What are USSD, IVR, STK… ?

What is USSD (Unstructured Supplementary Service Data)?
       It is used by operators to inform pre-paid cards users about their balance on a real-time basis. USSD is a capability of all GSM phones. It is generally associated with real-time or instant messaging type phone services. There is no store-and-forward capability that is typical of 'normal' short messages (in other words, an SMSC is not present in the processing path). Response times for interactive USSD-based services are generally quicker than those used for SMS.
       A sample USSD service is the bill status service accessed by dialling *141# or similar numbers in between * and #. USSD applications can be thought of as an IVRS (Interactive Voice Response System) without voice. Some of the USSD applications that we use regularly are:
· Alerts About special offers, services and news
· Balance enquiry
· Changing tariff plan and subscribing to various VAS services.
· Recharging using prepaid vouchers

What is IVR (Interactive Voice Response)?
        Interactive voice response (IVR) is a technology that allows a computer to interact with humans through the use of voice and DTMF tones input via keypad. In telecommunications, IVR allows customers to interact with a company’s host system via a telephone keypad or by speech recognition, after which they can service their own inquiries by following the IVR dialogue. IVR systems can respond with pre-recorded or dynamically generated audio to further direct users on how to proceed.

What is STK (SIM Application Toolkit)?
       SIM Application Toolkit (STK) is a standard of the GSM system which enables the Subscriber Identity Module (SIM) to initiate actions which can be used for various value-added services. STK has been deployed by many mobile operators around the world for many applications, often where a menu-based approach is required, such as Mobile Banking and content browsing.