Feature Description
Important | The PGW-C term used in this chapter denotes the EPS interworking functionality supported by SMF and must not be assumed as a standalone P-GW that is used in the LTE network. |
The dual connectivity enabled UEs support 4G LTE and 5G NR. Such UEs send a signal to the 4G Core Network, indicating that it’s a dual connectivity-enabled device.
When the Dual Connectivity New Radio (DCNR)-capable UE attempts to register in an MME, the MME sets the "UP Function Selection Indication Flags" IE with the DCNR flag set to 1 in the Create Session Request message. After the S-GW receives this IE over S11, it sends the IE over S5 to PGW-C. This IE transmission helps the SGW-C and PGW-C to select SGW-U and UPF, which supports dual connectivity with NR.
The SMF and PGW-C support Packet Data Unit (PDU) sessions from both the 4G-only capable device and the Option 3x capable device (NR and LTE radio connected to the EPC).
The SMF supports the following features and functionality:
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SUPI+IP session and affinity key for 4G to Wi-Fi handover
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Avoiding sending of 5G QoS for 4G-only UE (QoS Rules and QoS Flow Description)
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Handling 5GCNRS and 5GCNRI indication flags from S-GW and MME
DCNR-based UPF Selection
The SMF selects DCNR supported UPF for DCNR enabled session if DCNR is configured under query parameters. DCNR isn’t a mandatory query parameter for UPF selection. You can configure DCNR under the UPF Group profile.
For more information, refer to the UPF Node Selection section in the Policy and User Plane Management feature chapter.
Secondary RAT Data Usage Report Handling
The SMF and UPF track the usage on eNB to differentiate the NR or LTE usage for NSA devices. SMF receives usage data ports on the S5 interface in various messages and reports the usage towards CHF.
The SMF handles the periodic Secondary RAT Usage Data Report from MME over the S5 or S8 interface in the Modify Bearer Request, Delete Session Request, Delete Bearer Response, and Delete Bearer Command based on the Intended Receiver PGW-C (IRPGW) flag. SMF retains the Usage-Report if IRPGW = 1.
The SMF supports multiple instances of Secondary RAT Usage Data Report IEs. It stores reports until they are sent out to CHF based on the triggers. SMF sends out the stored secondary RAT usage data report when any of the charging triggers are met. You can configure the maximum number of stored secondary RAT usage reports under Charging Profile.
Presence Reporting Area
Presence Reporting Area (PRA) is an area defined within the 3GPP packet domain for reporting UE presence within that area due to policy control and/or charging reasons. For E-UTRAN, a PRA consists of a set of neighbor or non-neighbors tracking areas, eNBs and/or cells. You can enable the PRA functionality under the DNN profile.
The two types of Presence Reporting Areas that apply to an MME pool are UE-dedicated Presence Reporting Areas and Core Network preconfigured Presence Reporting Areas. SMF supports Core Network preconfigured PRAs.
4G to 2G or 3G Handover
During 4G to 2G or 3G handover, the SMF rejects the GTPv1 message request and reattaches with 2G or 3G for nondisrupted service continuity. If SMF drops the request, it delays the handover failure and subsequent attach to 2G or 3G.
Extended QoS Support with Legacy Interfaces
When the DCNR-enabled 4G devices connect to the SMF with the legacy interfaces and the Extended-BW-NR feature is negotiated with PCRF, the SMF sends the APN-AMBR values that are greater than 4.2 Gbps towards PCRF in the extended Bit-rates AVP.
If the SMF receives APN-AMBR values greater than 4.2 Gbps in the GTP-v2 messages from S-GW and the DCNR feature isn’t enabled, then the SMF limits the APN-AMBR value to 4.2 Gbps.
If the SMF receives extended QoS values from PCRF and the session isn’t DCNR-enabled, then the SMF limits the APN-AMBR value to 4.2 Gbps.