I. PDU Session Anchor: In the 5G (NR) system, each PDU session for a terminal (UE) must first complete the PSA (PDU Session Anchor); this task is performed by the UPF (User Plane Function) through the N6 interface of the PDU session (acting as a gateway connecting to the external DN (Data Network)). The PSA acts as the anchor point for each data session of the terminal (UE), managing data flow and establishing connections to services such as the internet. When the UE performs multiple services, the anchor point for each session in multiple PDU sessions is defined by 3GPP in TS23.501 as follows:
II. Multiple PDU Session Anchors: To support selective traffic routing to the DN or to support
In SSC Mode 3 as defined in TS23.501 Section 5.6.9.2.3, the SMF can control the data path of the PDU session so that the PDU session can correspond to multiple N6 interfaces simultaneously. The UPF terminating each interface is called a PDU session anchor. Each PDU session anchor supporting the PDU session provides access to different DNs.
Furthermore, the PDU session anchor assigned during PDU session establishment is associated with its SSC mode, while other PDU session anchors assigned in the same PDU session (e.g., for selective traffic routing to the DN) are independent of the PDU session's SSC mode. When PCC rules containing traffic steering enforcement control information influenced by the AF as defined in TS 23.503[45] clause 6.3.1 are provided to the SMF, the SMF may decide whether to apply traffic routing based on the DNAI included in the PCC rules (by using the UL classifier function or IPv6 multi-homing).
----The AF-influenced traffic steering enforcement control information can be determined by the PCF when requested by the AF via the NEF (as described in clause 5.6.7.1), or it can be statically pre-configured in the PCF.
----Selective traffic routing to the DN supports deployments where, for example, certain selected traffic is forwarded via the N6 interface to a DN "closer" to the AN serving the UE. This may correspond to:
