Paul Marshall

Founder & CCO


Find out what the difference is between eSIM and eUICC, IMSI and multi-IMSI, so you can differentiate and choose the most appropriate SIM for your IoT device. SIMs 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.

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:

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

SIM form factors gif

Which form factor is best for your device?

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.

Benefits of embedded SIMs

Embeddable SIMs are often the best choice for IoT devices for a number of reasons:

SIM terminology and functionality


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.

­The UICC is often called the SIM card or SIM.


SIM stands for Subscriber Identity Module. Although it’s often used to mean the whole physical card or chip, a SIM is technically one part of the UICC. It’s responsible for securely storing data, including:

A SIM passes identity and authentication information to a cellular modem. The modem performs the actual connection to the network.


An International Mobile Subscriber Identifier (IMSI) identifies a user or device on a particular network. The IMSI is one of the key data items stored on a SIM.

Traditional SIMs contain a single IMSI, provided by the selected network operator. The IMSI is installed onto the SIM during manufacture and ties the SIM to the operator’s network (the home network). Depending on the roaming agreements that the operator has in place, devices can roam onto other networks, but the only way to change network operator or service provider is to physically replace the SIM with a SIM from a new provider.

Replacing a SIM in a device is often not practical in IoT. Devices may:

Some providers offer multi-IMSI SIMs to offer more flexibility in connecting to different networks.


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

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.

Despite the benefits of multi-IMSI solutions over traditional SIMs, they have some disadvantages:

Read more about multi-IMSI solutions.


To provide a more robust and standardised method for remote global SIM provisioning, the GSMA introduced eSIM. This solution is designed to meet the needs of industries where fast time-to-market, low operational costs, low complexity, and high reliability are critical to successful deployments.

In the GSMA definition, eSIM stands for embedded SIM and refers to the Embedded SIM Remote Provisioning Architecture, introduced around 2013. The GSMA definition of eSIM encompasses the SIM itself and the systems required to provision and manage network profiles.

eSIM technology provides the same functionality and security as conventional SIMs, with enhancements that enable:

The GSMA chose to use the word embedded on the basis that this architecture was designed primarily to support devices with fixed SIMs. However, many IoT devices use removable SIMs and it’s as important to manage connectivity remotely for these devices as it is for fixed SIM devices. Fortunately, SIMs of all form factors can be enabled to use eSIM.


eUICC stands for embedded Universal Integrated Circuit Card (eUICC). It refers to the software component of eSIM that runs on a UICC and provides the capability to store multiple network profiles that can be provisioned and managed over-the-air (OTA). The term eUICC is often used to mean the whole physical card or chip.

Although it was envisaged that eUICC SIMs would primarily be in the form of soldered chips or implemented within a system on a chip (SOC), eUICC SIMs can be any form factor.


The term eSIM is often used to mean an embedded SIM (such as the MFF2), regardless of whether it is an eUICC-enabled SIM. Therefore, to avoid confusion, the term eUICC is often used instead of eSIM to refer to the GSMA eSIM architecture and solutions.

Read more about eUICC >>

Remote SIM Provisioning (RSP)

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.

RSP consists of two key elements:

The SM-DP and SM-SR systems may be implemented separately or in a single platform.

Read more about our Remote SIM Provisioning (RSP) platform >>

Network profiles

A network profile contains the operator subscription data, including the authentication credentials, as well as SIM-based software and applications. The Trusted Connectivity Alliance (formally the SIM alliance) has defined the content and structure for eSIM profiles to ensure interoperability between all parties in the eSIM ecosystem.

eUICC SIMs can hold multiple profiles, with each profile acting as a virtual SIM within the physical SIM container. Only one profile can be active (or enabled) at any time.

Two types of profile are used:

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.

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.

While an eUICC is an integral part of a device, each profile stored in the eUICC remains the property of the operator and is supplied under licence.

The number of profiles that can be stored on an eUICC is limited only by the memory available and the size of each operator’s profile. eUICCs require a minimum memory capacity of 512kb but some have a capacity of several megabytes.

Learn more about eSIM Localisation using eUICC >>

eUICC manufacturer (EUM)

eUICCs can only be manufactured by accredited organisations. An eUICC manufacturer (EUM) uses data from the operator and creates a personalised profile for each eUICC.


iSIM stands for integrated SIM. An iSIM is a non-removable eUICC SIM that is integrated into a secure enclave alongside the processor and modem on a system on a chip (SoC). The iSIM offers the potential for lower power consumption, footprint, and manufacturing costs over traditional SIMs.

AnyNet solution

Eseye has engineered and enhanced its AnyNet solution to create a multi-IMSI, eUICC-compliant offering. The AnyNet+ SIMs are available in all standard form factors.

Read more about the AnyNet solution.Eseye AnyNet+ SIM

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.