How it Works

The UPF initiates an N4 Association Setup request to set up an association with SMF.

The following is a high-level summary of how SMF selects the UPF node for the core network:

  • The SMF selects the UPF node for EPS and 5GS sessions based on the UPF selection policy configured under DNN profile configuration. The UPF selection policy defines a combination of the following parameters:

    • DNN

    • Network slice

    • Subscriber location

    • DCNR (only for EPS calls)

    • PDN/PDU subscription type

    • PDN/PDU session type

  • If the UPF selection policy is not defined under DNN profile configuration, then SMF selects the UPF based on the derived location DNN or requested DNN

  • The SMF enables you to define the UPF selection criteria which it uses to query the appropriate node.

  • If multiple UPFs match the selection criteria, then SMF selects the active UPFs and sorts them based on their priority and load information. The SMF then attempts to access the UPF one by one until the N4 Session Establishment is successful.

  • The SMF stores the load information provided by UPF and uses it in selecting the UPF for the new sessions. The SMF selects the less loaded UPF among the candidate (DNN-based) active UPFs.

  • The SMF considers priority and capacity configured statically against each UPF. In cases where UPF does not send the load information statically, the SMF uses the configured capacity to select the UPFs.

  • The SMF selects the UPF which is given more priority in a particular location. Both the UPF priority and UPF group priority are used to determine the final priority of UPF. For information on configuring the UPF group priority, see the Assign Priority for UPF Group section.