Description
The Fixed Network Cable Residential Gateway Globally Unique AMF Identifier (FN-CRG GUAMI) is a critical identifier defined in 3GPP Release 16 within the context of 5G fixed access over cable networks. It is a specific application of the broader GUAMI concept (defined in TS 23.003) for the Cable Residential Gateway (FN-CRG), which is the customer premises equipment for cable-based fixed broadband (e.g., DOCSIS) integrated into the 5G System. The FN-CRG GUAMI uniquely and globally identifies the specific Access and Mobility Management Function (AMF) instance that is responsible for the control plane mobility and connection management of a given FN-CRG device.
Architecturally, when an FN-CRG device (a type of Fixed Network Residential Gateway for cable) initializes and registers with the 5G Core Network, it needs to be assigned to an AMF. The network selection function (e.g., Network Slice Selection Function - NSSF) helps select an appropriate AMF based on subscription, network slice, and local policies. Once selected, that AMF's identity—its GUAMI—is associated with the FN-CRG. The FN-CRG GUAMI is structured according to the GUAMI format: it includes a PLMN Identifier (Mobile Country Code and Mobile Network Code) to ensure global uniqueness across operators, followed by an AMF Region ID, AMF Set ID, and AMF Pointer, which together identify the specific AMF instance within the operator's network.
How it works: The FN-CRG GUAMI is used in control plane signaling messages between network functions. For instance, when a Session Management Function (SMF) needs to communicate with the AMF managing a particular FN-CRG for PDU session establishment, it uses the FN-CRG GUAMI to route the N11 message correctly. It is also crucial during mobility events; if an FN-CRG's context needs to be relocated from one AMF to another (e.g., for load balancing or failure recovery), the GUAMI identifies the source and target AMFs. The identifier is stored in the Unified Data Management (UDM) as part of the subscriber's context, ensuring that subsequent registration updates or service requests can find the correct serving AMF. This precise routing is essential for the scalability and reliability of 5G core networks supporting millions of fixed devices.
Purpose & Motivation
The FN-CRG GUAMI was introduced to solve the addressing and routing challenges inherent in scaling the 5G core to support massive numbers of fixed residential gateways (like cable modems). In earlier generations and non-3GPP fixed networks, management of CPE was often done through proprietary or domain-specific protocols (like TR-069) with less granular, non-globally-unique identifiers. As cable access was integrated into the 5G core via the FN-CRG (starting Rel-16), it became necessary to manage these devices with the same principles of scalability, redundancy, and cloud-native statelessness applied to mobile user equipment.
The primary problem addressed is the need for unambiguous identification and efficient routing to the correct control plane entity (AMF) in a decomposed, service-based architecture where AMFs can be dynamically instantiated and scaled. Without a globally unique identifier, routing failures could occur in multi-vendor deployments or when operators merge networks. The GUAMI framework, applied specifically to the FN-CRG, ensures that any network function (SMF, UDM, another AMF) can precisely determine and communicate with the AMF instance serving a specific cable gateway, anywhere in the world. This enables advanced 5G features for fixed access: efficient mobility management (even for 'fixed' devices, there can be logical mobility or re-assignment), seamless AMF relocation for load balancing, and reliable service continuity during network function failures. It is a foundational element for treating fixed access as a true peer to mobile access within the 5G core.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (117 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the FN-CRG function was newly introduced, enabling a cable residential gateway to use a 5G Subscription Permanent Identifier (SUPI) formatted as a Global Cable Identifier (GCI) or Global Line Identifier (GLI), which takes the form of a Network Access Identifier (NAI). This allowed the FN-CRG to be uniquely identified within the 5G System for procedures like initial registration, supporting mobility and interworking with non-3GPP access. The release also defined the inclusion of the Globally Unique AMF Identifier (GUAMI) in related AN parameters and RRC messages for these access types.
