IMSI vs IMEI: 6 Key Differences and How They Work Together

The International Mobile Subscriber Identity (IMSI) is a unique identifier for a mobile subscriber within a network. It aids in authenticating and identifying users in the global telecommunications infrastructure. 

Typically including 15 digits, the IMSI is stored on a SIM card and used to track user activity within a cellular network. Its structure is divided into three parts: the mobile country code (MCC), the mobile network code (MNC), and the mobile subscriber identification number (MSIN). IMSI is not directly visible to end-users but helps network operators manage subscriber information. 

Its primary function involves routing calls and messages, allowing subscribers to roam internationally while maintaining consistent service. As an identifier, it assists in billing and access control, ensuring that the right services are provided to the appropriate parties. Its secure handling is vital, given its role in carrier operations and subscriber confidentiality.

The International Mobile Equipment Identity (IMEI) is a unique code assigned to a mobile device, which can be used to identify it. The IMEI number is used during manufacturing, assisting carriers and manufacturers in tracking the device. It is a 15-digit code consisting of a type allocation code (TAC), a serial number, and a check digit. 

The TAC identifies the model and origin of the device, while the serial number provides a unique identifier for each cellular based device. Unlike the IMSI, the IMEI is visible to the user and can be accessed through the device settings or by dialing codes. 

It is used for a range of applications, including tracking lost or stolen devices. Networks use this identifier to block devices reported as lost, protecting user data and mitigating unauthorized use. The IMEI ensures the equipment’s eligibility to access carrier services and offers security measures against theft and fraud.

This is part of a series of articles about Subscriber Identity Module

IMSI vs. IMEI: Understanding the Differences

While both serve as unique identifiers, they cater to different components of mobile connectivity—IMSI for subscribers and IMEI for equipment.

1. Association

The IMSI is linked to the identity of the mobile subscriber and their relationship with the network operator. It is associated with a mobile account, which is managed by the operator. This association enables tasks such as billing the correct user for their usage, granting network access, and tracking roaming activity. The IMSI allows the network to authenticate a subscriber no matter where they are in the world.

The IMEI is tied solely to the hardware of the mobile device. It identifies the physical handset and not the user or the account. This means even if a user switches SIM cards (and therefore IMSIs), the IMEI of the device remains constant. The IMEI is used for tracking devices globally, identifying their origin, and ensuring regulatory compliance for manufacturers.

2. Functionality

The IMSI is the key identifier used by the network to authenticate the user. When a device connects to a cellular network, the IMSI allows the network to verify whether the user has valid credentials and whether they are authorized to use the service. The IMSI also enables international roaming by ensuring the user can access services even when traveling abroad. Additionally, it helps billing systems accurately charge users for their voice, data, and text usage.

The IMEI ensures that the cellular device itself is recognized and accounted for within the network. This is critical for enforcing security policies, such as blacklisting lost or stolen devices. If a device with a blacklisted IMEI tries to connect to the network, it can be blocked to prevent unauthorized use. The IMEI also helps manufacturers and service providers track warranty claims, identify counterfeit devices, and support inventory management.

3. Structure

The IMSI and IMEI are both 15-digit numbers but differ in how they are structured and what information they encode.

The IMSI’s structure includes three main components:

• Mobile country code (MCC): The first three digits represent the country of the subscriber. For example, 310 represents the United States.

• Mobile network code (MNC): The next two or three digits identify the network operator within the country. For example, 260 represents T-Mobile in the U.S.

• Mobile subscriber identification number (MSIN): The remaining digits (up to ten) uniquely identify the subscriber within the operator’s network.

The IMEI’s structure, while also 15 digits, is formatted differently:

• Type allocation code (TAC): The first eight digits specify the device model and manufacturing origin. For example, 35299209 identifies the iPhone 8.

• Serial number: The next six digits provide a unique identifier for each device within the same model and batch.

• Check digit: The final digit is calculated using the Luhn algorithm to validate the IMEI’s accuracy.

4. Storage

The IMSI is securely stored on the SIM card, which is inserted into the mobile device. Because it resides on the SIM card, it can be easily transferred between devices. For example, if a subscriber upgrades to a new phone, they can retain their IMSI and associated account simply by moving their SIM card. This allows users to switch devices without losing their network identity. 

The IMEI is embedded in the hardware of the device during its manufacturing process. It is permanently linked to the device and does not change, even if the SIM card is swapped. This fixed storage location within the device’s firmware ensures that the IMEI can always be retrieved, providing a consistent and reliable way to identify the handset.

In devices that support eSIM (embedded SIM) technology, the IMSI is stored digitally within the device itself, rather than on a physical SIM card. The eSIM contains a rewritable software profile that includes the IMSI, allowing network operators to configure, update, or replace subscriber details remotely. Users can download and install profiles over the air without needing to physically change a SIM card.

