If you have a cellphone, you may know that you need a SIM card or the phone won’t work. But what exactly is the SIM card, and why do our phones need one?
A SIM card is also known as a subscriber identity module. It stores data for people who use GSM (Global System for Mobile communication) phones. GSM is used in most of the world, including Europe, Asia and Africa. Meanwhile, here in the United States, we use mainly CDMA (Code Division Multiple Access) phones. GSMs can use any carrier’s network. This lowers the price of phones and how much costs to manufacture them.
U.S. network carriers such as AT&T and T-Mobile use GSM, while the other carriers — including Sprint, Verizon and US Cellular — use CDMA. GSM phones are optimal for travelers because they can be used to make phone calls and send texts outside of the U.S. CDMA phones require WiFi to make free calls and send texts. They can be tracked by their ESN, also known as electronic serial numbers.
SIM cards are used to track phones for GSM phones since each SIM card has something called the ICCID (Integrated Circuit Card Identifier), which is made up of three numbers. The first number is an identifying number for the user, the second number is for the individual account, and the third number that comes from the first two numbers is used as extra security.
CDMA phones are tied to the carrier’s network once they are activated. Permission is needed to switch to a new phone if one uses a CDMA phone. Sometimes, CDMA carriers don’t accept new phones.
Without SIM cards, GSM phones won’t work. SIM cards consist of a chip with information, including the person’s phone number, an identification number and other data linked with the person using the phone. SIM cards can also contain up to 250 contacts and some text messages.
Photos are not contained on SIM cards so make sure, if you want to keep them, you back them up. The most important information SIM cards contain is the IMSI (International Mobile Subscriber Identity), along with the authentication key that validates it. The carrier provides this key.
During the startup, the phone obtains the IMSI and sends it to the network (kind of like a request), the network then looks at the IMSI’s known authentication key, generates a random number (A), signs it with the authentication key that then produces a new random number (B), sends A to the phone, which then forwards it to the SIM card. The SIM card uses its own authentication key to produce a new number (C), which is sent back to the network, and if the network verifies that C ends up being the same as A, the SIM card is determined as legitimate and gains access to the network.
A SIM card’s features depend on where you buy it — more specifically, what country. One feature includes whether the phone is locked to the carrier, which means you can switch over the SIM card as long as the phone is sold by the carrier.
SIM cards are inside the phone with the slot either on the side or where the batteries are. Sometimes, you can remove SIM cards with just your finger, while other times, you’ll need a tool (a paperclip or a credit card). There are also videos that can be helpful when you want to remove SIM cards.
The SIM card is very intricate but it is useful in cellphones, which is something most of us use on a daily basis.
Vaageesha Das is a rising 10th grader at Morgantown High School. Today’s information comes from: https://www.thestreet.com/techno
logy/what-does-sim-card-do-14796633; https://www.makeuseof.com/tag/
why-do-cellphones-need-a-sim-card/
