Serving Network

Description

The Serving Network is a fundamental concept in 3GPP architecture that defines the relationship between a user's home network and the network currently providing service. When a mobile subscriber is roaming, the Serving Network is the visited network that provides the user with access to the services of their home environment. This concept is central to the roaming architecture defined in 3GPP specifications, enabling subscribers to use their mobile services outside their home network's coverage area.

In the 3GPP architecture, the Serving Network encompasses the radio access network and core network elements that are currently serving the user equipment (UE). For a roaming user, this includes the visited Radio Access Network (RAN) and the visited core network components such as the Serving GPRS Support Node (SGSN) in 2G/3G or the Mobility Management Entity (MME) and Serving Gateway (SGW) in LTE, or the Access and Mobility Management Function (AMF) and Session Management Function (SMF) in 5G. The Serving Network provides the user with access to services by establishing connectivity to the Home Network, which authenticates the user and authorizes services.

The relationship between the Serving Network and the Home Network is governed by roaming agreements and standardized interfaces. The Serving Network is responsible for functions such as radio resource management, mobility management, and session management for the roaming user. It also enforces policies provided by the Home Network, such as quality of service (QoS) parameters and charging rules. The Home Network retains control over the subscriber's profile, authentication credentials, and service subscriptions.

Key components of the Serving Network include the radio access network (e.g., UTRAN, E-UTRAN, or NG-RAN), the core network elements that handle mobility and session management, and the policy and charging enforcement functions. The Serving Network interacts with the Home Network through interfaces such as the S6a/S6d for authentication and subscription data retrieval, and the S8/S9 interfaces for policy and charging control in evolved packet core networks.

The concept of the Serving Network has evolved with each generation of mobile technology. In 2G and 3G, the Serving Network was primarily defined by the Visited Public Land Mobile Network (VPLMN) and its SGSN and GGSN. In LTE, the concept expanded to include the MME, SGW, and Policy and Charging Rules Function (PCRF). In 5G, the Serving Network includes the AMF, SMF, and User Plane Function (UPF), with additional network slicing capabilities that allow the Serving Network to provide differentiated services based on slice selection.

The Serving Network concept is essential for enabling global roaming, which is a key feature of mobile communications. It allows operators to offer services to their subscribers even when they are outside the home network's coverage, while ensuring that the home operator maintains control over the subscriber's services and billing. This architecture also supports network sharing scenarios where multiple operators share the same radio access network, with each operator's core network acting as a Serving Network for its own subscribers.

Purpose & Motivation

The Serving Network concept exists to enable mobile subscribers to access services outside their home network's coverage area, a fundamental requirement for global mobility. Before this concept was formalized, mobile networks were isolated and subscribers could only use services within their home network. The introduction of roaming architecture in early 3GPP releases (and its predecessors in GSM) allowed operators to establish roaming agreements, enabling subscribers to use visited networks while maintaining their home subscription.

The Serving Network solves the problem of service continuity across network boundaries. It provides a standardized way for visited networks to authenticate and authorize roaming users, enforce home network policies, and route traffic back to the home network for services that require home network involvement. This architecture also allows the home network to maintain control over the subscriber's experience, including QoS, charging, and access to value-added services.

Historically, the Serving Network concept evolved from the GSM roaming model, where the Visited Location Register (VLR) and Mobile Switching Center (MSC) in the visited network served the roaming subscriber. In 3GPP releases, this concept was extended to packet-switched domains with the SGSN and GGSN, and later to LTE and 5G with more sophisticated core network functions. The introduction of network slicing in 5G further enhances the Serving Network concept by allowing the visited network to provide differentiated services based on slice selection, ensuring that roaming users receive the appropriate service characteristics.