Unstructured Supplementary Service Data

Unstructured Supplementary Service Data is a capability of all GSM phones. It is generally associated with real-time or instant messaging type phone services. There is no store-and-forward capability, such as is typical of other short-message protocols (in other words, an SMSC is not present in the processing path). Response times for interactive USSD-based services are generally quicker than those used for SMS.

USSD Phase 1, specified in GSM 02.90, only supports mobile initiated operation (pull operation). In the core network the message is delivered over MAP. USSD Phase 2, specified in GSM 03.90, supports network-initiated operation (pulls and push operation).

USSD is typically used as a 'trigger' to invoke independent calling services that don't require the overhead and additional usage costs of an SMSC, such as a callback service (e.g. cheaper phone charges while roaming), or interactive data service (e.g. stock quotes, sports results).

USSD is a standard for transmitting information over GSM signaling channels. It is mostly used as a method to query the available balance and other similar information in pre-paid GSM services. The function that is triggered when sending USSD is network-dependent and depends on the specific services the operator is offering.

Example USSD codes:

*101#
*109*72348937857623#

After entering a USSD code on your GSM handset, the reply from the GSM operator is displayed within a few seconds.

USSD is the base of some payment methods such as SharEpay, SWAP Mobile in South Africa, Mobipay in Spain, M-Pesa in Tanzania (but not in Kenya, where M-Pesa menus are provided by STK rather than USSD), and mPay in Poland.

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