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Mixed WLAN - No Protection

Although the "adaptive protection" that it looks like both Intersil and Broadcom are adopting would seem to eliminate the need for a "G-Only" mode, Broadcom appears to still be allowing that alternative. The feature goes by different names (Buffalo Tech calls it "Turbo" and Linksys uses a more descriptive "G-Only" moniker), but the action is the same, i.e. disabling the 11b protection mechanism.

Intersil, however, is a firm believer in protection and doesn't provide a way to disable it. While this might seem to put buyers of Interil-based products at a disadvantage, my testing shows that there's little to be gained by disabling protection, and that turning it off can cause more harm than good.

Figure 11 shows two Chariot runs for the Linksys WRT54G / WPC54G test pair, which uses the latest available Broadcom firmware. I again used the Linksys products because their absence of "throughput hopping" makes it easier to detect the changes that are of interest.

Broadcom New - Mixed vs. G-Only modes, No 11b Stations

Figure 11: Broadcom New - Mixed vs. G-Only modes, No 11b Stations
(click on the image for a full-sized view)

The screenshot shows two Chariot runs, one in "Mixed Mode" and the other in "G-Only" mode. Comparing the average throughput values for the two runs shows a mere 3% throughput gain in "G-Only" mode, hardly anything to write home about. But assuming you're the type who has to squeeze every last Mbps from their network connection, what's the harm in throwing the "G-Only" switch?

If you think back to Part 1, you'll remember that disabling protection doesn't prevent draft-11g stations and APs from hearing 11b data transfers, it just prevents understanding them. With protection disabled, 802.11b clients become just like any other 2.4GHz noise source and can interfere with your 802.11g WLAN's transmissions. The following tests once again illustrate this point.

Broadcom New - G-Only Mode. NETGEAR MA401 in range

Figure 12: Broadcom New - G-Only Mode. NETGEAR MA401 in range
(click on the image for a full-sized view)

Figure 12 takes the same Linksys test pair used in Figure 11, sets the AP in G-Only mode, then brings a NETGEAR MA401 station into range. (The other trace in the plot is a reference run in G-Only mode, but without any active 11b clients in-range.) Although the "throughput dropouts" are visually disturbing, a glance at the average throughput numbers for the two runs shows little real impact.

The effect, however, can differ from client to client, as the next test shows.

Broadcom New - G-Only Mode. NETGEAR WAB501 in range

Figure 13: Broadcom New - G-Only Mode. NETGEAR WAB501 in range
(click on the image for a full-sized view)

Figure 13 shows the same test runs as Figure 12, but this time using a NETGEAR WAB501 Dual-Band client, forced to 11b-only mode. I'm not sure what the WAB501 is doing, but this time it has a significant effect, knocking the average throughput down about 18% from test run to test run.

You also need to remember that the inability to understand (and potential for interference) works in the other direction too. Although I haven't run any tests to quantify the effect, Protection-disabled draft-11g stations will act as interference sources for 11b-based WLANs. Although you might have nice, new draft-11g stuff and think you're getting an extra boost from running "G-Only", you could be interfering with your next-door neighbor, who hasn't yet decided to upgrade his 11b WLAN!

Once again, the lesson here is that you run draft-11g gear with Protection off at your own risk. Although it may come across as somewhat paternalistic, maybe that's why Intersil doesn't give you this switch to play with.

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