What Is USB? USB-C vs USB 3, USB4 and Power Delivery
USB is a shared set of rules (a standard) for connecting computers and phones to the devices around them. USB-C is the name of a plug shape. Data speed (USB 2.0 to USB4) and power (USB PD) are set by separate standards. That is why cables with the same plug can do different things.
What you'll learn
- What USB defines, and who defines it
- Why it helps to think of "shape," "speed" and "power" separately
- The speed of each generation, and labels on the box such as "USB 10Gbps"
- Why a USB-C cable can charge a laptop but sometimes cannot show video
- Why a high-wattage charger does not damage a phone, and the EU common charger rules
Prerequisites: a rough idea of voltage (volts, V) and current (amperes, A). The only math is "power = voltage × current" (explained in words in the text).
In everyday life
Mice, keyboards, USB flash drives and phone chargers all connect by USB. Today it is also used to charge laptops and to connect external monitors.
Starting with the iPhone 15, released in September 2023, the iPhone's port became USB-C too. But the iPhone 15's top data speed is 480 Mb/s. The iPhone 15 Pro reaches 10 Gb/s (Apple). The shape is the same, yet the speed differs by about 20 times.
Common complaints such as "my USB-C cable shows nothing on the monitor" or "charging is slow" come from the same place. Let's see why, step by step, starting from how USB works.
How it works ①: USB is a shared set of rules for connecting
Before USB spread, a computer's ports differed for each kind of device. A mouse port and a printer port had different shapes and different insides. So a shared set of rules was created to connect any device through the same port. That is USB. Its first version, 1.0, came out in January 1996.
USB stands for Universal Serial Bus. "Serial" means sending 0s and 1s one at a time, in order. It needs fewer wires than sending over many wires at once.
USB is managed by USB-IF (USB Implementers Forum), a nonprofit group. The companies that created USB founded it in 1995. It also runs "certification." It tests whether products work as the standard says, and lets products that pass use its logo (USB-IF).
One handy feature of USB is that you can plug and unplug devices with the power on. When you connect one, the computer automatically checks what kind of device it is and makes it ready to use. This feature is called hot plugging.
How it works ②: "Shape," "speed" and "power" are separate rules
Much of the confusion around USB comes from three kinds of rules all being called "USB." They cover the plug shape, the data speed and how power is sent. In a table, they look like this.
| What the rule covers | Main names | Examples |
|---|---|---|
| Plug shape (connector) | USB Type-A, Micro-B, USB Type-C | The rectangular port on a computer, the port on Android phones from a few years ago, the port on today's phones and computers |
| Data speed | USB 2.0, USB 3.2, USB4 | 480 Mb/s to 80 Gb/s |
| How power is sent | Basic 5 V power, USB PD | A few W to 240 W |
USB Type-C (USB-C from here on) is a plug shape whose standard was finished in August 2014. It plugs in either way up, and it has 24 contacts (pins) inside. A USB-IF guide also states clearly that USB 3.2 is a speed standard, separate from USB-C.
The actual speed and power settle at the lowest ability among the two devices and the cable. If even one of them supports only an older standard, that becomes the limit.
How it works ③: Data speed depends on the generation
First, the unit of speed. One piece of information, a 0 or a 1, is called one bit (b). The number of bits sent per second is written "b/s." Mb/s means a million bits per second, and Gb/s means a billion bits per second.
| Standard name | Year | Top speed | Current name |
|---|---|---|---|
| USB 1.0 / 1.1 | 1996 / 1998 | 12 Mb/s | — |
| USB 2.0 | 2000 | 480 Mb/s | Hi-Speed USB |
| USB 3.0 (= USB 3.2 Gen 1) | 2008 | 5 Gb/s | USB 5Gbps |
| USB 3.1 Gen 2 (= USB 3.2 Gen 2) | 2013 | 10 Gb/s | USB 10Gbps |
| USB 3.2 Gen 2x2 | 2017 | 20 Gb/s | USB 20Gbps |
| USB4 | 2019 | 40 Gb/s | USB 40Gbps |
| USB4 Version 2.0 | 2022 | 80 Gb/s | USB 80Gbps |
The year is when the standard was finished. Products come out later. The speed is the standard's upper limit, and real file transfers are slower (USB-IF, Intel and others). USB4 was built on Thunderbolt, a standard that Intel contributed.
