USB, Bluetooth, NFC and docking: connecting a mobile device to everything else

Why this matters

This objective looks like a list of technologies and is really a list of negotiations. A USB-C cable between a laptop and a dock is agreeing power direction, wattage, data speed and whether video is carried at all — four separate decisions that can each fail independently while the plug fits perfectly.

That is why the faults here are so confusing to users and so decidable to a technician who knows the model. "It charges but the screen stays blank" is not a vague complaint; it names which of the four negotiations succeeded. The exam asks these as scenarios for exactly that reason.

Lesson 23 covers the USB and Thunderbolt standards in the depth domain 3 wants. This lesson stays on the mobile side: pairing, proximity, docking, and the configuration a technician actually performs.

The lesson

USB connector shapes against USB speeds, and why the two are unrelated

The single most useful thing to internalise is that the connector and the capability are independent. USB-C is a shape. It says nothing about whether the port behind it runs at 480 Mbps or 40 Gbps, whether it carries video, or how much power it can deliver.

The shapes worth knowing:

• USB-A, the rectangular host-side connector, in use for thirty years.
• USB-B and its variants, the squarer device-side connector found on printers and older peripherals.
• Micro-B, including the wider SuperSpeed variant on older external drives.
• USB-C, reversible, symmetric, and capable of carrying far more than USB.

The speeds are a separate axis that has been renamed repeatedly. What matters for diagnosis is the ranking rather than the marketing name: the original low- and full-speed tiers, then High Speed at 480 Mbps, then the SuperSpeed generations at 5, 10 and 20 Gbps, then the 40 Gbps tier shared with Thunderbolt. A device plugged into a port that supports a lower tier still works, just slower — which is precisely why a user reports "the external drive is slow" rather than "the drive is in the wrong port".

Practical identification: port colour is a hint and not a standard, the symbol beside the port is more reliable, and the system's own device information is definitive.

Pairing over Bluetooth, what a pairing actually stores, and how to remove it properly

Bluetooth pairing exchanges and stores a long-term key on both devices. That is the fact behind most pairing faults: removing the pairing on one side only leaves the other side holding a key for a partner that no longer recognises it, and the reconnection then fails in a way that looks like a hardware fault.

The correct procedure when a pairing misbehaves is therefore symmetrical:

1. Remove or "forget" the device on the phone or laptop.
2. Clear the pairing on the accessory, which is usually a button held for several seconds until the indicator changes pattern.
3. Put the accessory into discoverable mode.
4. Pair again, confirming the passkey on both ends where one is shown.

The other common faults are all about state rather than range:

• Already connected to something else. Many accessories hold one active connection; headphones connected to a phone will not appear to a laptop.
• Not in discoverable mode. Paired is not the same as discoverable, and a device that is already paired to something else will not advertise.
• Bluetooth disabled by a profile or a radio toggle, which airplane mode will also do.
• Interference or distance, which degrades audio quality before it breaks the link.

Profiles matter too: a device supports particular profiles for audio, hands-free operation, file transfer or input, and a pairing that succeeds but does nothing useful is usually a profile mismatch rather than a fault.

NFC: range, what it is used for, and why proximity is the security control

Near-field communication works over a few centimetres. That is not a limitation that was engineered around; it is the security property. An attacker has to be close enough to be seen.

Where it appears in scope:

• Contactless payment, where the phone presents a tokenised credential.
• Access badges and transit cards, the same technology at a different frequency of use.
• Tap-to-pair, where the NFC exchange hands over Bluetooth credentials so the slower, longer-range link can be established without a pairing dance.
• Tags, small passive stickers that trigger an action when read.

Troubleshooting is short because there is little to configure: NFC must be enabled, the antenna is usually near the top or centre of the back of a phone so position matters, a thick or metallic case will block it, and payment specifically requires the device to be unlocked and the payment application configured.

Docking stations and port replicators, including power delivery and display limits

A port replicator extends the ports a machine already has. A docking station is more: it can add displays, network, audio and power over a single connection, and it may include its own video controller.

Three things are being negotiated over that one cable, and each can fail separately:

• Power. USB-C Power Delivery negotiates a voltage and current. A dock rated below what the laptop draws under load will charge it slowly, or hold it steady while idle and lose ground while working. The symptom is a battery that falls during use despite being plugged in.
• Data. The USB tier the dock and host agree on decides whether the attached drive runs at full speed, and whether a gigabit network adapter can reach gigabit at all.
• Video. Video over USB-C requires DisplayPort Alternate Mode, or Thunderbolt, or a dock with its own display-link controller and driver. A port that does none of those will never output video regardless of the cable.

Display limits are set by the host, not the dock. A machine whose graphics supports two external displays will not drive three because the dock has three ports. Lesson 22 covers the bandwidth arithmetic behind that.

Diagnosing a dock that charges but does not carry data, or carries data but not video

This is the scenario the exam likes, and it is solvable by elimination because each symptom names which negotiation succeeded.

• Charges, no data, no video. Very often a charge-only cable. These are extremely common, physically identical, and contain only power conductors. Swap the cable first; it is the cheapest test available.
• Charges and data, no video. The host port does not support DisplayPort Alternate Mode, or the dock needs a driver, or the display cable beyond the dock has failed. Check whether the machine will drive that same display directly.
• Video and data, poor charging. The dock's power delivery is below the machine's requirement, or the supply feeding the dock is undersized.
• Everything works, intermittently. Suspect the cable or the connector, especially on a machine where the same port is used daily.
• Nothing at all. Try the other USB-C port on the machine: on many designs only some ports carry full functionality, and that is a documented specification rather than a fault.

The general principle, and the one worth carrying into domain 5: each symptom tells you which layer of the negotiation completed, so the fault is always above the last thing that worked.