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 PGW-C 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 PGW-C then processes the received Create Session Request and communicates with various SBI interfaces to receive the following information:

  • Subscription data from UDM by including PGW-C FQDN in the subscription request.

  • Policy information from PCF by sending SM policy create request. Policy information includes details, such as PCC rules and Session-AMBR.

  • 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 PGW-C sends the GTPv2 Create Session Response to the end user over S-GW and MME. This response includes:

  • PAA with IPv4 or IPv6 addresses that PGW-C IPAM module allocates

  • ePCO option with P-CSCF

  • DNS IPv4 or DNS IPv6 address based on DNN-Profile configuration

  • 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 PGW-C 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 PGW-C to allow GBE flow packets for the voice communication between the calling and called devices.

For prioritization of emergency and VoLTE calls during ICSR switchover and recovery, SMF includes the MP flag value in N4 and GTP messages. The flag values are listed in the following table.

MP Flag Values

Call Type

MP Flag Value

Emergency

1

IMS-Active

2

IMS-Inactive

3

WPS

1

Note
  • If message-priority gtpc is configured in WPS profile, then SMF sends the MP flag value as 1 for the WPS sessions. Earlier SMF used to send MP value as 0 in N4 and GTP header for WPS sessions.

  • If a call matches multiple call types, then SMF sends better priority, which is the lowest value. If the call matches with WPS and the message-priority is not configured, then message priority is not sent.

  • SMF marks session as IMS-inactive when dedicated bearer or modify procedure starts for establishment of QCI configured as IMS in DNN profile and the session is not marked as IMS-active.

  • SMF updates the session as IMS-active session after the IMS QCI bearer or flow is established.

  • SMF updates the session as IMS-inactive session after last IMS QCI bearer or flow is deleted.

  • SMF marks session as WPS session when dedicated bearer or modify procedure starts for an ARP that is configured in WPS profile.

  • SMF updates the session as WPS session after WPS bearer or flow is established.

  • SMF updates the session as IMS-active or IMS-inactive if an IMS QCI bearer exists when the last WPS bearer was deleted.

  • SMF updates the session as IMS-inactive, if an IMS flow is removed during handover, path-switch, or any other procedure.

  • Clear subscriber non-VoLTE CLI implementation is modified to use the QCI that is configured as IMS. If QCI is not configured, the earlier behavior of detecting non-VoLTE session is used. This behaviour implies that the session does not having bearer or flows with QCI as 1 or 2.