UE in idle mode. SMF determines the SSC mode for a session (we will discuss SSC modes in the next section). The SMF supports PCSCF discovery for voice sessions. The UPF The UPF is the main data plane NF in the 5G packet core. It connects to the gNB (radio) using the N3 interface and the data network or the internet on the data plane using the N6 interface. It is connected End-to-End Architecture Components, Concepts, Security, and Transport to the SMF on the control plane by the N4 interface. The user data traffic flows from the UE to the gNB to the user plane and exits to the data network. No other node in the 5GS is in the data path: NG-RAN Network N3-IWF Figure 2.6 - User plane reference points The UPF is the anchor point for intra- and inter-RAT mobility. In some cases, the UPF can also optionally allocate IP address prefixes in response to SMF requests. It carries out packet routing and forwarding and it can be involved in packet inspection, which is used to detect the application based on the service data flow template and the optional Packet Flow Descriptors (PFDs) received from the SMF. The UPF enforces gating, redirection, or steering policies on the traffic - these policies are received from the SMF but applied on the UPF, as it is directly in the traffic path. Hence, QoS enforcement UL/DL is done here and also supports reflective QoS marking in DL. The UPF plays an integral part in lawful interception and can forward data packets to law enforcement servers. The user plane also accounts for the volume of data packets and reports the same to the SMF. The UPF carries out downlink packet buffering, which applies to minimizing packet loss, mainly during handovers. It is responsible for sending and forwarding end markers to the NG-RAN node used in UE mobility scenarios. It also triggers downlink data notifications to the SMF when data arrives for UE in idle mode. The UPF is also responsible for responding to router solicitation and ARP requests; it sends the MAC address corresponding to the IP address in the