Sometimes referred to as “quick codes” or “feature codes”, USSD (Unstructured Supplementary Service Data) is a messaging protocol used in GSM-based cellular networks.

Like SMS (Short Message Service), USSD enables the transmission of short text messages (in the case of USSD, the maximum message size is 182 characters or 16 bits of data).

The main difference between USSD and SMS technologies is that USSD is session-based.

SMS works on a store-and-forward basis; i.e. when one device sends an SMS to another, no dedicated communication channel is opened, and there is always a short delay in the recipient getting the message. By contrast, when a device sends out a USSD request, a transparent channel is automatically established between sender and recipient. In this way, the recipient can respond to the sender’s requests in real time.

A USSD communication exchange usually starts by a query being sent from a mobile device (e.g. a request for an account balance reading). To send it, the user has to dial a specific short code. Once sent, a USSD gateway forwards the message to a USSD application, which responds to the request. The response then goes back to the USSD gateway, which displays that response on the sender’s device screen.

A USSD code is a short (usually four or five) series of digits for accessing information using the USSD protocol. The code usually starts with an asterisk and ends with a hashtag.

On a network, each unique code effectively unlocks a specific piece of information using an automated system. For instance, depending on the use case, the network might have specific codes configured for users to obtain an account balance, to check an order status or to check data usage on a connected device.