Terminology

The words this industry uses, and what they actually mean.

Cellular connectivity has an unusual amount of jargon, a fair bit of it used loosely and some of it used to mean different things by different companies. This page is the plain version. No sales pitch, just what each term means and what people commonly get wrong about it.

Kinds of SIM eSIM and form factors How it connects Reaching your device What is being sold to you

The five questions people arrive with

Is a data SIM the same as an IoT SIM?

No. A data SIM is usually a consumer product for a tablet, dongle or MiFi, sold on the assumption a person is nearby. An IoT SIM is built for equipment nobody attends, with longer life, better management and different network behaviour.

Is eSIM the same as eUICC?

No, though they are used interchangeably constantly. eSIM usually refers to a SIM soldered into the device instead of a removable card. eUICC is software that lets a SIM hold more than one operator profile and switch between them. You can have one without the other.

Do I need a private APN or a VPN?

They solve overlapping problems in different places. A private APN keeps your traffic off the public internet from the moment it leaves the device. A VPN builds an encrypted tunnel across the public internet after the fact, and needs a gateway that is itself exposed.

Does a multi-network SIM always pick the best network?

Not necessarily. Multi-network means more than one network is available. Whether your device moves to the strongest one depends on the profiles loaded, the operator preference list, the modem, the router firmware and any steering applied. It is worth asking specifically.

Why can I not just connect to my device over the internet?

Because almost all cellular SIMs sit behind carrier-grade NAT. Your device can reach out, but nothing can reach in, and there is no port to forward. This is the single most common surprise for people new to cellular.

Group one: Kinds of SIM

IoT SIM

(also seen as: M2M SIM)

A SIM designed for equipment rather than people. The practical differences are that it expects nobody to be standing next to the device, so it is built for long life, remote management, multiple networks and unattended reconnection. Usually more durable and available in soldered form factors.

Commonly confused: M2M SIM is largely the same thing under an older name. If a supplier uses one term and you use the other, you are almost certainly talking about the same product.

Data SIM

A loose term. In consumer retail it means a SIM for a tablet, dongle or mobile hotspot with a monthly data allowance. In business contexts people sometimes use it to mean any SIM that carries data rather than calls, including IoT SIMs.

Worth knowing: if you search for a data SIM you will mostly find consumer products. If your device is unattended equipment, what you want is an IoT SIM.

Spoke: Data SIM vs IoT SIM, in full

Roaming SIM

Two meanings depending on who is speaking. To a consumer it means a travel SIM that works abroad. In machine connectivity it usually means a SIM that connects through networks it does not natively belong to, often selecting whichever is cheapest or strongest in a given place.

Multi-network SIM

(also seen as: any network SIM / AnyNet, multi-IMSI, multi-operator, non-steered)

A SIM able to use more than one mobile network, sometimes described as an any network SIM. How that is achieved varies a lot and the differences matter: roaming agreements across partner networks, multiple IMSIs held on one SIM, or an eUICC holding several full operator profiles.

The important question: what actually happens when the current network fails? Ask about failover behaviour and how long recovery takes, not just how many networks are listed.

Spoke: What multi-network really means

Group two: eSIM, eUICC and form factors

This is the most muddled corner of the vocabulary, largely because two different things got the same name. It is worth separating the shape of the SIM from what the SIM can do.

Term	What it actually describes
2FF, 3FF, 4FF	Physical sizes of removable SIM card. Mini, micro and nano respectively.
MFF2	A shape. A SIM chip soldered onto the circuit board instead of slotted in. Survives vibration, heat and damp far better than a card in a holder.
eSIM	Used two ways. Strictly, an embedded SIM, so usually MFF2. Loosely, any SIM whose profile can be downloaded rather than shipped. Ask which is meant.
eUICC	A capability, not a shape. Software allowing a SIM to hold several operator profiles and switch between them remotely. A removable card can be eUICC. An embedded chip might not be.
iSIM	A shape again. The SIM function built directly into the device’s main processor, so there is no separate chip at all.
SGP.02 / SGP.22 / SGP.32	GSMA specifications for how remote profile switching works. SGP.02 is the older machine standard, SGP.22 is the consumer one, SGP.32 is designed for unattended IoT devices.

