Re: [real] RE: [MLUG] Wireless

[James Roe <EMAIL:PROTECTED>]

At 08:02 AM 2/10/2003 -0600, Shannon Spurling wrote:
>Let's clarify a couple of things on this wireless thing. Ethernet is a two 
>way street. Even if you get your cantenna to work, who is going to point 
>one back at you? There are two functions of the cantennna, or any 
>parabolic or other kind of signal collector/emitter, one is to focus and 
>strengthen the signal that you are transmitting. That you have well in 
>hand. The other is to collect and focus the signal coming back to you. At 
>the distances you are talking, you will probably need to have both ends 
>focused and pointed at each other.

Ummm, this is not strictly true that both ends of the link must be high 
gain.  It is the _product_ of the gains that figure into the link 
calculation (altho we usually add dBs, which are logarithmic).  One can 
achieve extreme distances (I've heard of 25 mile links) with WiFi powers if 
both ends are high gain, but I believe 1.8 miles or so are achievable with 
a more-or-less isotropic broadcaster on one end and a 18-20 dB gain antenna 
on the other.  Line-of-sight IS important at 2.4 GHz, though.

You can find a lot of talk on the Internet about cantennas,and most of it 
is misleading at best.  I caution everyone to be extremely leery of 
reported designs and test results as the folks doing it (bless their 
enthusiastic hearts! :) don't really understand the principles underlying 
antenna design.  Just a quick tutorial, the diameter of the can (a circular 
waveguide) must be greater than about 2.75 inches to permit the primary 
waveguide mode to propagate and LESS than about 3.75 inches to avoid the 
next higher mode (which complicates and confuses the radiation 
pattern).  You can find many examples of designs using much larger 
diameters than this but, believe me, any results they may observe are 
generally spurious.

My trip to the supermarket with a tape measure revealed that the family 
size of Campbell's soups was about 3.25 inches in diameter and made an 
excellent antenna.  However,  the gain of the bare can is not too great (6 
dBi or so if I remember correctly), but one can add a conical horn to the 
can to get a nice boost in gain.  To get 15 dBi the horn diameter must be 
close to one foot.  The horn itself can be made of 1/4 inch hardware cloth, 
but attaching it to the can is difficult.

On the other hand, a 19 dBi flat panel antenna can be purchased from a 
commercial source (http://www.fab-corp.com) for about $70 along with a 
cable to connect to your WiFi card or box for about $25.  I am using 
several of these flat panels looking at a dipole now for 11 Mbs links up to 
.5 mile (but I think I have plenty of margin - at worst, to get to 2 miles 
would cut the link to 2 Mbs) - but it IS line-of-sight (in one case we are 
shooting UNDER a tree limb).

Connecting to your WiFi card or box is a bit of a problem.  The cable from 
the antenna to your RF transceiver loses valuable signal strenth (which 
translates directly to range and data speed), so should be as short as 
possible.  The connectors used on various cards/boxes vary as well, and 
tend to be special order items.  If you want to use a card in the back of a 
desktop computer or a PCMCIA card (not all of which even have detachable 
antennas) you may not want to put the computer on the rooftop where the 
antenna must be located to get a good signal and running a long RF cable 
will likely negate the effects of the antenna anyway.  My solution was to 
use Access Points (I used Linksys Wap11s) for each computer.  The Wap11 has 
detachable antennas (hence I can connect a RF cable to it) and I can 
connect the Wap11 to the computer(s) via a CAT5 cable up to 100 meters 
long.  Therefore, I put the antenna connected via a short RF cable to the 
Wap11 on the roof (put the Wap11 in a weather-tight box) , run the CAT5 
cable down to a nice, comfortable location in the house and surf away.

Now, my little system is self-contained.  All the clients are using one 
brand of access points (Linksys) and are able to interconnect with each 
other.  Even though manufacturers claim to follow the 802.11b standard, 
that doesn't necessarily mean that one can intermix products.  Be aware!
.
Jim Roe




>  Who's going to do that for you over at campus? The other issue is that 
> you will need line of sight. At the frequencies we are talking about 
> here, buildings and trees knock out an appreciable portion of the range 
> of the signal. You would need a pretty good tower to get your line of 
> sight. Does the university put these on towers, or just mount them high 
> up on the edifice of certain buildings? I don't think their coverage on 
> the north east corner of campus is very good right now. I could be wrong, 
> but that's not a particularly populated area.
>
>
>Shannon Spurling
>WAN Engineer -Specialist
>
>MOREnet, Network Services, Core Network
>3212 LeMone Industrial Blvd.
>Columbia, MO 65201
>
>Main:(573) 884-7200   Fax:(573)884-6673
>
>EMAIL:PROTECTED
>EMAIL:PROTECTED
>-----Original Message-----
>From: Jeff [mailto:EMAIL:PROTECTED]
>Sent: Monday, February 10, 2003 2:31 AM
>To: EMAIL:PROTECTED
>Subject: [MLUG] Wireless
>
>Today I bought a Belkin Wireless Network Adapter from Staples (great deal 
>is going on for the next couple of days where you buy it for 60 bucks, and 
>get back 20 in mail in rebates), and I'm hoping to amplify it using a 
>cantenna or some such thing to hook my home pc into the wireless 
>Tigernet.  I haven't bought the parts for the cantenna yet, and am a 
>little afraid to start taking the thing apart, especially since I don't 
>know if I could get the cantenna to work with it or get it back together, 
>and want to be able to take the thing back in 13 days or less if this is 
>futile - I don't have a full line-of-sight, and am approximately 1.8 miles 
>from University Hospital (measured the trip to work the other day) by car 
>(probably a little less straight).  Have any of you all been able to 
>connect to the wireless network using similar type things at this 
>distance???  If so, what were you using, and how did you do it?  I'm 
>located right behind the Stephens Stables off of Old 63, and I think I may 
>end up having interference from Boone Hospital, and one or two building 
>between me and the main campus.  The box says that max distance @ 11 mbps 
>is 590 ft, @ 5.5 mbps is 984 ft, @ 2mbps is 1476 ft, and @ 1mbps is 1800 
>ft....
>
>I noticed in some of the archives that you all are working or mentioned on 
>working on a project to put wireless internet in the public library.  That 
>sort of cooperation with the city is great!
>
>I think something very important is going to be happening very soon in 
>downtown based on this: 
><http://www.gocolumbiamo.com/Council/feb3bills/ReportB.html>http://www.gocolumbiamo.com/Council/feb3bills/ReportB.html 
>
>
>Also, I've read that the wireless access on the main campus is just 
>Cisco.  That makes me curious if it would be agains a contract or 
>something to use other company's stuff...  because in the not too distant 
>future (sometime in first quarter of 2003) it looks like vivato is going 
>to be have wireless switches that are a heck of a lot more powerful than 
>the cisco stuff.  I don't know the pricing yet, as the product is not out 
>yet <http://vivato.net/>http://vivato.net/
>but from the few reviews and stories that I've read tonight and yesterday 
>- it looks like Vivato is going to be creating switches that act like 
>super powerful access points, and they will be able to reach miles, not 
>just feet....

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