Why the names got confusing
The same 5 Gb/s speed was renamed with each revision: USB 3.0, then USB 3.1 Gen 1, then USB 3.2 Gen 1. Each new version absorbed the old one and gave the older speeds new names too.
The "x2" in "Gen 2x2" means using two 10 Gb/s lanes at once. USB-C has two sets of fast wires, each with a pair for sending and a pair for receiving (four pairs in all). That allows this "two-lane" use.
To avoid confusion, USB-IF has asked since 2019 that boxes show the speed rather than the version number. Since 2022 it has used short labels such as "USB 10Gbps" (USB-IF).
How it works ④: 0s and 1s travel on a pair of wires
The faster a signal is, the more easily noise from its surroundings disturbs it. So USB pairs two wires and sends opposite signals on them. The receiver looks only at the "difference" between the two voltages to decide 0 or 1.
Noise tends to land equally on two wires that sit right next to each other. Taking the difference cancels that noise. This way of sending is called differential signaling. The comparison page "single-ended vs. differential signaling" shows in a table how it differs from sending on one wire.
USB 2.0 has only one such pair, and it takes turns sending and receiving. USB 3.x and later added separate pairs just for sending and just for receiving, so both can happen at once. The wires for fast signals are made with their width and spacing set precisely. "What Is a Printed Circuit Board?" explains why.
How it works ⑤: Power starts at 5 V and rises by agreement
Power (W, watts) is voltage times current. For example, 5 V at 1 A is 5 W. USB's power wire was originally 5 V. A USB 2.0 port gives 0.5 A (2.5 W), and a USB 3.0 port gives 0.9 A (4.5 W) (Intel).
USB-C can supply 1.5 A or 3 A (15 W) at the same 5 V. The connector has a "talking wire," called the CC wire. The charger sets a voltage on it to tell the other side how much it can supply (Microchip).
Charging a laptop takes much more power. So rules were made for the charger and the device to talk after connecting and raise the voltage. This is USB Power Delivery (USB PD). Its maximum was 100 W, at 20 V and 5 A. A revision in May 2021 extended it to 240 W, at 48 V and 5 A (USB-IF).
Raising the voltage instead of the current reduces the power lost as heat in the cable. For the same power, a higher voltage needs less current. It is the same idea as raising the voltage on power lines, explained in "AC vs DC Explained."
How it works ⑥: What is inside the cable changes what it can do
Cables with USB-C on both ends are not all the same inside. The simplest has only the power wire, the CC wire and the USB 2.0 pair. It has none of the fast pairs used by USB 3.x and later.
To send video to a monitor over USB-C, these fast pairs are lent to the video signal. The best-known example is DisplayPort, a video standard (VESA). A cable without fast pairs can charge, but it cannot send video this way.
A tiny chip inside the cable: the "e-marker"
An ordinary USB-C cable is built to carry up to 3 A. At 20 V, 3 A gives 60 W. To carry more, up to 5 A, the cable's plug must contain a small chip (USB-IF).
This chip works like a name tag on the power path. It tells the charger and devices facts such as "this cable handles up to 5 A, and up to so many Gb/s." The chip is called an e-marker (electronic marker). With an untagged cable, the charger holds itself to 3 A for safety. To charge at 240 W, you need an e-marked cable rated for 240 W.
Passive cables and active cables
Fast signals weaken and lose their shape over a longer cable. A passive cable has no circuits inside. For 10 Gb/s over a passive cable, the USB-C standard gives a guideline length of up to 1 m. To send fast signals farther, you use a cable with chips inside that clean up the signal. This is called an active cable.
The 80 Gb/s of USB4 Version 2.0 can run over today's 40 Gb/s passive cables. Newly defined 80 Gb/s active cables also work (USB-IF). Certified cables carry logos showing speed and power, such as "40Gbps" and "240W" (USB-IF).
How it works ⑦: The chips at work inside USB
On the computer side, a circuit watches the connected devices and decides when to talk with which one. This is called the USB controller. It is usually built into the same chip as the CPU, or into a chip next to it.
On the mouse or keyboard side, a small computer chip answers by the USB rules. Many of these chips are microcontrollers with a built-in USB circuit ("What Is a Microcontroller?").
