6.8 Mobile to External Network TrFO Call Establishment

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

Figure 6.8/1. Configuration during Call Setup of a Mobile to External Network Call

The description of Figure 6.1/1 (Configuration during Call Setup of a Mobile to Mobile Call) within clause 6.1 applies for the network and protocol entities involved in the External Network to Mobile Call scenario with following modifications:

No RNC-T is present – a party served by an external network is the terminating side of the call instead.

The terminating side CN nodes are Gateway nodes (Gateway MSC Server/Gateway MGW).

The Gateway MGW call context is no TrFO break equipment in general, i.e. T4 in general do not support the IuFP framing protocol. Appropriate interworking (in some cases transcoding) has to be performed between T3 and T4.

Therefore Figures 6.1/2 to 6.1/4. (the respective message flows for mobile to mobile call setup) apply in principle as well with appropriate modifications outlined below:

Codec negotiation

Step 1. Until 6., that give the codec negotiation phase in Figure 6.1/2, shall be applied with following modifications:

There is no terminating UE involved in this negotiation phase.

If the succeeding node of the Gateway MSC-Server doesn”t support OoBTC procedures for compressed voice types, the Gateway MSC-Server terminates the OoBTC procedures in order to enable transcoder placement at the edge gateway node.

The edge gateway node should accept the Codec Type MSC-O prefers and should not puncture out any Codec Mode. If TFO is to be supported then the Gateway MSC-Server shall supply the MGW with the codec list and the selected Codec Type in order that inband TFO negotiation may be performed. For further details see clause 5.5.