SIM cards

And eSIMs and iSIMs

Those fiddly little cards from your cellular provider that you have to put in your phone? Those are called SIMs or SIM cards. What are they? What purpose do they serve? Is there a better way? That’s our subject this time.

Before I get into that a friend shared how he ensures his Zelle transactions are going to his intended recipient. He always sends or requests a one dollar test transaction to make sure the payment is going to the right place. Once that transaction comes through, the rest of the transaction can proceed. Great idea!

Ok, first, what does SIM stand for? It’s a Subscriber Identity Module and identifies which cellular networks you’re allowed to connect with. More than one network? Sure, when you’re “roaming” (outside of your cellular provider’s service area) your SIM card lists the other networks with which your cellular provider has agreements to allow you to connect for service.

Printed on the card (and stored on the card’s memory) is the ICCID or Integrated Circuit Card IDentification - a long string that uniquely identifies that specific SIM card. You might be asked for this string when starting up service with a new SIM card.

Memory? The SIM card has a memory and can store stuff? Why is that? Back in the early days before our phones were “smart” and really didn’t do much besides make phone calls, SIM cards were big things, about the size of a credit card. They were used to store our contacts and our text messages. You can still store contacts and text messages on your SIM card today, but that feature is rarely used since we tend to save our contacts and text messages on our phones or in a cloud service. But the SIM cards still have memory and, if you look really hard on your phone, you can probably find how to save contacts and texts on your SIM card. But why? I don’t use that feature as it just confuses me. But the SIM card stores other things that are used to get you onto your cellular provider’s network, things that I won’t cover here. If you’re interested in those details, take a read of

Over time, phone manufacturers needed more of the space that was taken up by the SIM card, so the SIM card makers had to start shrinking them. First to a mini SIM which was roughly 1 inch by 0.6 inches, then to a micro SIM at about 0.6 inches by 0.5 inches, then a nano SIM at roughly 0.3 inches by 0.5 inches. Today’s phones use nano SIMs but shrinking that physical size didn’t change what’s stored on the SIM. Ultimately, the physical SIM card has been shrunk out of existence and replaced by an eSIM or “embedded” SIM which has no physical card! Most modern phones have both a nano SIM slot and an eSIM and, yes, this means you can have two different phone numbers and even two different cellular providers on one phone! I won’t go into how you manage such an environment here - Android is just getting that feature whereas Apple has had it for a while. If you want to see hw to handle it with an Apple device, see

How do you get the information your cellular provider needs to give you access to their network (the stuff that’s normally stored on a physical SIM card) into your phone if you have an eSIM? Unsurprisingly, it varies, largely depending on whether you have an Apple device or an Android - your cellular provider (or your retailer if you’re not buying from your cellular provider) will walk you through the process. But if you want a quick overview, see

For a pictorial view of the various sizes of the physical SIM cards as well as some of the history and even a more detailed discussion of eSIMs, see

For completeness, I’ll mention iSIM or Integrated SIM. It’s not in use by any phone manufacturer or cellular provider today as far as I know but it’s the next step in the SIM evolution. The iSIM is very small, about 1 millimeter by 1 millimeter, and integrated directly onto the main processing chip. It has a number of benefits over the eSIM including enhanced security and the ability for your cellular provider to send updates to your service directly to your iSIM. If you want to read more about it, see - the beginning of that article gives you a brief overview of iSIM.

That's all for this week's column. I hope this helps you understand SIM cards, their sizes, what they’re used for, and what’s in store for the future. Don't hesitate to write to me if you have questions!

As always, my intent is to help you understand the basics and equip you to search for more detailed information.

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