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What is a SIM card? Everything you need to know

A SIM card is an essential component to connect to your carrier's mobile network.
By
February 16, 2023
File photo of SIM cards.

SIM cards have been an integral yet forgettable aspect of owning a smartphone over the past few decades. For most users, the SIM card defines the basic commercial relationship they have with their carrier. But as we slowly inch forward, SIM cards are increasingly on their way out, losing importance to eSIM, especially thanks to Apple’s iPhone 14 series. If you ever wondered what exactly is a sim card, and whether you still need one in 2023, we’re here to answer your queries.

QUICK ANSWER

A SIM card is a small chip that you insert into your phone and allows you to connect to your carrier's network. You can then make phone calls, send messages, and use mobile data through your carrier. You still need a SIM card in 2023, but thanks to eSIMs and the proliferation of Wi-Fi, a physical SIM card is no longer as necessary as it once was to enjoy your new phone.


JUMP TO KEY SECTIONS

What is a SIM card?

HUAWEI Mate 20

SIM stands for “Subscriber Identity Module,” and a SIM card is essentially the card that holds your subscriber identity data. It’s the small chip-like card that you insert into your phone before you can begin making phone calls. A SIM is essentially an integrated circuit that holds the data needed to confirm your subscription identity to a carrier’s network.

Think of it like this: A SIM card is similar to a passport in some ways. A passport indicates that you are a citizen of a particular country, and through it, you can enjoy the rights available to citizens of that country. A SIM card is essentially the passport to your carrier, denoting that you are a subscriber to the carrier’s services. The carrier is reassured of your status as a subscriber, and lets you enjoy the services it offers to its subscribers.

A SIM identifies you as a subscriber of a network.

“SIM” and “SIM card” are often used interchangeably. SIM refers to the identification technology as a whole, while SIM card refers to the plastic card that holds the gold-colored electronic contacts that enables SIM tech.

What does a SIM card do?

Nokia 800 Tough review dual SIM and microSD card tray

The primary purpose of a SIM card is to identify you as a subscriber of a network. It’s the key that lets you into your carrier’s network. There’s more nuance and details at play here, but we’ll keep it simple for this explanation.

So for example, if AT&T has a good network in your region and you wanted to take advantage of it, you’d subscribe to AT&T’s network, and the carrier would then issue a SIM card to you. You can then physically insert this SIM card into your smartphone to access AT&T’s network to make phone calls, send SMS and MMS, and access mobile data through 5G and 4G.

Without the SIM card in place, your phone wouldn’t know what network it needs to connect to, and the network wouldn’t know that the phone in your hand belongs to a subscriber. You won’t get a phone signal in your phone without a SIM, so you will not be able to make phone calls.

How does a SIM card work?

A SIM stores several crucial pieces of data, including:

  • ICCID (Integrated Circuit Card Identification Number): This is a unique 18-22 digit code that is used to identify the physical SIM card itself. In common parlance, it is also called the SIM card number, but do not confuse it with your mobile number.
  • IMSI (International Mobile Subscriber Identity Number): This is a 14-15 digit code that forms the basis of identification for the subscriber.
  • Data related to security authentication such as the authentication key, LAI (Local Area Identity), and more.
  • Carrier-specific data such as SPN (Service Provider Name), SDN (Service Dialling Numbers), network identifiers, and more.
US Mobile Starter Kit with two SIM cards

Here’s a simplified flow of what happens when you insert a SIM card into your phone:

  • Your phone pulls the IMSI contained within the SIM card.
  • The phone identifies the mobile operator from the code and contacts it.
  • The phone passes the IMSI to the mobile operator for authentication.
  • The mobile operator searches within its database for the IMSI.
  • Upon successful location, the authentication key associated with the IMSI is also found. This is then utilized to help confirm the identity and authenticity of the SIM. There’s a fair bit of back and forth between the phone and the network operator in the authentication process, but these technicalities are outside the scope of this article.
  • Once authenticated, the mobile operator grants the phone access to its network.

The SIM is the most crucial piece in this whole puzzle, namely for the data it contains. The physical card is less important, and that is why alternatives like eSIMs have evolved to provide similar functionality.

Can a phone work without a SIM card?

