USB 3.0 Explained

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USB 3.0 Explained

A complete, readable guide to the SuperSpeed USB Specification





WHY USB 3.0?

USB 3.0, also known as SuperSpeed USB, is the next evolutionary phase of the Universal Serial Bus, inarguably the most successful and ubiquitous interface standard ever created. As computer hardware and peripherals continue to expand in capacity, speed, and portability, the interfaces that connect them must also advance in these areas.

Building off the success of USB 2.0, the third iteration of the Universal Serial Bus aims to tackle three pillars of interface design:

  • SPEED - Compared to the paltry 12Mbps speeds of USB 1.1 and the moderate 450Mbps of "High-Speed" USB 2.0, the "SuperSpeed" interface of USB 3.0 tries to live up to its name with a theoretical 5.0Gbps (5,120Mbps) of bandwidth. From high-definition displays to external Blu-Ray players and high-capacity hard drives, USB 3.0 is engineered to usurp and expand the market currently dominated by FireWire.

  • POWER - With an ever-expanding list of accessories and portable devices, bus-powered hardware has long been pushing the limits of what USB 2.0 could handle. First, the 3.0 specification allows up to 80% higher power consumption for devices running at SuperSpeed, and second, USB 3.0 includes an enhanced version of the USB-B connector called Powered-B, which allows USB accessories to draw power from peripheral devices as well as hosts.

  • CROSSOVER CONNECTIONS - In trying to establish a more robust ecosystem of USB devices, new features are implemented in the USB 3.0 specification to allow for cross-communication between hardware. Similar to FireWire and Ethernet specifications, the new SuperSpeed USB includes an established method of host-to-host communication (i.e. computer-to-computer) through a crossover USB A to USB A cable. Additionally, USB 3.0 builds on the "USB On-The-Go" principles of allowing portable devices (such as smartphones) to act as either a USB device or a USB host, dramatically increasing their feature set and usability with existing USB devices.


WHAT ARE THE SUPERSPEED CONNECTORS ?

There are five types of connectors in the USB 3.0 specification, falling into three basic groups:

USB A - The USB you know and love, only faster

Certainly the most recognizable form in computing today, the USB A connector remains largely unchanged from the 2.0 specification to the SuperSpeed 3.0. Hidden away internally are five additional pins which allow the SuperSpeed bandwidth, but the shape, size, and general appearance of the famous USB connector are just as they've always been.

Although not explicitly required by the specification, manufacturers of USB products are advised to distinguish 3.0 connectors with the use of blue-colored plastic in the connection. Short of an explicit label or wire branding, blue coloring is the best indication of a SuperSpeed-compliant cable.



USB B - Just a little more on the top
Unlike the host-connector above, the device-end connector of the USB world receives a physical upgrade in the 3.0 specification. The SuperSpeed USB B port is similar to the square-ish USB B of the past, but with an additional level protruding from the top of the connector. This added area contains the five additional pins used for SuperSpeed data transfers. The expanded profile of the connector and jack affect the cross-compatibility of these connections: USB B 2.0 connectors are compatible with USB B 3.0 ports, but not vice versa.

There is an offshoot of the Standard USB B connector as part of the USB 3.0 spec, similar in appearance but with the additional feature of providing bus-power from a device to an adaptor. Read more on Powered USB B below.



USB Micro AB - Here's where it gets confusing
The SuperSpeed standard makes a noble attempt to provide a new echelon of functionality to USB devices, but will undoubtedly stir up plenty of confusion with its Micro-A and Micro-B connectors. The underlying principle is that some USB products, including smartphones and tablets, should be able to function not only as devices (connecting to a desktop computer) but also as hosts (connecting to a printer, keyboard, or camera). To do this, a miniature connector set ("Micro") was designed to be cross-compatible with both host ("A") and device ("B") connectors. Learn more about USB Micro-A and USB Micro-B below.

HOW IS USB 3.0 BACKWARDS-COMPATIBLE WITH USB 2.0 ?

Since USB has proven itself to be the overwhelming favorite interface, a crucial pillar of the USB 3.0 specification addresses how the SuperSpeed interface must remain compatible with the millions of existing USB computers and devices in the marketplace. For the most part, it's good news: all USB 2.0 devices will work on USB 3.0 hardware, and (nearly) all USB 3.0 devices will work on USB 2.0 hardware.

As part of the requirement, all USB 3.0 ports include the necessary pins and logic to process USB 2.0 signals, and all of the receptacles are physically compatible with their earlier counterparts. This means that computers and infrastructure can be upgraded to USB 3.0 at any time, ensuring future-compatibility without compromising on existing functionality.

