The words you need to know

The small chip that identifies a device to a mobile network. Contains the credentials that tell the network who you are and whether you're allowed to connect. Without a SIM, a device has no identity on the cellular network. The SIM is separate from the device, you can move it, swap it, or provision it remotely.

The unique number stored on a SIM that identifies the subscriber to the network. 15 digits. The network uses the IMSI to look up your account, permissions, and roaming agreements. It's the identity of the SIM, not the device. One device can have different IMSIs if it has multiple SIM profiles.

The serial number of the SIM card itself, up to 22 digits, usually printed on the card. Where IMSI identifies the subscriber, ICCID identifies the physical (or virtual) card. Used by operators to manage SIM inventory and track which card is in which device.

The unique identifier of the device, not the SIM. 15 digits. If a SIM identifies who is connecting, the IMEI identifies what is connecting. Networks can block specific IMEIs, this is how stolen phones are blacklisted. In IoT, IMEI tracking lets you see exactly which physical device is on the network.

The phone number. The human-readable address of a SIM on the network, stored in full international format. Most people only know this one. Behind it sits the IMSI, which is what the network actually uses. The MSISDN is the public face; the IMSI is the internal identity.

Two codes embedded in the IMSI that identify which country and which operator the SIM belongs to. MCC 234 is the UK. MNC 30 is EE. Together they form the PLMN (Public Land Mobile Network) identity, the network's fingerprint. Roaming decisions are made based on these codes.

A SIM built into the device that can be reprogrammed remotely. No physical card to swap. Profiles are downloaded over the air. Increasingly common in consumer devices and essential in IoT, where physically swapping SIMs in thousands of deployed devices is not practical. The profile on an eSIM contains the same IMSI and credentials as a physical SIM.

A SIM that carries multiple IMSI profiles, allowing it to present itself as belonging to different networks in different countries. Used in global IoT deployments to avoid roaming charges, the SIM switches to a local IMSI rather than connecting as a foreign subscriber. The device thinks it's local everywhere.

A company that owns and operates its own cellular network infrastructure: towers, spectrum, core network. EE, Vodafone, O2, Three in the UK. AT&T, T-Mobile, Verizon in the US. MNOs are the source of the network. Everyone else is built on top of them.

A company that sells mobile connectivity without owning the underlying network. They buy capacity from an MNO and resell it, often with their own SIMs, pricing, and management tools. GiffGaff runs on O2's network. Many IoT connectivity providers are MVNOs. The network experience is the same; the commercial relationship is different.

When a device connects to a network other than its home network, usually in a different country. The visited network authenticates the SIM against the home network's records and charges for the traffic. Roaming agreements between operators determine where a SIM can connect and at what cost. In IoT, uncontrolled roaming can produce large unexpected bills.

When roaming, a SIM may be directed to a specific network (steered) or allowed to connect to whichever has the strongest signal (unsteered). Steered roaming gives operators cost control. Unsteered gives devices better coverage. For IoT in poor-signal environments, unsteered roaming often means better connectivity but less predictable costs.

The database at the heart of a mobile network that holds subscriber records: which SIMs exist, what services they're allowed, where they last connected. The HLR is the 2G/3G version; the HSS is its 4G/5G equivalent. When a SIM connects anywhere in the world, the visited network checks the home HLR/HSS to verify it.

The gateway between the mobile network and the internet (or a private network). When a device connects to the cellular network and wants to send data, it connects through an APN. The APN determines which network the data goes to, what IP address the device gets, and what security rules apply. Enterprise deployments often use a private APN to keep device traffic off the public internet.

An APN that routes device traffic directly into a company's own network rather than onto the public internet. Devices get private IP addresses. Traffic never touches the public internet. Used where security matters: healthcare devices, industrial equipment, financial terminals. Costs more to set up; significantly more secure.

The largest packet of data that can be sent in a single transmission over a network. Measured in bytes. If a data packet is larger than the MTU, it gets fragmented, split into smaller pieces and reassembled at the other end. Fragmentation adds overhead and can cause problems. On cellular networks, the MTU is typically 1500 bytes, but VPNs and tunnels reduce it further. Getting the MTU wrong is a common cause of mysterious connectivity failures.

The address assigned to a device on a network. On cellular, devices typically get a dynamic IP, a different address each time they connect. For devices that need to be reached from outside (a remote sensor you want to poll, for example), a static IP matters. Private APNs usually issue private static IPs. Public static IPs are available but cost more.

The commercial agreement that governs how much data a SIM can use, at what cost, and in which countries. IoT plans are structured differently from consumer plans, often pooled across many SIMs, with overage charges that matter when you have thousands of devices. Understanding your plan is essential before deploying at scale.

The generations of cellular network technology. 2G (GSM) introduced digital voice and SMS. 3G added mobile data. 4G (LTE) made mobile data fast enough to replace fixed broadband for many uses. 5G adds speed, lower latency, and the ability to connect many more devices in the same area. For IoT, the generation matters less than coverage and power consumption.

The technical standard behind 4G. The dominant global cellular standard for data. When a device shows "LTE" instead of "4G," it's the same thing. LTE is the baseline most IoT connectivity is built on today.

A version of LTE designed for IoT devices that don't need high data speeds but do need long battery life and the ability to work in poor signal conditions. Supports voice, moves between cells, and can handle devices that move around. Good for asset trackers, wearables, and devices that need to roam.

A cellular standard for IoT devices that send very small amounts of data infrequently, and need to run on a battery for years. Smart meters, environmental sensors, parking sensors. Very low power, very good at penetrating walls and basements, but low bandwidth and no voice. Devices stay in one cell, no handover between towers.

The slice of radio spectrum a network uses to transmit. Different bands have different properties: lower bands travel further and penetrate buildings better; higher bands carry more data but over shorter distances. A device needs to support the right bands to connect to a given network. This matters when deploying devices internationally. US and European networks often use different bands.

A rough measure of signal strength. Higher (less negative) is better. -70 dBm is good. -110 dBm is barely connected. A blunt instrument, it tells you how strong the signal is, not how clean it is. Useful for a quick check; not enough on its own to diagnose poor performance.

More precise measures of LTE signal quality. RSRP measures the power of the reference signal from the serving cell. RSRQ adds a measure of interference. Together they give a clearer picture than RSSI alone. If RSRP looks acceptable but performance is poor, RSRQ tells you whether interference is the problem.

The time it takes for a packet of data to travel from a device to its destination and back. Measured in milliseconds. 4G latency is typically 30–50ms. 5G aims for under 10ms. For most IoT applications, latency matters less than reliability. For real-time control systems, it matters a great deal.

How much data can actually be transferred in a given time. Measured in Mbps or Kbps. Distinct from the headline network speed, real-world throughput depends on signal quality, network congestion, and how many devices are sharing the same cell. A device showing 4G may still have low throughput if the cell is overloaded.