User-to-User Signalling Supplementary Service

Description

The User-to-User Signalling (UUS) supplementary service is a circuit-switched telephony service standardized by 3GPP that facilitates the transfer of a small amount of user-generated information between two subscribers. This transfer occurs transparently through the network using the signalling path, specifically within ISDN User Part (ISUP) or DSS1 (Digital Subscriber Signalling System No. 1) messages, rather than the voice bearer channel. The service is architecturally integrated into the core network's call control functions, residing within the Mobile Switching Centre (MSC) for mobile networks or equivalent exchanges in fixed networks. It operates by allowing the calling or called party to include a UUS information element within call establishment messages (like SETUP, ALERTING, CONNECT) or during the active phase of a call via FACILITY messages.

The service is subdivided into three distinct services: UUS1, UUS2, and UUS3, which a subscriber can subscribe to and use independently. The network's role is primarily to transport this user data transparently, performing basic checks on service subscription and length constraints but not interpreting the content. The data is typically limited to 128 octets (for UUS1 and UUS2 in call-associated mode) and can be used for various applications defined by the user or higher-layer applications, such as transmitting a calling card number, a short text note, or a service code. The signalling is carried either call-associated, where UUS data is embedded in call control messages, or call-unassociated, using separate signalling messages independent of call state, though the latter is less commonly implemented.

From a protocol perspective, UUS leverages existing call control protocols. In the mobile core network, it utilizes MAP (Mobile Application Part) for service control interactions with the HLR and CAP (CAMEL Application Part) for intelligent network interactions if needed. On the user-network interface (UNI), it uses protocols like DSS1. Its implementation requires support from both the terminal equipment (UE) to generate and display the information and the network to transport it. While largely associated with 2G (GSM) and 3G (UMTS) circuit-switched domains, its principles influenced later in-band signalling concepts, though it saw limited widespread consumer adoption compared to SMS.

Purpose & Motivation

UUS was created to address the need for simple, low-latency transfer of user-defined data in conjunction with a voice call, without requiring a separate data channel or service like SMS. In the early digital telephony era of ISDN and GSM, there was a growing demand for value-added services that could enhance basic voice calls. Prior to UUS, exchanging small amounts of data during call setup or a call required either verbal communication, a separate data call, or later SMS, which operated on a different store-and-forward signalling channel and incurred delay. UUS solved this by piggybacking data on the real-time call signalling path.

The historical context stems from ISDN, where the 'User-to-User Information' element was defined in Q.931 signalling. 3GPP adopted and standardized this as a supplementary service for GSM and UMTS to maintain feature parity with fixed ISDN networks and enable new service possibilities. It aimed to facilitate applications like secure caller ID (beyond CLI), transmitting account numbers for automated systems, or simple text exchange during a call attempt. However, its adoption was hindered by limited terminal support, the rise of ubiquitous SMS, and later IP-based messaging, confining it primarily to niche corporate and network-interoperability applications.