Dbm per Trans Rate Question

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

"Airhead" <(E-Mail Removed)> wrote in message
news:421a5170$0$22518$(E-Mail Removed) m...
> 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


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.

"Airhead" <(E-Mail Removed)> wrote in message
news:421a5f51$0$22518$(E-Mail Removed) m...
>
> "Airhead" <(E-Mail Removed)> wrote in message
> news:421a5170$0$22518$(E-Mail Removed) m...
> > 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
>
>
> 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

On Mon, 21 Feb 2005 15:17:18 -0600, "Airhead"
<(E-Mail Removed)> wrote:

>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?

I 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.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558

> >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?
>
> I guess you mean the tx power spec at:
> http://www.colubris.com/files/CN320%20Datasheet.pdf

You are correct.

> 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.

That makes sense.

> 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.
--
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 AE6KS 831-336-2558

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!!