Similarly, all USB 2.0 ports will be compatible with USB 3.0 signals, as all SuperSpeed devices are required to be capable of interfacing on their 2.0 pins alone. This means that a USB 3.0 hard drive (for example) can attach to a USB 2.0 computer using a 2.0 cable, or attach to a USB 3.0 computer using a 2.0 cable, or attach to a USB 2.0 computer using a 3.0 cable. While the device will not benefit from the higher bandwidth of the USB 3.0 interface, it is still required to function properly at full USB 2.0 speeds.

THE EXCEPTION: The one catch to backwards compatibility is that the SuperSpeed USB B male connector is not able to interface with a USB 2.0 B female jack. This situation will only occur with a device featuring a built-in (non-removable) USB Powered B connector, and is intentional because a USB 2.0 port is not capable of powering a USB 3.0 Powered B device. In any other situation involving USB 3.0 B and USB 2.0, there is compatibility either at the USB A end of the connection, or by using a USB 2.0 cable.


WHAT IS THE USB POWERED B CONNECTOR ?

A weakness of earlier USB specifications was the one-sidedness of the bus-powered option. Peripherals connected directly to a host machine were allowed to draw power from the host port and function without an external AC or DC power supply. There was no reciprocal for the device end of the run, however, so USB hardware on device-side (such as USB-Over-Ethernet units and USB-WiFi adaptors) required their own power brick. Particularly for in-wall installations and integrated products, this was a hurdle which presented any number of technical issues.

USB 3.0 seeks to remedy this situation with an offshoot of the USB B connector called the Powered B connector. When paired with a Powered B-compatible device, USB 3.0 accessories can draw their power from the device just like from a host. The Powered B male plug is only available hard-wired to devices that specifically support it; there are no Powered B cables.

The Powered B implementation is also related to the distinct form-factor of the SuperSpeed USB B connector. Standard USB B devices can connect to USB 2.0 hosts by using a USB 2.0 A-to-B cable, which is compatible with the USB 3.0 B port on the device. Powered USB B devices, on the other hand, will always have their cable hard-wired (non-removable), so there is no risk of attempting to run a Powered USB B device over a USB 2.0 cable, nor any risk of plugging a Powered USB B device into a USB 2.0 port.


WHAT ARE USB MICRO-A AND USB MICRO-B?

USB Micro-B:
This one is first on the list because it's the simplest to understand. The SuperSpeed Micro-B connection is similar in appearance to the classic USB 2.0 Micro-B, but with an additional set of pins to the side to host the SuperSpeed data transfer. The tapered part of the connector is backwards-compatible for USB 2.0-only devices and cables, and prevents a USB Micro-A cable from plugging into a Micro-B port. The USB Micro-B connector exists both as a male plug and a female receptacle.

USB Micro-A:
This male-only connector is nearly identical to the USB Micro-B connector, except that the larger portion does not have the tapered sides. This subtle but important distinction prevents a USB Micro-A cable from plugging into a USB Micro-B port, to ensure that a device-only peripheral will not attempt to be interfaced as a USB host. The USB Micro-A connector exists only as a male plug on a cable.

USB Micro-AB:
This female-only connector is identical in appearance to the USB Micro-A, but is distinctly named "Micro-AB" in the specification. The premise is that any USB On-The-Go device that can function as a USB host must also function as a USB device, therefore the USB port on the device must accept both Micro-A and Micro-B plugs.

It would be simpler all around if the Micro-AB connector were called Micro-A and understood to be compatible with Micro-B, but that is not the official nomenclature. How the manufacturers and public choose to refer to these connections remains to be seen.


WHAT'S THE BEST WAY TO IMPLEMENT USB 3.0 ?

Despite the scale and aggressive improvements of the SuperSpeed USB standard, it was fundamentally designed to be backwards-compatible with existing USB technology. The best approach to USB 3.0 is a progressive upgrade, particularly in terms of infrastructure and hard wiring. If installing USB through conduit or as a component of a larger assembly, there is no reason not to integrate USB 3.0 cables. End users will have little use for USB 3.0 until the connectors permeate the motherboard and peripheral markets, but given the choice between a strictly-2.0 enclosure and a 2.0/3.0 enclosure, more and more customers are leaning towards the future option of SuperSpeed bandwidth.

Written by Anthony van Winkle for DataPro International Inc.
Unauthorized duplication strictly prohibited.

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