6.13 Mobile to Mobile TrFO Call Establishment in GERAN AoIP mode

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

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

Figure 6.13/1: Configuration during Call Setup of a Mobile to Mobile Call in GERAN AoIP mode

Figure 6.13/1 shows the configuration for mobile to mobile call establishment in GERAN AoIP mode.

Following network and protocol entities are involved in the scenario, outlined in Figure 6.z/1:

BSC-T, BSC-O: terminating/originating BSCs.

MSC Server-T, MSC Server-O: MSC Servers, performing service, i.e. codec negotiation.

MGW-T, MGW-O: terminating/originating MGWs.

BSSAP, TICC:C-plane protocol incarnations, responsible for codec negotiation, controlling the respective interfaces (A, N_C), creating, modifying, removing etc. terminations and contexts.

NOTE: The following sequences are examples, further detailed call flows are described in TS 23.205 [6].

Figure 6.13/2: Call Setup. Mobile to Mobile Call in GERAN AoIP mode. Message Flow part 1

Figure 6.13/3: Call Setup. Mobile to Mobile Call in AoIP mode. Message Flow part 2

Figure 6.13/4: Call Setup. Mobile to Mobile Call in AoIP mode. Message Flow part 3

Codec negotiation

Steps 1. to 8. give the codec negotiation phase. The BSCs inform the MSC servers their capabilities (1. and 5.). The mobiles inform the network about their capabilities (2. and 6.). The MSC-Server performs codec negotiation according to clause 5.6.

Assignment procedure

RAN side terminations have to be seized (9. and 17.) before sending Assignment Request message (steps 10. and 18.). Assignment Request message contains the MSC server preferred codec list. BSC retains the decision to choose the final codec for the radio access and informs the MSC server about the codec to be used in the radio access in Assignment Complete message. Step 20 may then be required to modify the selected codec (SDP) if different from the preferred codec included at step 17.

NOTE: The BSC should offer codecs that it can support for the call and should therefore under normal circumstances select the MSC preferred codec.