What is a SIM card & how does it work?

What is a SIM card & how does it work?

Communication is one of the most important aspects in our lives as it helps us to stay connected with our loved ones, especially when travelling. One of the most common ways of communicating is through mobile phones and most phones need a SIM card to function.

This article takes us through what a SIM card is, how it works, the features, benefits and how to get a pay as you go SIM card at travSIM.

What is a SIM card?

A SIM card, which is also known as a subscriber identity module, is a portable memory chip that stores information about you as a cell phone user. It allows subscribers to use their mobile devices to receive calls, send SMS messages, or connect to mobile internet services.

The data that SIM cards contain include user identity, location and phone number, network authorization data, personal security keys, contact lists and stored text messages. Without a SIM card, some phones would not be able to make calls, connect to internet services.

Not all phones with SIM cards work the same. There are two distinct technologies used, such as ; GSM (Global System for Mobile communication) and CDMA (Code Division Multiple Access). GSM phones use SIM cards while CDMA phones do not.

History of SIM cards

SIM cards were developed in 1991 by Munich smart-card maker Giesecke and Devrient. The first SIM card was the size of a credit card. Since then, there have been several updates and iterations, making them smaller and smaller.

Today, SIM cards are ubiquitous, allowing over 7 billion devices to connect to cellular networks around the world.

According to reliable research, there were 5.4 billion SIM cards manufactured globally in 2016 creating over $6.5 billion in revenue for traditional SIM card vendors.

However, the introduction of embedded-SIM (eSIM) and remote SIM provisioning (RSP) from the GSMA may disrupt the traditional SIM card ecosystem with the entrance of new players specializing in digital SIM card provisioning and other value-added services for mobile network operators.

How does a SIM card work?

A SIM card contains a smaller chip that works as its micro controller. The chip contains a processor, memory, and security circuits. Your mobile device reads the chip when you stick the SIM in it.

Your SIM card is also where the International Mobile Subscriber Identity (IMSI) and its authentication key are kept.

When you turn your phone on, it obtains the IMSI from the SIM and sends this to your network. Your network then looks up the IMSI in its internal database, and searches for your IMSI’s authentication key.

This key is provided by your provider, and it connects your phone number with your device. This means that when someone dials your number, the call will go the exact phone that you have.

Types, sizes and design of SIM cards

If you don’t know how a sim card looks like, a SIM card basically looks like a small piece of plastic but plays such a big role in our daily communication.

Here are the different types of SIM cards and sizes you can use.

Standard/Mini SIM cards
They measure 25X15mm and are used in older and basic phones, which may also be referred to as old generation cell phones.

Micro SIM cards
They measure 15X12mm and are likely to be found in phones from 2010 upwards, such as Apple iPhone, iPad, Samsung Galaxy and many more.

Nano SIM cards
These measure 12.3X8.8mm and are used in newer and latest smartphones, such as Apple iPhone 5, iPad Air, iPad mini and many more.

eSIM/embedded SIM
Measure 6X5mm and the SIM card is installed in the phone already. The SIM cards are activated remotely by the network provider.

Standard SIM (0.98 x 0.59 inches)
Micro SIM (0.59 x 0.47 inches)
Nano SIM (0.48 x 0.35 inches)

SIM card compatible devices


SIM cards are always used on GSM phones, for CDMA phones, they are needed only for LTE-capable handsets. SIM cards can also be used in satellite phones, smart watches, computers, or cameras.

GSM networks are used by AT&T and T-Mobile in the U.S. (and most carriers around the world), and CDMA is used by Verizon and Sprint.

Difference between a SIM card and an eSIM

A SIM card is a chip that is physically installed or removed inside your phone with your provider’s plan, while an eSIM (embedded sim) is built-in to your phone and downloads your provider’s plan indirectly.

Here are some of the differences between SIM cards and eSIMS.

