Monday, December 15, 2008

WRT54GL Power Specs, Commands and Theoretical Limits

Everything that follows relates to the Linksys WRT54GL v1.1

First off, it is useful to install the "wl" package on the router. It has a lot more commands than wlc, and is the only way to adjust transmit power as far as I can tell.

Max transmit power is supposed to be 84mW = 19.24dBm
, though when using the configuration tool like so:
wl txpwr
The device outputs:
pwr in mw 127
pwr in mw after override adj 127
The values are mislabeled here, and are actually qdBm. But you can still set to max value like this:
wl txpower 1496
Also, "txpwr1" outputs more readable values and lets you set in dBm. I tested the two extremes with the Wi-Spy (a nifty USB network Analyzer), and saw a modest difference between the two settings. Look at the strong signal on channel 3 (screen captures are from the Chanalyzer 3.2 software - free from MetaGeek).

txpwr = 1:

txpwr = 1496:Though the difference between the two power levels was not particularly obvious, it was very repeatable. there's an 802.11b signal at this level as well, but has nothing to do with us.

[EDIT: there is a bug in the openWRT firmware that prevents the power from being set to anything greater than 19dBm, even though we set the value at nearly 32dBm. This may get fixed in the 8.09 version of OpenWRT]

Also note: The weaker 802.11g signal on ch3 is the another WRT54GL with the antennas removed. Both routers are the same distance from the Wi-Spy. (about 3ft in this case)

Receive sensitivity, according to the internet, is as follows:
Data Rate(Mbps) Sensitivity(dBm)
54 -74
48 -76
36 -82
24 -86
18 -88
12 -89
11 -90
5.5 -94
2 -97
1 -99
(from here)

If the above is true, it should theoretically be possible to make a 5km link with an antenna gain of as little as 13dBi. Nifty.

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