Wireless AP's query?

I know this is an odd request, and I don't know if i'm in the correct
group, but i'm trying to plan out a wireless network - can anyone
answer the following question?

Indoors, in an open space 802.11g what range to you think you'd get on
a roof mounted AP?

20 metres?

Am I making sense?

Found this to be a tough one, no manufacturer seems to want to pin
down a specific range for their APs?

Regards, James

Jellis <jellis14@talk21.com> hath wroth:

>I know this is an odd request, and I don't know if i'm in the correct
>group, but i'm trying to plan out a wireless network - can anyone
>answer the following question?

Yes, just about anyone can answer your question.

>Indoors, in an open space 802.11g what range to you think you'd get on
>a roof mounted AP?

See the FAQ at:
<http://wireless.wikia.com/wiki/Wi-Fi#Performance_and_Speed>
Intel's numbers for range are somewhat optimistic.

>20 metres?

Maybe. The problem is that there are tradeoffs. I'll try to outline
the basics. You get to Google for details.
1. Antennas make a huge difference in range. Bigger is NOT
necessarily better as the antenna pattern gets narrower with
increasing gain.
2. For gain,
6dB = 2 times the range
12dB = 4 times the range
24dB = 16 times the range
3. Coaxial cable is VERY lossy at 2.4GHz. Make an effort to keep the
coax cable runs to a minimum. Better yet, with a rooftop mount, put
the wireless access point in a weather proof box at the antenna and
run PoE.
4. Range and speed are inversely related. For everything else being
equal, if you go 4 times as far away, you're speed gets cut in half.
Actually, it's worse than that as you'll probably pickup reflections,
which will reduce the speed even more.
5. Reflections are a problem. The ability to handle reflections
gracefully is why MIMO is superior to 802.11g, which is in turn,
superior to 802.11b.
6. All data sheets lie. Cut all promises at least in half or more.

Before anyone can answer your question with a specific number, you
need to supply specific numbers to work with. Range can be calculated
but we need to know what you have to work with (topography, line of
sight, altitudes, etc) and what hardware you were considering.

You also need to untangle your question as to whether you really want
to talk to an indoor client from an outdoor antenna. If that's your
question, then some clue as to the composition of all the building
material in the way in needed. Also, be advised that the absolute
worst location for communicating to a rooftop antenna is directly
below the antenna. There's almost no RF sent in that direction.

>Am I making sense?

Barely. Not enough to answer or decode your question, but enough to
bury you in buzzwords so you can do some effective searching and
reading.

>Found this to be a tough one, no manufacturer seems to want to pin
>down a specific range for their APs?

That's like the gas mileage specification for cars. Your Km's will
vary. It really depends on a large number of parameters, some of
which you have no control over (interference, reflections). If you
want a number, you have to supply numbers with which to do the calcs.

I suggest you download and skim:
<http://802.11junk.com/jeffl/crud/Intel%20HotSpot%20Guide.pdf>
It's Intel's Wireless Hotspot Deployment Guide, which has evaporated
from their web pile. There's quite a bit in there on range, coverage,
gains, specs, and how things work. Well worth reading.

--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558