What do you know about phone SIM cards?

Have you ever wondered about the SIM card used in mobile phones? What are they and how do they work? Is there any way to use a mobile phone without a SIM card? Find the answer in this article.

1. Instructions for setting the PIN code for SIM phones
2. Instructions on how to view SIM / ICCID numbers on Android phones
3. How to check if the phone SIM you are using is a SIM card?

What is a SIM card?

There are two main types of phones for consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones use SIM cards while CDMA phones are not needed.

The SIM card is a small card containing the chip you need to insert into the GSM phone before operating. Without a SIM card, the GSM phone cannot access any mobile network because the SIM card contains all the important information.

CDMA operators have a list of all phones that can use their networks. CDMA phones are monitored by ENS (electronic serial number) so they do not need a SIM card. When operating, CDMA phones access directly to a specific service provider's network.

In the US, most mobile operators offer CDMA phones. However, in the world, GSM is the more popular technology by the legislature and industry that make suppliers promote using GSM.

What effect does a SIM card have?

What information does the SIM card hold? It contains the most important bits of data including IMSI (International Mobile Subscriber Identity) and IMSI authentication key (this key will be provided by the network provider).

In fact, SIM authentication works as follows:

1. On startup, the device receives IMSI information from the SIM card and forwards it to the network. You can understand this is the process of requesting access.
2. The network will retrieve IMSI information and find the known authentication key of IMSI in its internal database.
3. Then, the network will generate a random number A and sign it with an authentication key to create a new number B. This is the expected response if the SIM card is legitimate.
4. The phone receives the A number from the network and forwards it to the SIM card, signing it with its own authentication key to create a new number C. This number is forwarded back to the network.
5. If the A number of the network matches the C number of the SIM card, the SIM card is legal and granted access.

This data not only identifies the connection network, but also acts as a login information, allowing the phone to use the network.

Change phone with SIM card

SIM cards are actually quite convenient when switching phones because your subscription data is on the card. You can plug the SIM card into another phone and use it as usual. On the other hand, changing phones with CDMA networks is more difficult because the phone needs to register for a network.

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Each SIM card has a unique identification code called ICCID (Integrated Circuit Card Identifier), stored in the card and engraved on it. ICCID codes contain three numbers, including SIM card issuer identification number, personal account identification number and parity number calculated from the remaining two numbers for added security.

SIM cards are also capable of storing other information such as contact list data and SMS messages. Most SIM cards range in size from 32KB to 128KB. This capacity is mainly used when switching SIM cards to other phones, although this has become less important with the advent of backup applications. Therefore, SIM cards really have no effect other than granting access to specific networks.

What is SIM locked?

When your phone is locked to a specific network, it means that the phone is locked, not the SIM card is locked.

In fact, GMS operators develop software on their phones so that the phone can only accept the designated SIM card from the specific network. If the phone and SIM card do not match, the phone will not work.

Unlock the phone to eliminate this limit so that the phone can accept SIM cards from other networks. If you intend to sell the phone, you need to unlock it so that the buyer can use it with other networks.

Prepaid SIM

There is also another important point to consider which is prepaid SIM card. These prepaid SIM cards do not require you to register or sign a contract. It is often cheap and convenient when you travel internationally and want to avoid red zone roaming charges.

The rise of eSIM

With the development of technology, eSIM was born. This is a SIM that is embedded in the phone and activated by the network operator remotely. It has been used as a second SIM in some devices using two SIMs such as iPhone XS. However, you still need a SIM card as your primary SIM.

It will take some time for eSIM to become the standard. Until then take a look at some applications that help you manage SIM card data.