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Mixed WLAN - Station association

The test that really differentiates draft-11g products is how they function in "mixed" WLANs, i.e. those containing both draft-11g and 802.11b stations. If you remember from Part 1, the mechanism that dictates how 11g and 11b clients play together is called "Protection", and is a major influence on throughput when 11b clients are around. It's also an area that continues to evolve, with a major change in behavior dictated in the 6.1 version of the draft standard at the end of January.

Although the 11g draft standard does not require the use of protection, from what I can see from my latest round of testing, the trend seems to be toward automatic adaptive 11b protection. The main effect of this trend is it produces three distinct throughput levels for draft-802.11g devices:

  • Level 1 - No 11b clients sensed (Highest throughput)

  • Level 2 - 11b clients associated, but idle

  • Level 3 - 11b clients active (Lowest throughput)

Although exact throughput values and the percent change from level to level depend on implementation, virtually all devices I've tested so far exhibit this tri-level behavior. The only exception is Intersil's first available firmware, which has only two Levels, as illustrated below.


The basic test setup and methodology used for testing is described here.
All the following tests were taken under best-case signal conditions, with AP / router and client about five feet apart. WEP was not enabled unless otherwise noted.

Figure 7 shows a comparison plot of two Chariot runs. The bottom trace shows the results with the first-available Intersil client driver and AP firmware, and the top trace shows newer firmware released a few weeks later. During both runs, I plugged a NETGEAR WAB501 Dual-Band CardBus card (Atheros 5100X-based) into a second laptop sitting close by at about the 20 second point in the test run. I then shut down and removed the WAB501 at about the 50 second mark.

Intersil First vs. New - Temporary WAB501 Association

Figure 7: Intersil First vs. New - Temporary WAB501 Association
(click on the image for a full-sized view)

Two things are clear from the test runs:

  1. The first-available firmware showed no change in throughput with or without an 11b client associated. I attribute this to a non-adaptive implementation of 11b protection that effectively limits maximum throughput to a lower, "Level 2" value.

  2. The newer firmware implements an adaptive protection mechanism. This version of protection yields a higher "Level 1" throughput in what appears to be a protection "standby" or "sniffing" mode, but then drops to the same lower "Level 2" throughput once an 11b client is detected and associates with the AP.

By the way, don't associate any significance to that fact that the lower trace stops after one minute. I just was in a hurry and terminated the test when it was clear that things weren't going to change.

It turns out that I'd seen a similar behavior with the Broadcom-based products, which is easier to see with the Linksys implementation because it doesn't have the Buffalo Tech's "throughput hopping" behavior. Figure 8 shows you what I mean.

Intersil New vs. Linksys New - Temporary WAB501 Association

Figure 8: Intersil New vs. Linksys New - Temporary WAB501 Association
(click on the image for a full-sized view)

This plot shows the same experiment that I ran for Figure 7, but comparing the newer Intersil firmware with the newer Linksys-Broadcom firmware contained in the WRT54G router. I was a little sloppy with my plug and unplug times for the WAB501 11b client, so don't attach any significance to the difference in times of the throughput changes in Figure 8.

The main things to note here are that both Intersil and Broadcom seem to be pursuing similar "adaptive protection" strategies and that Broadcom presently seems to have a more efficient implementation, as shown by their higher throughput for both "Level 1" and "Level 2" states.

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