In 5G, a Network Slice Instance (NSI) is an end-to-end logical or virtual network created on top of shared physical infrastructure to provide specific customized services. These instances consist of Virtual Network Functions (VNFs) that ensure dedicated performance, security, and resource isolation (e.g., for IoT, high-speed, or low-latency applications). The support of SMF for NSIs is defined by 3GPP in TS23.501 as follows:
I. The SMF (Session Management Function) unit is a key control plane network function in the 5GC (5G Core Network), responsible for managing the entire lifecycle of Protocol Data Unit (PDU) sessions for end-users (UEs), including establishment, modification, and release. It acts as a central coordinator for session connectivity, IP address allocation, and selection/control of User Plane Functions (UPFs) to ensure Quality of Service (QoS) implementation.
II. SMF Application Instances: In the 5G system, the SMF can establish or modify sessions via the N4 interface, providing network instances to the UPF in the FAR and/or PDR. Specifically:
Network instances can be defined as: for example, used to separate IP domains, where multiple data networks allocate overlapping UE IP addresses when the UPF is connected to the 5G-AN, and for transport network isolation within the same PLMN.
Since the SMF can provide the network instance it selects for N3 CN tunnel information via N2, the 5G AN does not need to provide network instances to the 5GC.
III. SMF support for NSI specifically includes the following:
IV. UPF Support for NSI: The UPF can use the network instance included in the FAR, as well as other information such as external header creation (IP address portion) and target interface in the FAR, to determine the interface used for forwarding traffic within the UPF (e.g., VPN or Layer 2 technology).
