the selected codec at an Iu interface, the MSC server shall send a "Modify Bearer Characteristics" procedure to the attached MGW (action 1 and 12 in Figure 5.8.1/1). Upon completion of the "Modify Bearer Characteristics" procedure, the server node shall send a "RAB Assignment Request" to the radio access network (action 2 and 13 in Figure 5.8.1/1). The MSC server shall then wait to receive a corresponding "RAB Assignment Response" message from the radio access network (action 3 and 14 in Figures 5.8.1/2 and 5.8.1/3) before continuing the modification procedure.

An MSC server shall use the "Reserve Characteristic" procedure for the termination towards the preceeding node (with respect to the Modify Codec message) to perform the necessary bearer level modification. The MGW shall respond for that termination with the "Bearer Modified" procedure to indicate that the possible bearer modification to increase bandwidth was successful. The MGW shall not wait until the Iu UP initialisation is complete before replying with the "Bearer Modified" procedure. Each server shall not send forward the modify request to the succeeding node until the indication from its MGW that any necessary bearer level modification has been completed (BNC_Modified notification). The MSC server shall use the "Confirm Characteristics" procedure to confirm the modification at that termination.

An MSC server shall use the "Modify Characteristic" procedure for the termination towards the succeeding node (with respect to the Modify Codec message) to confirm the codec modification.

The specific handling of the Iu UP initialisation is described in clause 5.8.4.

Error Cases are described in clause 5.8.5.

Figure 5.8.1/1: Codec Modification Control Procedures

Figure 5.8.1/2: Codec Modification acknowledgement