It wasn't so long ago that the sub-notebook was thought of as an expensive toy: fine for people doing lightweight work on the move, but of no use for serious business applications.

How times have changed. Today's ultra portables are now hard on the heels of their bigger brothers, in terms of both performance and functionality, with processor speeds of up to 750MHz being installed in some of these thin notebooks. And interestingly, all the sub-notebooks we've reviewed here have both a modem and some form of LAN connectivity built in, implying that they're more than ready for life both on the road and in the office, thank you very much.

But the downside of installing powerful CPUs in such a thin form factor is the amount of heat generated; some of these systems do get fairly hot when running. Getting rid of all that internal heat is a major stumbling block, and the conventional sub-notebook design limits are rapidly approaching, with more and more hardware being fitted into a smaller and smaller space. Using advanced cooling solutions, such as heat pipes, miniature fans and a magnesium alloy chassis, helps to radiate and conduct the heat away from the internal components, but it's still a nightmare for designers.

The other flip side of these powerful processors is getting a battery to power them for any length of time. Intel's SpeedStep technology helps but this is still a major problem with current battery technology.

Here we've reviewed a cross section of sub-notebook systems, ranging from 'old technology' Celeron processor-powered ones to state of the art Mobile Pentium III gadgets, to see how - or if - these major design challenges have been overcome.

For the purposes of this feature, we've defined 'sub-notebook' as meaning a small, light notebook with a cut-down feature set; i.e. with no internal CD drive or floppy drive. Definitions of different types of notebook are tricky (some people might call these ultra-portables or mini-notebooks), but this works for us.

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