Re: Wireless Bridge
- From: Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 28 Apr 2006 10:16:24 -0700
jvolckaert@xxxxxxxxx hath wroth:
The multiple antenna splitter is interesting.
Would I have them pointing in different directions (or is it intended
to just gather more strength from one direction)?
The basic idea is to prevent the access point from switching between
antenna ports for every packet. Some of them just can't do it.
Antennas do NOT generate, create, or manufacture RF. The redirect it
in various ways and directions. You can't get something for nothing.
In transmit, a power divider would split the RF signal in half between
the two antennas. Each antenna would now have about -3.5dB less gain.
This may be a problem. In trade, it allows you to customize the
antenna pattern to be the combination of the two antennas. If both
antennas were pointed in opposing directions, you would get the rated
transmit gain of each antenna, minus the -3.5dB divider loss.
However, in receive, the loss is only -0.5dB. Each antenna acts with
it's normal gain, minus the -0.5dB.
Where things get messy is where the antenna patterns overlap. Your
highly directional dish and omni are going to be a problem. When
there are two paths to the power divider, the RF can either cancel or
reinforce depending on the phase of the RF signal. That happens every
half wavelength. Your combined pattern will look like a "fan" or
"daisy" with lots of weird lobes in odd places. The power splitter
trick only works if you have physical isolation between antennas.
That's great with sector antennas, and two highly directional
antennas. It won't work with an omni.
Since you mention
Hyperlink, I would like to add that I have ordered from them a few
times and would recommend them to anyone.
They're a mixed bag with me. Their return policy and proceedure is
abysmal. However, it's been about 2 years since I've bought anything
from them, so things may have improved.
With the lost packets I am thinking I'm just swamping the connection
with data which it can't keep up with.
"Swamping" is not a very useful term. My guess(tm) is that you're
feeding camera images at a faster rate than the wireless can handle.
Eventually, the buffer overflows (somewhere), and drops packets. Let's
do the math. You mumble that your video clips are 5 to 50 Megabytes
large (40 to 400Mbits). Your radios are 802.11b. That has a maximum
data thruput of 4.5Mbits/sec. My guess is you're getting much less
thruput thanks to the double hop (store and forward bridging) and
possibly lousy signal strength. I would give it an optimistic
1Mbit/sec. Instead of guessing, it would have been nice if you had
measured your thruput as I previously suggested. At 1Mbit/sec, you
can transfer one picture every 40 to 400 seconds. If you're feeding
images any faster than that, you're going to loose images.
I'm also wondering if I'm
nearing the MAC limit.
On which unit? The WAP11/DWL900AP+/MA102 boxes are limited to 32 MAC
addresses (including their own MAC addresses). Do you have 30 clients
connected through the bridge? I don't know about DWL-3200AP, but my
guess is well over 256 MAC's.
Does anyone know where to lookup the max number
of MACs for an AP?
It's difficult to find on the low end units. In fact, it's difficult
to find any data on low end radios. I did it with a proprietary AP
and client simulator software used for lab testing radios. I just
cranked up the number of simulated wireless associations until
something broke. Some access point gracefully discarded old MAC's in
order to make room for new associations. Most just locked up and
died. All the units with SNMP will display the MAC address table.
I don't think it's interference since I'm in a very
rural area. Trees are a bigger issue than interference.
Trees will ruin your signal strength and force the radios to slow down
in order to maintain a reasonable data rate. Other than trees, do you
have line of sight, Fresnel zone clearance, and have you done a site
survey for interference? You might be suprised.
Are you aware of any APs which have signal strength for a bridge?
Not off hand. I'll have to dig for that info. Most of them do not.
Have you tried Kismet to extract the signal strength and S/N from over
the air packets? You don't need to install Linux on a laptop. Just
boot a LiveCD such as Security Auditor:
http://www.remote-exploit.org/index.php/Auditor_main
Make sure your wireless card is supported:
http://www.remote-exploit.org/index.php/Auditor_dev_list1
--
Jeff Liebermann jeffl@xxxxxxxxxxxxxxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.
- References:
- Wireless Bridge
- From: jvolckaert
- Re: Wireless Bridge
- From: Jeff Liebermann
- Re: Wireless Bridge
- From: Jeff Liebermann
- Re: Wireless Bridge
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