Guide · 6 min read
USB standards, on four separate axes.
Nearly every USB argument comes from treating one label as if it described the whole port. Connector shape, data speed, power capability and alt modes are four independent specifications, and a port can be excellent on one and useless on another.
Axis 1 — the connector
USB-A (the rectangle), USB-B (the square, on printers), Mini-B and Micro-B (legacy), and USB-C (the oval, reversible, 24 pins). Shape tells you almost nothing else. A USB-C port can be 480 Mbps or 80 Gbps, 2.5W or 240W, video-capable or not. The only firm inference runs the other way: USB-A cannot carry a modern PD contract, because it has no CC pin, which caps it near 12–18W.
Axis 2 — data speed
| Technical name | Marketing name | Speed |
|---|---|---|
| USB 2.0 | Hi-Speed | 480 Mbps |
| USB 3.2 Gen 1 (was 3.0 / 3.1 Gen 1) | USB 5Gbps | 5 Gbps |
| USB 3.2 Gen 2 | USB 10Gbps | 10 Gbps |
| USB 3.2 Gen 2×2 | USB 20Gbps | 20 Gbps |
| USB4 | USB 40Gbps | 20 or 40 Gbps |
| USB4 Version 2.0 | USB 80Gbps | 80 Gbps |
The renaming history is the reason for the confusion: USB 3.0 was relabelled 3.1 Gen 1, then 3.2 Gen 1, without the hardware changing. The USB-IF's current logo scheme abandons version numbers for plain speed figures, so a port marked "USB 10Gbps" is unambiguous in a way "USB 3.2" never was. Charging is unaffected by any of this — a USB 2.0 cable carries 100W perfectly well, as the cable guide explains.
Axis 3 — power
| Power spec | Ceiling | How it is negotiated |
|---|---|---|
| USB 2.0 default | 2.5W (5V/500mA) | None — fixed |
| USB 3.x default | 4.5W (5V/900mA) | None — fixed |
| USB-C default | 15W (5V/3A) | Resistor on the CC pin |
| USB PD (SPR) | 100W (20V/5A) | Digital contract on CC |
| USB PD 3.1 (EPR) | 240W (48V/5A) | Contract plus a certified cable |
This axis is where the money is. Power capability is entirely independent of data speed, which is why a cheap USB 2.0 charging cable can feed a laptop and an expensive 40 Gbps cable might cap at 60W. The negotiation itself is covered in USB-C PD explained, the optional voltage-tracking extension in PPS explained, and the top tier in 240W USB-C.
Axis 4 — alt modes and tunnelling
A USB-C port can lend some of its high-speed pairs to another protocol entirely. DisplayPort Alt Mode is the common one: it is how a single cable drives a monitor, and its absence is why some laptops refuse to output video over USB-C. Thunderbolt 3, 4 and 5 ride the same connector with their own certification and bandwidth guarantees. USB4 tunnels DisplayPort and PCIe over the USB fabric rather than displacing it. HDMI Alt Mode existed and is effectively dead; analogue audio alt mode is gone too, which is why USB-C headphone dongles contain a DAC.
Practical follow-ons: e-marked cables for the chip that gates the power axis, USB-A vs USB-C for the connector decision, and the USB-C adapter hub for chargers that publish all four figures.
FAQ
Is USB-C faster than USB-A?
The connector is not a speed. USB-C ports commonly run faster because newer hardware uses them, but a USB-C port can be plain USB 2.0 at 480 Mbps.
Does a USB4 port charge faster than a USB 3.2 port?
Not necessarily. Charging depends on the PD implementation, which is specified separately. Check the port's watt rating, not its data generation.
What is the difference between Thunderbolt 4 and USB4?
Thunderbolt 4 is a certification with mandatory minimums — 40 Gbps, PCIe tunnelling, dual 4K displays. USB4 allows lower configurations, so a USB4 port may do less.
Why does my USB-C cable charge but not transfer data?
Because it is a charging-only cable with no data pairs wired, or a USB 2.0 cable where you expected SuperSpeed. Power and data are separate specifications.