On 28 Jul 2005 23:45:39 -0700, "Alex Bird"
<alex@redbeastie.fsnet.co.uk> wrote:
>
>David Taylor wrote:
>
>> How high gain? You should be able to get that working without
>> additional antennas with line of sight.
>
>This is overkill, we're in a built up area, no-one elses wi-fi or video
>sender should be able to break the phones - which will be used for
>phone in radio shows some of the time.
>Either 7 or 16dB yagis, more directional than gain-y.
Possible mistake. Don't use too much gain if you have a good view.
The problem is that you will pickup any and all forms of 2.4Ghz
interference along the line of sight of the antenna. Sure, you'll
have a strong signal from the other end of your link. However, you'll
also have a strong signal from any other device along the path.
That was one of my earliest screwups. I setup a point to point link
between two buildings in an industrial area. The distance was only
about 500ft, but I used a pair of 19dBi dish antennas. The initial
installation was a pair of 10ft poles on the rooftops. It worked fine
for about 6 months. Then the interference started. Eventually, I
traced it down to another wireless user about 3 miles away that was
inconveniently located along the line of sight. My solution was to
move one of the antennas from the rooftop to the side of the building.
This put the building at the other end of the link in the line of
sight, effectively blocking the source of interference.
If you think of the gain of the antenna in terms of distance, 6dB gain
is equal to twice the range. That means you will pickup interference
from twice as far away. Someone with a leaky microwave oven along
your line of sight could clobber your reliability.
There's another problem with too much gain too close. Most 802.11b/g
radios have rather poor receiver dynamic range. The current crop of
all direct digital conversion chipsets are great but sacrifice
receiver overload specs to gain sensitivity and low power consumption.
It's easy enough to tell if you have too much signal by just watching
the error rate.
This is balanced by the need to get a sufficiently good signal to
noise ratio, which implies a sufficiently high signal strength, in
order to get decent performance. Playing with the numbers, I'll
assume you want to get at least a 24Mbit/sec connection. The minimum
fade margin is assumed to be 20dB.
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)
Run the numbers at:
http://www.terabeam.com/support/calculations/som.php
using -79dBm for the receive sensitivity. Aim for a fade margin (SOM)
of about 30dB. See how much antenna gain it takes. With:
Distance = 0.016 miles (100ft)
Tx power = 15dBm
Tx ant gain = ??dBi
Rx and gain = ??dBi
Tx cable loss = 3dB (including connectors)
RX cable loss = 3dB
RX sens = -79dBm (for 54mbits/sec)
Doing the trial and error routine for antenna gain values until I get
30dB fade margin, I get antenna gains of only about 5.5dBi.
That means that you'll get reasonable performance at about half the
connection speed of 24Mbit/sec, with only 5.5dBi gain antennas.
Anything with more antenna gain just picks up more interference along
the line of sight with a possibility of overloading the receiver front
end. I'm gonna *GUESS* that the receiver will start to overload at
signal levels above -20dBm. (This is a guess and should not be
considered accurate). Using 15dBi gain antennas at both ends and
plugging back into the above URL, I find that the receive signal level
is -29dBm. Good enough, this should work.
Summary: Try to control your line of sight with high gain antennas.
Do the numbers first. Watch out of too much signal if you have a
short path length.
>Bridging and 'client mode' seem to be often names for the same
>proprietory and poorly supported mode, correct me if I'm wrong. This
>really puts me off!
Join the club. I once tried to scribble a document explaining the
various terms. I gave up. There's too much muddle. I'll try again,
but forgive me if I leave some holes. I didn't invent these terms so
don't blame me for the muddle. I'll only cover the bridging and
client modes as this can go on forever.
1. All wireless is bridging. Note the word ALL. A bridge connects
two networks by inspecting the destination MAC address and passing
only those packets that are know to have a destination address on the
other side of the bridge. There is no routing or IP addresses
involved in bridging.
2. An "ordinary" client radio is a bridge that is only capable of
bridging exactly one MAC address. Most client adapters fall into this
classification.
3. A "workgroup adapter" is a client radio that will bridge more than
one MAC address. Some (not all) game adapters fall into this
classification.
4. A "game adapter" is an ethernet to wireless bridge that may or may
not bridge more than one MAC address. Some do, some don't. Good luck
prying whether it does out of the support.
5. A "transparent bridge" or "wireless bridge" will pass multiple MAC
addresses usually using some form of proprietary bridging protocol.
That means that both ends of the bridge must have identical hardware.
Wireless clients may NOT connect to either end of the bridge.
6. A "WDS bridge" is similar to the "transparent bridge" but does not
require identical hardware. WDS allows simultaneous transparent
bridging and acting as an infrastructure access point. Wireless
clients may connect to either end of the bridge. WDS can be used to
extend the coverage of a network by adding additional radios.
--
Jeff Liebermann
jeffl@comix.santa-cruz.ca.us
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
AE6KS 831-336-2558
Linksys WET54G reliable? alternatives? 
Linear Mode
