Legacy Neighbor WLAN Tests - more

To test the behavior, the 655 was set to its "Bridge" (AP) mode, Auto 20/40 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 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 655 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.

Figure 22: CCA Test - Uplink, N starts first

Figures 22 and 23 show the uplink results, with very different behavior depending on who got on-air first. When the 655 and DWA-652 were running first, they definitely stopped using the 40 MHz channel as soon as the legacy WLAN started transmitting.

But as Figure 22 shows, they were too accomodating to the neighboring 11g WLAN. From the Cognio's spectrum display, it appeared that the D-Link pair were erratically transmitting. This is also shown by the fact that the 11g pair is achieving its normal ~20 Mbps throughput. Once the 11g WLAN stopped, however, you can also see that the D-Link pair resumed 40 MHz operation.

Figure 23, showing the Linksys pair transmitting first, shows the D-Link pair does better, but with high throughput variation. Other runs using the same test setup, however, showed erratic behavior more similar to Figure 22.

Figure 23: CCA Test - Uplink, G starts first

Figures 24 and 25 show the tests repeated, but running downlink.

Figure 24: CCA Test - Downlink, N starts first

The behavior is pretty much the same as for uplink, with the 11n gear taking it on the chin when a neighboring WLAN goes on-air in its extension channel. So the conclusion for this experiment is that this implementation of CCA virtually shuts down the draft 11n WLAN when legacy traffic is detected in the 11n WLAN's extension channel.

Figure 25: CCA Test - Downlink, G starts first