While comparing some products I have ran across this a few times and dont
really understand
the concept. Below is one example

Product Specifications:

17 dbm at 6 to 24mbs;
12 dbm at 54mbs

Seems to me it would be opposite but its evident I dont understand
something,
unless it means compared to distance. i.e. if you are close enough you can
get
54mbs at 12dbm but it takes 17 to get you 6 to 24 at a further distance?
Is this just a static statistic?

Thanks,
Airhead

Show quote Hide quote 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.

Show quote Hide quote 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

On Mon, 21 Feb 2005 15:17:18 -0600, "Airhead"
<campb***@alliancecable.net> wrote:

Show quoteHide quoteI guess you mean the tx power spec at:
  http://www.colubris.com/files/CN320%20Datasheet.pdf
You'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.

Show quote Hide quote You are correct.

Show quoteHide quoteThat makes sense.

Show quoteHide quoteIn 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!!

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