User-to-User Signalling Service 1

Description

User-to-User Signalling Service 1 (UUS1) is one of the three specific services that constitute the broader UUS supplementary service in 3GPP networks. It is defined as a call-associated service where the transfer of user information is initiated by the calling party and occurs during the call establishment attempt. Technically, the calling user's terminal equipment includes a User-to-User Information Element (UUIE) within the SETUP message sent to the network at the start of a call. The network, upon receiving this SETUP message, checks the caller's subscription for UUS1 provision and, if authorized, transparently forwards this information element within the subsequent ISUP Initial Address Message (IAM) towards the destination network.

The receiving network, after performing its own service validation for the called party (if required by network policy), delivers the UUIE to the called terminal, typically within the SETUP message presented to the called user. This allows the information to be available at the called device before the user answers the call. The content of the UUIE is not standardized by 3GPP and is for application-layer interpretation; it could be a text string, a numeric code, or other data up to the maximum allowed length. The service is strictly for information transfer from caller to callee during setup; no acknowledgement or response via UUS1 is defined within the call establishment phase.

Architecturally, UUS1 relies on the call control functions in the MSC or serving exchange to handle the UUIE. The HLR stores the subscriber's UUS service profile, indicating whether UUS1 is allowed. During call setup, the MSC may query this profile via MAP or use locally cached data. Protocol handling involves embedding the UUIE in the Protocol Discriminator and message type-specific fields of DSS1 (at the user-network interface) and in the User-to-User parameter of ISUP messages (within the core network). If the call fails to establish (e.g., busy, no answer), the UUS1 data is typically discarded, as its transport is contingent on the call setup proceeding.

Purpose & Motivation

UUS1 was designed to solve the specific problem of one-way, caller-initiated data transfer at the very beginning of a call attempt. Before its existence, a calling party had no standardized way to send contextual information (like the purpose of the call, an identity code, or a priority marker) along with the call setup itself. This was particularly useful for automated systems, telemetry, or business applications where the called equipment could use the received data to decide how to handle the call (e.g., route to a specific agent, display caller context, or trigger an automated response).

It addressed limitations of earlier systems where such context had to be communicated verbally after answer or via a separate channel, adding delay and complexity. By integrating data into the initial SETUP, UUS1 enabled faster processing and more intelligent call handling at the point of arrival. However, its success was limited by the need for both endpoints and all intermediate networks to support the feature, and it was largely superseded by richer SIP-based signalling in VoIP networks.

Key Features

• Caller-initiated user information transfer during call establishment phase
• User data is carried within the initial SETUP message and corresponding IAM
• Enables data delivery to the called party before call answer
• Requires subscription provisioning for the calling party
• Transparent network transport with subscription verification
• Maximum user data length defined by network and protocol constraints (typically up to 128 octets)

Evolution Across Releases

Defining Specifications