HOW SIM CARD WORKS

A SIM card, also known as a subscriber identity module, is a subscriber identity

module application on a smart-card that stores data for GSM/CDMA Cellular telephone subscribers. Such data includes user identity, network authorization data, personal security keys, contact lists and stored text messages.
Security features include Authentication and encryption to protect data and prevent eavesdropping.
Functionality of the SIM card
The SIM card performs
the following valuable functions:
1) Identification of a subscriber: The IMSI programmed on the SIM card, is the
identity of a subscriber. Each IMSI is mapped to a mobile number and

provisioned on the HLR to allow a subscriber to be identified.
2) Authentication of a subscriber: This is a process, where, using the
authentication algorithm (COMP128V3 for 2/2.5 G GSM, CAVE for CDMA and
Mileage for 3G) on the SIM card, a unique response is provided by each
subscriber based on IMSI, Ki (stored on SIM) and RAND (provided by network).
By matching this response with values computed on the network a legal
subscriber is logged on to the network and he or she can now make use the
services of the mobile service provider.
3) Storage: To store phone numbers and SMS.
4) Applications: The SIM Tool Kit or GSM 11.14 standard allows creating
applications on the SIM to provide basic information on demand and other
applications for m-commerce, chatting, cell broadcast, phonebook backup,
location based services etc.

Typical Diagram of Sim Card
A Sim Card have six pads that also corresponds to the six SIM connectors pins, but only five has totally have connection on the entire layout.
SIM DATA – this is a digital data that being stored on a SIM memory
SIM Clock – this is a clock frequency signal that being synchronize to the digital data to create data signal in order transfer or sends and receive data information.
SIM Reset – this is also a frequency signal that triggers or reset all synchronization process.
VSIM B+ Supply Voltage- This a power supply voltage used to activated the SIM circuit.
SIM Ground – a ground line voltage
The smartcard with Subscriber identity module application is generally known as
SIMCARD. But, In reality, the SIM is effectively a mass-market smart card.
PIN and PUK:
PIN –Personal Identification Number
2 PINs exist (PIN 1 and PIN2)
Limited attempts on PIN access
PUK –PIN Unblocking Code
Resetting PUK, resets PIN and the attempt counter
Too many attempts on PUK blocks use permanently.
Two ways of Storing Data in SIM
1. As GSM Files
The data used for Telco and GSM operation are all stored over the files.
Telco/operator can change the Data this file through RFM in a secure channel.
Only upon successful verification of file access condition a file can be read.
All files are protected by access conditions.
2. As application data within an STK application as instance data.
mChek stores all its secured encrypted information within application data. All the
information stored is in persistent objects. Only mChek Server can access these
data through mChek OTA platform.
                                                                                                         by
                                                                                                    Srikanth Sri
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