Page 77 - From GMS to LTE

Global System for Mobile Communications (GSM) 63

Table 1.7 SIM card properties.
CPU 8‐ or 16‐bit CPU
ROM 40–100 kB
RAM 1–3 kB
EEPROM 16–64 kB
Clock rate 10 MHz, generated from clock supplied by mobile device
Operating voltage 1.8 V, 3 V and 5 V. Modern devices use 1.8 V but support SIM cards with
higher voltage requirements as well

SIM-card
Data bus
RAM
(1–3kB)

System clock
Processor EEPROM
(16–64 kB)

Terminal ROM
I/O-interface (50 kB)
Address bus

Figure 1.50 Block diagram of SIM card components.

itself and not in the mobile device to protect the secret Ki key. If the calculation was
done in the mobile device itself, this would mean that the SIM card would have to hand
over the Ki to the mobile device or any other device upon request. This would seriously
undermine security, as tools like the one shown in Figure 1.49 would be able to read the
Ki, which could then be used to make a copy of the SIM card.
Furthermore, the microcontroller system on the SIM can also execute programs that
the network operator may have installed on the SIM card. This is done via the SIM
application toolkit (SAT) interface, which is specified in 3GPP TS 31.111 [33]. With the
SAT interface, programs on the SIM card can access functionalities of the mobile device
such as waiting for user input after showing a text message or sending or receiving SMS
messages without user intervention. While this functionality was used extensively by
network operators in the past for value‐added services, the SAT interface is now mainly
used for background tasks such as sending a notification to the network when the SIM
card detects that it has been inserted in a new device to trigger the transfer of welcome
and configuration messages. Furthermore, the SAT interface still plays an important
role in receiving ‘silent’ SMS messages from the network to update information on the
SIM card such as the list of preferred roaming networks.