- Including GUAMI as AN-parameters during registration for non-3GPP access TS 24.502CR0004
- GUTI unique across AMFs in an AMF SET TS 23.501CR0089
- Use of identifiers for mobility between GERAN/UTRAN and 5GS TS 23.501CR0017
- Partitioning of Identifier space to ensure success of Context retrieval for EPS Interworking TS 23.501CR0090
- Subscription Permanent Identifier TS 23.501CR0189
- Changed length and mapping of 5GS Temporary Identifiers TS 23.501CR0206
+ 23 more changes
In Release 16, the FN-CRG function was enhanced with the introduction of the Global Line Identifier (GLI) as a new type of Subscription Permanent Identifier (SUPI) for subscriber identification. This allowed for service area restrictions to be specifically applied to FN-CRG devices, and required alignment for the stop of enforcement of mobility restrictions in network elements acting on its behalf. Furthermore, the UDM was specified to derive the actual SUPI from a de-concealed SUCI when the SUPI type was a GLI.
- PEI for 5G-CRG and FN-CRG TS 24.501CR0937
- Line Identifier TS 29.507CR0084
- CableLabs, Charter Communications TS 23.316CR0032
- Line ID uniqueness TS 23.316CR0038
- Service Area Restrictions applicability for FN-CRG, and not FN-BRG TS 23.316CR0056
- Cable access related corrections TS 23.316CR1835
+ 25 more changes
In Release 17, the FN-CRG function was updated to support a new type of Subscription Permanent Identifier (SUPI) based on a Global Cable Identifier (GCI). This GCI-based SUPI, formatted as a Network Access Identifier (NAI), provides a globally unique identifier for subscribers on fixed cable networks. This addition enables the 5G system to natively identify and authenticate cable residential gateway users within the converged architecture.
- Group Identifier for Network Selection TS 23.003CR0636
- Corrections on the AF related identifier TS 23.501CR3064
- Avoid including both PAP/CHAP and EAP identifiers in PDU session establishment request TS 24.501CR2941
- Clarification on the setting of packet filter identifier value TS 24.501CR3300
- Network identifier is not specified TS 24.501CR3389
- Access Technology Identifier satellite NG-RAN TS 24.501CR3636
+ 5 more changes
In Release 18, the FN-CRG function was enhanced through the definition of a new Globally Unique AMF Identifier (GUAMI) type for inclusion in the AN-parameter. This addition, alongside the specific handling of identifiers like the Global Cable Identifier (GCI) and Global Line Identifier (GLI) as SUPI types, supports the unique identification and routing for fixed network residential gateways within the 5G system architecture.
- SNPN Identifier based N3IWF FQDN TS 23.003CR0687
- PIN identifiers TS 23.501CR4287
- Protecting the N3IWF/TNGF identifier information in the REGISTRATION REJECT message TS 24.501CR5932
- Resolving the EN related to N3IWF selection based on N3IWF identifier information in the REGISTRATION REJECT message TS 24.502CR0230
- Prefixed OI/TAI Identifier FQDN for N3IWF selection TS 24.502CR0223
- N3IWF selection for non-IMS services supporting extended home N3IWF identifier configuration and slice-specific N3IWF prefix configuration TS 24.502CR0248
+ 15 more changes
In Release 19, the FN-CRG function was enhanced to support the provisioning and handling of Non-3GPP Device Identifiers for devices connecting behind the gateway, including their storage in the UDR. This introduced new procedures for QoS differentiation, such as support for rejecting, suspending, and updating QoS rules based on these identifiers, along with clarifications for session management signaling and roaming support.
- Non-3GPP Device Identifier TS 23.003CR0708
- Definition of AIoT Device Permanent Identifier TS 23.003CR0713
- 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 reject QoS differentiation for non-3GPP device identifier(s) TS 24.501CR6926
- Procedure update for QoS differentiation of non-3GPP device identifiers TS 24.501CR6994
+ 19 more changes
Explore further
Broader topics and technologies where FN-CRG plays a role.
Defining Specifications
3GPP specifications that define or reference FN-CRG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.003 vj50 | Numbering, addressing and identification in 3GPP | Rel-19 |
| TS 23.316 vj30 | Wireline and Wireless Convergence Access Support | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.716 vg00 | Wireline and Trusted Non-3GPP Access to 5G Core | Rel-16 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.502 vj20 | 5G Core Access via Non-3GPP Networks; Stage 3 | Rel-19 |
| TS 29.507 vj40 | 5G Access & Mobility Policy Control Service | Rel-19 |
| TS 29.525 vj40 | 5G UE Policy Control Service Stage 3 | Rel-19 |
| TS 29.561 vj30 | 5G Interworking with External Data Networks | Rel-19 |