5. Mutability 

The IMSI is mutable because it is stored on a removable SIM card. Users can change their IMSI by replacing the SIM card with one issued by a different network operator or by acquiring a new SIM from their current provider. When using eSIM, the IMSI can be updated or replaced through software-based profile downloads, enabling easy network switching for international travelers or users with multi-carrier devices.

The IMEI is hardcoded into the device at the time of manufacture and cannot be altered without tampering with the hardware or firmware. Any attempt to change an IMEI is considered illegal in most jurisdictions, as it is often done to obscure the origin of stolen or counterfeit devices. This immutability ensures that the IMEI can be used reliably for identifying devices.

6. Use Cases

The IMSI is used primarily for managing subscriber-related operations. This includes verifying the subscriber’s identity during the authentication process, enabling access to the network, and supporting international roaming. The IMSI is also essential for billing, as it ensures that charges are accurately associated with the correct account. For instance, when a user makes a call or sends a text, the network uses the IMSI to log and bill the activity. Additionally, the IMSI is used for location tracking within the network, which is important for providing emergency services or optimizing network performance.

The IMEI’s use cases focus on the physical device. One of its primary applications is theft prevention. If a device is reported stolen, the IMEI can be added to a blacklist, preventing it from accessing any network, even if the SIM card is replaced. It is also used by manufacturers and regulators to track devices, ensure compliance with regional standards, and identify counterfeit products. It supports warranty claims, enabling manufacturers to verify the device’s authenticity and production details.

IMEI and IMSI: Combined Use 

While the IMEI and IMSI serve distinct roles in mobile communications, they are used together to enable network operations, improve security, and ensure proper device and subscriber management. This allows mobile networks to efficiently link devices to subscribers and deliver reliable services.

• Authentication and network access: When a mobile device connects to a network, both the IMSI and IMEI are transmitted during the initial handshake. The IMSI authenticates the subscriber, confirming their credentials and access rights, while the IMEI ensures that the device is eligible to operate on the network. This dual verification process helps prevent unauthorized access and ensures that both the user and their device meet the network’s requirements.

• Security and anti-fraud measures: The combined use of IMSI and IMEI improves security by allowing networks to identify potential risks. For example, if a device is blacklisted due to theft, its IMEI can be flagged, and the network can deny access regardless of the IMSI being used. Similarly, if a SIM card is reported stolen, the associated IMSI can be deactivated, preventing misuse even if the device’s IMEI is valid. 

• Roaming support: During international roaming, the IMSI ensures that the subscriber is authenticated and can access services in a foreign network, while the IMEI ensures compatibility and compliance with local regulations. Together, they enable roaming agreements to function smoothly, allowing subscribers to maintain connectivity across borders.

Note: In countries like Turkey, where permanent roaming is allowed for a maximum duration of 90 days, IMSI can be used to track the device, and if it exceeds the allowed time frame, the IMEI is banned from the network—even if you replace your SIM card you can no longer get service using the banned device. 

• Tracking and regulatory compliance: The IMSI helps operators track a subscriber’s activity within the network, while the IMEI provides information about the device itself. This data is valuable for regulatory compliance, such as maintaining lawful intercept capabilities or adhering to country-specific telecommunications laws. For example , some jurisdictions require operators to report both IMSI and IMEI details for devices active within their networks.

• Loss and recovery efforts: If a device is lost or stolen, operators can use the IMEI to locate or block the handset, while the IMSI helps identify the account tied to the device. This combined information can assist in device recovery efforts and ensure that unauthorized parties cannot use the stolen equipment with a new SIM card.

• Security: Coupling IMEI and IMSI together can also increase security, especially in IoT scenarios where devices are not personal devices. For example if an IMIS is locked to a specific IMEI, it is not valuable to steal the SIM, as it will not work on any device other than the one with the specific IMEI. This can prevent misuse of IoT SIM cards by employees or third parties.

Related content: Read our guide to multi IMSI

Managing SIMs at Scale for IoT and Connected Devices

IoT is constantly evolving, and as it evolves it introduces new requirements, technologies and business models that mandate quick adaptation from all elements of an IoT solution, including core network, connectivity management platform, and BSS.

floLIVE’s advanced CMP uses a Holistic CMP model rather than a Thin CMP model. This

includes all of the three major building blocks necessary for running a cost effective, scalable and profitable IoT business. 

Whether in the cloud or on the ground, with floLIVE mobile operators benefit from a highly advanced CMP that supports all cellular technologies, with an inherent IoT BSS that supports the most advanced business models required by today’s connected enterprises, a 3GPP-compliant core network that was built for IoT, and a technological infrastructure that’s ready for the future.

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