Unstructured Supplementary Services Data (USSD) allows for the transmission of information via a GSM network. Contrasting with SMS, it offers real time connection during a session. A USSD message can be upto 182 alphanumeric characters in length. Unstructured Supplementary Service Data allows interactive services between a MS and applications hosted by the Mobile Operator. These messages are composed of digits and the #, * keys, and allow users to easily and quickly get information/access services from the Operator.
USSD messages are simple to form and easy to send. User can directly enter the ussd string and press call to send the message. A typical USSD message starts with a * followed by digits which indicate an action to be performed or are parameters. Each group of numbers is separated by a *, and the message is terminated with a #. The USSD gateway in turn can interact with external applications based on the USSD command. This allows access to number of value added services via USSD.
USSD is a session oriented service, and can support a sequence of exchange of information. Phase 2 USSD also allows messages to be pushed onto a MS. It is several times faster than MO SMS messages since there is no store and forward of messages. The USSD gateway supports an open HTTP interface.
The USSD gateway will have an interface with the MSC over SS7. It uses MAP to receive and send USSD data from the HLR.
Generally the USSD functionality is implemented in the following modes:
Pull Mode, will handle Mobile Initiated USSD Requests.
Push Mode will handle network Initiated USSD Requests.
Characteristics of USSD:
A USSD message can be upto 182 alphanumeric characters in length
Unlike SMS, USSD is a session oriented service
Simple and easy to send. No need to go into any menus and options. Just directly entered on the default mobile screen.
USSD works on all GSM handsets of Phase II or later.
There will not be any latency in request and responce as we experience in SMS based services
The functionality will be the same even while roaming as the USSD messages always routed back to Home HLR.
USSD is supported by WAP, SIM Application Toolkit and CAMEL enabling scope for many applciations.
Works in two modes: pull mode and push mode.
USSD can be used as a WAP bearer.
Used for menu based content services like news, weather, sports etc
Used for prepaid callback service enabling prepaid roaming a better service
Used in location based content services
Used in SIM Application tool kit based applications.
Download specification : GSM ETSI 3.90 - Unstructured Supplementary Service Data (USSD) - Stage 2