Description
The Network Slice Simultaneous Registration Group (NSSRG) is a 3GPP concept that defines a set of one or more Network Slice Instances (NSIs) for which a User Equipment (UE) can maintain simultaneous registration states. This mechanism is crucial for enabling a single UE to concurrently access services from different network slices, each potentially tailored for distinct service types such as enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), or massive IoT. The NSSRG is managed by the Access and Mobility Management Function (AMF) in the 5G Core Network (5GC). When a UE initiates a registration procedure, it can indicate its requested Network Slice Selection Assistance Information (NSSAI). The AMF, in conjunction with the Network Slice Selection Function (NSSF), determines the allowed NSSAI and, based on network policies and subscription data, can assign the UE to a specific NSSRG. The AMF maintains the UE's registration context for all slices within the assigned NSSRG.
Architecturally, the NSSRG concept is tightly integrated with the 5G registration and session management procedures. A UE registered to an NSSRG does not need to perform a new registration procedure when establishing a Protocol Data Unit (PDU) session on a different network slice within the same group. This reduces signaling overhead and latency for service access. The AMF holds the common registration context for the UE across the slices in the group, while individual User Plane Functions (UPFs) and Session Management Functions (SMFs) are selected per PDU session based on the specific slice. The NSSRG identifier is part of the UE's context in the AMF and the Unified Data Management (UDM).
The operation hinges on the AMF's ability to support multiple Network Slice Identifiers (S-NSSAIs) under a single registration context. During the initial registration or a registration update, if the UE's requested S-NSSAIs belong to the same NSSRG as per network configuration, the AMF can accept them under one Registration Area and one 5G-Globally Unique Temporary Identifier (5G-GUTI). For mobility, if a UE moves to an area where not all slices in its NSSRG are available, the AMF may need to modify the allowed NSSAI, but the registration for the group remains intact for the available slices. This provides a balance between service continuity and network resource optimization. The NSSRG is a key enabler for efficient multi-slice device operation, which is fundamental to realizing the 5G vision of a single physical network infrastructure supporting a multitude of virtualized networks with different characteristics.
Purpose & Motivation
The NSSRG was introduced to solve the problem of signaling inefficiency and service interruption when a UE needs to access multiple network slices. Prior to its introduction, a UE typically maintained a registration state per network slice. Switching between services on different slices could require explicit registration procedures for each slice, leading to increased signaling load on the network and latency for the user. This was particularly problematic for devices requiring simultaneous access to slices, such as a smartphone running a high-bandwidth video stream (eMBB slice) while also controlling a low-latency industrial sensor (URLLC slice).
The creation of NSSRG was motivated by the need for a more scalable and efficient registration model in a sliced network. It addresses the limitation of a one-to-one mapping between UE registration and a network slice, which does not scale well as the number of slices and UEs increases. By grouping slices that a device is likely to use together, the network can maintain a unified mobility and connection management state, drastically reducing the control plane signaling associated with registration, deregistration, and periodic registration updates. This is essential for network performance and battery life of devices.
Historically, this concept evolved from the single registration model used in earlier mobile generations and the initial 5G slice-aware registration. NSSRG provides a middle ground, offering the simultaneous access benefits of multiple registrations with the signaling efficiency of a single registration. It is a foundational capability for advanced use cases like network slicing for enterprise services, where a device may need seamless access to both a corporate slice and a public internet slice. Its introduction in Release 17 reflects the maturing of network slicing implementations towards more complex and realistic deployment scenarios.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (722 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NSSRG (Network Slice Simultaneous Registration Group) function was newly introduced to enable a UE to be simultaneously registered with multiple network slices. This foundational capability supports features like delayed registration due to emergency services and the modification of the set of network slices for a UE, as detailed in the associated Change Requests. The introduction established the basis for managing slice-specific registration areas and the concept of Allowed and Configured NSSAI for slice access control.
