One of the editors looked up from the stack of papers in her lap that contained the vital statistics of the Summit7i Gigabit Ethernet switch from Extreme Networks as she finished her report. "And," she said, "the switch is only 4RU high." The editor, a member of the committee that decides on the Best of Show awards at Networld + Interop, was pitching it as her choice for Grand Best of Show last week. The size was one of a long list of reasons she thought this switch is important.

As it happens, this was only the latest of a number of size-related features that vendors have started designing into their products. Earlier this month, IBM sent me a new server that was only two rack units (RU) high, and the month before that Sun was telling me about one that's only one RU high.

The interest in proving that size does matter isn't restricted to servers and switches. At N+I this past spring, American Power Conversion went to great lengths to tell me all about its Net Shelter power protection racking system, and how few rack units the associated UPS and related equipment required. The idea is that you could include a whole network's worth of equipment, including power conditioning, in a single rack.

So what's a rack unit, and why does it suddenly matter? Once an obscure term that described a unit of space 19 inches wide and one-and-three-quarters inches tall, the RU is suddenly becoming one of the most important considerations in data center design. The reason is that networking equipment mounts in 19-inch racks. Increasingly, so do servers, switches, power management units, monitors and just about everything else you're likely to want in your server farm or switch installation.

This growth of things that can be mounted in racks has resulted in two things. The first is great convenience. Mounting all of your networking equipment in these metal frames gives you an orderly way to organize your data center or server room. Servers, switches and power supplies that work together can be mounted together, eliminating the spaghetti-like wiring snarls that frequently mark networking installations.

The second is that data centers started to become rank upon rank of metal racks. These collections of racks are expensive, and once the data center is filled, many companies have nowhere else to go. Conserving space within individual racks becomes vital, especially as demand for networking services and related items such as data storage grows.

The answer, obviously, is to cram more stuff into less space. But that's easier to suggest than to accomplish. Making a Gigabit Ethernet switch significantly smaller, while retaining the same or greater number of connections, while also keeping the same or better performance, is difficult indeed, and when it's accomplished the result is frequently a breakthrough in design. That's why manufacturers are so proud of their new one- and two-RU servers and their four-RU power supplies and switches.

Knowing that there will be smaller and smaller products, however, has limited appeal. More important is knowing how you plan to use the space in your data center and what you plan to put in it. There's a wide variety of opinion as to what equipment actually belongs in the controlled environment of a data center.

For example, many companies place virtually all of their networking equipment, regardless of function, in the same room. You'll find servers, switches, modem racks and tape drives next to one another. This is convenient for the data center staff since it's easy to reach any individual device, but is it really necessary? Might it be better to place critical high-value items in a protected environment, while placing other items, perhaps the modem racks, in another room?

Remember, even if you manage to find a space that meets all of the requirements for a data center, it's unlikely to remain that way for long. As your company's demands for more storage, greater bandwidth and increased services grow, so will the demands for increased rack space and, as a result, increased floor space.

On one hand, these demands can be met by equipment that gets ever smaller, but to really take advantage of these new products, you'll have to spend the money to replace most of what you have now. If you can't afford to replace everything regularly, then planning becomes more important. That means making sure you start out with room to grow and that you then have a plan for what to do when that room runs out. It always does.