USB 3.0

By | March 15, 2010


USB, an interface we are all familiar with, is changing (or changing again, if you remember USB 1.1). USB 2.0, which by now is as familiar as the electrical outlets on the wall, is getting a makeover that will bring it up to date with users’ demands for  increased bandwidth and peripheral support.




Turning the clock back, you might remember that the change from USB 1.1 to USB 2.0 entailed, most importantly, a forty-fold increase in speed from 12 Mbps to 480 Mbps. This speed increase put USB 2.0 in the ballpark with the FireWire of the time (400 Mbps IEEE 1394a), and made USB a practical interface for memory sticks, cameras, external USB drive enclosures, and so forth.

Nothing stands still for long however; FireWire 400 evolved to the faster FireWire 800 (IEEE 1394b), hard disk drive interfaces added the eSATA (external Serial ATA) interface, cameras started taking pictures with much larger megapixel counts, digital video overtook analog video, and everybody wanted to back up and transfer heaps of music and movies in digital form. USB 2.0 started to look old.

Enter USB 3.0. This specification adds a number of new features, most importantly:

  • a new architecture with strong backward compatibility
  • a much higher bandwidth with matching hardware
  • greater speed from more efficient data and protocol signalling
  • more sophisticated power management
  • higher power output on the USB port



USB 3.0 Architecture

USB 3.0, while an evolution from USB 2.0 in the sense that the user will see things as much the same, is at the same time revolutionary in its internal approach.

When USB started, it was in fact a two-speed interface: the "Low-Speed" USB, operating at 1.5 Mbps, and "Full-Speed" USB, operating at 12 Mbps. Low-Speed USB is the one everyone still uses all the time, and doesn’t even notice (the best thing you can say about an interface). But look at your mouse: that skinny little cable that doesn’t detach from the mouse (non-detaching cables are a characteristic of Low Speed USB) is what we are talking about. Full-Speed USB, arriving at the same time and with essentially the same wiring, was what put USB on the map. Later came USB 2.0, again running on the same wiring, but much faster; it is the standard today.
USB 3.0 breaks this pattern, because its new features are not actually implemented on the same bus as the older USB designs. Instead, USB 3.0 is really two buses in one, with two sets of wires and protocols. The USB 3.0 architecture diagram below shows what I am talking about: the new stuff, SuperSpeed USB, is shown in red, and the old stuff, USB 2.0, is shown in blue. They co-exist in parallel.

USB 3.0 architecture

Note however that the overall structure — host, hub, and peripheral — is the same for SuperSpeed USB and USB 2.0. Also note that although there are two buses coexisting in USB 3.0, they are passing through one set of "extended connectors" and "composite cables". And a final note: a USB peripheral cannot operate in both SuperSpeed and Non-SuperSpeed modes simultaneously; this limitation is not explicit in the diagram.
The upshot is that this design makes it possible for a great degree of forwards and backwards compatibility in USB 3.0. You won’t have to get rid of old peripherals just because you have new USB 3.0 ports.

Information supplied by Lava Computer MFG