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You are here: Wireless Wireless Features Does Wi-Fi MultiMedia (WMM) Really Do Anything? - Part 3 - Switching APs, Real World

Does Wi-Fi MultiMedia (WMM) Really Do Anything? - Part 3 - Switching APs, Real World

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Switching APs

So with these successes under my belt, I tried some different APs. I first tried an old Linksys WRT54G, updated with firmware that supports WMM. But no matter which clients or OSes I tried, I didn't get good results.

Figure 4 shows an example of what I saw, running a BK stream to an XP SP2 and VI stream to the Vista SP1 system. This same script ran fine with a D-Link DIR-655 as the AP, even though I was using a qWave QoS template with the XP SP2 system. But even when I switched to using DSCP coding on the BK stream, I still got similar results.

WRT54G WMM failure
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Figure 4: WRT54G WMM failure

Although the BK stream drops back when the higher-priority VI stream starts, it doesn't drop far enough to give the VI stream the 12 Mbps it needs. I also tried the WRT54G using two XP clients using DSCP QoS templates and got similar results.

Finally, I switched in a Linksys WRT160NL that just came in for review and paired it with another notebook running XP SP2, but with a wireless adapter using an Atheros AR5001X+ chipset. Figure 5 shows that the background stream once again dropped back, but the VI priority stream doesn't have enough bandwidth to satisfy the 10 Mbps stream rate that I set.

Linksys WRT160NL and Atheros adapter
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Figure 5: Linksys WRT160NL and Atheros adapter

I tried a few other combinations of stream rates and adapters and wasn't able to achieve the proper prioritization obtained with the DIR-655.

Real World

My final step was to try to show WMM providing improved video streaming robustness using simultaneous video and data streams. If WMM operates properly and the wireless connection has sufficient bandwidth to meet the needs of the video stream, WMM should ensure that bandwidth is taken from the lower-priority data stream so that the video stream always gets enough bandwidth to provide a trouble-free viewing experience.

I first checked the Wi-Fi Alliance's Certification database and searched for media servers with WMM certification. Even though this would only show media servers with built-in wireless, it would at least be a start. But the search results showed only media players.

Since media servers are built into virtually all NASes, I then asked Buffalo, Cisco, NETGEAR, QNAP and Synology if the media servers built into their NASes provided media streams with WMM-compliant priority tagging. Unfortunately, makers of media servers seem to be blissfully ignorant of WMM's third requirement—the source application supports WMM. I didn't hear back from QNAP, but Buffalo, Cisco, NETGEAR and Synology all told me that their NASes did not support WMM compliant priority tagging, or any other priority tagging, for that matter.

The responses I received seemed to indicate that NAS manufacturers don't understand the connection between WMM and media servers in general. Since WMM is a wireless technology and NASes are primarily wired products, NAS makers seemed unaware of the need to provide priority tagging in their media streams so that WMM could properly prioritize them.

Updated 6/15/2009
The Wi-Fi Alliance provided a clarification regarding source application priority tagging. A WFA Alliance technical contact said:

"source applications do not need to know about *WMM* tagging...they can do layer 3 DSCP tagging or layer 2 tagging.
So for example, a NAS that was connected by Ethernet could use DSCP tagging that [would be] converted to WMM tagging when it hits a WMM enabled AP.

I then discovered a reference to DLNA version 1.5 supporting WMM. Since some NAS manufacturers say their NASes are DLNA compliant, I started checking this lead. But this also turned out to be a dead end. Once again, since WMM is a wireless technology, only wireless products that are DLNA 1.5 compliant would implement it.

I did find, however, that Cisco's Linksys Media Hub and WD's New "White Bar" MyBook World are DLNA 1.5 certified. But the certification certs (Linksys, WD) show only media format interoperability and no indication of WMM. This probably makes sense, since Cisco told me that although DSCP marking on media packets is part of DLNA-1.5 QoS, this is an optional requirement.




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