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SuperSpeed Extends USB

USB 2.0 has served devices well for a decade. Now, USB 3.0 promises to extend the interface for the future.

The USB 3.0 specification, released in November, 2008, defines a new, SuperSpeed bus that operates in parallel with USB 2.0’s wires. The 5-Gbps signaling rate is over 10 times faster than USB 2.0’s top speed of 480 Mbps:

Bus Speed

Bus Signal­ing Rate (Mbps)

Channel Type

Max­i­mum Cable Segment (meters, typical)

Typical Use

Low

1.5

half duplex*

3

mouse, keyboard

Full

12

half duplex*

5

virtual COM port

High

480

half duplex*

5

mass storage, video

Super­Speed

5,000

dual sim­plex**

3

high-per­for­mance mass storage, video

* Both directions take turns on one pair of wires.
** Each direction has its own signal pair and ground wire.

Plus, unlike USB 2.0, SuperSpeed has a pair of wires for each direction so traffic can move in both directions at once. The first SuperSpeed devices will likely be drives and high-resolution video.

But you don’t need to worry that all of your USB 2.0 devices will soon be obsolete. USB 3.0 supplements, but doesn’t replace USB 2.0. A USB 3.0 host must support USB 2.0 speeds.

Where possible, USB 3.0 builds on USB 2.0. SuperSpeed devices use the same four transfer types, descriptors (with some additions), USB classes, and hub topology.

USB 3.0 also offers advances in power use. SuperSpeed devices can draw up to 900 mA per device, compared to 500 mA for USB 2.0. When reducing power is feasible, USB 3.0 defines new power-saving modes.

Every SuperSpeed device must support at least one USB 2.0 speed, but the device doesn’t have to fully function at that speed. The device might just return a message saying that the device needs a SuperSpeed host to perform its function.

USB 3.0 host and device controllers are available. Microsoft has announced that Windows 7 will add support for USB 3.0 in a service pack.