eSIM Series: How Does eSIM Technically Work?

Someone who understands the advantages of eSIM (eUICC) might wonder how it works technically.

eSIM (eUICC) is an unlocked SIM card that enables you to swap between carrier services "over-the-air" without the need to change the SIM card physically.

Let's take a peek at the technical innovations that make the eSIM available.

Before diving into the technicalities, a short history lesson of the SIM cards. The very first SIM card was produced in 1991 and was the size of a credit card. Over the years, the size of the SIM card has remarkably reduced.

Many of us believed that the SIM card could go smaller than nano (4FF), but we were wrong. Then came the embedded SIM, which is also known as the MFF2. Please don't confuse embedded SIM with an eSIM.

MFF2 is a SIM that that's soldered in the device at the time of manufacturing. It cannot be replaced or removed unless you want to pick apart the device. The embedded SIM is useful from a device design perspective.

But, what do you do when you need to change a subscription from one carrier to another?

An answer to this simple question marks the end of the nearly 25-year reign of the traditional SIM. In all its sizes, from regular (1FF), mini (2FF), micro (3FF), nano(4FF) and embedded (MFF2), the SIM progressed without any revolutionary developments until the introduction of eSIM.

This technological innovation allows the user to change the carrier subscription remotely without physically replacing the SIM. It sounds magical, but there are a whole new infrastructure and sophisticated servers/platforms to facilitate this technology.

Now that we have your attention, I'll describe how this subscription (carrier profile) change happens.

With the introduction of eSIM, The GSMA introduced two new servers that need to be placed within a carrier network.

First is the Subscription Manager Data Preparation server, commonly referred to as the SM-DP. The second is the Subscription Manager Secure Routing server, referred to as the SM-SR. They work in tandem to simplify the subscription changes on the eSIM.

The SM-DP is the database, or warehouse, where you store all the different carrier subscriptions that can be downloaded onto the eSIM. Including information like network authentication keys, IMSIs (the unique identification number of a subscriber), and all subscription-related data from carrier A, carrier B, etc.

The SM-SR ensures that all information passing from the SM-DP to the eSIM card, and vice versa, is secured and cannot be intercepted by any hackers.

The combination of both servers allows the eSIM's end-user to download new subscriptions (carrier profiles) securely, activate the downloaded subscriptions, and delete the unused subscriptions. Or swap between two downloaded subscriptions.

Imagine that a device is currently connected with Carrier A, and you want to connect to Carrier B.

The first step would be for the SM-DP to make Carrier B's subscription available for download. Once available, it is downloaded on the eSIM via the SM-SR. After downloading, the user will request the SM-SR to activate Carrier B's subscription. The SM-SR will then send a command to the eSIM for activation. After the activation command is successful, the device would connect to Carrier B, and the Carrier A subscription would automatically be deactivated.

Does it still sound like magic?

This process can be repeated, and a user can jump from one carrier to another with a simple push of a button.

As the market needs and use-cases of consumers, i.e. smartphones, watches, laptops etc., is different from IoT & M2M use cases, the GSMA has separated the consumer eSIM architecture from the M2M eSIM architecture. I'm describing the use case for the M2M market.

However, the concept for consumer eSIM is the same, apart from minor functional and architectural differences. Therefore the consumer devices case is another topic for another day.

If you’re interested in understanding the eSIM requirements for cellular modules, please see this blog post.