3. USSD Architecture Overview

3.1 High-Level Architecture

At a high level, a USSD Gateway sits between the mobile network’s signaling system and the external application environment.

• In legacy GSM/UMTS networks, the handset sends a USSD request that is processed by the serving MSC/VLR, and service logic involvement typically includes the HLR and other service control functions depending on operator design.
• In IMS-based deployments (LTE/5G), the USSD dialog is carried via IMS signaling to an IMS Application Server implementing USSI behavior.

Regardless of topology, the USSD Gateway function provides the same core outcome: it manages dialogs and connects signaling-originated USSD interactions to application services.