How it Works
The SMF generates a PDU session ID (pdu-session-id) upon receiving a Create Session Request from the 4G-only UE. The SMF validates if the request has the 5G-IWK indication without the PDU session ID in the Protocol Configuration option. The UDM provides the IWK indication to the SMF per DNN. The SMF does not use this indication for deciding whether the UE is 5G capable.
The SMF generates a pdu-session-id based on lbi. For the 4G Create Session Request, the SMF generates pdu-sesion-id=lbi+64 and for WiFI sessions it generates pdu-session-id=lbi+80.
The SMF allows you to configure the default NSSAI under the profile DNN, which it includes in sliceInfo IE in the Policy Create Request directed towards the PCF during session creation from 4g-only UEs. If the default slice is not configured, then SMF selects one of the configured slices. When the UE is DCNR capable, and the DCNR is enabled for the session, the SMF considers the UE capable of supporting dual connectivity. You can configure DCNR per DNN and other NFs, such as UPF.
The SGW notifies the DCNR support to PGW through the 'UPF Selection Indication Flags IE'.
DCNR-Based UPF Selection
SMF selects DCNR supported UPF for DCNR enabled session when DCNR is configured under query params. DCNR isn’t a mandatory query parameter for UPF selection. DCNR can be configured in profile network-element upf. Query parameters / Filter criteria configuration under nf-profile is enhanced to include DCNR support.
SecondaryRatData Usage Report Handling
SMF/UPF cannot differentiate usage in RAN, whether it is on NR or LTE for NSA devices while It is being tracked on eNB. SMF receives usage data ports on S5 interface in various messages, which it reports usage towards CHF.
SMF handles periodic Secondary RAT Usage Data Report from MME over S5/S8. The report is stored till it is sends out to CHF based on triggers mentioned below. SMF sends out the stored secondary RAT usage data report when any of the charging triggers are met.
UePresenceReporting Support
Presence Reporting Area is an area defined within 3GPP Packet Domain for the purposes of reporting of UE presence within that area due to policy control and/or charging reasons. In the case of E-UTRAN, a Presence Reporting Area may consist in a set of neighbor or non-neighbor Tracking Areas, or eNBs and/or cells. There are two types of Presence Reporting Areas: "UE-dedicated Presence Reporting Areas" and "Core Network pre-configured Presence Reporting Areas" that apply to an MME pool. SMF supports “Core Network pre-configured Presence Reporting Areas”.
2g/3g HO Support
During 4G to 2G/3G handover, SMF receives GTPv1 message, SMF has to drop the GTPv1 message, however, if SMF drops the request, it will delay the handover failure and subsequent attach to 2G/3G. In order to expedite the re-attach and reduce the UE service disruption, SMF supports the rejection of GTPv1 messages.