Unstructured Supplementary Service Data (USSD) is unique to GSM. It corresponds to a capability built into the GSM standard as a support for transmitting information over
the signaling channels of the GSM network: the mobile station user and an operator-defined application communicate in a manner that is transparent to the mobile station and to intermediate
network entities.
USSD provides bi-directional session-based communications, allowing development of operator-specific supplementary services and providing a variety of applications compatible with
nearly all legacy handsets.
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A unique set of characteristics
In comparison with the other GSM bearers (USSD, CSD, GPRS, voice), USSD provides a unique set of characteristics to build interactive services:
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USSD is similar to SMS as it uses the network's signaling path to exchange short strings of 182 characters (160 for SMS).
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USSD is similar to circuit switched data since it is session oriented (whereas SMS is "store & forward"). When a USSD session is established, the radio connection
remains open until the user, application, or time out releases it.
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USSD is similar to packet switched data since the USSD messages are handled in the NSS by MAP messages on the existing signaling network.
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USSD is similar to voice calls as the user simply dials a short code directly from the handset keypad to send
a USSD command and open a USSD session (e.g. #100#). Users do not need to access any particular
phone menu to access services through USSD.
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