Understanding IoT SIMs

The ultimate guide to SIM technology

Unlike M2M, IoT is a software and hardware-based technology. Cellular devices are activated by an IoT SIM which come in all shapes and sizes, types and form factors. But not all SIMs are built equally as some offer increased functionality and features. We get down to brass tacks in this article and explain the different SIM terminologies.

What is a SIM?

The UICC is often called the SIM card or SIM, but SIM stands for Subscriber Identity Module.

The Universal Integrated Circuit Card (UICC) is the hardware containing the SIM software and applications that enable a device to access cellular networks. A UICC contains a computer or microprocessor, its own data storage and software.

A SIM is technically just one part of the UICC. It’s responsible for securely storing data, including:

  • The Integrated Circuit Card Identifier (ICCID), which identifies the physical SIM itself
  • The International Mobile Subscriber Identity (IMSI), which identifies the mobile network to which the SIM is subscribed
  • Credentials and cryptographic security keys that identify and authenticate a subscriber on a network

A SIM passes a network profile containing the operator subscription data, including the authentication credentials, as well as SIM-based software and applications to a cellular modem. The modem performs the actual connection to the network.

What is an IoT SIM?

IoT SIM cards are designed for the specific purpose of connecting IoT devices and machines to the internet. They are typically used in connected devices such as sensors, trackers, smart meters, payment systems, and other non-consumer devices.

IoT SIMs are typically used with specialised IoT networks and agreements that prioritise coverage in areas where the IoT devices are deployed, and offer pricing plans tailored to IoT data usage.

Unlike consumer SIMs used in mobile phones and tablets, IoT SIM cards can come in various form factors, including traditional SIM cards, but they are increasingly being embedded directly into IoT devices. Embedded SIMs are soldered onto the device’s circuit board, making them more durable and tamper-resistant.

What are SIM form factors

The size and form of a physical SIM is known as its form factor (FF). Removable SIMs are available in a range of standard sizes ranging from 1FF, the first and largest SIM to be developed, to 4FF or nano-SIM, the latest and smallest SIM. There are also embedded SIM options—the MFF2 and USON-8.

Typical sizes of SIM form factors:

Form Factor


1FF (Not used in modern IoT devices)

85.6mm × 53.98mm × 0.76mm

2FF (Mini SIM)

25mm x 15mm x 0.76mm

3FF (Micro SIM)

15mm x 12mm x 0.76mm

4FF (Nano SIM)

12.3mm × 8.8mm × 0.67mm

MFF2 (Embedded, M2M SIM)

5mm x 6 mm x 0.9mm

USON-8 (Embedded SIM)

2mm x 2mm x 0.5mm

iSIM (Integrated SIM)

Less than 1sq mm

A triple-cut card contains 2FF, 3FF and 4FF form factors for the SIM. The required sized SIM can be snapped out.

AnyNet+ SIM form factors

Although the physical size differs, the capabilities of each form factor are the same. The choice of form factor depends on considerations such as the amount of space available for the SIM and the environment in which the devices will be used.

Prior to the introduction of remote network switching, removable SIMs had to be used in order to switch to a different network operator. The removable form factors, 2FF–4FF, are available in consumer or industrial-grade quality. Industrial-grade SIMs come with a thicker pin plating to safeguard devices from corrosion, vibrations, and other environmental factors, such as extreme temperature conditions (typically between -40°C and +105°C).

The MFF2 SIM is an integrated circuit that’s designed to be permanently soldered into an IoT device. It has eight electrical pins, which are the same as the eight gold contacts on removable SIMs.

The USON-8 is a plastic ultra-thin small outline no-lead package with 8 connectors. USON-8 chips are built into cellular modules or modems.

Network Profiles

A network profile contains the operator subscription data, including the authentication credentials, as well as SIM-based software and applications.

Two types of profile are used:

Operational (Step 2) profile

An operational profile (also known as a step 2 profile) provides a device with full access to the operator’s network.

Downloading and enabling a new operational profile means a device can switch to a different network operator without the need to physically change the SIM.

