Downstream Throughput Vs. Range
The real value of the Azimuth system is its ability to automatically perform throughput vs. range tests with as little as 1 dB attenuation increments in a RF-quiet test environment. Think of this as the ability to change the distance between a STA and AP in very small, repeatable steps with no interference from other wireless LANs, microwave ovens, etc. What results is the ability to see what the products themselves are doing without environmental effects.
For the sake of brevity, I'm going to condense a lot of testing into a few very interesting plots. Figure 16 shows a comparison of Airgo Gen 3, Broadcom Intensi-fi and Marvell TopDog downstream throughput vs. range using the Netgear RangeMax 240, Linksys Wireless-N and Netgear RangeMax Next Gigabit products respectively.
These plots were created using an Azimuth MIMO Rate vs. Range script that interfaced with IxChariot to generate the data traffic and the Azimuth system to control signal attenuation. The test used the same modified IxChariot throughput.scr script that I used to generate the 0 dB max throughput plots. Each data point in the following plots represents the average throughput value measured by IxChariot from a 5 second test run. The tests also used a 5 second pause between changing the attenuators and starting the IxChariot run.
Keep in mind that since these plots represent a single data stream, you won't be seeing the maximum throughput rates we saw earlier in the IxChariot plots. Instead, we're looking to see what happens to throughput as we attenuate the signal level in 1 dB steps, to simulate increasing distance between AP and STA.
The Buffalo Nfiniti is not included in Figure 16 because I couldn't get it to operate reliably downstream. I had a difficult time keeping the STA associated with the AP, and when I could get a run started, it would quit with around 60 -70 dB of path loss.
Note: I was not able to repeat the Azimuth tests using the new 126.96.36.199 card driver, but instead ran a quick walk-around test in my home using IxChariot to generate simultaneous up and downstream traffic. I found that the Buffalo AP and STA remained connected at all locations and changed link speed as they should. Although it's interesting to note that I didn't see any 11g or 11b rates among the various rates shown by the XP Zero Config Wireless Connection Status utility.
Figure 16: Downstream Rate vs. Range comparison
The significant points from Figure 16 are:
- The Linksys Wireless-N appears to be unable to make the shift to 802.11g rates. This is confirmed by Figure 17, which shows the link data rates reported by the Linksys client driver.
Figure 17: Linksys Wireless-N Reported Data Rate vs. Range
- Both the Marvell and Broadcom-based products shift down from maximum rates sooner and more quickly than the Airgo Gen 3 based product.
- The Marvell-based product appears to have better rate adaptation and can shift both to 802.11g and even 11b rates as shown in Figure 18.