(*121*1234#) 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, mobile-money services, 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 a message 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),
          enhance mobile marketing capabilities,
          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.
        • On some pay as you go networks, once a user performs an action that costs money, they see a USSD message with their new balance.
        • USSD can also sometimes be used to refill user’s money balance on phone (SIM card to be exact) 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.
        • USSD is sometimes used in conjunction with SMS: the user sends a request to the network via USSD, and the network replies with an acknowledgement of receipt e.g.
          “Thank you, your message is being processed. A message will be sent to your phone.”
        • Subsequently, one or more Mobile Terminated SMS messages communicate the status and/or results of the initial request.
        • In such cases, SMS is used to “push” a reply or updates to the handset when the network is ready to send them.
        • In contrast, USSD is used for command-and-control only.