Everything you wanted to know about USB 2.0 but were afraid to ask

USB 2.0 promises to deliver incredibly fast data transfers (up to 480 Mbps), providing organizations with new external storage options. James McPherson dissects USB 2.0 technology so you'll be ready to merge 2.0 into your existing PC world with ease.

External storage is a handy thing to have around, but it never seems to be fast enough or large enough. Floppies gave way to Zip disks, which have mostly succumbed to CD-R, but nothing seems to beat a spare hard drive for instant backups or installations. For the longest time, this required a significant investment in removable drive bays or the standardization on SCSI to use external enclosures. The original USB offered a minor panacea, but transfer rates weren't high enough to really make it a replacement for network storage. Twelve megabits per second theoretical bandwidth would only realize 1 MBps of transfer, restricting USB 1.1 primarily to the world of peripherals. This roadblock can now be overcome with the introduction of USB 2.0.

Check out my product review of Evergreen's new Pocket HotDrive to see USB 2.0 in action.

A little USB history
Apple pioneered high-speed, low-cost external storage with its introduction of FireWire, later standardized as IEEE 1394. At 400 Mbps and with the ability to handle 63 devices, it offered enough bandwidth to support digital video editing. Only gigabit Ethernet and Ultra Wide SCSI-3 or better could match it for bandwidth at the time of its introduction.

USB originally was intended to support a wide variety of devices across a spectrum of transfer rates. One of the key selling points of USB 1.1 was that 127 devices could be connected to a single controller. Controllers were relatively inexpensive, and more than one could be installed on a motherboard to provide plenty of bandwidth. Thus, device manufacturers have capitalized on USB technology by hitting the market with any device that could function within the bandwidth limitations.

Now the PC industry has been working to improve the USB standard to meet the needs of its users with version 2.0. With more raw bandwidth than IEEE 1394, many see USB 2.0 as a contender for the speed crown. However, this challenge is a misinterpretation of the specification, as USB 2.0 has its own goals separate from those of IEEE 1394.

USB 2.0 nuts & bolts
USB 2.0 controllers fully support USB 1.1 devices and transfer rates. Most USB 2.0 devices are equipped with a 1.1 mode to provide backward compatibility, but this will vary between manufacturers. USB 2.0 is supported by Mac OS 8.6 or greater and Windows 98SE, Me, 2000, or XP. USB 2.0 has just been introduced to Linux on the development kernel; however, it is in its first version and is largely untested.

USB 2.0 introduces a number of new transfer modes at much higher bandwidth: 480 Mbps total bandwidth for High-Speed mode, 12 Mbps for Full-Speed mode, and 1.5 Mbps for Low-Speed mode. I must reiterate, however, that this is theoretical bandwidth and not functional bandwidth. The USB bus reserves approximately 10 percent of the bandwidth for administrative tasks, like device polling, and to ensure a degree of redundancy.

Like all packet modes of communication, USB has overhead. There are a number of bits used to indicate sequencing, transfer mode, source and destination devices, and any optional information. This reduces the total bandwidth to the effective bandwidth. The header size for USB 2.0 is approximately four times that of USB 1.1 (173 bytes vs. 45-46 bytes), so the greatly increased bandwidth compensates for the additional overhead.

Increasing the granularity of the signaling and meshing the faster USB 2.0 signals in with the existing USB 1.1 rates provides the bandwidth increase. While this high clock rate nominally increases the cost of the USB 2.0 controller, the improvement of manufacturing processes tends to offset this. The one weakness may be that not all USB 1.1 cables will be of sufficient quality to operate at USB 2.0 speeds. During the transition period, I would advise you to verify that any cabling between USB 2.0 devices was provided with them or was specifically rated for USB 2.0 use. Most cables are expected to function without difficulty, but with increased clock rates comes increased sensitivity to outside interference, and shielding is one of the easiest places to scrimp.

USB 2.0 has a number of transfer modes defined to support different types of devices. Furthermore, devices can use custom transfer rates as long as the drivers are written properly to establish the connection between the hub and the device. The asynchronous transfer mode has several modes designed to guarantee delivery of data at up to 15 MBps. The isochronous transfer mode provides even more bandwidth.

The low recommended asynchronous bandwidths are to allow multiple devices full access to the bus. Speeds of 15 MBps (120 Mbps) are more than sufficient for most peripherals. A typical computer with an Ethernet card (100 Mbps), a hard drive (120 Mbps), a 24x CD-RW (28 Mbps) drive, and a 40x CD-R (47Mbps) drive running continuously will still have 100 Mbps remaining to operate other bandwidth-intensive devices like printers and scanners and will almost never reach the point of interfering with mice or keyboards.

Isochronous mode is intended for real-time streaming data like audio and video. Unlike digital video (DV) editing, where you want to transfer data in its entirety, Web cams, digital video recorders, speakers, and microphones perform worse when the stream is interrupted. Because isochronous mode is time sensitive, it is capable of virtually monopolizing the bus, claiming up to 400 of the 480 Mbps in the fastest of the predefined transfer rates. These devices will likely be at the heart of most problems with USB 2.0, as they could easily interfere with the operation of keyboards, mice, or other vital peripherals.

Integrating USB 2.0 devices into your inventory should not be difficult if a modicum of care is taken. Backward compatibility will likely ensure ready adoption and ease of implementation. Any organization rolling out USB 2.0 devices would be advised to examine each installation to ensure there are no misunderstandings. Users, or even technicians, who are inattentive could easily connect a USB 2.0 device to a USB 1.1 hub and hamstring their own performance. Overall, USB 2.0 should merge into the existing PC world with no more than a slight ripple.

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