KVM Switches

Basics and Working Principles


What are KVM switches? KVM stands for Keyboard Video Mouse - do not confuse with Linux Kernel Virtual Machine. These switches are hardware solutions to operate several computers or servers from a few number, or even only one, keyboard, monitor and mouse user set. This keyboard, monitor, and mouse set is called console. This goal is reached without using special software running into the connected computers.

What is an IP KVM switch? IP KVM switches allow you to access their console port remotely through a TCP/IP access. It may be simply from another room over your LAN, or from anywhere in the world across the Internet. Users take remote control of computers or servers by using a web browser or a small utility software similar to remote control tools. However, there are major differences between IP KVM switches and remote control software. IP KVM switches don't need a server program running into the computer, the control is possible even at BIOS level, and it is generally possible to power off and power on the computers under control.

How does a KVM switch work? On the computer side, the KVM switch emulates a monitor and a PS2 and/or USB keyboard and mouse set. Thus, each computer or server "thinks" it is connected to a keyboard, monitor and mouse at all times. On the local console side, the KVM switch drives the standard keyboard mouse and monitor in the same way as a computer does. When you select a computer, its video, keyboard, and mouse ports are replicated on the local console ports allowing you to operate the computer normally. Some KVM switches can switch the audio ports.

KVM operation: Most of time, several methods permit to configure and to operate the KVM switch: front panel push buttons; special keyboard strikes or "hot keys"; on screen display (OSD) menus.

Cables: All connections can be made with special cables or simple twisted-pair cat5 cables. In case of twisted pair you will need for each computer a small device called dongle, or computer module, whose purpose is to capture the keyboard, mouse, and video signals, and make the adaptation permitting to carry them into the cat5 cable. Of course this device has a cost that is higher than the cost of special combo cables used with conventional KVM switches. The advantage to using cat5 cables is that connections can be much longer than with traditional cables, and the cabling is more flexible.

Remote access: Very often, console peripherals are connected directly to the KVM switch, limiting the user installation area to a few meters. To expand the user area, the consoles can be attached remotely, up to a few hundreds of meters with cat5 cable, or farther by using a TCP/IP connection.

What are the characteristics of a good KVM switch? KVM switches are complex devices that vary a lot in quality and performance. Low-price switches often have poor video quality, poor USB and PS2 emulations, wrong video bandwidth figures, and tiny controllers incapable of providing simultaneous emulations on all ports. You get what you pay for. To know if a KVM switch is of good quality, you should check the following points:

  • quality of USB and PS/2 emulations that will make it compatible with all computers
  • quality of the video switch that must handle hundreds of MHz without spoiling the video
  • no cross talk between computers
  • ability to plug new peripherals and new computers alive with automatic configuration
  • keyboard and mouse emulations maintained under all conditions, even if the KVM switch is powered off
  • handiness of commands and hot keys to prevent user fatigue
  • speed of switching process, to not wait 10s for video and peripherals coming back after each switching
  • quality of On Screen Display menus
  • capability to cascade or daisy-chain several devices to increase the overall capacity of the system
  • protection of ports against transients and over voltages
  • possibility of updating the firmware