As high definition video becomes increasingly popular in business, you may have wondered (or even worried) about the impact it will have on your network. How much bandwidth does it take to deliver HD video to a XenDesktop user? And what can be done to keep bandwidth consumption under control?

Surprisingly, there’s considerable confusion about what “HD” really means. Many people would expect the HD designation to guarantee a certain minimum image quality. But in fact that’s one key dimension that’s missing from the definition. HD is fundamentally a statement about resolution and, less consistently, frame rate. The lower end of HD is generally accepted to be “720p24″. That’s shorthand for a vertical resolution of 720 visible horizontal scan lines (and an implied horizontal resolution of 1280 pixels), progressive scanning, and 24 frames per second. It makes no statement about image quality, which is truly “in the eyes of the beholder” and highly dependent upon the compression technology that’s used.

Cable companies and satellite television providers are notorious for degrading image quality while still claiming to deliver “HD” video. The image detail of HD streaming video from sites like YouTube, Hulu, and Netflix is likewise far below that of broadcast HD television. But is that a problem? OK, it may be disappointing if you’re relying on those services to bring the best of Hollywood to your big screen HDTV. But for HD business videos being viewed on a computer monitor, I think most people are very pleased with the quality now available from these streaming video services.

Video quality evaluation is an important but tricky problem. Even if a video meets all of the criteria to be labeled “HD”, the perceived quality could be disappointing. The most common method of objectively evaluating video quality is to calculate the signal-to-noise ratio (SNR) and the peak signal-to-noise ratio (PSNR) between the original video signal and the signal coming out of the system being tested. This is an imperfect approach and so this area continues to be actively researched. One noteworthy tool for objectively measuring video quality is ClearView from Video Clarity. Miercom used this tool in their comparison of competing VDI products since subjective measurements would be dismissed by many readers.

So what is the video quality threshold for HD video? Since this is not well defined, a practical answer in the context of desktop virtualization comes from looking at the compression schemes used for HD video streaming over the Internet. In the context of watching movies, by far the most popular compression scheme for HD video streaming is H.264 at a video bit rate of at least 2.5 Mbps. But 2.5 Mbps may not be representative of the “HD” videos that users in your organization need to watch. Go to YouTube and play an HD video. Right-click on the video and select “Show video info”. You’ll find that a typical business video marked as “720p HD” may have a frame rate of only 15 fps and a bit rate of just 500 Kbps, far less than an HD movie trailer.

XenDesktop supports 720p24 HD video playback. First of all, it supports screen resolutions of 1280×720 (and much higher). And, effectively if not literally, it refreshes the entire image with each frame (progressive scanning) rather than every other horizontal scan line. Thirdly, XenDesktop’s default frame rate is 24 fps, increasable to 30 fps (that is actually the maximum frame rate and will be dynamically reduced if the network connection cannot support it). On top of that, XenDesktop supports full color depth (True Color, which is 16.7 million colors).

Let’s go down another layer. There are actually two different ways that XenDesktop delivers videos (and this is true of XenApp, too). The foundational approach is server-rendered video. That means the video frames and accompanying audio are generated on the server in the data center, and then technologies such as compression and caching are used to get the data over the wire to the Citrix Receiver on the user device. What’s highly attractive about this approach is that it works with any media player, any media format, any user device and any reasonable network connection. But video rendering can be resource intensive. To increase server scalability, it is extremely valuable to be able to redirect video rendering to the user device. So we look for opportunities to optimize video playback for the most popular and resource-heavy media players and formats by means of multimedia redirection. That’s why HDX MediaStream includes Windows Media Redirection and Adobe Flash Redirection.

HDX Adaptive Orchestration is the intelligence that ties these HDX MediaStream technologies together. The SmartRendering feature of HDX Adaptive Orchestration considers the media player and format, the network connection and the user device to decide how best to deliver the video. And policy settings give the system administrator control over this adaptive behavior.

Server-rendered video is delivered over the Thinwire virtual channel. We recommend using Progressive Display to apply lossy compression to the video images. The higher the image quality you choose, the more bandwidth and CPU will be consumed. You can create policies to set the level of compression according to the network connection (e.g. for a range of client IP addresses) and XenDesktop will automatically adjust the frame rate based on available bandwidth, up to whatever maximum you specify. (Be careful with thin clients to choose a maximum frame rate that they can comfortably handle without pegging the CPU.) For devices that are LAN-connected to the data center, I recommend configuring Progressive Display to use Medium Compression on moving images. For most WAN connections, High Compression works best.

Bandwidth consumption for server-rendered video ranges from about 800 Kbps to several Mbps for high quality full screen video.

When one of our multimedia redirection technologies is used, the Citrix Receiver receives the native already-compressed media stream. In many cases, this is more bandwidth-efficient than rendering the video on the server and recompressing it. For a lot of business videos, the bandwidth consumption will be in the range of 200 Kbps to 1 Mbps. But a 720p HD video will often have a native bit rate of over 6 Mbps! On WAN connections where bandwidth consumption is a concern, you might prefer to render the video on the server (despite the CPU hit) and squeeze it down using Progressive Display. Obviously the video clarity will be diminished, but that trade-off may be exactly what you need.

So, what if users watch HD-encoded videos but they’re on a network connection where the available bandwidth is less than the bit rate of the video? Windows Media Redirection will only kick in if the available effective bandwidth (that is, the bandwidth after taking network latency into account) is above the SmartRendering bandwidth threshold. The default for that threshold is just 0.5 Mbps, suitable for most business videos, but if you have users who will be watching HD videos then I’d recommend bumping that threshold up to 8 or 10 Mbps (see Citrix Knowledge Center article CTX124777).

All of this gets even more complex and interesting if you have a shared pipe, as in the case of a branch office. How can you estimate how much bandwidth you need? How can you minimize your bandwidth requirements? How does Citrix Branch Repeater help? Let’s explore the branch office video use case in a future blog post.


Derek Thorslund

Citrix Product Strategist, HDX