Unstructured Supplementary Service Data

When a user sends a message to the phone company network, it is received by a computer dedicated to USSD.

Messages sent over USSD are not defined by any standardization body, so each network operator can implement whatever is most suitable for its customers.

The vendor's "check balance" application hides the details of the USSD protocol from the user.

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.

[5] In such cases, SMS is used to "push" a reply or updates to the handset when the network is ready to send them.

An SMSC is not present in the processing path, so that the store-and-forward capability supported by other short-message protocols such as SMS is not available.