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How to design and tune a WLAN to support mobile VDI clients

Editor's note: In the first two part of this series on networking for virtual desktop infrastructure (VDI), we explore testing the network for VDI application performance

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and relying on converged storage for a VDI environment. In this final part of the series, we look at building WLANs that can handle VDI mobile clients.

As more enterprises look to enable mobile devices as Virtual Desktop Infrastructure (VDI) clients, engineers must tune the wireless LAN to accommodate all of these devices and dynamic applications. There are several ways to tune a wireless LAN for VDI.

Need enough bandwidth for mobile VDI clients? Avoid overlapping channels

VDI tends to be bandwidth intensive, so if you want to use a wireless network to support mobile VDI clients, you will need to be sure your network performs optimally. One way to do this is to avoid deploying your wireless access points in a way that makes them susceptible to interference from existing wireless hardware. If two wireless access points are placed within range of each other on the same channel, both the performance from both will be degraded.

Simply avoiding a channel that is already in use is not enough however. Each Wi-Fi specification has a degree of overlap between channels. If you want to get optimal performance from your wireless hardware, it is important to use non-overlapping channels. For example, the non-overlapping channels for 802.11B are 1, 6, 11 and 14 (channel 14 is not used in North America). Likewise, the non overlapping channels for 802.11 G/N networks are 1, 5, 9 and 13. Keep in mind that the available channels, as well as the degree of overlap between channels, varies by region.

The easiest way to avoid channel overlap is to use a laptop running Wi-Fi Stumbler. Wi-Fi Stumbler is a free tool that analyzes Wi-Fi channel use. You can run this tool through a Java-enabled Web browser on a laptop and then walk around your facility checking to see which wireless networks are detected in various parts of the building.

Perform site surveys to build a VDI-ready WLAN

Checking for overlapping wireless networks is only one step. A site survey is also critically important. The goal of a site survey is to determine optimal placement for your wireless access points.

To perform a site survey, pick out a few strategic locations for your wireless access points. One by one, place a wireless access point at the locations that you have chosen and then test to see what kind of coverage the access point provides before moving on to the next test location.

Generally speaking, access points should be placed up high and away from any structural metal or large metal objects. One thing to keep in mind is that most of the access points that are rated for indoor use use omnidirectional antennas. Although omnidirectional antennas do a good job of providing widespread coverage, there is almost always a dead zone immediately below the wireless access point. This happens because the radio signals emitted from an omnidirectional antenna initially travel horizontally. The radio waves spread out vertically as they get further away from the access point, but typically leave a dead zone immediately beneath the access point. This dead zone can not only affect users who are directly underneath the access point, but also users on lower floors.

As you perform your site survey, be aware of things that can interfere with the wireless signal, but that might not be causing interference right at this moment. Access points that support 802.11 B/G/N use 2.4 GHz signals. These signals are susceptible to interference from microwave ovens and older cordless telephones. Wireless 802.11A access points and some 802.11N access points work on a 5.8 GHz frequency. This frequency range is susceptible to interference from newer cordless telephones.

Beware of wide channel, it could be a VDI WLAN pitfall

Earlier I spoke about the problem of channel overlap. Channel overlap occurs because wireless access points divvy the 2.4 GHz or 5.8 GHz spectrum into a series of channels. In the case of 802.11N, each channel is 20 MHz wide. By using two separate radios and other techniques, 802.11N can provide up to 150 Mbps of raw throughput.

Many wireless vendors offer a feature known as wide channels. Wide channels expand each channel to 40 MHz (essentially using two channels instead of one) in order to achieve a theoretical 300 Mbps of wireless bandwidth.

Unfortunately, there are two potential problems with using wide channels. First, using wide channels increases the chances of interference from other wireless hardware. Second, and more importantly, Bluetooth devices (which operate in the 2.4 GHz band) are likely to interfere with wide channel Wi-Fi.

One of the most frustrating aspects of designing and tuning a wireless network is that all of your hardware can be undone by someone in close proximity to your facility carelessly deploying wireless hardware. As such, the tuning process should not be thought of as a onetime operation. Instead, you should periodically audit the airwaves to make sure that your wireless network is not suffering a performance degradation as a result of someone else’s new wireless network.

ABOUT THE AUTHOR:
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer, he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies.

This was first published in January 2012

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