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.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (121 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the 5GSM (5G Session Management) function was newly introduced as the core protocol for handling PDU sessions, defining procedures like PDU session establishment, modification, and release. It introduced specific coordination mechanisms with the 5GMM (5G Mobility Management) layer, where 5GSM messages are piggybacked within 5GMM transport messages for parallel execution. The release also established foundational elements such as procedure transaction identities, security reliance on 5GMM, and the inclusion of parameters like DNN and S-NSSAI in 5GSM requests.
- Revision on AMF transport behaviour of 5GSM message TS 24.501CR0209
- Clarification on 5GSM cause_#46 out of LADN service area TS 24.501CR0598
- Correction for editor's note on further 5GSM causes and further minor issues TS 24.501CR0025
- Corrections on inconsistent descriptions for 5GSM and 5GMM TS 24.501CR0108
- Correction on UE behaviour for 5GSM congestion control TS 24.501CR0113
- Correction to 5GSM/ESM coordination TS 24.501CR0164
+ 26 more changes
In Release 16, the 5GSM function was enhanced with new capabilities for Multi-Access (MA) PDU sessions and the introduction of specific 5GSM cause values for improved error handling, such as cause #54 for "PDU session does not exist." The release also provided clarifications and corrections on the handling of 5GSM parameters and procedures, including the coordination with 5GMM during service requests and the inclusion of 5GSM causes in PDU session modification and release requests.
- Indicate 5GSM cause when initiating 5GSM procedure for error handling TS 24.501CR2283
- Ethernet header compression for CP CIoT – 5GSM aspects TS 24.501CR2323
- 5GSM cause values #27, #50, #51 and #70 for SINE_5G TS 24.501CR1249
- Handling of indication to the 5GSM sublayer in case of 5GSM message not forwarded to SMF due to service area restrictions TS 24.501CR0789
- Correction on the descriptions of 5GSM parameters and capabilities TS 24.501CR0955
- Exception in suspension of 5GSM procedures in case of ongoing 5GMM procedures: Service request procedure initiated during connected mode TS 24.501CR1047
+ 28 more changes
In Release 17, the 5GSM function introduced several key enhancements for session management robustness and new service support. These included new procedures and clarifications for handling redundant PDU sessions, UE-requested PDU session release with a specific cause, and the protection of 5GSM messages. The release also defined new 5GSM cause values, such as for UAS services, and refined the handling of back-off timers, session collisions, and procedures during network-triggered events like fallback or SOR.
- DNN setting in the 5GSM sublayer TS 24.501CR2860
- 5GSM cause value of PDU session establishment reject for UAS services TS 24.501CR3629
- 5GSM protocol update for redundant PDU sessions TS 24.501CR3671
- Handling of 5GSM procedures when fallback is triggered TS 24.501CR2505
- The suggestion on back-off timer for 5GSM #29 TS 24.501CR2585
- Addition of 5GSM cause #37 TS 24.501CR2782
+ 32 more changes
In Release 18, 5GSM enhancements included updates for LADN (Local Area Data Network) procedures per DNN and S-NSSAI, and introduced specific handling for UEs with S-NSSAI location validity information. The release also defined UE behavior for coordination with EPS bearer parameters and extended the re-attempt indicator for equivalent SNPNs. Furthermore, it introduced mechanisms for custom throttling of temporary failed 5GSM procedures and refined the interaction between the 5GSM entity and upper layers for URSP handling.
- 5GSM procedure updates for LADN per DNN & S-NSSAI TS 24.501CR5013
- 5GSM operation for a UE with S-NSSAI location validity information TS 24.501CR5594
- 5GSM coordination: UE behaviour in case of missing EPS bearer parameters TS 24.501CR4500
- Extending Re-attempt indicator IE & 5GSM congestion re-attempt indicator IE for equivalent SNPN TS 24.501CR5113
- Correction in the 5GSM sublayer state transition in terms of the PDU SESSION MODIFICATION REJECT message including 5GSM cause value #43 TS 24.501CR5241
- Interaction between a 5GSM entity and upper layers for URSP handling TS 24.501CR5355
+ 4 more changes
In Release 19, the 5GSM function saw clarifications and corrections to its operation, including updates to the handling of specific cause values and the 5GSM status message. The release also provided a new specification for 5GSM sublayer behavior upon receiving a congestion control indication for CIoT user data from the 5GMM sublayer. Furthermore, corrections were made to the inclusion and setting of specific information elements, such as the 5GSM capability IE for PDU session modification and the SDNAEPC within that capability IE.
- Correction on the 5GSM cause reason TS 24.501CR6399
- Correction on inclusion of 5GSM capability IE for PDU SESSION MODIFICATION REQUEST message TS 24.501CR6774
- Correction related to the setting of MPQUIC-IP and MPQUIC-E in the 5GSM capability IE TS 24.501CR6793
- 5GSM sublayer operation upon receiving an indication that the CIoT user data was not forwarded due to congestion control and a timer value from the 5GMM sublayer TS 24.501CR6767
- Clarification regarding providing replaced S-NSSAI in 5GSM signalling TS 24.501CR6938
- Update to the handling of 5GSM status with #43 TS 24.501CR6969
+ 1 more changes
Explore further
Broader topics and technologies where 5GSM plays a role.
Defining Specifications
3GPP specifications that define or reference 5GSM, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TS 29.214 vj20 | Policy and Charging Control over Rx | Rel-19 |
| TS 38.508 vj11 | 5G NR UE Radio Transmission & Reception | Rel-19 |
