VoLTE Mobile-Terminated (MT) Call Creation Call Flow

This section describes the VoLTE MT call creation call flow.

VoLTE MT Call Creation Call Flow
VoLTE MT Call Creation Call Flow Description
StepDescription
1UE requests for establishment of the IMS default bearer, PDU session, through PCF. After establishing the session, the UE receives the P-CSCF addresses from PCF.
2P-CSCF (AF) sends the NPCF policy authorization update to PCF.
3PCF sends the NPCF SM Policy control update notify request to SMF+PGW-C.
4SMF+PGW-C sends the NPCF SM Policy control update notify response to PCF.
5SMF sends ChargingDataUpdateReq by including Multi-Unit-Usage with Rating-Group-Id that are received as part of Charging_Description of Sm_PolicyControl_UpdateNotify_Request to install PCC Rules.
6CHF provides ChargingDataUpdateResp with Multi-Unit-Information for received Rating-Group values in requested message. CHF also provides params changes for Session-Level and Rating-Group values.
7SMF sends N4 Session Modification Request to the UPF by including Create ULPDRs and Create ULFARs. Create ULPDRs include SDFs and QER Info which are received as part of PCC Rule Installation.
8UPF responds back with N4 Session Modification Response to SMF by including Created ULPDR and Created ULFAR. Create ULFAR contains UL Tunnel Information of UPF for the dedicated bearer creation.
9SMF+PGW-C sends the GTPv2 create bearer request to S-GW.
10S-GW sends the GTPv2 create bearer request to MME.
11MME sends the bearer setup request and session management request to eNodeB.
12RRC connection reconfiguration starts between UE and eNodeB.
13eNodeB sends the bearer setup response to MME.
14UE initiates a direct transfer toward eNodeB.
15eNodeB sends the session management response to MME.
16MME sends the GTPv2 create bearer response to S-GW.
17S-GW sends the GTPv2 create bearer response to SMF+PGW-C.
18SMF+PGW-C sends the N4 session modification request or response to UPF+PGW-U.
19SMF+PGW-C sends the NPCF SM policy control update request to PCF.
20PCF sends the NPCF SM policy control update response back to SMF+PGW-C.
21PCF sends the NPCF policy authorization notify request to P-CSCF (AF).
22Establishes SIP-signaling and voice call between end-users through UE and P-CSCF (AF).