When you plug in your monitor to your KVM switch or your extender, usually you expect it to work without problems and without any hangups. I have seen a growing problem while troubleshooting KVM switches, and this is very few people have an understanding of what DDC is, and many people do not know that exchanging of DDC information is usually a key factor in if your video will transmit or not with your KVM switch or extender.
Taking a step back though, DDC stands for Display Data Channel. There are VESA standards that define exactly what DDC is, but basically it is how your monitor communicates with the video card it is attached to, to notify the video card of what type of resolutions it is capable of and some other data. They store the working resolutions and the monitor and graphics card now have a common ground to display on.
The problem arises when you have a piece of hardware in the middle of the monitor and your graphics card, and this hardware does not send the DDC information to the graphics card. This will cause no video to display, or in some cases it will cause the computer to revert back to its lowest resolution. Often hard wiring the monitor and rebooting your computer takes care of this problem by letting the EDID information be exchanged, and thus having a resolution they agree on.
There are also products like a DDC ghost, which emulates DDC information, and software programs which will let you manipulate video card driver EDID information and force resolutions other than the ones in the monitors EDID information.