How it Works
A 5G mobile device with LTE access requests voice services to communicate with PGW-C over S-GW and MME resulting in the establishment of a PDU session. The P-GW supports a non-GBR bearer with QCI flows as 5 for VoLTE sessions. This support allows IMS signaling along with P-CSCF, DNS IPv4, or DNS IPv6 addresses for end users. For mobile-originated (MO) or mobile-terminated (MT) calls, the Application Function (AF) provides policy authorization details to the PCF. The PCF then converts these details to GBR flows and PCC rules and sends them to PGW-C. The PGW-C then converts the GBR-flows to dedicated bearers by establishing the dedicated bearer creation procedure with UE. The PGW-C provisions the GBR with the QCI flow as 1 to UPF. By this provisioning, the UPF supports voice communication between the calling and called devices over IMS network elements.
As per the E-UTRAN Attach procedure, the MME triggers the GTPv2 Create Session Request to PGW-C over S-GW. This request includes the EPS Bearer Identity (EBI) value, ePCO options for P-CSCF and DNS IPv4 or DNS IPv6 containers, PDN-Type, and PAA options for IPv4 or IPv6 allocated address for end users. The P-GW then processes the received Create Session Request and communicates with various SBI interfaces to receive the following information:
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Subscription data from UDM by including PGW-C FQDN in the subscription request.
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Policy information from PCF by sending SM policy create request. Policy information includes details, such as PCC rules and Session-AMBR.
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Online and offline charging information from CHF by sending the charging create data request.
After communication with SBI interfaces, which are based on the local SMF profile configuration, the P-GW sends the GTPv2 Create Session Response to the end user over S-GW and MME. This response includes:
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PAA with IPv4 or IPv6 addresses that PGW-C IPAM module allocates
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ePCO option with P-CSCF
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DNS IPv4 or DNS IPv6 address based on DNN-Profile configuration
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Non-GRB with the QCI flow as 5 for IMS signaling
For an MO or MT call, if the PCF is provisioned for GBR with the QCI flow as 1 for end users, the P-GW converts these GBR flows to the dedicated bearer creation. The GBR flows include the flow information and the PCC rules in the SM Policy Update Notify Request. The dedicated bearer is created by sending GTPv2 Create Bearer Request to UE over S-GW or MME. Another S5-U tunnel is created between S-GW and P-GW to allow GBE flow packets for the voice communication between the calling and called devices.