Fixed Part

Description

The Fixed Part (FP) is a term used in 3GPP specifications, particularly those dealing with the integration of other radio technologies like DECT (Digital Enhanced Cordless Telecommunications). It denotes the stationary infrastructure of a wireless network system. Architecturally, the FP is contrasted with the Portable Part (PP), which is the user's mobile terminal. The FP typically consists of Radio Fixed Parts (RFPs), which are the base stations or access points, and a central control unit or network controller that manages multiple RFPs.

How it works involves the FP providing the radio coverage and network attachment point for PPs. The FP handles critical functions such as radio resource management, call control, mobility management within its coverage area (e.g., handover between its own RFPs), and interconnection to a core network (like the PSTN or a 3GPP core via interworking functions). It manages the air interface protocols, authenticates PPs, and encrypts communications. In a 3GPP interworking scenario, the FP acts as an access network, with its controller translating between DECT-specific signaling and 3GPP core network protocols.

Its role was to define a clear functional and architectural boundary in specifications for cordless and wireless local loop systems. By specifying the FP's responsibilities separately from the PP, standards could ensure interoperability between infrastructure from one vendor and handsets from another. In the broader 3GPP ecosystem, specifications detailing FP requirements ensured that when such access technologies were used, they could be integrated in a standardized manner to provide services through the 3GPP core network.

Purpose & Motivation

The term Fixed Part originated in cordless telecommunication standards like DECT to clearly separate the network infrastructure from the user equipment. Its adoption into 3GPP specifications served the purpose of facilitating the standardization of interworking between 3GPP mobile networks and existing, widely deployed cordless systems. This allowed operators to leverage DECT for indoor coverage or fixed wireless access while integrating subscriber management and services with their GSM/UMTS core.

It addressed the need for a standardized architectural model when incorporating non-3GPP radio access technologies. Before such definitions, integration was proprietary and limited. Defining the FP and its interfaces (e.g., the 'A' interface between FP components, the air interface to the PP, and the interworking interface to the core) created a blueprint for consistent implementation. This supported scenarios like using DECT as a radio in the local loop (RLL) to provide telephone service, where the FP connects to the traditional telephone network, or later, to a 3GPP IMS core for VoIP services.