On Tue, 25 Jul 2006 19:03:01 GMT, John Navas
<spamfilter0@navasgroup.com> wrote:
>>He left off one important word. There are only 3 _non-overlapping_
>>channels. These are 1, 6, and 11. Channels in between tend to
>>overlap these channels. DSSS (direct sequence spread spectrum)
>>spectra is about 22Mhz wide. Each FCC channel is 5MHz wide, which
>>means that wi-fi spectra occupies approximately 5 channels.
>
>That's overly simplistic.
><http://en.wikipedia.org/wiki/IEEE_802.11#Channels_and_international_compatibili ty>
"Close enough for this context."
(Sorry, I couldn't resist).
> Since the spectral mask only defines power output restrictions up to
> ±22 MHz from the center frequency, it is often assumed that the energy
> of the channel extends no further than these limits. In reality, if
> the transmitter is sufficiently powerful, the signal can be quite
> strong even beyond the ±22 MHz point.
(etc...)
This is correct and a serious problem for co-located radios, such as
at WISP sites. However, having more than one radio on different
channels is a rather unusual situation for the typical home user.
Let's play with the numbers and see how it works.
Let's take a typical wireless nightmare situation. Your municipal
government has just announced that they will use your tax dollars to
provide free wireless to the multitudes by installing an access point
on a street light outside your house. You look out the window and
there's the antenna on the opposite side of the street, 50ft away. A
quick check with Kismet shows that they're on Channel 6, insuring that
you're stuck with Ch 1 or Ch 11.
Quiz question: Will the interference generated by the municipal
xmitter on Ch 1 or 11 be enough to cause a problem and how badly?
Let's pretend they municipal wireless vendor is clueful and does not
install maximum overpower 1 watt radio alligators. Let's say 200mw or
+23dBm. The usual omni antenna has +8dBi gain, but the light pole is
well over the head of the nearby home, resulting in perhaps only +4dBi
gain at street level. I'll guess about
+23dBm + 4dBi = +27dBm EIRP.
At 50ft, the path loss at 2.4GHz is:
loss = 36.56 + 20Log(MHz) + 20Log(miles)
= 36.56 + 20Log(2400) + 20Log(0.01)
= 36.56 + 67.6 - 40 = 64dB
I'll assume a typical access point with a yucky 2dBi rubber ducky
antenna. Therefore, the rx signal level on Ch 6 at the receiver input
is:
+27dBm -64dB +2dBi = -35dBm
The 2nd lobe of the sinx/x spread spectra is guaranteed by an agency
of the federal govenment to be -50dB below the center power peak 22MHz
on either side, or the middle of ch 1 or ch 11. Therefore, the
interference signal at the receiver on Ch 1 or ch 11 will be:
-35dBm + -50dB = -85dBm
If the user is trying to use 802.11b at 5.5Mbits/sec the required
receiver sensitivity for 10^-5 BER is about -86dBm. The receive level
has to be at least 11dB higher (fade margin) at 5.5Mbits/sec or the
receiver will be completely unreliable. That means the operating
receive signal level will be -75dBm.
-85dBm worth of interference isn't going to do too much to a -74dBm
receive level. That's 11 dB of margin. The home owner can safely
operate his home network without fear that his tax dollars are being
used to construct a municipal jamming system.
Obviously, this is a contrived situation, but I think fairly realistic
and common. By moving the interfering transmitter closer to the users
receiver, the situation will deteriorate rapidly. Inverse square law
says that if I cut the distance in half, the signal level will
increase 6dB. Assuming the receive needs a S/N of about 11dB to
operate, if the original 50ft distance were reduced to about 14ft, the
noise on ch 1 or ch 11 will be sufficient to screw things up rather
badly. Same with a substantial increase in xmit power.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
jeffl@comix.santa-cruz.ca.us
#
http://802.11junk.com jeffl@cruzio.com
#
http://www.LearnByDestroying.com AE6KS
Re: 11 Channels 
Linear Mode
