One of the secrets behind Citrix’s success in delivering 3D graphics and video over limited bandwidth connections is how we dynamically adjust to the network and the endpoint to optimize the user experience. This is what we call Adaptive Display. Our HDX engineers are working on further enhancements to Adaptive Display for mobile use cases as part of the Project Avalon Excalibur release, and the results are very impressive.

Adaptive Display has its roots in a technology called Progressive Display that we introduced several years ago in Presentation Server 4.5. The concept was simple:  When you’re moving 2D or 3D graphics around on the screen, fluidity of motion is more important than image quality. There’s no benefit in sending each frame at full resolution on a low bandwidth connection if it results in hesitant, jerky motion or even a worse, a “slide show”. So Progressive Display used lossier compression on moving images and less lossy compression (potentially even lossless compression) for still images. The level of compression needed would depend, of course, on the available network bandwidth so Progressive Display provided policies to address different types of connections. In addition, Heavyweight Compression (arithmetic encoding) was introduced as an option to provide even more bandwidth savings for the same image quality, at the expense of higher CPU utilization. And other technologies like Queuing & Tossing and Dynamic Frame Rate Capping further adjusted the delivery of graphics and video to squeeze the best possible user experience out of a narrow bandwidth connection.

Progressive Display was a bit tricky to set up properly. I often worried that some customers might not be getting the full value out of this feature due to incorrect or incomplete configuration. So with XenDesktop 5.5 and XenApp 6.5 HRP1, we introduced Adaptive Display. The basic premise remains the same as with Progressive Display, but Adaptive Display is self-tuning, even as network conditions change. Zero configuration.

With Excalibur, we’ll introduce the next generation of Adaptive Display. Our SuperCodec, which determines which compression algorithm to use for different parts of the screen, has been enhanced with H.264-based Deep Compression from XenDesktop HDX 3D Pro. Citrix was the first VDI vendor to leverage H.264 codec technology with the introduction of XenDesktop HDX 3D Pro three years ago. We’ve continued to enhance our H.264 technology to deliver higher frame rates, better image quality and best-in-class bandwidth efficiency, as you’ve seen in our XenDesktop 5.6 Feature Pack 1 release of HDX 3D Pro which came out in June. And now we’re extending our use of H.264 beyond high-end 3D graphics so that all users of XenDesktop and XenApp will benefit from the latest video codec technology. Our “secret sauce” is how we tune H.264 on-the-fly to dynamically adapt to changing network conditions.

Achieving 30 frames per second on a LAN connection isn’t a big challenge, but 3G is another story. With early Excalibur code in the lab we’re seeing 18 frames per second on 800 Kbps 3G connections, more than double what was previously possible with such limited bandwidth. And WAN link scalability has increased over 100% with no reduction in video quality. Remarkably, our new technology can even deliver a good experience playing HD videos over low speed connections by reducing the bit rate an order of magnitude. For example, we can take an 8 Mbps HD video and transrate it down to 800 Kbps so that a user on a 3G connection will still get a decent experience.  And we can leverage multimedia hardware decoding on mobile devices to offload the CPU and conserve battery power.

A glimpse into the future of Adaptive Display is possible with the Project Avalon Excalibur Tech Preview release and the accompanying Citrix Receiver for Windows. Although we haven’t yet integrated Adaptive Display with Desktop Composition Redirection, which is the out-of-the-box default setting in the Tech Preview, you can enable Adaptive Display by switching your Windows 7 virtual desktop (configured with two vCPUs) from Aero mode to Basic mode. This will give you a preview of what’s coming in the mobile versions of Citrix Receiver.

With smooth motion, high frame rate video and graphics, on any device, over any network connection, HDX Adaptive Display is sure to play an increasing role in enabling mobile workstyles.

Derek Thorslund
Director of Product Management, HDX