Every month, you pay a bill for cell phone service. Somehow, that access needs to make it to your device. The SIM card, or “subscriber identity module,” connects your phone with your phone bill.
While you might expect authentication to happen over a network with a simple username and password, cell phones predate the Internet, so SIM cards can seem a little old-fashioned. Still, the industry is trying to modernize itself while maintaining backward compatibility with older devices, and that has led to the rise of many different SIM card formats. The newest is iSIM, the latest advancement in a never-ending quest to save space in your smartphone.
Ye olde physical SIM card
Common physical SIM card sizes.
Enlarge / Common physical SIM card sizes.
Wikipedia
Physical SIM cards have been around forever. They’re the hunks of plastic you get from your phone carrier and slide into your cell phone. A small gold chip printed on the card—much like those on credit cards—makes electrical contact with the slot in your phone. Even these physical cards have their own standards and have shrunk over time; there’s Mini SIM, Micro SIM, and Nano SIM, which all use the same chip with differing amounts of plastic around it.
Going back even further, there was a credit card-sized “Standard SIM” without any of the diminutive prefixes. If you want to understand why SIM cards work the way they do, it’s worth looking at how these smart cards previously operated. Many of the earliest cell phones in the 1980s were fixed car phones or 12-pound luggable devices with shoulder straps and lead acid batteries. (The first cell phone, the Motorola DynaTac, was a lot smaller, but it had a half-hour battery life. Practical phones needed to be much bigger.)
A pre-SIM “smart card” for the Motorola PRX C451 from the incredible “BrickCellphones” YouTube channel. See that white arrow and slot on the phone body? That’s where the card goes.
Enlarge / A pre-SIM “smart card” for the Motorola PRX C451 from the incredible “BrickCellphones” YouTube channel. See that white arrow and slot on the phone body? That’s where the card goes.
BrickCellphones
Since phones were less portable at that time, you could take your “smart card” with you instead (they didn’t use the “SIM” standard yet). The card contained your billing information and address book so you could just slot it into any phone and make a call. It used a credit card form factor because you used it a lot like a credit card—you might be expected to insert it into multiple phones each day to pay for phone service. (If you’ve ever noticed how a SIM card chip and a chipped credit card look the same, they are based on the same standard, ISO/IEC 7816.) The card slots in early phones were also very easy to access, making frequent card swaps as easy as using a credit card terminal. You could rent a car, stick your credit-card-sized SIM into the car phone, and off you went.
You’ve probably gotten one of these breakout cards before, but the full-sized card is a standard, too.
Enlarge / You’ve probably gotten one of these breakout cards before, but the full-sized card is a standard, too.
Wikipedia
SIM cards were introduced in 1991, but the multi-device card-swapping paradigm of 1980s smart cards was built into the design. Eventually, phones were made to be the same width as a credit-card-sized SIM, and as preferences shifted to pocketable, personal devices that people carried around all day, it was clear that full-sized cards were becoming a limiting factor—and a waste of space. The SIM card was cut down to a Mini SIM, and instead of being an easily swappable card, it became a semi-permanent, fidgety little chip that went into a hidden compartment. They’ve been getting smaller ever since.
SIM cards are computers. They have a CPU, RAM, ROM, flash storage, and an operating system stored under those gold contacts. They get a couple of volts from your phone, boot up, and start communicating. Today, SIM cards use something like the ARM SecurCore SC300, which is a 32-bit, ~20MHz Cortex M3 core with somewhere between 8KB and 30KB of RAM (Yes, this means your wallet is also full of ARM chips). They usually have around 265KB of storage space, which stores important data like your International Mobile Subscriber Identity (IMSI) number—the number that connects your phone to your phone number and allows billing—and other bits that set up your phone to connect to a specific network.
A X-ray of a SIM card. You can see the actual ARM chip in the middle.
Enlarge / A X-ray of a SIM card. You can see the actual ARM chip in the middle.
Wikipedia
SIM cards can also store some user data like your address book and SMS messages, and while that data is useful if you’re in the 1980s days of SIM swapping, modern smartphones—with their new-fangled Internet connections—make the portable storage aspect of SIM cards obsolete. At the end of the day, you’re storing about 256KB of data on a 12×9 mm card, which is ridiculously bad data density. MicroSD cards are about the same size and can store up to 1TB, or about 4.2 million times more data, in the same space.
While many phones still use SIM cards for regional compatibility or budget reasons, flagship phones and smaller devices like smartwatches are killing the physical SIM card. In an industry that can’t abide spending a few millimeters of space on a headphone jack, slotting a plastic chip into your device just to provision you for service is unacceptable.
The eSIM: A SIM card, but soldered onto your motherboard
An eSIM chip. It’s a SIM card in a soldered-on chip package.
An eSIM chip. It’s a SIM card in a soldered-on chip package.
