Subscriber

Description

In 3GPP terminology, a Subscriber is a fundamental business and technical concept representing an entity—typically a person or organization—that has entered into a Subscription agreement with a service provider. This entity may be associated with one or more users who actually consume the services. The subscriber holds the contractual relationship, is responsible for payment, and has the authority to subscribe or unsubscribe services, register users authorized to use those services, and define usage limits for those users. This separation of subscriber (payer/manager) from user (consumer) is crucial for flexible service models in mobile networks.

The subscriber concept is central to the architecture of the IP Multimedia Subsystem (IMS) and other 3GPP service enablers. In IMS, the subscriber is identified by an IP Multimedia Private Identity (IMPI) and may have one or more IP Multimedia Public Identities (IMPUs). The Home Subscriber Server (HSS) stores subscription-related information, including service profiles, authentication vectors, and authorization data. When a user attempts to access a service, the network authenticates the subscriber and checks whether the requested service is allowed based on the subscription profile. This ensures that only authorized entities can use services and that billing can be correctly applied.

The subscriber also plays a key role in policy and charging control (PCC). The Policy and Charging Rules Function (PCRF) or, in 5G, the Policy Control Function (PCF) retrieves subscriber-specific policies from the Subscription Profile Repository (SPR) or Unified Data Repository (UDR). These policies determine quality of service (QoS) levels, charging rules, and bandwidth limitations. For example, a subscriber may have a premium subscription that allows higher data rates or access to specific services like VoLTE. The network enforces these policies dynamically based on the subscriber's profile and current network conditions.

In the context of supplementary services, such as Explicit Call Transfer (ECT), the subscriber is the entity that subscribes to the service and can invoke it. The term "Subscriber A" is used in specifications like 22.072 and 22.091 to denote the served mobile subscriber who invokes the ECT service. This highlights the subscriber's active role in managing and using services, beyond just being a passive account holder.

From a management perspective, subscriber data is maintained in various network elements: the HSS in IMS and EPS, the UDR in 5G, and the Equipment Identity Register (EIR) for device-related information. Operations such as subscription management, authentication, and authorization rely on accurate subscriber data. The 3GPP specifications (e.g., 21.905, 22.101, 23.228) define the subscriber concept and its interactions with network functions, ensuring interoperability across vendors and operators. As networks evolve towards 5G and beyond, the subscriber concept remains foundational, adapting to new service models like network slicing, where a subscriber may have slice-specific subscriptions.

Purpose & Motivation

The Subscriber concept exists to clearly separate the entity that holds the contractual relationship with the service provider from the users who actually consume the services. This separation is essential for flexible business models, allowing a single subscriber (e.g., a corporation) to manage multiple users (employees) and control their access and usage. It also enables accurate billing and policy enforcement, as the network can apply rules based on the subscription rather than individual users.

Historically, early mobile networks (2G) had a simpler model where the subscriber and user were often the same person, identified by the SIM card. However, as services became more diverse and complex, especially with the introduction of IMS in 3GPP Release 5, the need for a more sophisticated subscriber model emerged. The subscriber entity allows for multiple users under one subscription, each with potentially different service permissions and limits. This is particularly important for enterprise customers, machine-to-machine (M2M) communications, and multi-device scenarios.

The subscriber concept also solves the problem of authentication and authorization in a multi-service environment. By centralizing subscription data in the HSS/UDR, the network can authenticate the subscriber once and then authorize access to various services based on the subscription profile. This avoids the need for separate credentials for each service and simplifies the user experience. Furthermore, it enables operators to offer personalized services and charging plans, driving revenue and customer loyalty.