Software Media Servers
So how about software media servers? Well, perhaps the most promising route would be via Windows Media Center (the version built into Vista) streaming to Media Center Extenders like the Linksys DMA2200, Xbox360 or other devices. With Vista providing both the media source and player and both ends implementing qWave, surely this would be an promising configuration.
I didn't have any Windows Media Extenders to try. But I fired up a copy of Vista Home Premium SP1 and the Windows Media Center that it includes, on an Ethernet-connected test system. I transferred some test video files to the Vista machine, then had Windows Media Center add the files to its library. I next associated the Vista SP1 notebook that I had used in my IxChariot testing with the D-Link DIR-655, which had WMM enabled.
Once I had everything connected, I opened Windows Media Player 11 on the Vista notebook and used its Library browser to find and play a 720p WMV demo file from the Windows Media Center system. After it played for awhile, I started a large file download on the notebook.
The video played pretty well for the most part, but had some stuttering in spots while I was running the file download. But I wasn't sure whether WMM was working and there just wasn't enough bandwidth to go around, or whether WMM just wasn't being used.
So, to find out, I fired up Wireshark and ran a packet capture on the server machine while the video played on the wireless notebook. Wireshark revealed that the Media Center / Player pair used TCP for the file play and not a streaming protocol like UDP or RTP. The packet capture also confirmed that the DSCP tag was set to 0, which is the "default" coding of "best effort" (lowest-priority).
To ensure that I wasn't interpreting Wireshark incorrectly, I ran a capture on one of my IxChariot tests that used BK and VI DSCP priorities. Figure 6 shows a packet from the notebook running the VI priority stream properly tagged with DSCP = 5. The same capture showed the BK priority stream tagged with DSCP = 1 (not shown). So qWave or not, Vista, Windows Media Center and Windows Media Player didn't appear to implement any form of media priority tagging.
Figure 6: Wireshark showing DSCP Service Class 5 (VI) tagging
My last shot was to download TwonkyMediaManager and to try to see if it would provide a priority-tagged media stream. Maybe I had something wrong in my setup, but it didn't seem to be able to play even unmolested VOB files ripped from a DVD. It also didn't seem to even want to index a Quicktime 720p .mov file. So I quickly abandoned that experiment.
There is no doubt that the Wi-Fi Alliance has been successful in getting wireless product manufacturers to support WMM in the majority of today's Wi-Fi products. And WMM seems to provide a benefit for wireless VoIP (or Skype) phones that properly handle incoming and outgoing voice streams with WMM-compliant tagging and processing.
But my experiments show that WMM provides no benefit for improving video (or audio) streaming robustness by shifting available bandwidth from low-priority data traffic to higher-priority media streams. The only way that this will change is if media servers—both standalone and NAS-embedded—implement WMM-compliant priority tagging, something that, at least until this article, they weren't even aware that they needed to do. And it also appears that some Wi-Fi product manufacturers, like Cisco, may have some work to do on their WMM implementations so that they properly allocate bandwidth among prioritized streams.