STMicroelectronics
So SIM cards started as big credit cards, but with the “nano SIM” form factor, all of the useless plastic was trimmed off, and we were left with the “smart” gold part. Going smaller required a change to the electrical connection, and that’s where eSIM, or embedded SIMs, come in. eSIMs take all that SIM card stuff and solder it right to your device’s motherboard in a form factor that’s a lot smaller than the physical card standard.
eSIMs are really just SIM cards in a new electrical package. They still use a 32-bit ARM chip with a few KB of RAM and storage, but they are permanently attached to your motherboard rather than a removable slot. As a chip, the chip package size and motherboard connection can be freely changed without worrying about compatibility with other devices. You get to control the chip size and the size of the bits it connects to. So eSIM started at 6×5 mm, almost half the size of a 12.3×8.8 mm nano SIM, and continued to shrink. Today, you can get them in 2 mm squares.
It’s not just about the size of the card, though; eSIM means you can also get rid of the SIM card reader, which means you no longer need a SIM card tray, a way to open it, and an opener tool. If you’re making a waterproof device, you no longer have to worry about making the SIM card opening water-resistant—you can just get rid of it. Those are all precious space (and cost) savings. eSIM cards debuted on smartwatches; the first was the Galaxy Gear S2 Classic 3G in 2016, and the chip landed in the Apple Watch Series 3 in 2017.
An STMicroelectronics ST33G1M2 eSIM chip inside the Samsung Galaxy Watch 3.
Enlarge / An STMicroelectronics ST33G1M2 eSIM chip inside the Samsung Galaxy Watch 3.
iFixit / Ron Amadeo
The first eSIM phone was the Google Pixel 2 in 2017. The iPhone started moving to eSIMs with iPhone XS in 2018, and Samsung started in 2021 with the Note 20. Some of these devices supported eSIM and physical SIMs at the same time, but today, it’s common for flagship smartphones to not have a SIM slot at all.
Not being able to remove your SIM card does lead to some complications. eSIMs have to be provisioned entirely over the Internet, which requires more carrier buy-in. Apple removed physical SIMs with the iPhone 14, though, and that was a major wake-up call to carriers that were slow to adopt the previously optional technology.
On modern carriers like Google Fi, setting up or transferring your eSIM is as easy as tapping a few buttons in the carrier app. It would be nice if every carrier was like this. Compatibility issues still exist, with some carriers not supporting eSIMs on certain models of phone, though the situation is getting better every day. If you travel internationally and are used to picking up a random SIM card at the airport, that’s still tough to do with eSIMs. SIM cards have a 30-year legacy of universal compatibility, and eSIMs aren’t quite there yet.
The ever-smaller iSIM future
Doing away with bulky SIM cards, slots, and trays is a good start for space savings, but we can do better than a chip on a motherboard. The modern foundation for devices smaller than a laptop (and, increasingly, laptops as well) is the SoC, or system on a chip. Rather than individual components for a CPU, GPU, modem, audio, and controller hubs, the SoC asks, “What if we just packed everything into one chip?” The one-chip solution does away with tons of chip packages and motherboard routing, unifies cooling, and saves space.
The result is lower power usage and, thanks to a smaller board footprint, more room for battery, and that has brought a revolution in portability and battery life to phones and basically enabled smartwatches to exist. The rule for what you include in the SoC is basically “anything that is universal across all models,” and that has meant everything other than storage and RAM, which is often changed to offer multiple tiers of the same base product.
Qualcomm is doing this iSIM project with the tech company Thales, which made this lovely graphic.
Enlarge / Qualcomm is doing this iSIM project with the tech company Thales, which made this lovely graphic.
Thales
It makes a lot of sense, then, to throw the SIM silicon into the SoC, too. This iSIM, or “Integrated SIM,” converts the SIM “card” from a chip on the board to just one more component in the SoC. Being part of the SoC means the eSIM can be extremely small. The above graphic shows it as less than one-millimeter square, a change from the normal 12×8.8 mm nano SIM size. Plus, that’s just right now; as part of the SoC, eSIMs will continually shrink. SoCs in flagship smartphones and smartwatches are made with state-of-the-art manufacturing technologies resulting in ever-smaller transistors, and that now includes the eSIM silicon as well.
Kigen—a SIM business that spun out of Arm—tells manufacturers that iSIMs are “98 percent smaller” and “50 percent cheaper” and that they use “up to 70 percent less power” than eSIM, which is a compelling sales pitch. The software piece is already figured out, and iSIMs have all the same activation gotchas as eSIMs. You’ll need over-the-Internet provisioning since iSIMs are a permanent part of your device, but iSIMs will use the same provisioning system as eSIMs, so any carrier that has eSIMs figured out will be fine with iSIMs. The difference should be invisible to users.
While eSIMs are prevalent, there aren’t any iSIM phones yet. Since the switch from eSIM to iSIM is all upside, anyone who’s OK with an eSIM-only phone should switch as soon as possible. The first major chip vendor to offer iSIMs as an option is Qualcomm with the Snapdragon 8 Gen 2. I’m sure other vendors will jump on the iSIM train soon, too, and then we’re done. SIM cards can’t get any smaller.
