User-to-User Information

Description

User-to-User Information (UUI) is a circuit-switched telephony supplementary service standardized by 3GPP that facilitates the exchange of a small, user-generated data payload between two endpoints as part of the call establishment or teardown process. Unlike the voice bearer path, UUI is carried transparently within the call control signaling messages, specifically in the ISDN User Part (ISUP) or the Q.931/Q.932 protocols used in the core network. The service defines a UUI element, which is a sequence of octets (bytes) with a specified maximum length, that can be inserted by the calling party and delivered to the called party. The network's role is primarily to transport this information element without interpretation or modification, although network nodes may need to check length constraints and service subscription profiles.

Architecturally, UUI operates within the signaling layers of the circuit-switched domain. When a user initiates a call and includes UUI, the User Equipment (UE) or terminal places the data into the appropriate SETUP or FACILITY message. This message traverses the signaling network (e.g., via the MSC). The serving MSC verifies that the originating subscriber is authorized for the UUI service and that the information length complies with network limits. It then copies the UUI element into the outgoing ISUP Initial Address Message (IAM). The receiving MSC performs similar checks for the terminating subscriber before including the UUI in the SETUP message sent to the called party's terminal. The service can be configured for one-way transfer (from caller to callee) or two-way exchange, and it can be sent at call setup (UUI Setup), during an active call (UUI Mid-Call), or at call release (UUI Release).

Key components involved include the UE, which must support UUI generation and reception; the Mobile Switching Center (MSC), which acts as the service switching point, enforcing subscription and network policies; and the signaling protocols themselves (e.g., DSS1, ISUP). The role of UUI in the network is to enable enhanced services without requiring a separate data connection. It provides a simple, in-band signaling mechanism for context transfer, which is crucial for integrating telephony with intelligent network services, corporate PBX features, or emergency services (e.g., passing location or medical data). Its operation is defined across multiple 3GPP specifications covering service description (22.087, 23.087), protocol details (24.087, 25.415), and vocabulary (21.905).

Purpose & Motivation

UUI was created to extend the functionality of basic circuit-switched voice calls by allowing the exchange of application-layer data alongside call signaling. Prior to its introduction, creating services that required context sharing between callers (like screen pops in call centers, secure callback tokens, or passing a customer ID) required complex, out-of-band data systems or proprietary implementations. UUI solves this by providing a standardized, network-transparent channel within the ubiquitous call setup procedure, enabling interoperability between equipment from different vendors and across network boundaries.

The historical context lies in the evolution of ISDN and intelligent network services in the 1980s and 1990s. As business and value-added services became more sophisticated, there was a clear need to pass information about the call's purpose or the caller's identity. UUI, standardized from 3GPP Release 4 onwards, addressed this need for GSM and UMTS CS networks. It overcame the limitation of the signaling system being a pure 'dumb pipe' for call routing, turning it into an enabler for service innovation. While largely superseded by IP-based mechanisms in IMS, UUI remains relevant for legacy CS services and specific regulatory applications.