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:
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The SMF selects the UPF node for EPS and 5GS sessions based on the following parameters:
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DNN
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DCNR (only for EPS calls)
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pdn-type-subscription
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pdn-type-session
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The SMF/PGW-C enables you to define the UPF selection criteria which it uses to query the appropriate node.
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If multiple UPFs match the SMF's 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.
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The SMF stores the load information provided by UPF and uses it in selecting the UPF for new sessions being established. The SMF selects the less loaded UPF among the candidate (DNN based) active UPFs.
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The SMF considers priority and capacity configured statically against each UPF. In cases where UPF does not send the load information statically, configured capacity is considered while selecting the UPFs.