The 3500L isn't the best router for wireless modding, since it doesn't have external antennas that can be upgraded, like the ASUS RT-N16. But among the additional controls it exposes, DD-WRT lets you adjust transmit power from 0 to 251 mW (Figure 5).
Figure 5: DD-WRT Advanced Wireless settings
I don't know how or whether DD-WRT calibrates the control so that you really are adjusting power in mW. So I ran a check using the Cognio (acquired by Cisco awhile ago) Spectrum Expert card and software.
Figure 6 shows two traces, measured with the Spectrum Expert notebook sitting about 6 feet from the WNR3500L and a test notebook running a continuous IxChariot throughput script to generate a constant signal. The higher trace is with 3500L's power set to 251 mW and the lower with it set at the default 71 mW.
Figure 6: WNR3500L at 71 and 251 mW DD-WRT power settings
If my math is correct, an increase of 71 to 251 mW is 5.5 dB. So if the 3500L can produce that much power, that's the difference that we should see between the two traces. Eyeballing the larger image shows approximately a 7 dB difference, which is close enough, considering the measurement method.
So it looks like the power control actually does work. It's interesting to note that the DD-WRT Wiki recommends using only a maximum setting of only 84 mW, which would provide only an insignificant 0.7 dB increase.
So did boosting power all the way up to 251 mW help? The tables below say "sorta". They show three test runs using the default 20 MHz bandwidth mode: with factory firmware at 100% transmit power (the default); DD-WRT at 71 mW (the default); and DD-WRT at 251 mW (the maximum). I used our standard one minute IxChariot test, with the 1 minute average throughput shown in the tables.Downlink table 2.4 GHz / 20
|WNR3500L - Factory Firmware||45.5||40.5||43.2||26.3||10.0||2.0|
|WNR3500L - DD-WRT 71 mW (default)||59.6||50.2||42.56||21.8||1.6||1.4|
|WNR3500L - DD-WRT 251 mW||27.1||23.9||20.7||18.5||1.7||1.2|
In each table, I've highlighted the best result in each test location and if results are within 1 dB, I highlight both. For downlink, the default DD-WRT settings provide a nice throughput boost in Locations A and B. But as signal levels drop, the advantage quickly fades.
Cranking power all the way up significantly reduces throughput in the stronger signal locations (A - D) and, worse, provides no significant advantage in the weakest signal locations E and F.Uplink table 2.4 GHz / 20
|WNR3500L - Factory Firmware||54.0||28.9||41.5||16.2||5.6||2.3|
|WNR3500L - DD-WRT 71 mW (default)||64.0||21.6||28.5||2.9||0.5||0.6|
|WNR3500L - DD-WRT 251 mW||57.4||17.6||17.2||5.9||1.6||0.6|
The uplink results are a bit puzzling. Since the 3500L is receiving, I wouldn't expect any significant difference across the three test runs. But the table definitely shows a difference. Would this difference make an improvement in wireless performance? Probably not.
Faithful DD-WRT fanboys and girls might dismiss my findings as biased, incomplete, inaccurate or all the above. But I really did approach this experiment with an open mind and used the same test methods I use for all my wireless router testing. In the end, the results are what they are. But with DD-WRT constantly changing, your experience may be completely different than mine months, or even weeks from now.
There are many reasons to flash your router with an alternative firmware and DD-WRT is certainly one of the most evolved. But it seems that performance improvement shouldn't be a primary goal for your change. Because, if it is, you may be very disappointed.