A variety of KVM switches are available, all with assorted feature sets and prices. The simplest KVMs are nothing more than glorified A/B switches, while the advanced ones go as far as being accessible via IP.

Multiple platform support is your first consideration. If you're running a collection of Intel x86, Sun Microsystems and Apple Macintosh hardware, make sure the KVM you choose supports connections to all of them since each of these computers use different connectors for the keyboards. However, newer Macs and x86s can get along well if both use VGA for video and USB for the keyboards. Some manufacturers, such as Tripp Lite, sell little adapters so you can plug a Sun unit into a x86 KVM. Other vendors, like Aten International, make two or three different KVM switches to accommodate various machines.

There are different ways to switch the active screen on a KVM. The low-cost, low-tech methods are to push buttons or turn a rotary dial on the switch. Those who remember the old A/B/C/D printer switches of yore will understand the disadvantages of button KVM switches. If the switch is not close to the head, you'll have to walk over, switch monitors and walk back. Some KVMs support the weight of a 15-inch CRT monitor, and in a small machine-room rack, a pushbutton device may be sufficient.

For those who want more flexibility or plan on joining a large number of computers, an on-screen menu or hot-key–based KVM is a smarter choice. On these models, a special key combination will switch screens or activate a menu letting you switch computers without removing your hands from the keyboard. Some menu-based switches will even let you set and/or permanently display the computer's name or an identification string. Four machines running the same OS look so similar it's easy to mix them up. You wouldn't want to reboot the Web server instead of the test server accidentally. Some switches also let you display which devices are powered on or off.

The scalability of these devices is a grand factor. If you are at a small shop or have limited rack space at a collocation facility, you may not need more than 16 connections. However, if you want to control every device in a 2,000-square-foot machine room, more planning is needed.

Many KVM switches let you stack or cascade multiple switches into one logical unit. Find out the maximum number of feet allowed between switches, because when you're dealing with collections of racks, cabling between KVMs must sometimes be run under raised flooring. Likewise, the distance between individual computers and the switch can have limits that will vary from manufacturer to manufacturer. Some may require proprietary cabling. Avocent Corp. and other vendors sell extender cables (over Category 5 cable) to run between the KVM and a head, even as far as 1,000 feet.

Greater Intelligence

The most advanced KVM switches let you do more than just control multiple machines. One advanced feature of Avocent's DSR4160 is user authentication. On these models, a user must log into the KVM switch before using it. You can also set up ACLs, letting users look at or touch some computers and modify settings, while keeping users off other machines.

Logging support lets you see who's using the KVM. Authentication can be made against back-end databases, such as a Microsoft SQL database for the KVM switches, or with a Windows NT domain. Some products, like Aten MasterView, support only one KVM-lock password.

Obviously, authenticating against a domain requires network access. As such, vendors are taking advantage of the network and offering KVM switching over IP, which is similar to using Symantec Corp.'s pcAnywhere. You can remotely access the KVM switch and all devices connected to it. This feature is especially helpful for telecommuters, including the administrator who's home in bed during the 4 a.m. crisis.

Another benefit of KVM over IP is that distance limitations are lessened because cascading isn't necessary. You can also have multiple users control devices on the same switch as opposed to using the "one operator, one head" model of traditional KVM switches.

KVM over IP requires more setup than is involved with traditional KVM switches, which are usually just install and play. Management or authentication servers must be installed, and individual clients trying to connect to the KVM will need specialized software or a Web browser plug-in--in case the vendor did full-screen video in a Java applet, for instance. Remote users should use a VPN connection to access the switch.

Pay attention to the price of KVM-over-IP switches. You may be required to buy a management server and licenses to use IP. This could raise the cost of your KVM switch purchase several thousand dollars. Still, the advantages of connecting multiple machines without installing custom software on the server, such as pcAnywhere, can be very attractive.

Trials and Tribulations

No KVM switch is perfect. Occasionally, the mouse or keyboard will stop working. Some switches don't like if you turn on the computer before you turn on the switch, so unplugging the switch might require you to reboot every device connected to it. Menu or hot-key switches don't always let you change the hot key.

On some of the KVMs in our labs the menu is accessed by pressing the control key twice quickly. When we use SSH (Secure Shell) to access a Unix box, we often hit control, which usually results in a few choice words being muttered as the KVM-switch menu pops up. Finally, the disadvantage of KVM switches is that they make using multiheaded displays or comparing two screens side by side more difficult.