Served Subscriber (UUS Supplementary Service)

Description

The Served Subscriber is a concept defined in 3GPP specifications for the User-to-User Signalling (UUS) supplementary service, which allows a mobile subscriber to send and receive short user-to-user information to and from another subscriber over the signalling channel. The served subscriber is the subscriber who has a provision of the UUS supplementary service and who activates it. This concept is essential for determining which party controls the service and how the service is invoked and managed within the network.

UUS is defined in 3GPP TS 22.087 (Stage 1), TS 23.087 (Stage 2), and TS 24.087 (Stage 3). The service is divided into three distinct services: Service 1, Service 2, and Service 3. Service 1 allows the served subscriber to send user-to-user information to the remote party during call establishment and clearing. Service 2 allows the served subscriber to send user-to-user information to the remote party after call establishment. Service 3 allows the served subscriber to send user-to-user information to the remote party during call establishment and clearing, and also to receive user-to-user information from the remote party. For Service 1 and Service 2, the served subscriber is always the calling subscriber. For Service 3, either the calling or the called subscriber can be the served subscriber, depending on who activates the service.

The served subscriber concept is crucial for the operation of UUS because it determines the direction of the user-to-user information flow and the responsibilities of the network elements. When a subscriber activates UUS, they become the served subscriber for that particular call or session. The network uses this information to route the user-to-user signalling messages appropriately and to apply the correct charging and security policies. The served subscriber may also be subject to specific service restrictions or permissions based on their subscription profile.

In terms of architecture, the served subscriber interacts with the UUS service through the mobile station (MS) or user equipment (UE). The network elements involved include the Mobile Switching Center (MSC), Visitor Location Register (VLR), Home Location Register (HLR), and the interworking functions with other networks. The UUS service is typically implemented using the User-to-User Information (UUI) element within the signalling messages, such as the ISDN User Part (ISUP) or the Session Initiation Protocol (SIP) messages in IMS. The served subscriber's identity and service profile are stored in the HLR and are used by the MSC/VLR to authorize and manage the service.

The concept of the served subscriber has evolved over releases, but the definition has remained largely unchanged from Rel-4 through Rel-19. This stability reflects the maturity of the UUS supplementary service and its continued relevance in providing supplementary communication capabilities. The served subscriber concept ensures that the service is provided consistently and that the rights and obligations of the parties involved are clearly defined.

Purpose & Motivation

The Served Subscriber concept exists to clearly identify the party that has control over the User-to-User Signalling (UUS) supplementary service in a call. UUS allows subscribers to exchange limited user information over the signalling channel, which is useful for applications such as call centre context sharing, automated call distribution, and supplementary service control. However, to manage this service effectively, the network must know which subscriber is authorized to activate and use the service, and how the user-to-user information should be handled.

Before the introduction of the served subscriber concept, there was ambiguity about which party could invoke UUS and how the service should be charged and managed. The served subscriber definition resolves this by specifying that for UUS Services 1 and 2, the calling subscriber is always the served subscriber, while for Service 3, either the calling or the called subscriber can be the served subscriber. This clarity is essential for service provisioning, call handling, and interworking with other networks.

The concept also supports the flexible deployment of UUS across different network architectures, including GSM, UMTS, and LTE. By defining the served subscriber, 3GPP ensures that UUS can be offered consistently and that the service can be controlled and billed correctly. This is particularly important in roaming scenarios, where the served subscriber may be in a visited network but still needs to access their subscribed services.