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The truth about Citrix HDX and WAN acceleration in XenDesktop 5.5

Citrix built WAN acceleration into HDX to improve performance and lower the cost of XenDesktop 5.5. But does this mean VDI shops can eliminate third-party WAN accelerators?

One of the biggest challenges with VDI is delivering an acceptable end user experience. Ideally, a VDI-based endpoint should perform on par with traditional desktops and should increase productivity while reducing administrative overhead.

But WAN connections create numerous problems for virtual desktop infrastructure (VDI) sessions, ranging from slow response due to latency to slow screen updates caused by limited bandwidth. This means companies that want to deliver VDI-based virtual desktops to remote users over a WAN have to invest in third-party WAN acceleration hardware to resolve performance issues.

The cost of server-hosted VDI is already high, so to lower the cost and put VDI on par with physical desktops, vendors have to lower or eliminate some infrastructure costs.

One way Citrix has done this is by building WAN acceleration directly into its remote desktop protocol technologies, called HDX. The new version of HDX with WAN acceleration became available with release of XenDesktop 5.5 in August 2011.

By building WAN acceleration into HDX, Citrix claims customers no longer a need to buy third-party WAN acceleration products.

But to say this eliminates the need for WAN acceleration is a questionable claim at best. The latest iteration of HDX may not gain any additional performance from hardware-based WAN acceleration, but you'll still need third-party WAN acceleration products for other things.

Most enterprises that run a WAN use it to support remote sites, mobile workers, connections to data centers and so on. When it comes down to it, VDI traffic may only be a small part of the overall load on the WAN. For many, a traffic control and acceleration solution is necessary, and it normally comes in the form of a WAN optimization and acceleration device.

In those situations, Citrix's improved HDX offers little advantage and, in some cases, may harm performance due to the fact that the WAN acceleration device may add latency by trying to accelerate the HDX traffic using compression and decompression technology.

However, for those deploying XenDesktop 5.5, the performance enhancements will work for remote employees, regardless of there being a WAN accelerator in the mix. Nevertheless, I wouldn't deploy XenDesktop 5.5 with the expectation that it eliminates the need for a WAN accelerator.

More HDX enhancements
While you may still need a third-party WAN accelerator in your data center, other improvements make the new version of XenDesktop worthwhile, including the vDisk technology Citrix acquired from RingCube.

The vDisk feature stores users' preferences, data and applications in a separate container on a Microsoft Virtual Hard Disk (VHD) file. This feature speeds up performance in several ways and also allows users to create customizable/personalized desktops. The performance boost comes from the VDI environment not having to recreate a complete virtual desktop, allowing its last state to be quickly reloaded, instead of being reassembled using a layer cake style approach, common to most VDI implementations.

Other XenDesktop 5.5/HDX improvements include accelerating the real-time protocols behind voice, video and audio for virtual desktops.

There's also Citrix's HDX MediaStream Flash Redirection technology, which detects when a virtual desktop user attempts to execute an application that uses Flash and instantly determines if the endpoint is capable of running it. Flash Redirection shifts the workload from the server top the endpoint and gives users better performance by executing locally.

Additionally, Citrix claims XenDesktop 5.5 HDX includes a six-fold boost in scanning and printing speeds, as well as the ability to calibrate QoS for branch offices based on user needs, such as prioritizing real-time traffic like voice and video, for example.

Ultimately, Citrix's improvements to XenDesktop 5.5 and HDX enhance the end user experience, but administrators should carefully evaluate the real-world performance of the product before redesigning any WAN connectivity.

Frank Ohlhorst is an IT journalist who has also served as a network administrator and applications programmer before forming his own computer consulting firm.

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