Unstructured Supplementary Service Data

USSD messages are up to 182 alphanumeric characters long. 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. When a user sends a message to the phone company network, it is received by a computer dedicated to USSD. The computer's response is sent back to the phone, generally in a basic format that can easily be seen on the phone display. Messages sent over USSD are not defined by any standardization body, so each network operator can implement whatever is most suitable for its customers. USSD can be used to provide independent calling services such as a callback service (to reduce phone charges while roaming), enhance mobile marketing capabilities or interactive data services. USSD is commonly used by prepaid GSM cellular phones to query the available balance. The vendor's 'check balance' application hides the details of the USSD protocol from the user. On some pay as you go networks, such as Tesco Mobile, once a user performs an action that costs money, the user sees a USSD message with his or her new balance. USSD can also be used to refill the balance on the user's SIM card and to deliver one time passwords or PIN codes. Some operators use USSD to provide access to real-time updates from social-networking websites like Facebook and Twitter. Wikipedia uses USSD to send articles to some feature phones.