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 faster than services that use SMS.

While GSM is being phased out in the 2020s with 2G and 3G technologies, USSD services can be supported over LTE, 5G, and IMS.

Uses

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 their 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 including Facebook and Twitter. Between 2012 and 2018, the Wikipedia Zero project provided access to Wikipedia articles via USSD.

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:

"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.