What is a SIM card and why is it so important? It connects you to your carrier network, stores contact information, and more.
struggling with SIM cards can be frustrating when upgrading to a new cell phone or backing up a backup. Have we not gone far enough with technology that such a thing should no longer mean anything? What is a SIM card and what does it do?
Is there a way to use a cell phone without needing it? Keep reading to find out.
What is a SIM Card?
In the mobile world, there are two main types of phones available to consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones use SIM cards while CDMA phones do not work.
SIM cards are small cards containing a chip that you need to insert into a GSM phone before it can work. Without a SIM card, the GSM phone will not be able to touch any mobile network. The card contains all the important information.
By comparison, CDMA carriers keep a list of all phones that can use their network. Phones are tracked by their ESN (electronic serial number) so they do not need SIM cards. Once activated, the CDMA phone is directly connected to the network operator.
In the United States, many mobile carriers provide CDMA phones. The two main variants are AT&T and T-Mobile, both of which offer GSM phones. Internationally, GSM is the most popular technology for landslides, due to the influence of the legislature and industry that has shifted providers to the use of GSM.
What does a SIM Card do?
What information does the SIM card hold? The most important data fragments include IMSI (International Mobile Subscriber Identity) and IMSI verification key. The carrier offers this key.
If you are interested in the nitty-gritty, SIM verification goes as follows:
- When started, the phone receives the IMSI on the SIM card and transmits it to the network. Think of this as a “request for access.”
- The network picks up the IMSI and looks at its internal website for that known IMSI authentication key.
- The network generates a random number, A, and signs it with a confirmation key to create a new number B. This is the response I would expect if the SIM card was valid.
- The phone receives an A from the network and transfers it to the SIM card, signing it with its confirmation key to create a new number, C. This number is returned to the network.
- If the network number A is the same as the SIM card number C, then the SIM card is called valid and access is granted.
Long story short: this data not only determines which network to connect to but also serves as “login details” that allow the phone to use the specified network.
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How to Check SIM Card Number?
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Switch Phones With SIM Card
For this reason, SIM cards are actually simpler when it comes to switching phones. Since your registration data is on the card itself, you can connect the SIM to a different phone and everything will be fine. Conversely, switching phones with a CDMA carrier is extremely difficult as the phone itself is a registered network service.
Each SIM card has a unique identifier called ICCID (Integrated Regional Card Identifier), which is stored on the card and recorded on it. ICCID consists of three numbers. There is an SIM card identification number, the unique identification number for each account, and a rating digit calculated from the other two numbers for added security.
SIM cards can also store other information, such as contact list data and SMS messages. Most SIM cards have a capacity of between 32KB and 128KB. Transferring this data mainly involves removing the SIM card from one phone and inserting it into the other, though this is less important with the appearance of backup apps.
However, SIM card storage is now limited to internal phone storage capacity, so SIM cards have no function other than grant access to certain networks now.