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USB—IT'S NOT JUST A BUS, IT'S AN ADVENTURE

by Tom Cantrell

Start ı Nearvana ı Waiting for Baudot ı Not So Fast ı VolksPort ı Going Mobile ı Sources and PDF

NOT SO FAST

Just as mistaken as the notion that 2.0 will blitz the market overnight is the idea that it will replace 1.1 gear. Look at it this way. Every 2.0-capable host that does get installed provides a new home for 1.1 devices as well.

Even the staunchest USB 2.0 promoter isnıt going to argue that you need a 480-Mbps mouse or keyboard. Going further, and against conventional wisdom, Iıd argue there are a lot of middle-of-the-road applications that can get by just fine with 1.1. Why? There are two reasons.

First is a smart feature of 2.0, speed matching. In 1.1, a low-speed connection propagates upstream through intervening hubs clogging up the final connection at the PC itself. By contrast, a 2.0 hub is designed to accommodate whatever data rate the device prefers while communicating upstream towards the PC at high speed.

Second, remember that from the point of view of a USB peripheral the connection is point-to-point. It doesnıt matter how efficiently the peripheral uses cable bandwidth, because it doesnıt have to share it with anyone. Thus, thereıs little to be gained from boosting peripheral USB bandwidth much beyond the bandwidth of the device itself.

Consider the typical low-cost inkjet printer. At a few pages-per-minute throughput, letıs say bandwidth into the paper hopper is about 1 MBps (e.g., a 20-MB color picture takes 20 s to print). Of course, the CPU inside the PC always wants to do I/O much faster than poky mechanical devices. Thatıs why buffering (i.e., speed matching) is so important. In the case of the inkjet printer, the file is actually spooled to disk at a higher speed.

Yes, thereıs a lot of protocol overhead for USB, and the connection gets more crowded as you head upstream. But, remember that the entire bandwidth is available on the particular cable that connects each peripheral. A 12-Mbps USB connection fully dedicated to efficient (bulk in USB speak) transfers probably delivers close to the 1 MBps the printer can ultimately digest.

What would having a 480-Mbps 2.0 port in the printer buy you? If you donıt turn off the spooling, the user experience (the time from clicking Print until the system comes back) is still the same (the time to write the spool file to disk). None of this makes the printer heads fly or ink spew any faster, so the paper wonıt appear in the hopper any sooner.

If you do turn off spooling, it means youıll need a huge buffer somewhere down the line. Yes, RAM is cheap, but 20 to 30 MB in a sub-$100 inkjet is pushing it. And even if the printer can buffer a complete page, what happens when you try to print two?

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