Description
User-to-User Signalling Service 3 (UUS3) is a standardized supplementary service defined within the 3GPP Core Network domain, specifically for Circuit-Switched (CS) telephony. Its primary function is to provide a mechanism for the transparent transport of a limited amount of user-generated signaling information between two User Equipments (UEs) during the establishment, active phase, or release of a mobile-originated or mobile-terminated circuit-switched call. This transport occurs within the existing signaling messages of the call control protocol (e.g., within SETUP, ALERTING, CONNECT, or DISCONNECT messages), without requiring the setup of a separate bearer traffic channel. This makes it highly efficient for short data exchanges related to the call itself.
Architecturally, UUS3 operates within the Call Control (CC) entity of the UE and the network's Mobile Switching Centre (MSC). The service is invoked by the calling UE, which includes UUS information elements in its call setup message. The MSC, upon receiving this, checks the subscription and service compatibility before forwarding the UUS data transparently towards the called party within the relevant ISDN User Part (ISUP) or Bearer Independent Call Control (BICC) signaling. The receiving UE's CC entity extracts this information and presents it to the appropriate application. The service defines three types of operation: type 1 (during call establishment), type 2 (available during the entire call), and type 3 (network-specific).
The key components involved are the UUS information element, which contains the user data and a protocol identifier to specify its format, and the service logic within the MSC for authorization and transparent routing. Its role was crucial in pre-IMS networks for enabling interactive value-added services. For instance, it could be used to send a calling card number, a service code, or authentication tokens directly within the call signaling, allowing the receiving application to process this data before or during the call connection. This mechanism provided a seamless way to integrate telephony with data services before the widespread adoption of packet-switched bearers for in-call data.
Purpose & Motivation
UUS3 was created to address the limitation of traditional voice-centric circuit-switched networks, which lacked a standardized, in-band method for applications to exchange signaling data. Prior to its standardization, implementing interactive services like calling card validation, menu-driven services, or client-server interactions during a call required either complex Dual-Tone Multi-Frequency (DTMF) post-connect signaling or the establishment of a separate data channel, which was inefficient and slow. The historical context is the evolution of GSM and UMTS networks, where operators sought to introduce intelligent network (IN) and value-added services without overhauling the core circuit-switched architecture.
It solved the problem of service transparency and interoperability by providing a clean, protocol-defined 'side channel' within the native call control signaling. This allowed network operators and third-party service providers to develop applications that could communicate necessary data (like user identities, service keys, or transaction identifiers) directly at call setup time. This was particularly important for prepaid systems, virtual private network (VPN) services, and customized alerting services, where the network needed additional information from the caller before deciding how to route or handle the call. UUS3 essentially bridged the gap between pure voice telephony and the emerging demand for integrated data services within the constraints of the existing CS core.
Key Features
- Transparent transport of user data within call control signaling messages (SETUP, CONNECT, etc.)
- Operation defined for call establishment (type 1), entire call duration (type 2), and network-specific use (type 3)
- Includes a protocol identifier to distinguish different user data formats and applications
- Requires subscription checking and service authorization by the MSC
- Does not require establishment of a traffic bearer channel for the data transfer
- Supports both mobile-originated and mobile-terminated call scenarios
Evolution Across Releases
Introduced the User-to-User Signalling Service 3 as a supplementary service for UMTS and evolved GSM networks. Defined the basic architecture with three service types, the UUS information element structure, and the procedures for invocation, network authorization, and transparent transport through the MSC using CAP or MAP.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.087 | 3GPP TS 23.087 |
| TS 24.087 | 3GPP TS 24.087 |