What is a Subscriber Identity Module (SIM)?
A Subscriber Identity Module (SIM) is a microchip that securely stores a subscriber’s International Mobile Subscriber Identity (IMSI) and its associated authentication key, which together identify and authenticate a device on a mobile network. In the context of the Internet of Things (IoT), a SIM card allows cellular-connected devices to establish a secure connection with a mobile network operator.
Types of SIM Cards
Plastic SIMs: Form factors have evolved over time to accommodate shrinking device designs. The original full-size SIM, known as the first form factor (1FF), was the size of a credit card. It was followed by the mini-SIM (2FF), micro-SIM (3FF), and nano-SIM (4FF), each progressively smaller while retaining the same chip architecture. Today, many IoT deployments use triple-cut SIMs, which package mini, micro, and nano sizes into a single plastic card, allowing the user to break out the required size for their hardware.
Embedded and Integrated SIMs: Hardware constraints in IoT devices led to form factors that do not require a physical, removable slot. An embedded SIM, specifically the MFF2, is soldered directly onto the device’s circuit board during manufacturing. This design increases durability against vibration, corrosion, and environmental factors. An integrated SIM (iSIM) goes a step further by embedding the SIM functionality directly into the device’s primary processor or system-on-a-chip (SoC), eliminating the need for a separate hardware component entirely.
What the SIM Does
The primary function of a SIM is network authentication and secure identification. A SIM functions as an isolated microcontroller with its own dedicated secure operating system. It holds the unique International Mobile Subscriber Identity (IMSI) number and a root authentication key. When an IoT device attempts to connect to a cellular network, the network issues a random challenge. The SIM uses its internal cryptographic engine and the root key to calculate a response, verifying the device’s identity to the network without ever exposing the key itself.
Beyond initial authentication, the SIM holds the data that guides ongoing network selection and security operations. It securely stores data including the Integrated Circuit Card Identifier (ICCID), local session keys, and the Public Land Mobile Network (PLMN) selector list. This prioritized list tells the device which roaming networks to look for first, which is critical for maintaining connectivity in mobile or global IoT deployments. Finally, the SIM generates the cipher keys required to encrypt all over-the-air data traffic between the device and the cellular base station, preventing unauthorized interception.
Multi-Network SIM Technologies
eUICC (Embedded Universal Integrated Circuit Card): Often referred to as an eSIM, this GSMA-standardized technology enables remote SIM provisioning, allowing organizations to change network profiles over-the-air (OTA) without physically swapping the hardware component. eUICC is the software functionality inside a SIM rather than the shape of the SIM itself, so an eSIM is a SIM system, embedded or not, that includes eUICC technology, and a removable plastic card can be eUICC-compliant. GSMA specifications differ by device type: SGP.22 addresses consumer devices, where a profile change is initiated locally on the device, while SGP.32 addresses IoT, where a remote manager handles profiles across devices that have no screen or user.
Multi-IMSI: A Multi-IMSI SIM holds multiple International Mobile Subscriber Identities on a single card, each one the identity a mobile network operator uses to grant a subscriber access to its network. Because the card holds more than one identity, the device can automatically switch between operator profiles based on location, signal strength, or a detected network fault, providing localized rates and broader coverage. Identities can also be switched or updated over the air, without the cost of downloading an entirely new profile.
Velocity IoT’s Global Connectivity Approach
Velocity IoT delivers multi-network connectivity using both multi-IMSI and eUICC technology, so devices can reach multiple carrier networks and profiles can be managed remotely as a deployment evolves. Our team works with you on connectivity strategy based on your deployment requirements, covering SIM technology, network selection, coverage planning and device compatibility. Getting started is straightforward. SIMs are available through Digi-Key with no minimum order, and the Velocity Flex platform is self-service with no contract required. For custom plans, deeper platform capabilities, or enterprise-grade control, contract-based options are available as well. Have questions about optimizing SIM technology for your IoT deployment? Contact Us.