If a UICC connected to an MT uses USIM Application Toolkit (USAT), some USAT features can be provided by the MT itself, whereas other features can be implemented in the TE. This applies especially to MTs with limited capabilities, where the user interface could be provided by the TE. If there are multiple entities inside the TE providing USAT services, for the purpose of this specification, the TE is visible as one entity handling AT commands and responses on the interface to the TA/MT, see Figure 12.1-1.

The AT commands for definition of the USAT profiles and transmission of USAT proactive commands, USAT terminal responses and USAT envelope commands between the TE and UICC are specified in the subsequent clauses. Compared to APDUs, where the MT is the initiator, USAT defines logic where the UICC is the initiator of the USAT proactive commands and the MT sends responses to these commands. Due to that, USAT proactive commands are sent in unsolicited result codes, whereas the associated responses are transported in subsequent AT commands.

These examples provide a sequence of interactions where USAT commands and responses are transmitted in AT commands. As it would make the examples unreadable, USAT data is not given in its hexadecimal representation (which is found in TS 31.111), but in abstract textual form. For readability, only a subset of USAT facilities typically provided in a profile is given in the examples. Resulting or triggering actions on the MT-UICC interface are also indicated in the examples. First, the TE reads the MT default profile: The TE removes some USAT facilities in the MT by writing a new MT profile, which contains those facilities that are to be handled by the MT: Now the TE tries to write a TE profile that conflicts with the MT profile. The MT responds with the conflict information and rejects the command. The next attempt to write a TE profile is successful and the TE activates the profiles. The UICC sends a SET UP MENU proactive command, resulting in the command being transferred to the TE and the latter confirming it with a terminal response; thereafter the proactive session ends. A selection by the subscriber in the menu results in an ENVELOPE being sent to the MT and forwarded to the UICC. The UICC responds with the next proactive command DISPLAY TEXT. After the terminal response, the proactive session ends. The TE defines UICC profile download options. The definition is effective for the next UICC start-up. The UICC restarts. The MT reports the start-up. During start-up the profile download definition cannot be altered and the MT responds error code 14 (SIM busy). The UICC is awaiting PIN verification. During PIN verification the profile download definition cannot be altered and the MT responds error code 14 (SIM busy). The UICC becomes active and the profile download definition can be changed. The TE enables the downloaded profile. The TE reads the TE profile: Now the TE tries to write a MT profile that conflicts with the TE profile. The MT responds with the conflict information and rejects the command. The next attempt to write a MT profile is successful. The TE reads the List of MT only facilities: Now the TE tries to write a TE profile that conflicts with the MT only facilities. The MT responds with the conflict information and rejects the command. The next attempt to write a TE profile is successful.