PDU Session Anchor (PSA) in 5G Systems

In 5G (NR) systems, the PSA (PDU Session Anchor) is the UPF (User Plane Function). It acts as a gateway connecting to the external DN (Data Network) via the N6 interface of the PDU session. As the anchor point for user data sessions, the PSA manages data flow and establishes connections to services such as the Internet.

I.There are three PSA modes: SSC Mode 1, SSC Mode 2, and SSC Mode 3.

• SSC Mode 1: In this mode, the 5G network maintains the UE connection service. For IPv4, IPv6, or IPv4v6 class PDU sessions, the IP address is reserved. In this case, the User Plane Function (UPF) acting as the PDU session anchor remains unchanged until the UE releases the PDU session.
• SSC Mode 2: In this mode, the 5G network can release the connection to the UE, i.e., release the PDU session. If the PDU session was used to transmit IP packets, the allocated IP address will also be released. One application scenario for this mode is when the anchor UPF requires load balancing, allowing the network to release connections. In this case, the PDU session can be transferred to a different anchor UPF by releasing the existing PDU session and subsequently establishing a new one. It uses a "disconnect + establish" framework, meaning the PDU session is released from the first serving UPF and then a new PDU session is established on the new UPF.
• SSC Mode 3: In this mode, the 5G network maintains the connection provided to the UE, but some impacts may occur during certain processes. For example, if the anchor UPF changes, the IP address assigned to the UE will be updated, but the change process ensures that the connection is maintained; that is, a connection to the new anchor UPF is established before releasing the connection with the old anchor UPF. 3GPP Release 15 only supports Mode 3 for IP-based PDU sessions.

II. The main uses of the PDU session anchor point include:

• Data Termination Point: The PSA is the UPF where the PDU session terminates its connection with the external data network.
• Data Routing: It routes user data packets between the user equipment (UE) and the external DN.
• IP Address Allocation: The PSA is associated with an IP address pool. The UE's IP address is allocated from this pool, either by the UPF itself or through an external server (e.g., a DHCP server). The Session Management Function (SMF) manages this address pool.
• Data Path Control: The SMF controls the data path of the PDU session, selects the PSA, and manages the termination of the N6 interface.