1. Technology
Send to a Friend via Email
You can opt-out at any time. Please refer to our privacy policy for contact information.

Discuss in my forum

What is USB 3?


Question: What is USB 3?

Apple added USB 3 to its latest Mac offerings. What is USB 3 and will it work with my older USB 1.1 and USB 2 devices?


USB 3 is the third major iteration of the USB (Universal Serial Bus) standard. When it was first introduced, USB provided a truly remarkable improvement in how peripherals connected to a computer. Previously, serial and parallel ports were the norm; each required a detailed understanding of both the device and the computer hosting the device in order to set up the connection properly.

While there had been other attempts at creating an easy-to-use connection system for computers and peripherals, USB was perhaps the first to successfully become a standard on just about every computer, regardless of the manufacturer.

USB 1.1 started the ball rolling by providing a plug-and-play connection that supported speeds from 1.5 Mbit/s to 12 Mbits/s. USB 1.1 wasn't much of a speed demon, but it was more than fast enough to handle mice, keyboards, modems, and other slow-speed peripherals.

USB 2 upped the ante by providing up to 480 Mbit/s. Even though the top speeds were only seen in bursts, it was a significant improvement. External hard drives using USB 2 became a popular method of adding storage. Its improved speed and bandwidth made USB 2 a good choice for many other peripherals as well, including scanners, cameras, and video cams.

USB 3 brings a new level of performance with a new data transfer method called Super Speed, which gives USB 3 a theoretical top speed of 5 Gbits/s.

In actual usage, a top speed of 4 Gbits/s is expected, and a continuous transfer rate of 3.2 Gbits/s is achievable.

That's fast enough to prevent most of today's hard drives from saturating the connection with data. The old adage that external drives are slower than internals is no longer always the case.

Raw speed isn't the only improvement in USB 3. It uses two unidirectional data paths, one to transmit and one to receive, so you no longer need to wait for a clear bus before sending information.

USB 3 Architecture

USB 3 uses a dual-bus system that allows USB 3 traffic and USB 2 traffic to operate over the cabling simultaneously. This means that unlike earlier versions of USB, which operated at the top speed of the slowest device connected, USB 3 can zip along even when a USB 2 device is connected.

USB 3 also has a feature common in FireWire and Ethernet systems: a defined host-to-host communications capability. This capability lets you use USB 3 with multiple computers and peripherals at the same time. And specific to Macs and OS X, USB 3 should speed up target disk mode, a method that Apple uses when transferring data from an older Mac to a new one.


USB 3 was designed from the onset to support USB 2. All USB 2.x devices should work when connected to a Mac equipped with USB 3 (or any computer equipped with USB 3, for that matter). Likewise, a USB 3 peripheral should be able to work with a USB 2 port, but this is a bit more dicey as it depends on the type of USB 3 device. As long as the device isn't dependent on one of the improvements made in USB 3, it should work with a USB 2 port.

So, what about USB 1.1? As far as I can tell, the USB specification doesn't list support for USB 1.1. But most peripherals, including modern keyboards and mice, are USB 2 devices. You'd probably have to dig pretty deep in your closet to find a USB 1.1 device.

USB 3 and Your Mac

Apple chose a somewhat interesting way to incorporate USB 3 into its Mac offerings. The MacBook Pro, MacBook Pro with Retina display, and MacBook Air only have USB 3 ports. There are no dedicated USB 2 ports, as you'd commonly find in the PC arena. Apple used the same USB A connector most of us are familiar with; the difference is that the USB 3 version of this connector has five additional pins that support the high-speed operations of USB 3. This means that you must use USB 3 cabling to get USB 3 performance. If you use an old USB 2 cable that you found in a box under your table, it will work, but only at USB 2 speeds.

You can recognize USB 3 cabling by the logo embedded in the cable. It consists of the letters "SS" with the USB symbol next to the text. For now, you may only find blue USB 3 cables, but this may change, because the USB standard doesn't require a specific color.

USB 3 isn't the only high-speed peripheral connection that Apple uses. Most Macs have Thunderbolt ports that can operate at speeds of up to 10 Gbps. But for some reason, manufacturers still aren't offering many Thunderbolt peripherals, and the ones they do offer are very expensive. For now, at least, USB 3 is the more price-conscious approach to high-speed external connections.

©2014 About.com. All rights reserved.