USSD and Service Codes

USSD stands for Unstructured Supplementary Service Data. )It is a protocol which is used by GSM cellular telephones to communicate with the service provider's computers.

USSD today is being used to provide a vast number of services. Such as menu-based information services, mobile banking, WAP browsing, location-based content services and many more.

How does USSD work ?

Information through USSD is sent directly from a sender’s mobile handset to an application handling the USSD service. The USSD service might locate in the sender’s mobile network or in a visited mobile network as well. 

To send  USSD information, there need to establish a real-time “session” between the mobile user and the USSD application platform. This real time session is initiated when the USSD service is invoked. So it is obvious that USSD is not working as SMS. Both can be distinguished from each other because SMS has a store and forward mechanism while USSD has a real time session initiation. As a result of the real time session, data is being sent back and forth between the mobile user and the USSD application platform until the USSD service is completed. More over this makes USSD more responsive when compared to services which use SMS.

What are USSD service codes ?

Specific 'MMI strings' (MMI stands for Man Machine Interaction) or telephone numbers can be dialed from a user’s “mobile station” (mobile handset) to initiate USSD services. These numbers can be named as USSD Service Codes as well. 

3GPP TS 22.090 specifies the standard format for USSD service codes. According to the standards the code format should be as follows;

“1, 2 or 3 digits from the set (*, #) followed by 1X(Y), where X=any number 0-4, Y=any number 0-9, then, optionally “* followed by any number of any characters”, and concluding with # SEND”.

So the application developers can build and install services ( eg: a menu driven service) within a particular mobile network and let the users obtain their service via USSD. 

Image courtesy : http://blog.arrivedok.mobi/tag/ussd/