What is a SIM card? Everything you need to know

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.

What is a SIM card?

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.

“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?

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.

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?

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 SMS-based text messages, or connect to a carrier’s network.

As for the physical SIM card, solutions like eSIM and iSIM mean you don’t 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

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?

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.

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

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?

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.

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