sim command

SIM COMMANDS

The mobile equipment communicates with the SIM using standard ISO 7816-3 Operating Procedures (a copy of parts 1, 2, and 3 can be found here).

This transmission protocol uses Application Protocol Data Units (APDU), which can be either commands or responses. These are sent across the electrical interface between the SIM and the mobile equipment, or the SIM-ME interface for short.

CLA

INS

P1

P2

P3

Data

Command APDU format.

There are five fields in an APDU command. The class of instruction (CLA) is always A0 for GSM. The instruction code (INS) indicates the particular command to be performed. P1, P2, and P3 are parameters for the command, with P3 containing the length of the Data segment, if any.

Data

SW1

SW2

Response APDU format.

The response to a command is returned in three fields. The Data portion, if any, contains information requested in the command. SW1 and SW2 are status words indicating the success or failure of the command.

A number of commands are defined for GSM SIM cards, including functions to read and write data, confirm security features, and run the GSM authentication algorithm. Completing an entire GSM procedure may require a series of APDU command/response pairs.

 

COMMAND	INS	P1	P2	P3
SELECT	A4	00	00	02
STATUS	F2	00	00	length
READ BINARY	B0	offset (high)	offset (low)	length
UPDATE BINARY	D6	offset (high)	offset (low)	length
READ RECORD	B2	record number	mode	length
UPDATE RECORD	DC	record number	mode	length
SEEK	A2	00	type/mode	length
INCREASE	32	00	00	03
VERIFY CHV	20	00	CHV number	08
CHANGE CHV	24	00	CHV number	10
DISABLE CHV	26	00	01	08
ENABLE CHV	28	00	01	08
UNBLOCK CHV	2C	00	00 (for CHV1) 02 (for CHV2)	10
INVALIDATE	04	00	00	00
REHABILITATE	44	00	00	00
RUN GSM ALGORITHM	88	00	00	00
SLEEP	FA	00	00	00
GET RESPONSE	C0	00	00	length

SIM command coding.

 

SIM FILE SYSTEM

The SIM maintains information in a series of "files" that are organized hierarchically, much like the operating system of a personal computer. These files may be for administrative purposes or be specific to the GSM user application. Data in these files are accessed through APDU commands sent to the operating system.

A file is composed of a header and a body. The header contains structure and attribute information and is managed by the operating system. The body, which is optional, contains the actual data. Files are uniquely identified by a two-byte hexadecimal number.

First Byte	GSM file type
3F	Master File
7F	Dedicated File
2F	Elementary File under the Master File
6F	Elementary File under a Dedicated File

GSM file type identifiers (first byte).

There are three different types of files on a SIM: a master file (MF), dedicated files (DF), and elementary files (EF). There is one master file on a SIM, which holds all the other files in a tree-like structure. Dedicated files are headers that hold hierarchical trees of elementary files, but don抰 have data of their own. GSM defines two dedicated files immediately under the MF, DF_GSM

containing GSM application files and DF_TELECOM containing the application service features.

Elementary files (EF) have both a header and a body, and come in three flavors. The first, called transparent, is a binary file that can store information of varying length in any location. This is a raw, unstructured, random access file. The second is called linear fixed and stores data records that all have the same fixed length. The third is termed cyclic and is designed to store records in chronological order. It also uses fixed-length records, but when the last entry is full the next record overwrites the oldest entry.