5.1 Network Model

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

The codec negotiation mechanism (OoBTC) is designed to work in the general situation where more than two call control (CC) nodes need to participate in the codec negotiation. The codec negotiation mechanism works as follows:

– Originating CC node: sends its list of supported codec types and options, listed in order of preference.

– Transit CC nodes: if needed, analyse the received list of options, delete unsupported options from the list and forward the list. No modification is done to the preference levels of any of the listed codecs.

– Terminating CC node: analyse the received list of options with their associated priorities and selects the supported option with highest indicated priority appropriate for the call.

Figure 5.1/1 illustrates the architecture for Rel-4 for UMTS to UMTS TrFO connection. The transit network may exist for calls between PLMNs or between islands of mobile CNs separated by transit networks. This figure is a basic illustration, OoBTC shall apply to other access technologies where the OoBTC procedures are supported, i.e. not limited to this figure. The negotiation occurs at call set-up phase, and possibly later on in the call due to other changes such as handover or relocation. However, as described in the next clause, it shall be possible to modify the selected codec at any moment during the active phase of the call.

Further detail of the Call & Bearer Separation for 3GPP is described in [8].

Figure 5.1/1. Basic Architecture for UMTS to UMTS TrFO Connection

The following clauses describe successful call establishment scenarios using the codec negotiation mechanism.