On Mon, 6 Nov 2006 16:30:23 -0500, "Neil H." <th@s.unlisted> wrote:
>I'm not sure what you mean by "sniffer."
Anything that can be used to located access points, clients,
interference, noise sources, transmitters, etc. It's a generic term I
borrowed from ham radio transmitter hunting.
>My antenna at this computer is just
>a small rectangular frame on the USB device -- I presume it's a loop
>antenna.
Nope. Most USB devices have a 1/4 wave meandering line monopole on a
ceramic substrate. The ceramic has a high dielectric constant which
drastically shorten the required length (and size). There are
limitations to what can be done with a corner reflector, but they work
well enough for general purpose transmitter hunting. I kinda prefer
my salad bowl dish idea:
http://802.11junk.com/jeffl/antennas...ish/index.html
However, it's not a perfect parabola and also tends to attract too
much attention from the authorities. For real site surveys, I use a
30ft telescoping fiberglass window washing pole with a 15dBi dish and
an MMDS downconverter on top.
>I know I don't have a perfectly clear line of sight. I'm in an
>apartment house with a steel frame, and there are at the least a lot of
>trees between me and that address. It is only slightly hilly around here.
The real problem with not having line of sight is reliability. You
can usually find a spot that works. The problem is that it will not
remain working as objects along the line of sight move around. There's
also the problems of Fresnel Zone diffraction and reflections off the
ground and objects.
>The shown signal strength from my own router/AP in the next room is -30 dBm.
>How does that compare with the usual sort of network inside a house? The
>utility generally shows my link quality at 100%, and 54 Mbps.
-30dBm is a rather strong signal for wireless. That should work as
long as you don't have a bunch of reflections.
>I notice that
>when I move the antenna around, the link quality drops temporarily, then
>recovers and returns to 100%. Occasionally the Tx rate drops to 24 Mbps
>briefly -- I presume that makes no difference since I'm only using this to
>connect to my cable modem, which has less throughput than that anyway,
>right?
The drop in link quality was cause by multipath or reflections. There
will be places in the room where the incident (direct) signal cancels
the reflected signal, thus reducing the SNR. There's also the problem
with inter symbol interference, where the reflected signal arrives
somewhat after the incident signal and clobbers the next arriving
packet. 802.11g has a rather large inter-symbol delay to take care of
this very real problem, but it's not large enough to handle all
possible reflections.
>The four 2WIRExxx signals I'm picking up right now are -70, -86, -78
>and -90. What I'm getting from the house a mile away is -94. I understand dB
>is on some sort of logarithmic scale, so those numbers aren't really as
>close to my own AP as they look, correct? What would be a reasonable minimum
>signal strength for a reliable link? And do those numbers suggest that at
>*their* end they're getting about the same signal strength from me, or isn't
>that necessarily so?
That's going to be a problem as there is no single answer. The signal
levels in -dBm you mention have different effects at different speeds.
The following chart is the receiver sensitivity at various speeds.
|
http://wireless.wikia.com/wiki/Wi-Fi...ance_and_Speed
This is from a Dlink DI-624.
Mbps Type PER dBm
54 OFDM 10% -68
48 OFDM 10% -68
36 OFDM 10% -75
24 OFDM 10% -79
18 OFDM 10% -82
12 OFDM 10% -84
11 CCK 8% -82
9 OFDM 10% -87
6 OFDM 10% -88
5.5 CCK 8% -85
2 QPSK 8% -86
1 BPSK 8% -89
So, if you have a -70dBm signal, you could theoretically no faster
than 36Mbits/sec (for a thruput of about half that or 18Mbits/sec).
Unfortunately, this is the *BEST* case approximation. Add a bit of
interference, some noise, a few reflections, and some marginal
hardware, and you'll probably end up at 24 or 18Mbits/sec. It's also
not very useful running at a PER of 10% as you will see substantial
retransmissions. However, we'll pretend that everything is perfect so
I won't complicate the estimates. Your -94dBm signal level will not
even work at 1Mbit/sec. I suggest you read the following sections on
fade margin and range/speed.
|
http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
|
http://wireless.wikia.com/wiki/Wi-Fi...ance_and_Speed
It may not make much sense at this point, but is as simple as I can
make it and still include the calculations.
Also, it might be useful to know that a 6dB increase in power or
sensitivity will yield a doubling in range. 12dB is 4 times, 18dB 8
times. If you look at the above table, the difference in dB between
54Mbits/sec and 1Mbit/sec is 21dB. That means you can go 16 times as
far at 1Mbit/sec than at 54Mbits/sec. You can also use this to
predict your range. If you can go 100ft at 1Mbit/sec, you'll only be
able to go 6.3ft at 54Mbits/sec.
As for how fast you need to go to run your cable modem, I would need
to know your cable modem speed. I'll assume 6Mbit/sec. You'll need
at least a 12Mbit/sec wireless connection to equal this. However, you
will have reflections, interference, and the attendent
retransmissions, so something somewhat faster would be the required
minimum speed. Methinks 18Mbits/sec would be a safe minimum. Looking
at the table, your recevie signal level will need to be over -82dBm or
you will be slower than your cable modem.
>I'm not really terribly concerned about being hacked, but I'm not carefree
>about it either. I do have WEP on, of course.
Nobody worries about security (and backups) until AFTER they have been
hacked or had their computer trashed.
--
# 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
"Link quality"? (newbie question) 
Linear Mode
