31 series - USAT architecture and protocol stack
The USAT is a set of commands and procedures, which allows the UICC to run applications that are specific to the card issuer (ie the network operator). Typical applications include information- and location-based services, and mobile banking.
As shown above, the UICC application typically uses the USAT to communicate with an external application system. To do this, it instructs the ME to send data to the application system, and wait for the response. The data can be sent by various mechanisms, such as SMS or GPRS, and are secured using the USAT security procedures.
The UICC application also uses the USAT to communicate with the ME's user interface. For example, it can instruct the ME to display a menu, and wait for the user's selection. In this situation, the protocols terminate in the ME.
A C API allows network operators not only to write C language applications, but also to download them after the card has been issued. There are also API's for Sun Java Cards, with separate specifications for cards running a USIM or an ISIM.
The ETSI specifications can be downloaded here.
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