On Fri, 17 Dec 2004 08:06:40 -0500, "Ethan" <(E-Mail Removed)>
wrote:
>I have a customer with two YAGI (about 18 inches long) antennas connected to
>two Linksys WRT54G V2 access points. The distance between the two sites is
>approximately 1548 feet, the antennas are mounted on the roof of each
>building, line of sight is good. The cabling from the linksys box to the
>antenna is approximately 20 feet, it is heavily shielded.
I guess you've installed alternative firmware in the WRT54G to act as
a transparent bridge. Wireless routers normally do not talk to each
other unless you're using WDS.
Ok, let's play with the numbers. My guess(tm) is that the 18" long
Yagi antennas are about 12dBi gain. I'm also guessing that the 20ft
of coax is LMR-400 and that you have a pigtail somewhere in the
system. From:
* 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)
I'll guess that you're connected at about 12Mbits/sec (or your users
would also be complaining about the speed) for a sensitivity of
-84dBm.
Plugging values of:
Distance = 0.25 miles (1500ft)
TX power = +15dBm
RX sens = -84dBm (at 12Mbits/sec OFDM)
TX ant gain = 12dBi (yagi)
RX ant gain = 12dBi (yagi)
TX coax loss = 4dB (including connectors and pigtail)
RX coax loss = 4dB (same as tx)
into:
http://www.ydi.com/calculation/som.php
I get a fade margin (SOM) of 23dB. Ok, this thing will work as
specified.
>Their are six users at the remote site who need access to a server running
>terminal services at the host site. Users are complaining that they
>constantly get dropped and disconnected. How can I test the throughput of
>the connection?
The built in diagnostics are close to useless. I like to use ping.
Well, actually I use fping, which is a replacement for ping that
numbers the packets. Download a copy of fping 2.09 at:
http://www.kwakkelflap.com/fping.html
Here's the results of pinging a neighbors wireless link (slightly
edited):
fping 192.168.1.50 -c -t 1000 -b-
Pinging 192.168.1.50 with 32 bytes of data every 1000 ms:
Reply[1] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[2] from 192.168.1.50: bytes=32 time=10 ms TTL=127
Reply[3] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[4] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[5] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[6] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[7] from 192.168.1.50: bytes=32 time=10 ms TTL=127
Reply[8] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[9] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Reply[10] from 192.168.1.50: bytes=32 time=10 ms TTL=127
Reply[11] from 192.168.1.50: bytes=32 time=0 ms TTL=127
Note that the latency is normally zero, but every once in a while, it
creeps up to 10msec. That's because I've got some interference from
another access point in the neighborhood. (Actually, a wireless TIVO
abomination that downloads TV junk all day long). Whenever there's a
collision, the packet has to be resent, and the latency increases.
Under ideal conditions, the latency should be constant. If you're
getting interference, have a marginal signal, or have a sick radio, it
will show up here.
>Does the linksys box have any utility to inidcate the signal
>level?
Of course not. If it did, people would call support and demand to
know why the signal strength doth suck. Give the customer as little
information as possible seems to be the trend among the cheapo
wireless vendors. Diagnostic output only breeds complaints.
You can extract the signal strength and s/n ratio by sniffing the
wireless traffic with a portable machine using Netstumbler or Kismet.
I'm not sure how well it will work with a transparent bridge setup.
Since such a bridge does not respond to probes, I'm not sure it will
work at all. However, it's worth a try.
>They need a speedier connection, what would you recommend?
Oh, so they are having speed problems. Well, try this simple
experiment. Block *HALF* the antenna with a piece of RF absorbent
material. A wet towel will work. Black carbon doped intergrated
circuit foam is better. No sheet metal please. Effectively, you've
reduced the signal about 3dB. If you lose connection, you have a
marginal system with some type of excessive loss. You can also do
this by rotating the antenna the number of degrees specified on the
datasheet for half the 3degree beamwidth, again reducing the signal by
about 3dB. If the signal just dies, you again have a marginal RF
signal. What you're measuring is the fade margin. You should have at
least 10dB of fade margin. The calcs show that you have 20dB so you
should be able to do cruel and unusual things to the antenna system
and still have it work. It's it's very critical, then there's
something wrong with the RF system.
Offhand, my guess is that you have either:
1. Interference from other radios. Change the channel.
2. Water in the coax cable or connectors. This is fatal and
difficult to remove.
3. A crappy antenna. I hate Yagi's for 2.4Ghz.
If you want more speed, and don't mine spending real money, I suggest:
http://www.proxim.com/products/bwa/p...e20/index.html
http://www.proxim.com/products/bwa/p...e60/index.html
Getting rid of the coax cable is a big plus for reliability and
weather resistance. Getting rid of the yagi will make me happy.
If you don't wanna spend quite that much money, one of these:
http://www.ydi.com/products/bridges/index.php
http://www.ydi.com/products/bridges/bridge-in-a-box.php
should work. Again, no coax cable and no yagi.
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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