How Does The SIM work-How Does The SIM Card Connect To Networks-What is eSIM-IMSI

The SIM(Subscriber Identity Module) card is made of silicon and gold, coated with some other metal to keep it in contact with the phone. The chip has a processor, memory, and security circuits that act as its microcontroller, which is widely known as the 2nd processor in our phone.

SIM's

Standard:- SIM cards measure 25x15mm and are used in older and basic phones.

Micro:- SIM cards measure 15x12mm and are mostly found in 2010s phones and older.

Nano:- SIM cards measure 12.3x8.8mm and are used in new smartphones.

Each SIM card comes with a unique identifier called an Integrated Circuit Card Identifier (ICCID).

ICCID:-

Integrated Circuit Card Identifier, is a basic account number of 19 digits. This number includes sections like Issuer Identification Number or IIN, Personal Account Identification, Check Digit, etc.

How Does The SIM work

Sim Architecture

• VCC:- For connecting to the phone's power supply.

• RST:- Used for setup, unlocking the sim for resetting and deleting old service provider information, and adding new service provider information.

• CLK:- provides the SIM with a clock signal for its processor.

• Not connected:-Manages connectivity and reserved for future uses.

• I/O:- It is the interface between the phone and the SIM.

• VPP:- wireless connectivity 

• GND:-Act as the motherboard of SIM.

• RAM:- It contains 1kb to 8kb.

• ROM:-it contains  64kb to 512kb and it stores SIM card Operating System and 250 contacts, some messages.

• EEPROM:-  Electrically Erasable Programmable Read Only Memory, It stores phone book, SMS settings, and security key information. and it contains 16kb to 512kb.

• CPU:- To Perform Operations.

Each SIM card has its unique identification code so that the phone company's central database can identify it.

SIM Inner Part

The SIM card also contains a unique 64-bit number known as IMSI (International Mobile Subscriber Identity). In addition, the SIM card also contains a unique authentication key.

IMSI (International Mobile Subscriber Identity):-

It is used to identify an individual operator's network. Usually, it consists of 109 digits. Its first 3 digits indicate the Mobile Country Code or MCC, the next 2 to 3 digits indicate the Mobile Network Code or MNC, and the next digits indicate the Mobile Subscriber Identification Number or MSIN.

Authentication Key:

It is 128-bit which is used for authentication of SIM cards on the mobile network. Each SIM has a unique authentication key assigned by the operator at the time of personalization. The authentication key is also stored in the carrier network's database.

How Does The SIM Card Connect To Networks

• When you turn on your phone, it gets the IMSI from the SIM and sends it to your network. Your network will look up the IMSI in its internal database and will also search for your IMSI authentication key.

• After the network, we call "Xxxxx" and sign it with your authentication key to generate a new number "Yyyyy".

• The network sends an “Xxxx” to your phone, which forwards it to the SIM. The SIM signs with its authentication key to create a new "Zzzz" number.

• "Z" is then sent back to the network.

• If the network number “Yyyyy” matches the “Zzzz” of the SIM card, the SIM card is declared legitimate and you are given access to the network to make calls and use data.

What is eSIM

The industry is likely to shift to eSIM. This is where the SIM is embedded in the phone and activated remotely by the carrier, meaning you don't have to fiddle with tiny cards anymore. It's already in use for a second SIM on some dual-SIM devices, such as the iPhone XS. You still need a card for your main SIM though.