How to choose a Cat5 KVM extender

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Cat5 cable is the obvious choice for long distance KVM transmissions within a building. Many buildings are already flood wired with Cat5 cable for the Ethernet infrastructure and Cat5 cable is inexpensive, industry standard and very easy to install.

Cat5 KVM extenders are now widely available and generally work by converting the analogue RGB signal to differential analogue signals for transmission over 3 of the twisted pairs within the cable. The fourth pair in the cable is used for transmission of the PS/2 keyboard and mouse data and sometimes also for serial and audio transmission.

Extenders of this sort work very well in most applications for distances from 20 meters to 300 meters (depending on the model) and at resolutions up to 1600 x1200. It is always a good idea to test a Cat5 KVM extender before purchasing a large number because the extended picture will never be exactly the same as the original, and the acceptability of a displayed image is very subjective.

Electrical noise can also result in degradation of the extended picture and grounding differences between the transmitter and receiver may result in a flickering image, moving bars on the screen or problems with the keyboard and mouse.

If you are extending KVM between buildings or areas supplied by different mains phases, or if you environment is very electrically noisy (e.g. an industrial area with motors), a Cat5 KVM extender will not be suitable. In these instances a fibre KVM extender, or KVM over IP may be a suitable alternative.

Another common problem associated with the extension of analogue video using Cat5 cable is that of skew, or colour drift. Contrary to popular opinion, this problem can be found in Cat5 cable as well as Cat5e and Cat6 while some Cat5e and Cat6 cables do not show any real effects of skew at all.

The effect of skew can be seen as a coloured shadow around objects on a video screen. It occurs when the pairs of wire within the cable are twisted at slightly different rates to reduce crosstalk between pairs. Because of these differences between wire pairs, video signals for different colours travel different distances before they reach the remote receiver. When one colour signal arrives behind the others because the wire is longer, you get a red, green, or blue shadow around the objects on your video screen. You are more likely to experience skew on longer cable runs (greater than 50 meters).

There are low skew Cat5 cables available on the market specifically for use with video extension products. There are also standalone skew compensation devices or delay lines available to correct skew on existing cable runs.

Many new Cat5 KVM extenders have automatic or manual skew compensation built in to the receiver and these are the best devices to choose where you think you may experience video problems due to skew.

Another factor to consider when choosing a Cat5 KVM extender is whether the extender is dual access. If you need a keyboard, monitor and mouse near your computer as well as at a distance, or, if you need to connect your computers into a local KVM switch as well as grant remote access to a single remote user, you will need a dual access KVM extender.

You also need to consider the sort of keyboard and mouse you will be using and whether you need to extend any of the other ports on your computer. Many extenders will transmit bidirectional audio as well as KVM, ideal for presentation environments. Some extenders will also transmit serial data for the remote connection of touch screens, serial printers, modems and other serial devices.

Very recently, KVM extenders have become available that can transmit USB data. These fall broadly into two categories; transparent USB extenders, and USB extenders for keyboard and mouse data only.

Transparent USB extenders will allow you to extend the USB connection to a wide range of peripherals including touch screens, USB drives and security devices, while USB extenders for keyboards and mice are limited to just these. USB is a difficult protocol to transmit over long distances as it has a high data rate and is very susceptible to the effects of noise. For this reason KVM extenders that also transmit USB tend to be limited to maximum distances of 50 or 100m.

It is always wise to test a USB KVM extender with your computer and peripherals before purchasing a large number because not all USB devices will work correctly with them. A firmware upgrade may be available to solve this, but it is always best to identify and address any potential problems as early in a project as possible.

Other popular and useful features to look out for include high density rack mounted local extender options for applications where you are extending KVM from many computers in the same area, and multi video support for extending from dual and quad head graphics cards.

Other factors to consider
Support
Buy your extender from a company you trust to advise you on the best solution for your application—if a company only sells a very small variety of extenders they are unlikely to be able to do this. Look for a supplier who will advise you on how to install your extender and who is prepared and able to help you if problems arise. What hours is technical support is available, what experience do they have, and can you speak directly to a technical support engineer or project manager? Ask about installation, warranty and maintenance options and make sure these suit your specific requirements.

Try before you buy
Request a demonstration or evaluation:

• If your application is mission critical
• If you will require a large number of extenders
• If you will be interfacing non-standard equipment
• If you will be daisy chaining multiple components
• If you are unsure if the product is right for you

There is always an outside possibility that you will experience problems and incompatibilities when installing a KVM extender in a new application. These problems are usually minor and can often be overcome by correctly configuring the equipment or by a firmware upgrade. It is always better to be prepared. If you are unsure of the best extender for your application, find out if you can try more than one product, and make sure you can send a product back if it does not meet your expectations.