Samsung Galaxy S23 Ultra Green Calling On Phone
Harley Maranan / Android Authority

The absence of a SIM doesn’t actually stop you from using a phone or connecting to the internet on a phone. Phones can be enjoyed without a SIM in place, with the caveat that certain network functions will understandably not work. For instance, you can still connect to a Wi-Fi network and access the vast majority of internet functions on your phone without inserting a SIM. You can not, however, make phone calls, send text messages, or connect to a carrier’s network.

As for the physical SIM card, solutions like eSIM and iSIM allow us to not need the physical card anymore, while still allowing access to all relevant SIM information to ensure a successful connection to a mobile network.

How many types of SIM cards are there?

There are four main types of SIM cards on the basis of their physical size, although only the smallest retains relevancy in current times. Beyond them, there are two other types of SIMs that people should be aware of.

Full-size SIM, mini-SIM, micro-SIM

GSM Micro SIM Card vs. GSM Mini Sim Card Break Apart
By Justin Ormont - Own work, CC BY-SA 3.0, Link

The full-size SIM was the first format of SIM cards, issued all the way back in the 1990s. It was the size of a credit card, although the total area of the gold-colored contacts was the same as we see now.

Mini-SIM and micro-SIMs were introduced later on, with the intention to reduce the size of the plastic holding the gold-colored contacts and allow for smaller mobile handsets to come into existence. As mentioned, they retained the same area for the gold-colored contacts as the full-size SIM. In the transition phases between two sizes, mobile operators would issue the bigger size SIM but offer an easy way to “break” the SIM into its smaller size.

Nano SIM

This is the current size of SIM cards that most popular smartphones accept. It’s the smallest size possible while maintaining backward compatibility, as the size of the card is practically just the gold-colored contacts and a very thin layer of plastic.

However, phone manufacturers find this small size enough of a liability to consider removing it completely. For instance, the iPhone 14 series completely removed the SIM card slot and opted for an eSIM-only solution in the US.

eSIM (Embedded SIM)

Advancements in technology have allowed SIMs to be programmed remotely, allowing the SIM to take the form of a chip built into your phone itself. There is no physical card for the SIM for you to insert into your phone anymore. The eSIM configuration profile can be installed onto a phone by the network (usually by providing a QR code that the subscriber can scan), so you don’t even need to pop open the SIM slot anymore.

eSIM support started off rather slow but has built up steam in the past few years. And with the launch of the iPhone 14 series, Apple removed the SIM slot entirely on phones sold in the US, which skyrocketed eSIM adoption in the region as well as across the globe. For better or for worse, most phones and most popular carriers provide eSIM options nowadays.

iSIM (Integrated SIM)

iSIM shrinks the SIM down even beyond what eSIM could achieve. It integrates the SIM directly into the modem chip or the SoC of the phone. So you’re no longer reserving the (relatively) larger space that an eSIM needed. While this technology has been slow to pick up on smartphones, it has great scope in IoT devices.

SIM vs eSIM: What’s the difference?

iPhone eSIM setup
Apple

When trying to understand the difference between a SIM and an eSIM, note that the SIM within this context usually refers to a nano-SIM card. It does not refer to the SIM technology per se — as an eSIM also includes SIM technology, but just not available physically for the user to access. An eSIM is a digitized SIM, after all.

SIM cards are different from eSIMs primarily in their physicality: you can touch a SIM card, but you cannot touch an eSIM. The eSIM resides within the phone and cannot be physically removed. SIM cards exist outside of the phone and are inserted into the SIM slot present on the phone. The biggest advantage that eSIM brings is space saving for phone manufacturers, as they can now eliminate the SIM slot without crippling network functions on the device.

eSIMs are also theoretically easy to transfer across devices. If you lose your phone, you can be issued a new eSIM on your new phone without needing to visit your carrier. You can also quickly and easily swap between different carriers, by just enabling the relevant carrier profile.

Not all operators and all phones support eSIMs.

However, network operators often have convoluted processes for issuing eSIM configurations, so there can be pain points coming in from your carrier and their outdated workflows. In comparison, you can usually just swap a physical SIM card from one phone to another within a few easy seconds. If you lose your phone, your carrier will have to ship you a new SIM card (which can take days to arrive), or you will have to walk into a carrier store yourself. To swap between carriers, you do have to remove the SIM and pop a new one in — but it’s a very reliable method that doesn’t fail.

