Technical Details
Most GSM phones have USSD capability. It is generally associated with real-time or instant messaging services. There is no store-and-forward capability, as is typical of other short-message protocols like SMS—in other words, an SMSC is not present in the processing path.
USSD Phase 1, specified in GSM 02.90, only supports mobile-initiated ("pull") operation. In the core network, the message is delivered over MAP. USSD Phase 2, specified in GSM 03.90, supports network-initiated ("push") operation as well.
Read more about this topic: Unstructured Supplementary Service Data
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