5G Session Management

Description

5G Session Management (5GSM) is a control plane protocol defined within the 5G System (5GS) architecture. It operates between the User Equipment (UE) and the Session Management Function (SMF) to manage Packet Data Unit (PDU) Sessions, which are the fundamental connectivity service providing user plane connectivity between the UE and a specific Data Network (DN). The 5GSM protocol is carried within the Non-Access Stratum (NAS) signaling messages between the UE and the Access and Mobility Management Function (AMF), with the AMF transparently relaying 5GSM messages to and from the SMF. This separation of the session management signaling from the access and mobility management signaling (handled by the 5GMM protocol) is a key architectural principle of 5G, allowing for independent scaling and evolution of session and mobility management functions.

The primary function of 5GSM is to establish, modify, and release PDU Sessions. A PDU Session establishment procedure involves the UE sending a PDU Session Establishment Request to the network, which triggers the SMF to select a User Plane Function (UPF), establish the necessary N4 session with the UPF, and negotiate the required QoS parameters. The 5GSM protocol conveys critical information such as the PDU Session ID, the selected Data Network Name (DNN), the requested Single-Network Slice Selection Assistance Information (S-NSSAI), and the PDU Session Type (e.g., IPv4, IPv6, IPv4v6, Ethernet, or Unstructured). It also manages the activation, modification, and deactivation of QoS Flows within a PDU Session, mapping service data flows to specific QoS Flows with defined 5G QoS Identifier (5QI) values, Guaranteed Flow Bit Rate (GFBR), and Maximum Flow Bit Rate (MFBR).

Beyond basic session lifecycle management, 5GSM supports advanced features essential for 5G's service-based architecture. It enables the negotiation and management of Protocol Configuration Options (PCO), which allows the SMF to provide the UE with parameters like DNS server addresses. It also supports UE-requested PDU Session Modification procedures, allowing the UE to request changes to the session's characteristics, such as adding or removing a QoS Flow. Furthermore, 5GSM procedures are tightly integrated with network slicing; the S-NSSAI included in the 5GSM signaling ensures the PDU Session is established within the correct network slice instance, providing the intended service characteristics and isolation. The protocol also handles the release of PDU Sessions, either initiated by the UE or the network, ensuring proper cleanup of resources in the SMF, UPF, and UE.

Purpose & Motivation

5GSM was created as part of the new 5G Core (5GC) architecture to address the limitations of the Evolved Packet System (EPS) session management protocol used in 4G/LTE. In EPS, session and mobility management were more tightly coupled within a single protocol (ESM and EMM, respectively). The 5G design principle of service-based architecture and network function disaggregation necessitated a clean separation between the Access and Mobility Management Function (AMF) and the Session Management Function (SMF). 5GSM exists to provide a dedicated, efficient protocol for session management that can operate independently of the underlying access technology (3GPP access like NR or non-3GPP access like Wi-Fi) and can scale to support the massive diversity of 5G use cases.

The protocol solves the problem of managing complex, service-specific connectivity sessions with dynamic QoS requirements. Previous systems had more rigid QoS models. 5GSM is designed to support a wide array of PDU Session types (beyond just IP), including Ethernet and Unstructured types, which are critical for vertical industry applications and fixed-mobile convergence. It also provides the signaling foundation for network slicing, allowing a single UE to have multiple concurrent PDU Sessions, each potentially in a different network slice with entirely different performance and security characteristics. This was a significant evolution from the EPS bearer model, enabling true service-based connectivity.

Furthermore, 5GSM was motivated by the need for enhanced efficiency and flexibility. By separating session management signaling, the network can optimize the placement and scaling of the SMF independently from the AMF. The protocol supports UE-triggered session modification, giving applications more direct control over their network resources. It also simplifies the integration of non-3GPP access, as the same 5GSM protocol is used regardless of how the UE attaches to the 5GC, providing a unified session management experience. In essence, 5GSM is the enabling protocol that translates high-level service requests into specific, actionable user plane connectivity rules within the 5G network.

Key Features

• Manages the lifecycle (establishment, modification, release) of PDU Sessions
• Supports multiple PDU Session Types: IPv4, IPv6, IPv4v6, Ethernet, Unstructured
• Negotiates and manages QoS Flows with granular 5QI, GFBR, and MFBR parameters
• Integrates with network slicing via S-NSSAI for session isolation and service-specific treatment
• Enables UE-initiated session modification for dynamic service requirements
• Operates independently of access technology (3GPP and non-3GPP) via NAS transport

Evolution Across Releases

Defining Specifications