Mobile connectivity: Wi-Fi, cellular data, tethering and hotspots

Why this matters

A phone has two independent routes to the internet and switches between them constantly without telling the user. Most "my phone has no internet" calls are really "my phone is on the wrong one of the two", and the fix is a setting rather than a repair.

The exam tests this as configuration and as fault finding, and both depend on one idea: at any moment, the device has chosen a route, and that choice is visible if you know where to look. A technician who can state which interface is carrying traffic has already done most of the diagnosis.

It also matters because the wrong answer costs money. A device that silently falls back to cellular while a user downloads a system update will spend someone's data allowance, and a hotspot left on will do the same in the background for hours.

The lesson

Joining and forgetting wireless networks, and what a saved profile keeps

When a device joins a wireless network it saves a profile, and that profile is more than a password. It holds the network name, the security type, the credential, and usually flags for automatic joining and for whether the network is treated as metered.

Two consequences follow, and both are examinable:

• A saved profile is why a device reconnects on its own, including to networks the user no longer wants. Turning Wi-Fi off does not remove the profile; the device reconnects the moment it is turned back on.
• A stale profile is why a device fails to join after the password changes. The device keeps presenting the old credential, the access point rejects it, and the device reports a vague authentication error rather than a wrong password.

The correct action in both cases is to forget the network rather than retrying. Forgetting deletes the profile so the device asks again from a clean state. This is the most common single fix in mobile wireless support, and it is the right answer to a surprising number of scenario questions.

Hidden networks are worth a note: not broadcasting the name is not a security control, since the name is transmitted by every client that looks for it. A hidden network must be added manually, with the exact name and security type, and a typo produces the same symptom as a wrong password.

Cellular generations, bands and why coverage is not the same as capability

Cellular networks are described by generation — 3G, 4G LTE, 5G — but a device's actual performance is decided by which bands it supports and which the network uses in that location. Two phones with identical generation support can behave completely differently in the same building.

What a technician needs to be able to say:

• The generation indicator is the device's current attachment, not its capability. A 5G phone showing LTE is doing the correct thing where 5G is unavailable.
• Bands matter more than generation for compatibility. A device bought abroad may attach only to some of a local carrier's spectrum and appear to have poor coverage everywhere.
• The SIM identifies the subscriber; the device identifies the hardware. Physical SIM and embedded SIM differ in how the profile is provisioned, but the role is identical. A SIM swap that produces no service usually means the profile is not activated rather than that the device is faulty.
• Signal bars are a summary and a poor one. The measured values — signal strength and signal quality — are available in the device's own status screens and are the numbers to record.

Preferred network type settings can force a device to a particular generation. That is occasionally the fix for a device that has strong 5G signal and unusable throughput, because it is holding a congested cell.

Tethering and mobile hotspots, and the battery and data costs of each

Tethering shares a phone's cellular connection with another device. It comes in three forms and the differences are practical rather than theoretical:

• Wi-Fi hotspot. The phone becomes an access point. Convenient, supports several clients, and the most expensive in battery terms because two radios are running continuously.
• USB tethering. The phone appears as a network adapter over the cable. Fastest, most reliable, and the phone charges while it works. This is the right answer when a single laptop needs a reliable connection.
• Bluetooth tethering. Lowest power, lowest throughput. Useful for light traffic when battery matters more than speed.

The costs to warn a user about: hotspot mode drains a battery in a few hours, laptops treat a hotspot as an ordinary network and may start large downloads on it, and carriers frequently meter tethered data separately from on-device data. Marking the connection as metered on the client is the control that prevents the expensive version of this.

Airplane mode, radio toggles, and diagnosing a device with one radio disabled

Airplane mode disables all radios at once: cellular, Wi-Fi, Bluetooth and usually NFC and location. Modern devices then allow individual radios to be turned back on while airplane mode remains active, which produces the confusing state at the heart of many support calls — airplane mode on, Wi-Fi working, cellular dead, and a user who is certain they did not change anything.

The diagnostic habit that resolves this quickly is to enumerate the radios rather than trusting a summary:

1. Is airplane mode on?
2. Is Wi-Fi on, and is it connected or merely on?
3. Is cellular data on — separately from the phone having signal?
4. Is Bluetooth on?
5. Is there a per-application restriction on cellular use?

Each of those is a separate switch, and the last is the one users never find. A device with full signal and no internet in one application, while others work, is nearly always an application-level data restriction or a data saver setting.

Roaming, data limits and the settings that quietly spend money

Roaming is the device attaching to a network other than its home carrier's. The device does this automatically, the charging is set by the carrier, and the user usually finds out afterwards.

The settings that matter, and what each does:

• Data roaming. Off by default on many devices, and the reason a phone has calls but no data abroad. This is a configuration answer, not a fault.
• Data limits and warnings. The device can warn at one threshold and cut data at another. Useful, and also the cause of "my data stopped working on the 28th of the month".
• Data saver. Restricts background data for most applications, which produces selective failures that look like application faults.
• Per-application cellular permission, which is the finest-grained version of the same idea.
• Automatic updates over cellular, which is the single largest unexpected consumer of an allowance.

The professional answer to "why was my bill so large" is to look at the device's own per-application data breakdown before blaming the carrier. It is built into both major mobile platforms, it is itemised, and it usually names the cause in one screen.