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Wireless Reviews

Legacy Neighbor WLAN Tests

My last set of tests was to check the Clear Channel Assessment (CCA) mechanism that is supposed to ensure "neighbor friendly" operation with legacy WLANs. (See this page for an explanation of CCA).

For this test, I configured the 854T as an AP, set to 300 Mbps mode and Channel 1 with no encryption. This made Channel 1 the primary channel and Channel 5 the extension.

I then set up a second wireless LAN using a Linksys WRT54G router (original rev) and WPC54G V3 CardBus card. The WRT54G had most recent firmware and was set to its default Mixed mode and Channel 5—the extension channel. I also operated it as an AP and connected it to the same switch that the 854T was connected to.

I then set up two IxChariot throughput.scr scripts to drive traffic to both 11n and 11g test pairs. I tested both uplink and downlink traffic and alternated between having the 11n and 11g WLANs start first. (If you need a diagram of the test setup, this one showing a similar test setup should give you the basic idea.) I also used the Cognio Spectrum Expert to monitor spectrum use.

CCA Test - Uplink, N starts first
Click to enlarge image

Figure 21: CCA Test - Uplink, N starts first

The results in Figure 21 were a mirror image of those obtained in my DIR-655 testing. (Open this link in another window for easy comparison.) Essentially, the 854T / WN511B combo crushed the throughput of the neighboring legacy WLAN operating on its 40 MHz extension channel. It didn't matter whether I ran up or downlink or whether the 11g or 11n STA got on-air first. When the 11N WLAN was operating, the 11G WLAN was reduced from around 22 Mbps down to around 3 Mbps.

In contrast, the CCA implementation in the D-Link DIR-655 virtually shuts down the draft 11n WLAN when legacy traffic is detected in the 11n WLAN's extension channel.

Closing Thoughts

While the WNR854T / WN511B takes top downlink throughput honors among the four draft 11n routers tested so far, it doesn't rate a buy recommendation like I gave to the D-Link DIR-655. It's not just the neighbor unfriendliness that gives me pause, but other things that I've touched on throughput the review, i.e.

  • Ho-hum routing features with not even manual QoS for prioritizing multimedia over data traffic
  • No WPS for easy, secured WLAN setup
  • Significant wireless control omissions
  • High throughput penalty for legacy STAs in mixed WLANs
  • Hit-the-wall throughput drop-off

Simply put, for about the same money, the DIR-655 is a better deal. I personally, however, would still wait for more, and less expensive, dual-band 11n options before taking the plunge.

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