SIM Access Control Class

Access between the Mobile Phone and GSM Network can be controlled through the SIM Card. There is one file inside the SIM named EF ACC 6F78 (Access Control Class) which contain parameters to control this.

15 classes are defined inside this file. The first 10 classes are randomly allocated for normal subscribers, the rest 5 classes are allocated for specific high priority users. The class allocation
is depent on the requirements of Mobile Operator.

In some cases, Mobile Operator will enable the Class based on the last digit of IMSI. For example, if one SIM has "0" as the last digit, then Class 0 will be enabled. With this method the Mobile Operator
can manage the usage of Radio Access channels.

The size of EF ECC is 2 bytes (16 bits) which each bit represents the enabled class, except for the 3rd bit which set to "0" (disable). The 3rd bit represent Class 10.

Below is the figure of class allocation in this file :

Byte 1:
b8 b7 b6 b5 b4 b3 b2 b1
15 14 13 12 11 10 09 08 Number of the ACC (except for bit b3)

Byte 2:
b8 b7 b6 b5 b4 b3 b2 b1
07 06 05 04 03 02 01 00 Number of the ACC

In normal condition the radio site will give access with SIM with classes 0- 9 enabled. In other condition radio site will give priority for classes 11- 15 enabled SIM. Some examples for high priority users
are Emergency Call, Police Department, etc.

References :
- Universal Mobile Telecommunications System (UMTS); LTE; Characteristics of the Universal Subscriber Identity Module (USIM) application (3GPP TS 31.102 version 8.5.0 Release 8)
- Digital cellular telecommunications system; Service accessibility (GSM 02.11 version 5.0.0)