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Unstructured Supplementary Service Data (USSD) is a protocol used by GSM cellular telephones to communicate with the service provider's computers. USSD can be used for WAP browsing, prepaid callback service, location-based content services, menu-based information services, and as part of configuring the phone on the network.
USSD messages are up to 182 alphanumeric characters in length. Unlike Short Message Service (SMS) messages, USSD messages create a real-time connection during a USSD session. The connection remains open, allowing a two-way exchange of a sequence of data. This makes USSD more responsive than services that use SMS.
The user composes some message—usually rather cryptic—on the phone keyboard. The phone sends it to the phone company network, where it is received by a computer dedicated to USSD. The answer from this computer is sent back to the phone. The answer could be seen on the phone screen, but it is usually with a very basic presentation. The messages sent over USSD are not defined by any standardization body, so each network operator can implement whatever it finds suitable for its customers.
USSD can be used to provide independent calling services such as a callback service (e.g. cheaper phone charges while roaming) or interactive data service (e.g. stock quotes, sports results).
USSD is commonly used by pre-paid GSM cellular phones to query the available balance. The vendor's "check balance" application hides the details of the USSD protocol from the user.
Other protocols and interfaces