What is an International Mobile Subscriber Identity (IMSI)?
IMSI is a 15-digit unique number stored on the SIMs itself. It is used by the cellular network to identify mobile networks and users.
The first three digits are the mobile country code (MCC). The next 2–3 digits are the mobile network code (MNC), indicating the subscribers’ home network code. The remaining digits identify the line of service, or what is also known as the subscriber and their plan.
For example, if a Cellular IoT device deployed in the field attempts to establish a connection to a network, the network identifies it by the IMSI, while the mobile station performs a location update.
Why is the IMSI Important for Cellular IoT?
As a definitive identifier of the line of service, the IMSI is critical in device authentication.
Each IMSI has a secret key (Ki) attached to it. Both the key (Ki) and the IMSI are stored on the SIM. Together, they create a key-in-lock combination that authenticates the device on a mobile network. It enables the operator to identify and verify information on the customer such as their data plan and data usage.
Beyond authentication, the IMSI is also useful for network roaming. Since the IMSI is linked to your home network, when a Cellular IoT device is deployed in a different country or region, other networks can identify it by its IMSI and determine agreements and roaming rates.
Current Challenges with IMSIs in Cellular IoT Deployments
The IMSI and key (Ki) provide an effective solution for consumer devices, which rarely need to use multiple SIM cards or lines of service.
In Cellular IoT deployments, your needs are a little different.
Single Network Limitations
If a utilities operator wants to deploy Smart meters across Europe, they face connectivity challenges if they rely on a single-IMSI SIM. Because mobile network operators (MNOs) are region specific, the Smart meters will only be able to connect in the home network region. Outside that, the utilities operator will have to negotiate several roaming agreements with each deployment location, or struggle to find coverage.
Single-IMSI SIMs are also vulnerable to that the regional MNOs infrastructure problems. Outages may cause unnecessary downtime for your devices, negatively impacting your business. Should the MNO choose to increase rates, you are also locked into paying higher prices.
A Cellular IoT SIM with a single IMSI can only connect to the network defined in its country code and network code. With a regional MNO, your device might be fine within the home area. If it is on the move or deployed to another area, it will struggle to stay connected.
For example, a cold chain use case where cargo may need to be tracked internationally across countries with different operators. Your MNO’s list of roaming partners might also not always be the best option.
Solution: Multi-IMSI eUICC eSIMs
Embedded universal integrated circuit card (eUICC)-enabled eSIMs are loaded with several IMSIs. This allows subscribers to seamlessly switch carriers when the Cellular IoT device moves to a new region.
It promises flexibility for global deployments because devices can find and connect to local service providers. This bypasses coverage restrictions of a particular carrier, or the added roaming expenses of another.
Some eUICC-enabled eSIMs also allows for remote provisioning over-the-air (OTA) to change profiles and network providers. If one profile won’t work for a deployment, you can provision the SIM with a new profile OTA without manually swapping out the SIM card. Otherwise, every time you add a new profile, operators have to integrate their SM-SR to the SIM, chalking up administrative costs.
So even if you are designing a stationary device like an EV charging station, you might want to market the device in more than one city around the world. Why not install a SIM that would work everywhere?
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