Follow Me function node

Description

The Follow Me function node (FFN) is a service control entity specified in 3GPP TS 23.094. It is a core component of the Follow Me (FM) supplementary service, which provides personal mobility by allowing a subscriber to be reached via a single, persistent number regardless of their physical location or the terminal they are using. Architecturally, the FFN resides within the network's service layer, typically interfacing with the Home Location Register (HLR) for subscriber data and the Mobile Switching Center (MSC) or Gateway MSC for call control. Its primary role is to execute the service logic for incoming calls destined for the FM subscriber.

When a call is placed to a subscriber's Follow Me number, the call is initially routed to a designated network node, often a specific MSC. This node then queries the FFN for routing instructions. The FFN consults its internal database or linked subscriber profile, which contains the user's current active contact number or address. This profile is dynamically updated by the subscriber, for example, via USSD codes or a web interface, to reflect their current preferred destination (e.g., their office phone, mobile phone, or voicemail). Based on this information, the FFN instructs the MSC to forward the call to the specified contact address.

The FFN's operation is integral to realizing seamless personal mobility. It handles the translation between the logical, user-centric Follow Me number and the physical, network-routable address. This decouples a user's identity from a specific device or line. The node also manages service triggers, interacts with charging systems for service-specific billing, and ensures that the forwarding logic complies with the subscriber's predefined rules, such as time-of-day routing or caller-based filtering. While conceptually a standalone function, its capabilities were later subsumed and enhanced by broader IP Multimedia Subsystem (IMS) service architectures and Voice Call Continuity (VCC) features, which offer more flexible and IP-based personal mobility solutions.

Purpose & Motivation

The FFN was created to address the growing need for personal mobility in early 3GPP networks, specifically within the circuit-switched domain. Prior to such services, a user's telephone number was intrinsically tied to a specific physical line or SIM card, making it difficult to maintain a consistent point of contact while moving between locations or devices. The Follow Me service, enabled by the FFN, solved this by introducing a layer of indirection, allowing users to define a single, persistent identity.

The motivation stemmed from business and professional needs where individuals required a constant, reachable number. The FFN provided the network-based intelligence to make this possible without requiring changes to the calling party's behavior or the underlying core network routing protocols for basic calls. It represented an early step toward decoupling user identity from network topology, a concept that became fundamental to later all-IP communication services. While its implementation was specific to a particular supplementary service, it laid conceptual groundwork for more advanced unified communication and number portability features.

Key Features

• Manages the service logic for the Follow Me supplementary service
• Interfaces with HLR for subscriber data and MSC for call control
• Dynamically translates a persistent Follow Me number to a current routable contact address
• Supports subscriber-controlled profile updates via network signaling (e.g., USSD)
• Enables call forwarding based on user-defined rules and preferences
• Integrates with charging systems for service-specific billing

Evolution Across Releases

Defining Specifications