The one that catches people: eUICC is sold as freedom from lock-in, and it does let you change mobile operator without touching the device. But the eUICC platform itself is written onto the SIM at manufacture, so changing your eSIM management provider is a different and much harder question. Worth asking which specification is in use and who controls the platform.

Spoke: eSIM vs eUICC vs MFF2, in full

Group three: How it connects

APN

(Access Point Name)

The setting that tells a mobile network where to send your device’s traffic once it has connected. Think of it as choosing which door the data leaves through. Get it wrong and the device attaches to the network but nothing works.

Private APN

An APN that routes your traffic to your own destination rather than out to the open internet. This is the mechanism behind most secure cellular deployments, because it means the device never sits on the public internet in the first place.

Ask how you get one. Most providers treat a private APN as a configuration project: a request, a setup fee, a lead time and often a minimum number of SIMs. Whether it is a product you order or a setting you control makes an enormous practical difference.

Steering

When a provider influences which network your device attaches to, usually to favour cheaper wholesale agreements. A non-steered SIM lets the device choose for itself, which normally means it picks the strongest signal.

Permanent roaming

Most roaming agreements assume a device is visiting temporarily. Some countries restrict devices that roam indefinitely, and local operators may cut them off after a period. It affects a minority of territories but the consequences can be severe, up to replacing SIMs in the field.

Spoke: Permanent roaming, where it applies and where it does not

Group four: Reaching your device

CGNAT

(carrier-grade NAT)

Mobile networks do not have enough public addresses for every device, so they put thousands of devices behind shared ones. Your device can start a connection outwards, but nothing on the internet can start one inwards. This is why you cannot simply browse to your router’s address.

Static IP

An address that stays the same rather than changing each time the device connects. Useful for allow-listing and for finding equipment reliably. Note that static does not automatically mean reachable from the internet, and a static private address is a different thing from a static public one.

Port forwarding

Telling a router to pass incoming traffic on a particular port through to a device behind it. Familiar from home broadband, and mostly useless on cellular, because CGNAT means there is no incoming traffic to forward in the first place.

VPN

An encrypted tunnel across the public internet, letting you reach equipment as though it were on your own network. The standard answer to the CGNAT problem, and a workable one, with two costs: you have to build and maintain it, and the gateway where the tunnel terminates sits on the public internet where it can be found and must be kept patched.

Spoke: Private APN vs VPN, in full

Layer 2 network extension

Rather than tunnelling to your devices, your own network is extended to include them, so they behave as though they are plugged into it directly. Their addresses come from your addressing plan and there is no separate network to bridge.

Group five: What is being sold to you

Different companies in this market do genuinely different things, and the marketing language does not always make it obvious which.

Term	What it usually means in practice
Managed connectivity	The broadest term available and it tells you very little. It can mean anything from reselling airtime with a dashboard to operating a network. Worth asking what is actually being managed and by whom.
MVNO	A company selling mobile service using another operator’s radio network. Some MVNOs run their own core systems, many resell someone else’s entirely. The difference determines how much they can change for you.
Mobile core	The systems deciding who may connect, where their traffic goes and what it may reach. A provider running its own core can change these things directly. One that does not has to ask whoever does.
Connectivity management platform	A dashboard for SIMs: activate, suspend, check usage, set alerts. Useful, but it manages the SIM rather than the network the SIM sits on.
Device management platform	Software for the equipment rather than the connection: configuration, firmware, diagnostics. Historically bought separately from connectivity, which is why many deployments run two systems that do not talk to each other.

Where SIMSY sits, since you are on our website. We operate our own mobile core rather than reselling somebody else's, which is why things that are ordinarily a request with a setup fee, such as a private APN or your own IP addressing, are settings you control yourself.

That is the only sales point on this page. Everything above is true regardless of who you buy from, and we would rather you understood the vocabulary than took our word for anything.

Still not sure what you need?

If you would rather skip the vocabulary entirely, the plain-language explanation covers what this is for and whether it fits what you are building.

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