Description
The Serving Network Identifier (SN) is a fundamental parameter within 3GPP architectures, serving as a unique label for the network currently providing service to a User Equipment (UE). It is constructed from the Mobile Country Code (MCC) and Mobile Network Code (MNC) of the serving Public Land Mobile Network (PLMN). This identifier is not just a static label; it is dynamically used by the network and the UE during critical procedures. For instance, in authentication and key agreement (AKA) procedures, the SN is an input for generating security keys, ensuring that security contexts are bound to a specific network, which prevents key reuse across different networks and enhances security.
Architecturally, the SN is utilized across multiple network domains. In the core network, it is used by the Mobility Management Entity (MME) in 4G or the Access and Mobility Management Function (AMF) in 5G to identify the network context for a UE. It is also signaled between network nodes, such as between the MME/AMF and the Home Subscriber Server (HSS) or Unified Data Management (UDM), during procedures like location update or authentication. In the radio access network, while not directly used in air interface protocols, its derived information influences the selection of network slices and tracking areas.
The role of the SN extends to mobility management, particularly during handovers between different PLMNs. It helps in identifying the target network and ensuring that the correct network policies and security parameters are applied. Furthermore, in network sharing scenarios, where multiple operators share radio access network infrastructure, the SN helps distinguish the core network of each operator, ensuring subscribers are correctly associated with their home operator's services and billing systems. Its consistent use from 3G through to 5G and beyond underscores its importance as a stable, foundational element of 3GPP system identity management.
Purpose & Motivation
The Serving Network Identifier was created to address the fundamental need for unambiguous network identification in mobile telecommunications. As networks evolved from single-operator deployments to complex, multi-operator environments with roaming and network sharing, it became critical to precisely identify which network was serving a subscriber at any given time. This identification is necessary for routing signaling messages, applying correct subscriber profiles, and ensuring lawful interception and billing accuracy.
Historically, without a standardized, unique serving network identifier, systems faced challenges in handling inter-PLMN mobility and security. The SN solves these by providing a consistent, standardized way to reference the serving network across all 3GPP interfaces and protocols. It enables the security architecture, particularly the Authentication and Key Agreement (AKA) protocol, to generate network-specific keys, mitigating risks like replay attacks across networks. Its introduction formalized a key piece of data that was implicitly needed but not always explicitly standardized in early mobile systems, thereby improving interoperability and security in multi-vendor, multi-operator deployments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (218 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, new procedures and clarifications were introduced for the Serving Network (SN) function to enhance identifier handling and mobility. This included support for subscription identifier privacy, refined usage of temporary and permanent identifiers during inter-PLMN mobility and EPS interworking, and specific handling for network sharing and equivalent PLMNs. The release also clarified the use of identifiers received during PDN connection establishment and corrected identifier usage in registration and resume procedures.
- Subscription identifier privacy suppport TS 31.102CR0778
- Clarification to Subscription identifier privacy TS 33.501CR0145
- Use of identifiers for mobility between GERAN/UTRAN and 5GS TS 23.501CR0017
- Network sharing prioritised PLMN handling TS 23.501CR0056
- Inter-PLMN mobility when N26 is not used TS 23.501CR0070
- Non-Allowed Area as criterion for Cell Reselection or trigger for PLMN Selection TS 23.501CR0076
+ 64 more changes
In Release 16, key enhancements for the Serving Network (SN) function included the formal definition of an SN Identifier for standalone non-public networks (SNPNs) and expanded support for network interworking. This enabled new access control and service scenarios, such as allowing a UE to access PLMN services via an SNPN and vice-versa. The release also clarified and aligned procedures for PLMN selection, capability signaling, and rate control, specifically emphasizing the role of the serving PLMN.
- Support for USIM configuration of RLOS PLMN list TS 31.102CR0847
- Security requirements for Inter-PLMN User Plane Security (IPUPS) Function TS 33.501CR0754
- SUCI computation: implementers' test data for network specific identifier-based SUPI TS 33.501CR0847
- Temporary Identifier usage at interworking TS 36.413CR1643
- Introduction of Conditional PSCell Change for intra-SN without MN involvement TS 37.340CR0210
- Support of SN Resume during the RRCResume procedure TS 38.401CR0112
+ 50 more changes
In Release 17, the SN function was enhanced to support secure mobility and service continuity between Standalone Non-Public Networks (SNPNs) and Public Land Mobile Networks (PLMNs), including for emergency calls when no acceptable PLMN cell is available. It also introduced capabilities for simultaneous data service from a PLMN and a Private Network Integrated NPN (PNI-NPN), and for the reporting of Tracking Area Identities (TAIs) corresponding to the Selected PLMN. Furthermore, the release included corrections for switching 3GPP access between an SNPN and a PLMN.
- Informative guideline on supporting session/service continuity between SNPN and PLMN when using N3IWF TS 23.501CR2563
- User Plane Remote Provisioning of UEs if PLMN as ON TS 23.501CR2802
- Mobility support between SNPNs and between SNPN and PLMN TS 23.501CR2826
- Add attribute to configure an identifier of a TraceJob TS 28.622CR0134
- Introducing multiplier coefficient to adjust high priority PLMN search timer TS 31.102CR0948
- Satellite E-UTRAN in PLMN selector TS 31.102CR0956
+ 28 more changes
In Release 18, specific enhancements were made to clarify and uniquely identify the Serving Network (SN) function, particularly for non-3GPP access and inter-network signaling. This included resolutions on the temporary identifier used during trusted non-3GPP access and ensuring the PIN ID is unique within a PLMN. Furthermore, alignment was provided for the use of PLMN IDs in network function discovery and service-based interface headers, such as for the `3gpp-Sbi-Originating-Network-Id`.
