UPF Selection Algorithm

The SMF determines the UPF node based on an algorithm.

The following figure depicts the UPF node selection workflow.

UPF Node Selection Workflow

The SMF lists the UPF nodes based on the priority assigned to the node. When there are multiple nodes with the same priority value, then the SMF selects a UPF experiencing the lowest level of load. The load parameter is applied only for UPFs that have the same priority.

When load is not available as a selection criteria, then SMF selects a random UPF when there are multiple UPFs with the same priority.

The SMF stores UPF order list based on priority. When a failure occurs, the SMF selects the next entry in the list based on failure handling template (FHT) configuration.

If priority is not available as a selection criteria and load is available as a selection criteria, then SMF selects least loaded UPF from the list of selected UPFs.

Important

The SMF performs UPF selection during initial call establishment and handover procedure. For more details on the handover, see the Co-located UPF Selection During Handover section.

When the subscriber location is used as the UPF selection parameter, the SMF uses the priorities that are set for the UPF and the UPF group to choose the best suitable UPF.

The following is an example to understand the UPF selection logic.

Assume two UPF groups and two UPFs with the following configurations.

  • UPF groups:

    • UpfGrp1:

      • Location Area Group List:TAI1

      • Slice list

      • PDN type list

    • UpfGrp2:

      • Location Area Group List:TAI2

      • Slice list

      • PDN type list

  • UPFs

    • Upf1:

      • Priority: 500

      • Capacity: 1000

      • Upf Grp List: ((UpfGrp1, priority: 10), (UpfGrp2, priority: 30))

    • Upf2:

      • Priority: 500

      • Capacity: 1000

      • Upf Grp List: ((UpfGrp1, priority: 20), (UpfGrp2, priority: 5))

A combination of UPF group priority and UPF priority is used for selecting the UPF having more preference (less priority) in a particular location.

The SMF selects upf1 for location TAI1 as upf1 is with less priority. Similarly, upf2 is selected for TAI2 based on the UPF priority and UPF group priority.

The SMF also provides the capability to configure DNN profile based on UE location, that is, TAI or ECGI. The location-based DNN profile allows mapping of location area group with DNN profile where location area group specifies the TAI or ECGI group.

For TAI-based UPF selection, it is mandatory to first select the DNN profile based on the UE location through location-dnn-profile configuration. Then, use the UPF selection policy (for example, DNN and slice selection criteria) defined in the selected DNN profile.

For configuration details, see the Select Location-based DNN Profile section.