Unstructured supplementary service data  is a capability associated with GSM based mobile phones with the ability to process real time data. Messages sent via USSD cannot be stored and sent later or manupulated. This implies that it is session oriented.  Messages can be sent from the cell phone screen page directly. Response times of applications using USSD is less compared to other SMS since USSD is session oriented. USSD allows for virtual home network where the commands sent via USSD are routed back to Home Location Register of the home mobiles network. Sim application toolkit and the wireless application protocol complies with USSD.  This concept is the same as that while roaming. USSD is basically a trigger rather than some application and it can be used to sent text between the user and various applications. Basically it enables other applications to function based on USSD. USSD is something like telnet while on the other hand SMS is like a regualr mail.

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