- Initial registration not accepted due to serving network not authorized TS 24.501CR0301
- Update to 5GS registration type IE and introduction of a new 5GS update type IE TS 24.501CR0426
- LADN indication from UE at registration TS 24.501CR0065
- NW slicing and delayed re-registration due to emergency services TS 24.501CR0124
- Registration procedure triggered by a change of UE Radio Capability TS 24.501CR0278
- NW slicing and delayed registration due to emergency services, reject PDU session request TS 24.501CR0290
+ 109 more changes
In Release 16, the NSSRG function was enhanced through the introduction of network slice-specific authentication and authorization, which includes a new "Pending NSSAI" provided to the UE during registration while this procedure is ongoing. This release also added the capability for the network to reject a registration due to no available allowed S-NSSAIs and introduced a condition to avoid redundant registration procedures during inter-system change. Furthermore, it defined a generic UE configuration update trigger for registration changes.
- Enhancement on slice interworking--501 TS 23.501CR0850
- Further detailing of 5G LAN group management TS 23.501CR1052
- Introduction of Slice-Specific Authentication and Authorisation TS 23.501CR1174
- Adding UDR NF Group ID association functionality TS 23.501CR1384
- Update NRF descriptions to support AF Available Data Registration as described in TS23.288 TS 23.501CR1406
- Assistance indication for WUS grouping TS 23.501CR2053
+ 135 more changes
In Release 17, the NSSRG function was enhanced to support the Simultaneous Use of a Network Slice attribute as defined by GSMA NG.116, allowing a UE to be simultaneously registered to the same network slice over different accesses. This release also introduced support for different slices over different Non-3GPP access within the group and enabled configuration of Network Slice AS Groups for radio resource management purposes.
- Support of different slices over different Non 3GPP access TS 23.501CR2525
- Network Slice restriction based on NWDAF analytics TS 23.501CR2567
- TS23.501 KI#1 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2679
- TS23.501 KI#2 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2680
- Support for UE-Slice-MBR TS 23.501CR2706
- Support of 5GC assisted cell selection to access network slice TS 23.501CR2719
+ 156 more changes
In Release 18, the NSSRG function was enhanced to support **graceful/gradual termination of PDU sessions during network slice decommissioning** and **network slice replacement**. It also introduced optimizations for **time validity policies for a network slice** and improved network control of UE behavior for a network slice.
- Update for UPF registration and event exposure related context concluded in FS_UPEAS TS 23.501CR3723
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Service area provisioning and LADN aspects for enhanced group management TS 23.501CR3914
- Improved network control of the UE beahviour for a network slice TS 23.501CR3939
- KI#3, NEF exposure for handling PDU Session Type change and managing temporal invalidity/validity condition for a group of UEs TS 23.501CR3964
- Group MBR TS 23.501CR3982
+ 238 more changes
In Release 19, enhancements for the NSSRG function included the introduction of support for slice change based on an AF request, allowing the Application Function to modify the set of network slices for a UE. This release also provided further clarifications and corrections on procedures, specifically for AF-triggered slice replacement and its interaction with a Partially Allowed NSSAI, as well as the handling of Configured NSSAI during such slice modification procedures.
- Exposure enhancements for static UE IP address assignment and 5G VN group's User Plane Security Policy TS 23.501CR5492
- Support of Slice change based on AF request TS 23.501CR5764
- Abnormal case in Registration procedure for handling Paging subgroup ID for LP-WUSPS TS 24.501CR6693
- Low Power Wake UP Signal with Paging Subgrouping - registration TS 24.501CR6737
- Allow configurable 5G registration retries for some lower layer failures TS 24.501CR6677
- Updates to registration procedure TS 24.501CR6950
+ 53 more changes
In Release 20, the NSSRG function was updated with the removal of editor's notes specifically related to VFL server registration, providing a cleaner specification text. This change refined the documentation without altering the core technical procedures or capabilities of the Network Slice Simultaneous Registration Group. The update focused on editorial clarity within the existing framework.
- Removal of editor's notes related to VFL server registration TS 23.501CR6488
Explore further
Broader topics and technologies where NSSRG plays a role.
Defining Specifications
3GPP specifications that define or reference NSSRG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |