9.11 Mobile Terminated CS Call

3GPP43.318Generic Access Network (GAN)Release 16Stage 2TS

The description of the procedure in this clause assumes the MS is in GAN Iu mode; i.e., it has successfully registered with the GANC and GA-RRC is the serving RR entity for CS services in the MS.

Figure 51: Mobile Terminated CS Call

1. A mobile-terminated call arrives at the MSC. The MSC sends a RANAP Paging message to the GANC identified through the last Location Update received by it and includes the TMSI if available. The IMSI of the mobile being paged is always included in the request. Note that the paging message may also be sent from the SGSN to the GANC (i.e., if the Gs interface is present between MSC/VLR and SGSN).

2. The GANC identifies the MS registration context using the IMSI provided by the MSC. It then pages the MS using the GA-RRC PAGING REQUEST message. The message includes the TMSI, if available in the request from the MSC; else it includes only the IMSI of the MS. The message includes the CN Domain identity (CS or PS).

3. The MS responds with a GA-RRC INITIAL DIRECT TRANSFER message containing the Paging Response message. The message includes the CN Domain identity (CS or PS). The CS domain GA-RRC sublayer entity in the MS transitions to the GA-RRC-CONNECTED state.

4. The GANC establishes an SCCP connection to the MSC. The GANC then forwards the paging response to the MSC using the RANAP Initial UE Message. Subsequent NAS messages between the MS and core network will be sent between GANC and MSC using the RANAP Direct Transfer message.

5. The MSC may optionally authenticate the MS using standard UTRAN authentication procedures.

6. The MSC normally updates the security configuration in the MS, via the GANC, as described in clause 9.8.

7. The MSC initiates call setup using the Setup message sent to the MS via GANC. GANC forwards this message to the MS in the GA-RRC DL DIRECT TRANSFER message.

8. The MS responds with Call Confirmed using the GA-RRC UL DIRECT TRANSFER after checking it’s compatibility with the bearer service requested in the Setup and modifying the bearer service as needed. If the Setup included the signal information element, the MS alerts the user using the indicated signal, else the MS alerts the user after the successful configuration of the user plane. The GANC forwards the Call Confirmed message to the MSC.

9. The MSC initiates the assignment procedure with the GANC, which triggers the setup of the RTP stream (voice bearer channel) between the GANC and MS, same as steps 8-13 in the MO call scenario described in clause 9.10.

10. The MS signals that it is alerting the user, via the Alerting message contained in the GA-RRC UL DIRECT TRANSFER. The GANC forwards the Alerting message to the MSC. The MSC sends a corresponding alerting message to the calling party.

11. The MS signals that the called party has answered, via the Connect message contained in the GA-RRC UL DIRECT TRANSFER. The GANC forwards the Connect message to the MSC. The MSC sends a corresponding Connect message to the calling party and through connects the audio. The MS connects the user to the audio path.

12. The MSC acknowledges via the Connect Ack message to the GANC. GANC forwards this message to the MS in the GA-RRC DL DIRECT TRANSFER. The two parties on the call are connected on the audio path.

13. Bi-directional voice traffic flows between the MS and MSC through the GANC.