- Edge Relocation within the same hosting PLMN's EHEs TS 23.501CR3820
- KI#4: Support for Centralized NSACF in a PLMN with multi-service areas TS 23.501CR3822
- PIN identifiers TS 23.501CR4287
- Rel-18 CR TS 28.622 Enhance the ManagementDataCollection to support request management data per PLMN TS 28.622CR0308
- Resolution of editor notes related to the temporary identifier used during trusted non-3GPP access. TS 33.501CR1766
- NWDAF discovery with overlapping Serving Areas TS 23.501CR4037
+ 22 more changes
In Release 19, enhancements to the Serving Network (SN) function focused on improving support for Non-3GPP Device Identifiers, including their definition, handling in session management for QoS differentiation, and storage in the UDR. The release also introduced clarifications and procedures for NF discovery and selection by a target PLMN, including token-based authorization for such indirect communication scenarios. Furthermore, it provided new mechanisms like the Operator-controlled-LSP-PLMN and PeriodicSearchTimerNonLSP to govern network selection behavior.
- NF discovery and selection by target PLMN TS 23.501CR5399
- UDR enhancement supporting Device Identifier of non-3GPP Devices connecting behind a UE/5G-RG TS 23.501CR5547
- Definition of identifiers of N3GPP device behind UE/5G-RG TS 23.501CR5749
- Support of UE-satellite-UE communications when serving satellite changes TS 23.501CR5518
- Introducing Operator-controlled-LSP-PLMN and PeriodicSearchTimerNonLSP TS 31.102CR1081
- Token-based authorization for indirect communication scenarios when NF is selected at target PLMN TS 33.501CR2135
+ 23 more changes
In Release 20, the primary update to the Serving Network (SN) identifier function involved a security correction for roaming scenarios, specifically addressing the format of PLMN ID claims within access tokens. This change replaced appended PLMN ID claims with PLMN ID-specific claims to resolve a misalignment with the technical specifications, thereby enhancing the security framework between the home environment and the serving network. The modification aimed to bolster the secure infrastructure between operators, minimizing the need for the home environment to trust the serving network for security functionality.
- Correction of misalignment with TS 29.510: Replace appended PLMN ID access token claims with PLMN ID specific claims in roaming TS 33.501CR2214
Explore further
Broader topics and technologies where SN plays a role.
Defining Specifications
3GPP specifications that define or reference SN, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 21.133 v1400 | 3G Security Requirements | Rel-5 |
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 22.105 vj00 | Telecommunication Services Framework | Rel-19 |
| TS 22.811 v1700 | Network Selection Mechanisms Overview | Rel-7 |
| TR 22.975 v1310 | UMTS Numbering and Addressing Requirements | Rel-4 |
| TS 23.048 v1400 | Secured Packets for UICC Remote Management | Rel-5 |
| TS 23.060 vj00 | GPRS Service Description Stage 2 | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.173 vj00 | Multimedia Telephony Service and Supplementary Services in IMS | Rel-19 |
| TS 24.404 v1700 | Communication Diversion Services (CDIV) | Rel-7 |
| TS 24.504 v8m0 | Communication Diversion Services Stage 3 | Rel-8 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 25.322 vj00 | RLC Protocol Specification | Rel-19 |
| TS 26.110 vj00 | 3G-324M Multimedia Codecs for Circuit Switched Networks | Rel-19 |
| TS 28.622 vk20 | Telecommunication Management; Generic NRM Information Service | Rel-20 |
| TS 29.205 vj00 | BICC Protocols for Bearer-Independent CS Core Network | Rel-19 |
| TS 31.102 vj40 | USIM Application Specification | Rel-19 |
| TS 31.115 vj00 | Secured Packet Structure for UICC Applications | Rel-19 |
| TS 32.422 vk00 | Telecom Management: Trace Control & Configuration | Rel-20 |
| TS 32.425 vj00 | E-UTRAN Performance Measurements | Rel-19 |
| TS 32.808 v1800 | Common User Profile Storage Framework | Rel-8 |
| TS 33.102 vj10 | 3G Security Architecture Specification | Rel-19 |
| TS 33.224 vj00 | Generic Push Layer (GPL) Specification | Rel-19 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TS 33.825 vg01 | Security for 5G URLLC Services | Rel-16 |
| TS 36.322 vj00 | E-UTRA Radio Link Control Protocol Specification | Rel-19 |
| TS 36.323 vj00 | PDCP Protocol Specification | Rel-19 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.423 vj10 | X2 Application Protocol (X2AP) Specification | Rel-19 |
| TS 36.463 vj00 | XwAP Protocol Specification | Rel-19 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | Rel-19 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.322 vj00 | NR Radio Link Control (RLC) Protocol | Rel-19 |
| TS 38.323 vj00 | Packet Data Convergence Protocol (PDCP) | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.804 ve00 | Study on New Radio Access Technology; Radio Interface Protocol Aspects | Rel-14 |
| TS 43.068 vj00 | Voice Group Call Service (VGCS) Stage 2 | Rel-19 |
| TS 43.069 vj00 | Voice Broadcast Service (VBS) Stage 2 | Rel-19 |