User-to-User Signalling Service

Description

The User-to-User Signalling (UUS) supplementary service enables a subscriber to send and receive user-defined information to and from another subscriber within the context of a call. This information is transported transparently through the network, meaning the network does not interpret or modify the user data. The UUS service is defined in 3GPP TS 22.087, with stage 2 procedures in TS 23.087 and stage 3 in TS 24.087. It is a supplementary service that can be activated, deactivated, registered, and interrogated like other supplementary services. The UUS service is composed of three distinct services, referred to as UUS Service 1, UUS Service 2, and UUS Service 3. These services differ in the timing and direction of the user-to-user information transfer relative to the call setup. UUS Service 1 allows the exchange of user information during the call setup phase, before the call is fully established. UUS Service 2 permits the exchange of user information after the call is established, during the active call phase. UUS Service 3 enables the exchange of user information during the call release phase, after the call has been cleared but before the signalling connection is fully released. Each of these services can be independently requested and controlled by the subscriber. The UUS service is typically used in applications such as call centres, where additional data (e.g., customer account information) needs to be exchanged alongside the voice call, or in private networks for signalling between user terminals. The service is available in both circuit-switched (CS) and packet-switched (PS) domains, although its usage in PS domain is limited. The UUS service is not intended for high-bandwidth data transfer; it is designed for small amounts of user information, such as a few bytes to a few hundred bytes per message. The service is controlled by the UUS supplementary service, which manages the subscription and the individual UUS services. When a subscriber has the UUS supplementary service provisioned, they can handle the UUS services independently within a call, meaning they can choose to use Service 1, 2, or 3 as needed. The network ensures that the user-to-user information is delivered reliably and in sequence, if required. The UUS service has been part of 3GPP specifications since Release 4 and has remained stable, with no significant changes to its definition across releases up to Release 19. This stability reflects its mature design and continued relevance in specific niche applications.

Purpose & Motivation

The UUS service was created to address the need for a standardized mechanism to exchange small amounts of user-defined information between subscribers during a call, without requiring a separate data connection. Before UUS, such information exchange would require either in-band signalling (e.g., DTMF) or a separate data call, both of which were limited and inefficient. In-band signalling using DTMF is slow, unreliable, and limited to a small set of tones, while a separate data call adds complexity and cost. UUS provides a transparent, out-of-band channel for user information, integrated with the call control signalling. This allows applications such as call centres to pass customer data, or private networks to exchange control information, seamlessly within the voice call. The service was designed to be flexible, with three distinct services to cover different phases of the call, giving subscribers fine-grained control. It also solves the problem of associating user data with a specific call, as the information is tied to the call context. The UUS service has been particularly useful in enterprise environments where PBX features require the exchange of additional information between terminals. Over time, with the advent of IP-based communication and rich data services, the use of UUS has diminished, but it remains a standardized feature for backward compatibility and specific use cases.