Bootstrap profiles

A bootstrap profile (also known as a provisioning profile) enables a device – at a minimum – to access a cellular network in order to communicate with the RSP system when it first starts up. The RSP system can then download and activate an operational profile for the device to use.

Most providers supply an IMSI in the bootstrap profile that provides access to the operator’s network and roaming agreements. This enables the device to connect as soon as it starts up.

Remote SIM Provisioning

Remote SIM Provisioning (RSP) is the secure management of network profiles on eUICC-enabled devices using over-the-air commands.

RSP uses secure communication channels to download, install, activate, and delete network profiles on devices. To ensure security and interoperability, all eUICCs and RSP systems must achieve GSMA certification to operate within the eSIM ecosystem.

The history of IoT SIMs

The first SIM – introduced in the early 1990s – was the size of a credit card, had little memory and required a 5V voltage supply.

In response to requirements for ever-smaller, more efficient devices, new versions of the SIM shrank in form factor from Mini to Micro to Nano. The new versions offered better performance, functionality and security – and a reduced voltage requirement of 1.8V.

In the early 2010s, the embedded SIM was introduced to overcome issues with removable SIMs. Issues included the space required on devices for the socket, the potential for damage or theft and the cost and complexity in managing SIMs separately to the devices.

The iSIM is the next step on this evolutionary path – completely removing the requirement for a separate SIM (card or chip) by integrating the SIM functionality into a System-on-Chip (SoC).

Types of IoT SIM

Global IoT SIMs are designed to connect to cellular networks in various countries and regions and can support multiple network operators, which is essential for single-SKU production for global deployment, or for devices that might move across borders.

There are different ways to achieve global connectivity:


A Multi-IMSI SIM can store multiple IMSIs, enabling devices to switch to different networks without physically changing the SIM.

For most IoT deployments, a single MNO doesn’t provide the flexibility to handle global installations and single SKU products, mobile devices, gaps in coverage, roaming restrictions, and changes in the commercial and service landscape in mobile telecommunications. Connectivity providers offer multi-IMSI solutions with different levels of sophistication, functionality, and security. Some solutions can use over-the-air updates to download additional IMSIs and remotely manage the IMSIs on a SIM.


One of the major barriers to successful IoT deployments is the difficulty in switching devices to different mobile networks once the devices are in the field. With changes in legislation around data sovereignty and updates to IoT roaming agreements, permanent roaming using traditional or multi-IMSI IoT SIMs isn’t a feasible option in many situations.

The introduction of embedded SIMs was a step change for IoT but it intensified the need to manage SIMs remotely – if you want to change to a different network, you can’t just swap one embedded SIM for another.

In response, the GSMA developed the eSIM specification. This uses an eUICC SIM, which can store multiple operator profiles, and a Remote SIM Provisioning (RSP) solution to enable connectivity providers and end users to manage SIMs remotely.


iSIM stands for “Integrated SIM”. An iSIM is a non-removable eUICC SIM where cellular connectivity and subscriber identity management functionality are integrated into a System-on-Chip (SoC).

Unlike traditional physical SIM cards that need to be inserted or embedded into a device, moving the SIM into a SoC reduces the footprint required on the device for connectivity components.

Choosing a connectivity solution for your device

For any IoT device to deliver on its promise, it must have access to a secure, reliable connection. Are you looking to produce and deploy IoT devices but not sure which connectivity solution you need?

Our article, “Cellular IoT Connectivity: What Business Leaders Need to Know” provides valuable insights for achieving success in cellular IoT. It covers topics such as selecting the optimal network, maximising coverage and uptime, and addressing connectivity design considerations for your device.

In this article

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Combining the best of Eseye’s multi-IMSI technology with an enhanced approach to the eUICC standard – available in all standard form factors. Get the most advanced IoT connectivity solution on the market today.

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Paul Marshall

Founder & CCO


Paul is one of Eseye’s co-founders. With a background in senior design engineering, Paul’s focus is on ensuring his development, operations and support teams deliver solutions that work faultlessly in the field.

Paul was co-founder of CompXs, with Ian Marsden, and developed the world’s first IEEE 802.15.4 radio. Before CompXs, Paul was in senior radio design at Philips.

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