UPF stands for User Plane Function, and it is a critical component of 5G networks. The UPF is responsible for handling the user data, such as voice and video, as it travels between the user equipment (UE) and the internet or other network services.

The UPF is part of the 5G core network architecture and is responsible for performing several important functions, such as packet forwarding, quality of service (QoS) enforcement, and traffic filtering. It also plays a crucial role in enabling features such as network slicing, which allows multiple virtual networks to be created on a single physical network.

In addition to its role in managing user data, the UPF is also responsible for managing the flow of control data between the UE and the network. This includes tasks such as session management, policy enforcement, and mobility management.

Functions of UPF as per 3GPP TS 23.501:

• Packet forwarding: Responsible for routing data packets from the UE to their intended destination.
• Quality of Service (QoS) enforcement: Ensures that packets are delivered quickly and reliably, while also prioritizing certain types of traffic.
• Traffic filtering: Identifies and blocks certain types of traffic based on predefined policies.
• Network slicing: Allows different virtual networks to be created on a single physical network, each with its own set of QoS policies and parameters.
• Session management: Sets up and tears down data connections between the UE and the network.
• Policy enforcement: Ensures that network policies and rules are followed by all network components.
• Mobility management: Determines the best route for data packets to take as the UE moves between different cells or base stations.
• Data forwarding: Forwards data packets between the UE and external network services.
• QoS policy maintenance: Maintains QoS policies and other parameters between different UPFs.

There are 4 distinct reference points of UPF in 5G networks

N4: The N4 interface is used to connect the UPF to the Access and Mobility Management Function (AMF). This interface is used for session management, which involves setting up and tearing down data connections between the UE and the network.

• Session management: The N4 interface is used to set up and manage data sessions between the UPF and the SMF. This includes establishing a connection between the two functions, exchanging session-related information, and tearing down the session when it is no longer needed.
• QoS policy management: The N4 interface is used to exchange QoS-related information between the UPF and the SMF. This includes information about the QoS policies that are applied to data sessions, as well as any changes or updates to those policies.
• Policy enforcement: The N4 interface is used to enforce network policies and rules, as defined by the SMF. This includes ensuring that data sessions comply with the appropriate QoS policies, as well as any other network policies that are in place.
• Mobility management: The N4 interface is used to support the mobility of the UE as it moves between different parts of the network. This includes selecting the best route for data packets to take, as well as ensuring that the connection remains active as the UE moves between different cells or base stations.

N3: The N3 interface is used to connect the UPF to the Serving Gateway (SGW). This interface is responsible for packet forwarding, QoS enforcement, and traffic filtering.

• Data forwarding: The N3 interface is used to forward data packets between the UPF and the RAN. This includes data packets that are sent and received by the UE, as well as control packets that are used to manage the connection.
• QoS enforcement: The N3 interface is used to enforce QoS policies that are defined by the network operator. This includes prioritizing certain types of traffic, such as voice or video, to ensure that they are delivered with low latency and high reliability.
• Mobility management: The N3 interface is used to support the mobility of the UE as it moves between different cells or base stations. This includes selecting the best route for data packets to take and ensuring that the connection remains active as the UE moves between different parts of the network.
• Packet inspection: The N3 interface can be used to perform deep packet inspection (DPI) on data packets that are being forwarded between the UPF and the RAN. This can be used to identify and analyse specific types of traffic, such as video or voice, and to apply appropriate QoS policies to that traffic.

N6: The N6 interface is used to connect the UPF to the Data Network (DN). This interface is responsible for forwarding data packets between the UE and external network services.

• Data forwarding: The N6 interface forwards user data packets between two UPFs. When a UE moves from one PDU session to another, the data needs to be forwarded between the two UPFs, and the N6 interface is responsible for this.
• QoS Enforcement: The N6 interface is used to enforce Quality of Service (QoS) policies that are defined by the network operator. The QoS policies may prioritize certain types of traffic, such as voice or video, to ensure that they are delivered with low latency and high reliability.
• Policy Enforcement: The N6 interface is used to enforce network policies and rules that are defined by the network operator. This includes ensuring that data sessions comply with the appropriate QoS policies, as well as any other network policies that are in place.
• Mobility management: The N6 interface is used to support UE mobility as it moves between different PDU sessions. This includes selecting the best route for data packets to take and ensuring that the connection remains active as the UE moves between different parts of the network.

N9: The N9 interface is used to connect two UPFs in different network functions. This interface is used for data forwarding between the UPFs, and for maintaining QoS policies and other parameters.

• Data forwarding: The N9 interface is responsible for forwarding user data packets between the UPF and the DN. This includes all data traffic that originates from or is destined to the UE.
• Network Address Translation (NAT): The N9 interface can be used to perform Network Address Translation (NAT), which allows the UPF to map private IP addresses used by the UE to public IP addresses that are routable on the Internet. This is necessary because the Internet uses public IP addresses, while many UE devices use private IP addresses.
• Packet filtering: The N9 interface is used to filter incoming and outgoing packets based on a set of defined rules. This helps to ensure that only authorized traffic is allowed to pass through the UPF.
• Charging: The N9 interface can be used to collect information about data usage and generate charging records. This information can be used to bill subscribers for their data usage.