  • eSIM cannot be physically damaged or lost and a SIM card can.
  • SIM cards are limited and eSIMs are not.
  • eSIMS are not available for older phones and SIM cards are.
  • SIM cards are easier to hack and eSIMs are not.
  • eSIMs can store multiple cellular profiles and SIM cards cannot.

Although they are both different, they also have similarities.

  • They both connect your phone to your mobile provider’s network.
  • They both may be used in many different devices (not just phones).
  • They both identify your phone and the type of plan is used.
  • They both can be used simultaneously in a dual-SIM phone.
  • They’re both technically SIMs.

Functions of a SIM card

So far, we get the idea of what a sim card is and what it does. Here are some functions which SIM cards perform.

  • Identifies the subscriber– The IMSI on the SIM card is the identity of the subscriber.
  • Authenticate the subscriber– This is where , using the authentication algorithm in the SIM card, a unique response is provided by each subscriber based on IMSI and RAND (provided by network).
  • Storage– Store phone numbers and SMS.
  • Applications– The SIM tool kit standard allows creating applications on the SIM to provide basic on demand and other applications for m-commerce, chatting, cell broadcast, phone backup and so on.

Benefits of SIM cards

SIM cards have several important benefits.

• SIM cards make upgrading your phone easy.
• They are very useful for travelers.
• They allow the user to use multiple phones with just one account, although using one phone at a time.
• You can download your contact information to your computer by purchasing a SIM card reader.
• SIM cards also allow the user to store contact information on the chip itself.
• They allow the user to transfer their services and personal information to a new phone without having to manually rewrite every file or purchase a new service plan.
• They are conveniently stored behind the battery of most phones, preventing damage, and keeping the user’s data plan safe.
• SIM cards have advanced functionality.
• They also eliminate switching costs.

How to get a SIM card

Ordering a travel SIM card at travSIM is a easy and convenient process.

Step 1: Shopping cart– Select the products you want to order by clicking the “Add to Cart” button.

Step 2: Till– Please log in with your email address and password if you already have a customer account, otherwise please register as a new customer.

Step 3: Check 3rd billing address / select payment method– Check your billing address and enter your desired Mode of payment. By clicking the button “Next” you will be taken to the next order step.

Step 4: Check 4th shipping address / select shipping method– Check your shipping address and specify your desired Shipping type. By clicking the button “Next” you will be taken to the next order step.

Step 5: Complete order process / general terms and conditions and revocation instruction– You will receive an overview of your order: the selected products, the shipping and billing address and your contact details.

Further information: Order process

Features of a SIM card

Some features of SIM cards include:

• International mobile subscriber Identity (IMSI)
• Location area identity
• Authentication key
• User identity
• Location and phone number
• Network authorization data
• Personal security keys
• Contact lists
• Stored messages
• Applications
• Personal identification number (PIN)
• Personal unblocking number (PUC)

Common SIM card problems

  1. Physical damage
  2. Network issues
  3. Calls and messages being delayed
  4. Bad sound quality
  5. Pin code requested frequently
  6. Welcome to network message frequently
  7. No SIM card contacts showing
  8. Airplane mode activated suddenly
  9. SIM card is not recognized
  10. You have cut your SIM message

SIM card FAQs

1. What is the difference between a SIM card and an SD card?
While SIM cards store data related to cellular connectivity, Secure Digital (SD) cards store other information, such as pictures, music, and cell phone apps.

2. What is stored on a SIM card?
SIM cards contain data specific to the user, such as their identity, phone number, contact lists, and text messages.

3. Does taking out the SIM card delete everything?
Removing the SIM card stops the phone from communicating with the network but doesn’t erase the email and contact lists already on the phone.

4. What is the difference between a physical SIM and an eSIM?
A physical sim is a small piece of plastic used in a mobile device that connects to a network carrier. An eSIM is emdedded onto a device and there are no physical sim cards involved.

5. Does an eSIM work without the internet?
eSIM doesn’t always require an internet connection. The connection is required when setting up profile and using it to surf the web.