Chargers and phones contain a chip that handles the USB PD negotiation. This is called a PD controller. Based on the result, it switches on the power path and instructs the circuit that changes the voltage. "What Is GaN (Gallium Nitride)?" explains the parts that handle high power in a small charger.
Along the path of fast signals, a chip is sometimes placed to revive a weakened signal. One that only reshapes and amplifies the wave is called a redriver.
The other kind first reads the 0s and 1s, then sends them again as a clean signal. This is called a retimer (Texas Instruments). It copes better with badly weakened signals, but it costs more than a redriver.
The port you touch with your fingers is a place where static electricity easily jumps in. So parts near the port drain off static to protect the chips (electrostatic discharge, ESD).
Rules around the world: the EU "common charger"
The EU (European Union) has made a rule that charging ports must be USB-C. It is Directive (EU) 2022/2380. From December 28, 2024, devices sold in the EU must have a USB-C charging port. It covers mobile phones, tablets, digital cameras, headphones, handheld game consoles, earbuds and more. Laptops follow from April 28, 2026 (European Commission).
The rule covers more than the port shape. Devices that fast-charge above 5 V, 3 A or 15 W must support USB PD. Sellers must also offer a way to buy the device alone, without a charger. The aim is to cut charger waste and let people reuse the chargers they already have.
Products sold outside the EU are not covered by this rule. Still, many makers sell the same models worldwide, so USB-C has spread to devices sold in other countries too.
Common myths
Myth: All USB-C cables are the same
In fact, cables with the same shape differ inside. Some reach only USB 2.0 speed, some carry up to 3 A (60 W), and some carry 5 A for 100 W or 240 W. The shape name and the speed and power standards are set separately, so you cannot tell them apart by looking. The surest way is to check the "Gbps" and "W" numbers on the certification logo.
Myth: A charger with high wattage will damage your phone
In fact, a USB PD charger supplies only 5 V right after you connect it. The phone picks the voltage from the list the charger offers and requests it. A charger's wattage is the most it can supply, not an amount it forces in. So if you charge a 20 W phone with a 100 W charger, only the agreed amount flows. This holds for products built to the standard, though. Poor-quality products without certification may not behave this way.
Myth: A higher version number means a faster speed
In fact, "USB 3.2" covers three speeds: 5 Gb/s, 10 Gb/s and 20 Gb/s. A 5 Gb/s product from the USB 3.0 days is also USB 3.2 Gen 1 under today's names. The number alone does not tell you the speed, so check the Gbps figure when you buy.
By the numbers
| Number | What it is |
|---|---|
| 1995 / 1996 | USB-IF founded / first USB version, 1.0 |
| 12 Mb/s → 80 Gb/s | Top speeds of USB 1.1 (1998) and USB4 Version 2.0 (2022). About 6,700 times faster |
| About 2 min 50 s / 8 s | Shortest theoretical time to send a 10 GB file at 480 Mb/s and at 10 Gb/s |
| 5 V | USB's basic voltage. It is also 5 V before the USB PD negotiation |
| 2.5 W / 4.5 W / 15 W | Power at 5 V from a USB 2.0, USB 3.0 and USB-C (3 A) port |
| 60 W / 100 W / 240 W | Limit over a 3 A cable / USB PD limit until May 2021 / today's limit (48 V, 5 A; the range called EPR) |
| December 28, 2024 / April 28, 2026 | Dates a USB-C charging port became required in the EU: mobile phones and other devices / laptops |
The 10 GB times count 1 byte as 8 bits and leave out protocol overhead. In practice it takes longer. You can try other file sizes and speeds with the data transfer time calculator.
Learn more
- Related articles: "What Is GaN (Gallium Nitride)?" (small fast chargers) / "How Wireless Charging Works" / "How Lithium-Ion Batteries Work" / "What Is a Printed Circuit Board?" (wiring for fast signals)
- Terms: USB Power Delivery / differential signaling / retimer / hot plugging / isochronous transfer (how audio and video are sent) / SerDes (circuits for fast serial links)
- Compare in a table: single-ended vs. differential signaling / NRZ vs. PAM4 / PCIe 3.0 vs. 4.0 vs. 5.0 vs. 6.0 / I2C vs. SPI vs. UART
- Try the numbers: data transfer time calculator