Paul Marshall

Founder & CCO


Before we can get into what a multi-IMSI SIM is, we must first define the terms SIM and IMSI.

What is a SIM?

A SIM (Subscriber Identity Module) card is the encryption device for the network and a file repository. The file repository contains the rules governing how the device can connect to a network. This includes a file containing the IMSI.

What is an IMSI?

An IMSI (International Mobile Subscriber Identity) is a globally unique number it is used to authenticate the device on a GSM or UMTS network.

IMSI is the acronym for International Mobile Subscriber Identity: a globally unique number that cellular networks use to identify and authenticate every user and/or device on a GSM or UMTS network. IMSIs are used in any network that connects with another and are stored on the SIM card.

A 16-digit IMSI consists of three elements:
• Mobile Country Code (a three-number code for the home country)
• Mobile Network Code (identifier of the home network operator)
• Subscriber Number (unique identifier assigned by the mobile network operator).

What is a multi-IMSI SIM?

A multi-IMSI SIM contains many IMSIs, each of which can connect to a different network. There is usually one bootstrap IMSI that comes pre-loaded, although more advanced SIMs can accommodate multiple bootstraps, and some SIMs can be updated with additional IMSIs over-the-air.

How does a multi-IMSI SIM work?

Multi-IMSI SIMs are often used in IoT because they are capable of maintaining a high level of connectivity and therefore device uptime. They are able to offer a fallback solution because there are multiple IMSI profiles – and connection options – stored on a single SIM. This means that it can switch networks to respond to circumstances like network failures and roaming restrictions, and ensure the IoT device remains connected to the best available network.

Further, multi-IMSI SIMs can be used to optimise connectivity depending on where the IoT device is located. For example, an asset tracking device needs to frequently change connection in order to regularly transmit data. Here the SIM can be set up to dynamically switch IMSI profiles to the nearest and most highly available network. Multi-IMSI SIMs offer a good solution for IoT estates that operate in multiple countries for this reason.

Benefits of multi-IMSI IoT SIMs

For most IoT deployments, zero-touch, out-of-the-box connectivity is essential. SIMs must be able to connect to a network from any location and if they can’t connect to a local network, they should be able to roam onto another network. Almost every mobile network provider claims to have hundreds of roaming agreements with other providers to give them global coverage. But not every roaming arrangement works in practice.

Problems may only be discovered when IoT devices, particularly static devices can’t connect in foreign countries, even with a roaming agreement in place. These issues can take months to resolve – especially for smaller operators.

A key capability of multi-IMSI SIMs is that they can maintain high connectivity by switching networks in response to circumstances such as network faults and roaming restrictions.

Switching IMSIs doesn’t change from the current service provider but allows the SIM to localise to another network to which the provider has access, either by agreement or through interconnects.
But not all multi-IMSI SIMs are equal. Some provide a limited capability to swap IMSIs based on pre-configured rules. Others provide a fully functional solution that ensures devices can connect out-of-the-box and remain connected for their lifetimes.

Bootstrap options

No single IMSI can connect everywhere in the world. Devices either need different SIMs for different locations, or SIMs must be pre-loaded with multiple bootstrap IMSIs. Some multi-IMSI SIMs use a different pre-configured bootstrap IMSI depending on where the device is installed.

A better solution is for the SIM to dynamically determine which IMSI is best to use from its available bootstrap IMSIs. That way it can connect even if it encounters unexpected problems – without any external intervention.

Using bootstrap IMSIs from Tier 1 providers – well-established network providers with extensive, verified roaming agreements – provides the best guarantee that every device will connect out-of-the-box, anywhere in the world.

Intelligent connectivity

IoT SIMs should connect to the network that provides the highest level of connectivity. This means that a SIM might need to switch networks to avoid permanent roaming restrictions, or in the event of network faults or poor service.

Ideally, the SIM should have built-in intelligence so it can switch autonomously to another network when required. For example, if another network has comparable signal but lower costs. It should also be capable of receiving new IMSIs and commands to enable, disable and delete IMSIs from the connectivity management platform.

If a SIM fails to connect to a new network, there must be a way to recover or the device will remain unconnected. The SIM should have a fallback capability so that it can always revert to an IMSI that can connect, such as one of the bootstrap IMSIs.

An IoT connectivity management platform allows estate and device managers to manage this connectivity solution and add/remove IMSIs over-the-air throughout a device’s lifecycle.

Improved IoT security

Some providers use a single security domain on their multi-IMSI SIMs, with the same access keys used for all IMSIs.

A more secure method is to use separate security domains and keys by downloading and storing complete profiles for each network rather than just the IMSIs. This is possible if the provider has interconnects to the networks.

The SIM should be capable of receiving software and security updates over-the-air so that security can be managed centrally and maintained throughout the IoT estate.

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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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