USSD

Unstructured Supplementary Service Data (USSD) is a communication protocol used by GSM mobile devices to interact with their service provider's computer systems in real time. It enables interactive, text-based sessions that do not require an Internet data connection. USSD is session-based, so the user can navigate a sequence of prompts, and the conversation ends when the user sends a termination command or the session times out.

A USSD session is started with a short code, typically beginning with an asterisk and ending with a pound sign, such as *123#. During the session, the user sends requests and receives immediate text responses. The maximum length of an interactive USSD screen is commonly about 180 characters, though this limit varies by network.

Technical context: USSD operates over the signaling or control channel of the cellular network (not over the IP data channel). It is supported on many GSM and newer networks (2G/3G/4G), and works on feature phones as well as smartphones without data connectivity.

Common uses: prepaid balance checks, mobile top-ups, and mobile money services often rely on USSD; also subscription management, service configuration, and account verification.

Advantages and limitations: advantages include real-time response, low data requirements, and broad device compatibility; limitations include lack of encryption by default, dependence on operator implementation, and non-end-to-end security; not available on all networks or roaming; session can be interrupted by network issues.

History: USSD was standardized within GSM specifications in the late 1990s and has since become a staple of many operators' service menus, especially for financial and provisioning services.