10.7 Codec

23.2833GPPMission Critical Communication Interworking with Land Mobile Radio SystemsRelease 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

10.7.1 Information flows for codec

10.7.1.1 IWF codec reconciliation request

Table 10.7.1.1-1 describes the information flow IWF codec reconciliation request from the IWF to the MCPTT server.

Table 10.7.1.1-1: IWF codec reconciliation request

Information element	Status	Description
MCPTT group ID	M	The MCPTT group ID for which a codec change is requested.
Codec type	M	Type of the requested codec

10.7.1.2 IWF codec reconciliation response

Table 10.7.1.2-1 describes the information flow IWF codec reconciliation response from the MCPTT server to the IWF.

Table 10.7.1.2-1: IWF codec reconciliation response

Information element	Status	Description
MCPTT group ID	M	The MCPTT group ID for which a codec change was requested.
Result	M	Result indicates success or failure of the requested codec change.

10.7.2 IWF transcoding

The IWF can be used to transcode voice packets in transit between the LMR and MCPTT systems. In this scenario, the MCPTT system can operate its own vocoder type and the LMR system can operate its own vocoder type. The type of vocoder used on the LMR side is outside the scope of the present document.

When operating in this mode, the IWF converts voice media formats between the two sides. Vocoder negotiation is according to procedures in the present document.

10.7.3 Codec negotiation by the LMR system

10.7.3.1 Description

An MCPTT group may be configured to use an LMR speech codec, such that speech can be carried end to end between all group members in both LMR and MCPTT system without transcoding.

An LMR system can support more than one speech codec; for example P25 supports both a full rate and a half rate speech codec. Circumstances within the LMR system might require that the codec in use within a group is changed according to the needs of the LMR system.

Figure 10.7.3.1-1 below illustrates a procedure which allows the LMR system to change the speech codec within an MCPTT group that is connected to the LMR system via the IWF.

Pre-conditions:

1. Group members have affiliated to the MCPTT group in both the LMR system and in the MCPTT system

2. A permitted LMR codec has been negotiated for use by MCPTT group members

3. MCPTT group members support the requested second LMR speech codec

NOTE 1: The exception condition created if the IWF does not support trancoding and the MCPTT client does not support the requested LMR codec is outside the scope of the present document.

4. The LMR system requires a change to an alternative speech codec.

Figure 10.7.3.1-1: Codec reconciliation procedure

1. The IWF sends a codec reconciliation request to the MCPTT server on behalf of the LMR system.

2. The MCPTT server checks that the requested codec is permitted for the MCPTT group.

3. The MCPTT server sends a codec reconciliation request to all of the affilliated MCPTT client(s) to negotiate the use of the speech codec requested by the LMR system.

4. The MCPTT client replies with a codec reconciliation response to the MCPTT server, indicating acceptance of the new speech codec.

5. The MCPTT server sends a codec reconciliation response to the IWF.

6. Further transmissions in the MCPTT group use the new codec in the media plane.

NOTE 2: The time at which the new codec is first used by a transmitting party is outside the scope of the present document.