User-to-User Signalling Service 2

Description

User-to-User Signalling Service 2 (UUS2) is the complementary service to UUS1 within the UUS supplementary service family. While UUS1 facilitates caller-to-callee data transfer at call setup, UUS2 enables the called party to send user information back to the caller during the call establishment phase. This transfer is initiated by the called user's terminal and occurs after the called party receives the call setup request (SETUP) but before the call is fully connected. Technically, the called terminal includes a User-to-User Information Element (UUIE) in its response message sent back towards the caller—typically the ALERTING message (indicating the called terminal is alerting the user) or the CONNECT message (indicating call answer).

The network transports this UUIE transparently in the reverse direction within the corresponding ISUP messages, such as the Address Complete Message (ACM) for ALERTING or the Answer Message (ANM) for CONNECT. The calling user's terminal receives the UUIE embedded in the ALERTING or CONNECT message from the network and can present the information to the user or application. Like UUS1, the content is user-defined and not interpreted by the network, which only performs service subscription checks. The service is call-associated, meaning the data transfer is intrinsically linked to the successful progression of the call establishment signalling sequence.

From an architectural standpoint, UUS2 activation requires the called subscriber to have the service provisioned in their HLR profile. The serving MSC for the called party validates this before allowing the UUIE to be included in the outgoing response message. On the protocol layer, the UUIE is inserted into the DSS1 ALERTING or CONNECT message on the user-network interface. For network transit, it is mapped into the User-to-User parameter of the relevant ISUP message. UUS2 can be used independently of UUS1; a call could involve only UUS2, only UUS1, or both, depending on subscriber services and terminal capabilities. The service is particularly useful for providing ringback tones with data, sending caller authentication responses, or indicating the reason for a delayed answer.

Purpose & Motivation

UUS2 was created to address the need for bidirectional data exchange during call setup, allowing the called party to respond to the caller with contextual information. Prior to UUS2, a called user or device had no standardized mechanism to send data back to the caller before the voice path was established, apart from in-band tones or announcements after answer. This was a limitation for interactive voice response systems, secure callback procedures, or situations where the called party wanted to indicate status (e.g., 'I will answer in 30 seconds' or 'Enter your PIN').

It complemented UUS1 to form a simple request-response mechanism entirely within the signalling channel, reducing post-answer interaction time. The service aimed to enhance automated telephony applications and human-to-machine communication in the pre-VoIP era. However, similar to UUS1, its adoption was constrained by the need for widespread terminal and network support, and it remained a niche feature primarily used in specific corporate or network-interworking scenarios before being overshadowed by IP-based signalling capabilities.

Key Features

• Called-party-initiated user information transfer during call establishment
• User data is carried within ALERTING or CONNECT messages and corresponding ISUP ACM/ANM
• Enables response or context from the called party before or upon answer
• Requires subscription provisioning for the called party
• Operates transparently with network subscription verification
• Supports independent use from UUS1, enabling flexible call-associated signalling patterns

Evolution Across Releases

Defining Specifications