4. Session Flow Lifecycle

USSD interactions are dialog-based, and the gateway’s core responsibility is to keep that dialog consistent across multiple steps, enforcing timeouts and mapping subscriber inputs to the correct application session.
Note: exact MAP operation usage may differ between deployments (HLR-anchored, MSC-anchored, SCP-triggered, gateway-anchored). For normative primitives and parameters, refer to [TS 29.002]  and USSD service descriptions in [TS 23.090] / [TS 22.090].

4.1 Mobile-Originated USSD (MO / “Pull”)

1. Subscriber initiates a session by dialing a USSD code (e.g., *123#).
2. The serving network node triggers a TCAP dialog toward the USSD service logic (HLR/SCP/USSD Gateway depending on deployment).
3. The gateway creates a session context and routes the request to the appropriate application endpoint.
4. The application returns either: