Fixed Network

Description

The Fixed Network (FN) within the 3GPP context is the architectural and functional domain representing traditional wireline telecommunications infrastructure. It is not a single network but a conceptual grouping that includes various fixed access technologies (such as DSL, fiber, and cable), transport networks, and core network elements like fixed-line switches and broadband network gateways (BNGs). The FN provides the foundation for services like residential broadband internet, voice over IP (VoIP), and IP television (IPTV). Its inclusion in 3GPP specifications, starting from Release 99, acknowledges the need for standards that facilitate interworking and convergence between mobile networks (like UMTS and later LTE/5G) and these established fixed networks.

Architecturally, the FN interfaces with the mobile core network through defined reference points. Specifications like TS 25.423 and TS 25.931 detail the Iu-Flex and Iur-F interfaces for connecting Radio Network Controllers (RNCs) in UMTS to core network nodes, considering scenarios where these nodes might be served by fixed transport networks. The FN provides the reliable, high-bandwidth backhaul and transport necessary for mobile traffic, especially as data demands increased. It acts as the 'pipe' connecting radio access network sites (Node Bs, eNodeBs, gNBs) to the core network elements like the MSC, SGSN, and later MME/AMF.

Its role evolved from being a separate transport medium to an integral part of the overall service delivery architecture. In management contexts (specified in TS 32.833), the FN is considered a managed domain with its own performance and fault management requirements, separate from but coordinated with the mobile network management. The FN's importance lies in its capacity and ubiquity; it often forms the economic backbone for carrying mobile traffic, especially in dense urban areas and for network aggregation points. The concept underpins fixed-mobile convergence (FMC), where services are delivered seamlessly across both network types.

Purpose & Motivation

The FN concept was introduced into 3GPP to formally recognize and standardize the interaction between emerging 3G mobile networks and the pervasive existing fixed telecommunications infrastructure. Prior to 3GPP's work, mobile and fixed networks often evolved independently with proprietary or ad-hoc interworking solutions. The primary problem addressed was the need for efficient, standardized transport and interconnection. Mobile networks, particularly as they transitioned to packet-switched architectures with UMTS, required high-capacity, reliable backhaul links to connect base stations to core network controllers and gateways—a role naturally fulfilled by fixed networks (e.g., leased lines, fiber, DSL).

Another key motivation was enabling fixed-mobile convergence at a network level. By defining the FN as a domain within the 3GPP system architecture, it became possible to specify common management procedures (fault, configuration, accounting, performance, security - FCAPS), quality of service (QoS) mapping across domains, and seamless service continuity. This was crucial for operators who owned both fixed and mobile assets, allowing them to optimize infrastructure and offer bundled services. The inclusion in Release 99 set the stage for later innovations like IP Multimedia Subsystem (IMS), which is access-agnostic and relies on both fixed and mobile IP connectivity for delivering multimedia services.

Key Features

• Provides standardized transport and backhaul for 3G/4G/5G RAN nodes to core network
• Enables fixed-mobile convergence (FMC) at architectural and management levels
• Defines interfaces (e.g., Iu, Iur over fixed transport) for interconnection with mobile core
• Includes management domain definitions for integrated OAM of combined networks
• Supports QoS mapping and policy enforcement across fixed and mobile domains
• Serves as the foundation for broadband residential access in converged scenarios

Evolution Across Releases

Defining Specifications