USB 2.0 boosts bus speeds

By Jeff Chang
Network World, 04/29/02

Over the past five years, PC users, network managers and developers have grown to see the value of the Universal Serial Bus standard. USB offers universal plug-and-play and extreme ease of use. When a USB peripheral is connected to a USB-enabled PC, the system will autodetect and autoconfigure the device. USB also eliminates the need for multiple I/O standards, simplifying PC connectivity.

However, the original USB specification, Version 1.1, had one drawback - lack of bandwidth. With a top speed of 12M bit/sec, bandwidth sharing has long been a major issue with USB. That is about to change.

Enter USB 2.0

The USB specification was recently updated to improve the performance and usability of PC peripherals, opening the door to a world of high performance and high bandwidth, such as mass storage, digital video and broadband access. The speed of USB 2.0 has been increased to 480M bit/sec, a 40-fold improvement over Version 1.1.

Perhaps the most important feature of USB 2.0 is that it is forward- and backward compatible with existing USB 1.1 devices and systems. The connectors, software, cables and other external product aspects are all the same. Existing USB 1.1 peripherals will continue to work with PCs equipped with USB 2.0, and vice versa.

Inherently, USB is a master-slave (or host-peripheral) architecture. The PC host is in charge of all the devices attached to it. It initiates all communications, and devices do not "speak" unless they are spoken to. This type of architecture leaves much of the system intelligence on the PC, letting the USB peripherals be designed and implemented in a cost-effective manner - an important feature for the cost-sensitive PC market.

At the electrical level, USB is a four-wire serial interface - power, ground and two data lines for differential signaling. There are three distinct speeds supported by the USB specification:

Low-speed - 1.5M bit/sec.
Full-speed - 12M bit/sec.
High-speed - 480M bit/sec.

Having three speeds gives developers the flexibility to choose between multiple price/performance levels when implementing their designs. For example, hardware devices for low-speed applications such as mice and keyboards can be scaled back in cost to provide the minimum functionality required.

A typical USB transaction has three phases:

Token packet.
Data packet.
Handshake packet.

The packets consist of encoded bit fields, and devices will respond to packets addressed only to them. The host also sends out a start-of-frame packet every millisecond in full-speed mode and every 125 msec in high-speed mode to keep the entire bus synchronized. The host will access multiple devices within a single frame to optimize bandwidth allocation.

There are three pieces to the puzzle that need to be in place in order for USB 2.0 to become a ubiquitous interface on PCs:

1. Host hardware support - USB 2.0 host hardware is available now in the form of PCI add-on cards or PCMCIA cards. Chipset vendors are currently evaluating integrated USB 2.0 products with their development partners, and should be in full production by the fall. Chipset integration is a key driver for adoption rates.

2. Host software support - USB 2.0 host software is available today from Microsoft for Windows XP and Windows 2000. Although these drivers are not native to the shrink-wrapped product on the store shelves, they are available through Windows Update, a feature in Windows that lets users download upgrades to their software packages.

3. USB 2.0 peripherals - USB 2.0 peripherals are available today. There are a number of certified devices that are in production, and many more will hit store shelves in 2002.

Although mainstream adoption is not expected until the second half of next year, many OEMs will ship USB 2.0-enabled PCs between now and then.

USB 2.0 boosts bus speeds

Enter USB 2.0

Related Links