Global Line Identifier

Description

The Global Line Identifier (GLI) is a critical identifier introduced in 3GPP Release 16 as part of the 5G System's support for Fixed Mobile Convergence (FMC) and wireline access integration. It is defined across several specifications, including TS 23.003 (Numbering, addressing and identification), TS 29.561 (Interworking between 5GS and wireline access), and TS 31.102 (UICC/USIM application). The GLI serves as a permanent, globally unique label for a specific physical fixed access line, such as a Digital Subscriber Line (DSL), a Passive Optical Network (PON) fiber line, or a cable modem connection.

Architecturally, the GLI is assigned by the fixed network operator and is associated with the network termination equipment at the customer premises (e.g., the Residential Gateway or Optical Network Terminal). In the 5G convergence scenario, the User Equipment (UE)—which could be a 5G residential gateway—uses the GLI during registration and authentication procedures over the fixed access network. The GLI is conveyed to the 5G Core Network (5GC) within the Access Network parameters during the N2 interface signaling (e.g., in the NGAP Initial UE Message or in the authentication signaling).

How it works involves several network functions. Upon receiving the GLI, the Access and Mobility Management Function (AMF) can use it to correlate the access session with subscriber data. The Authentication Server Function (AUSF) may utilize the GLI, potentially alongside other credentials, for authentication and authorization. Crucially, the GLI enables the 5GC to apply specific policies for that fixed line via the Policy Control Function (PCF). For example, it allows the network to identify that the access is coming from a trusted, fixed broadband subscription and apply appropriate QoS policies, charging rules, or service entitlements that differ from those for mobile access.

Its role extends beyond simple identification. The GLI is a key enabler for seamless service experience across access types. It allows operators to offer unified service plans where a subscriber's services, security policies, and quality of experience are consistent whether connected via a 5G radio or a home broadband line. Furthermore, it aids in network slicing for fixed access, where a slice instance can be associated with a specific set of physical lines identified by their GLIs. This deep integration of fixed line identity into the 5G core is a foundational step for truly converged networks.

Purpose & Motivation

The GLI was created to solve the problem of identity and policy disjoint in converged network architectures. Prior to its standardization, fixed and mobile networks operated largely in silos. A fixed broadband line was identified by operator-specific, non-global identifiers (like circuit IDs or PVC/VPI values), and the mobile core had no standardized way to recognize or trust a fixed access leg. This hindered the deployment of truly unified authentication, billing, and service policies across access domains.

The motivation stemmed from the industry drive towards 5G Fixed Wireless Access (FWA) and deeper Fixed Mobile Convergence. Operators wanted to leverage their 5G core to manage all types of access, including wireline, to reduce operational costs and create new bundled services. A globally unique, standardized identifier for the fixed line was a prerequisite for this vision. It addresses the limitation of proprietary or access-technology-specific identifiers that could not be reliably interpreted by a common mobile core network.

By providing the GLI, 3GPP enabled several key use cases: secure authentication of a residential gateway over a fixed network using 5G credentials (like a SIM), application of subscriber-specific QoS and bandwidth policies based on the physical line capabilities, and accurate charging for converged services. Its introduction in Release 16 was a direct response to operator requirements for a single, agile core network capable of managing heterogeneous access, a central tenet of the 5G system design.