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What is a SIM Card? How it works? Different Formats of SIM Card !

The struggle with SIM cards can be an annoyance when upgrading to a new cell phone or reverting to a backup. Haven’t we come far enough with technology that such a thing shouldn’t matter anymore? What is a SIM card and why is it so important? Is there a way to use a mobile phone without requiring one? Keep reading to find out.

What Are SIM Cards?

In the world of mobile phones, there are two primary phone types that are available to consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones are the ones that utilize SIM cards while CDMA phones do not.

SIM cards are the small cards which contain a chip that must be inserted into GSM phones before they will work. Without a SIM card, a GSM phone won’t be able to tap into any mobile network. The card is what holds all of the critical information.

For comparison sake, CDMA carriers keep a list of all phones that are allowed to use their network. Phones are tracked by their ESN (electronic serial number) so they do not require SIM cards. Once activated, a CDMA phone is tied directly to that particular carrier’s network.

In the United States, most mobile carriers provide CDMA phones. The two exceptions are AT&T and T-Mobile, who both provide GSM phones. Internationally, GSM is the most popular technology by a landslide. Why? Mostly due to legislature and industry influence that nudged providers towards using GSM.

How Do SIM Cards Work?

What sort of information does a SIM card hold? The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. This authentication key is provided by the carrier.

If you’re interested in the nitty-gritty, SIM authentication goes like this:

On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key. The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate. The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network. If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

Each SIM card has a unique identifier called the ICCID (Integrated Circuit Card Identifier), which is stored in the card and engraved upon it. The ICCID contains 3 numbers: an identifying number for the SIM card issuer, an identifying number for the individual account, and a parity digit that’s calculated from the other two numbers for extra security.

SIM cards are also capable of storing other information, such as contact list data and SMS messages. Most SIM cards have a capacity between 32 to 128 KB. Transferring this data mainly involves removing the SIM card from one phone and inserting it into another, though this has become less important with the advent of backup apps. However, SIM card storage is now dwarfed by internal phone storage capabilities, so SIM cards really have no use other than to grant access to specific networks now.

Formats and sizes of SIM cards:

SIM cards are available in various sizes. At the beginning of the mobile era SIM cards in the credit card format of 85.60 x 53.98 x 0.76 mm (LxWxH) were the market standard, but the increasing miniaturization of devices has seen ever smaller formats being created.

Today's mobile phones feature Mini-SIMs are (25 x 15 x 0.76 mm), Micro-SIMs (15 x 12 x 0.76 mm) and Nano-SIMs (12.30 x 8.80 x 0.67 mm).

Dual SIM technology:

Some mobile phones have dual SIM capability or can be operated with dual SIM adapters. This allows two SIM cards to be used simultaneously, so that one mobile device can have two different contact numbers.

These are either active simultaneously or the mobile phone can switch between them. These mobile phones make it possible, for example, to combine, yet strictly separate, both work and personal communications on a single device.

 This data not only determines which network to connect to but also acts as the “login credentials” which allow a phone to use said network.
For this reason, SIM cards are actually quite convenient when it comes to switching phones.
Since your subscriber data is on the card itself, you can plug the SIM into a different phone and all will be well. On the other hand, switching phones with a CDMA carrier is more difficult since the phone itself is the entity that’s registered with the network.

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