Citrix’s RAVE (Remote Audio & Video Extensions) technology, by capturing the video streams and rendering on the client endpoint, provides excellent results for streamed HD videos on a virtual desktop.

While the solution works very well in a LAN environment, the limited bandwidth, higher latency and packet loss can deteriorate the user experience over the WAN. HD video that plays smoothly in corporate office with LAN connection may become choppy and even frozen for a remote worker in home office.

In XenDesktop 5.5 and Receiver 3.0 for Windows, we have optimized the RAVE technology to provide smooth video delivery under WAN connection. Two enhancements are made to accomplish this goal: End-to-end flow control, as well as frame dropping.

In the past, RAVE sends media samples down to the client as fast as it could, without taking the available bandwidth into consideration. This can congest the limited bandwidth in WAN scenario, hence affecting the playback flow, eventually freezing the video. With the new end-to-end flow control, an acknowledgement is sent back to the host for each sample processed. The host can then adjust the flow and send out the samples in a steady pace, without flooding or starving the client.

When the bandwidth is less than the video bitrate, RAVE will also drop the delta video frames, which allows a smooth playback by transmitting less data through the network.

The following graph shows a comparison of network usage when playing a HD 720p 6Mbps video over a 3Mbps, 100ms latency WAN connection:

Figure 1 – Network usage on XenDesktop 5.0


Figure 2 – Network usage on XenDesktop 5.5


Network usage is quite jaggy on XenDesktop 5.0. On XenDesktop 5.5, it is a lot steadier, hence provides a more efficient usage of the bandwidth. For the end user on a WAN connection, the HD video would play choppy and eventually freeze up on XenDesktop 5.0. On XenDesktop 5.5, the HD video will play stable and smooth towards the end. Our testing shows with flow control and frame dropping, RAVE can tolerate around 100-150ms round-trip latency, and can run at around 50-60% of the required bandwidth for typical HD 720p video.

Other cool Windows Media features also in XenDesktop 5.5 and Receiver 3.0 include:

  • Improved support for HD content including 1080p videos
  • VMR9 support which gives us better rendering performance on the client (uses DirectX 9 instead of DirectDraw), with fallback
  • Better disconnect/reconnect & Smooth Roaming support
  • Vista/Win7 volume mixer controls integration
  • Better audio prioritization
  • Prerolling which enables the client to pre-buffer some data before playback starts
  • More HDX diagnostics for RAVE