Also, note that not all operators and all phones support eSIMs seamlessly, while a more blanket statement can be made for physical SIM cards. Many large operators continue to support both eSIMs and physical SIMs, and many recent phones come with support for both as well — with the notable exception of the iPhone 14 in the US, which is eSIM-only. If your phone is eSIM-only, then international roaming will also be a pain point, as you can’t just pop in a new local SIM card for cheap when you are traveling abroad — eSIM support is usually seen on larger, expensive carriers, though this situation is evolving.

How to insert a SIM card into a phone

Google Pixel 6 SIM Card Slot
C. Scott Brown / Android Authority

Despite the wide diversity of smartphones, the process of inserting and removing a SIM card is largely the same.

  • Locate the SIM slot. It is usually present on the phone’s mid-frame and looks like a closed cupboard drawer.
  • Locate the pinhole near the SIM card slot.
  • Use the SIM removal tool that came with your phone, or a small paperclip, to press into the pinhole. This will eject the SIM slot tray.
  • You can now insert the SIM card into the SIM slot tray.
  • There are likely to be markings present on the SIM slot tray to indicate which side of the tray should be facing the screen. Insert the tray back in that orientation.

And that’s it. You can now easily and quickly swap SIMs across your phones.

Note that phones also have microphone holes that look similar to the pin holes for SIM removal. These mic holes are usually present on the top and bottom edges of the mid-frame. Inserting the SIM removal tool into the mic holes could potentially damage the microphone if you apply excessive pressure. However, light pressure or accidental insertion shouldn’t damage the microphone as there are safeguards in the design. Still, be careful with the process.

Can I use two SIM cards on a phone?

ROG Phone 2 SIM Card Easter Egg

Most recent smartphones offer dual-SIM connectivity in some format. Generally speaking, phones routinely launch with either dual nano-SIM card support, or support one nano-SIM card and one eSIM active profile. Either way, there is a good chance you can use two SIM cards on your phone. The SIM card tray can either be a longer tray that accommodates two SIM cards (as shown in the picture above), or it could accommodate two SIM cards behind each other with their gold contacts facing outwards.

However, dual-SIM support can be a bit more hit-and-miss for US residents, as its usage is not very popular in this region as compared to the rest of the world. You can explore these dual-SIM Android smartphones if you are looking to run two SIMs on a single device. Note that even the eSIM-only iPhone 14 series in the US can run dual eSIMs.


FAQs

SIM cards are backwards compatible through the use of adapters. You can also use punches to cut out a smaller SIM card for a larger-sized card. However, your carrier will easily issue you a newer SIM card for a nominal fee, and we recommend that you choose that option for the additional security and benefits present in newer, higher-storage SIM cards.

No, you do not need a new SIM card to upgrade from 4G to 5G. The SIM upgrade was needed during the upgrade from 3G to 4G as the older SIM lacked certain capabilities. This is not the case for switching from 4G to 5G. However, ensure that your phone supports 5G, and that your carrier offers 5G in your region.

SIM cards do not usually go bad, but they can stop working due to poor maintenance and habits. If you remove them very frequently, the contact points could get scraped and fail. They can also fail due to corrosion and water damage. Bending a SIM card will also destroy the contact lines and cause failure.

Yes, SIM cards can be swapped between devices, as long as they are the same size. If sizes mismatch, you may need an adaptor or a punch, as the case may be.

It depends. On iPhone 13 and older, you can easily use a SIM card from an Android phone by simply popping it in to the SIM slot. However, the newer iPhone 14 series is eSIM-only in the USA, so you will need to migrate your eSIM configuration profile. You can do so during the iPhone 14 setup process, or contact your carrier for further assistance.

iPhone 13 and older iPhones come with SIM card support. However, iPhone 14 series sold in the US does not support a SIM card as the series is eSIM-only. iPhone 14 sold outside the US still retains SIM card support.

While it is technically possible to use a SIM card in an eSIM-only iPhone 14, we do not recommend users do so. The process involves disassembly and complicated motherboard resoldering, and thus, is best avoided by most people.