Dbm per Trans Rate Question

I think I answered my own question,, these specs were for Transmit Power Control for 802.11h in th 5ghz band...evidently made for Europe. The Ap listens on the channel and adjusts its power down if there is another detected 5ghz channel, or it uses DFS and jsut changes channels automatically.

Wish I could delete this post.......I am not sure what it is. If you want to take a look check out a colubris ap spec sheet

I guess you mean the tx power spec at:

'll find that almost all the FCC type certification reports have different power levels at different data rates. In general the higher data rates cause the signal to be splattered over a slightly wider frequency bandwidth resulting to lower average power levels. The higher speeds are also belching tx power for shorter xmit times than those at lower speeds. There's also the problem of making the xmit AGC (automagic gain control) deliver constant output at all modulation types and speeds. T'is not easy. Looking at some random FCC type certification test reports, the power output seems to drop about 3dB from 6Mbits/sec OFDM down to 54Mbits/sec OFDM. Also, not that the Colubris specs allow a +/- 2dB variation. That's about what the accumulated AGC and watts-guesser accuracy yields.

For calculating the range, you need to also consider the receiver sensitivity which varies considerably with modulation type and speed. The following was lifted from the DI-624 datasheet but is close enough for most similar 802.11g radios: * 54Mbps OFDM, 10% PER, -68dBm) * 48Mbps OFDM, 10% PER, -68dBm) * 36Mbps OFDM, 10% PER, -75dBm) * 24Mbps OFDM, 10% PER, -79dBm) * 18Mbps OFDM, 10% PER, -82dBm) * 12Mbps OFDM, 10% PER, -84dBm) * 11Mbps CCK, 8% PER, -82dBm) * 9Mbps OFDM, 10% PER, -87dBm) * 6Mbps OFDM, 10% PER, -88dBm) * 5.5Mbps CCK, 8% PER, -85dBm) * 2Mbps QPSK, 8% PER, -86dBm) * 1Mbps BPSK, 8% PER, -89dBm) That's a -20dB drop in sensitivity between 6Mbit/sec OFDM and

54Mbit/sec OFDM. Adding that to the 3dB loss in xmit average power, we get a 23dB difference. At 6dB equals half the range, for a 23dB difference, we get about 1/16 the range at 54Mbits/sec as we would at 6Mbits/sec. Yeah, speed and distance can be traded.

distance?

You are correct.

higher

modulation

That makes sense.

speed.

enough

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In the case of the Colubris 802.11G the receive sensity is -65dbm at

54mbs and -85dbm at 6mbs. a -20dbm difference just like your above chart. The Xmit power for 6-24mbs is 17dbm and 11.5dbm for 54mbs. A difference of -6.5dbm. Now Im curious as to how I benefit by knowing this, other than calculating a link budget where I need 54mbs throughout the link or is it useful for something else? Forever learning! As always thanks for your valuable insight!!