On Sun, 2 Sep 2007 10:59:11 -0700, "Mountain Mike^^"
<rukidding@aol.com> wrote:

>I'm just playing around with setting up a "mesh" wifi access for my entire
>neighborhood, and would like to discuss theory if possible.

Why use a mesh? The only real benifit to a mesh network is the cost
savings on the wired backhaul to the ISP. Since you're probably only
going to have one or two backhauls, there's not much savings. Instead,
you get the deal with store and forward packet repeaters, that eat
airtime, mutual interference problems, bandwidth bottlenecks, complex
routing algorithms, and performance issues. More specifically, you
only use mesh when you absolutely must use mesh.

Light reading:
<http://pdos.csail.mit.edu/roofnet/doku.php>

Reality. Be sure to read this one carefully, especially the part
where there's typically 50% or less probability of delivering a packet
intact, and the reliability even at 1Mbit/sec.
<http://pdos.csail.mit.edu/roofnet/doku.php?id=interesting>

The commercial version of Roofnet:
<http://meraki.com/products/outdoor/>

>Am I right in that using a router as an AP only (or bridge) would cut the
>bandwidth in half?

Not quite. One hop will cut the *MAXIMUM* thruput in half. Let's
pretend that you're very lucky and get a 36Mbit/sec association
(that's a wireless connection). Maximum thruput is about half or
18Mbits/sec for TCP (assuming no interference, etc). However, if you
shove that through a mesh network or store-n-forward repeater, it gets
cut in half again to perhaps 9Mbits/sec. I say perhaps because it's
usually more than half if there are a number of mesh nodes nearby. If
you add a 2nd hop, it gets cut in half again or 4.5Mbits/sec *MAXIMUM*
TCP thruput.

What you need is a good overdose of reality. Setup a mesh network in
a closed room, where everyone can hear everyone else. A dedicated
repeater or WDS bridge will suffice. Now, try to measure performance
from end to end using IPerf (instructions on request or Google this
newsgroup for my posting on IPerf). If you turn off the repeaters,
you should get something near the maximum theoretical performance.
Turn on the repeaters, and watch the thruput drop radically along with
a corresponding increase in retransmissions and errors. Try it.

>And, for experiments sake, I'd like to get an idea of what kind of hardware
>to buy to allow access for, say, 100 homes, in a 6 block area.

Ummm... 100 customers? Every time it goes down, you want 100 phone
calls? Have you perhaps been snorting, injesting, or smoking
controlled substances? Never mind the technology. Do you think you
can handle the complaints and service calls? Whatcha gonna do when
some clueless user comes home with a virus or worm and eats all your
bandwidth? How about the not so trivial problem that customers will
tend to call you first, before calling their computer guru (who
charges money), simply because you're cheaper? Do your really need
this headache?

Anyway, the loading is easy:
100 light email and web users
10 business users
1 file sharing user
It doesn't matter how many access points or backhauls. One user can
hog the whole system unless you have monitoring and traffic management
in place.

Ummmm.... How big is a "block" in feet or meters? Is that a city
block or country block? Tall apartments of single family suburbia?

>I know I'd need to convert the AP for outdoor use (I'd like to play with
>cheaper consumer stuff), which I can do using an oudoor box and power over
>the cat5, right?

Right. It's called PoE (power over ethernet) or 802.3af. There are
vendors that will sell you the complete package, or just the pieces.
For example:
<http://www.hyperlinktech.com/web/power_over_ethernet_weatherproof_enclosures.php>

>Although I'd probably design a conduit to carry the signal
>and 12V seperately for easier maintenance. Any idea the wattage necessary?
>I'm thinking of using PSU's from scrap computers.

The delivered power is limited by the resistance of the power wiring.
That's why PoE is normally done at 48VDC, so that the effects of the
wire resistance is minimal. Junk power supplies are only going to
supply 12VDC, make a bunch of noise, probably smoke the cable if you
short it, and are not terribly reliable. Look into real PoE adapters.

>Then, how about the repeaters? I need the same brand usually, correct?

Repeaters are not very well defined in the IEEE 802.11 specs. The
result is substantial incompatibility among vendors and versions.
That's one nightmare I suggest you avoid. Repeaters are also a basic
component of mesh networks, which I previously suggested are a bad
idea.

>I know about the LOS issues, and plan on doing all roof mounts, and USB
>client adapters.

Keep planning. You'll find that USB is limited to 16ft maximum cable.
You can get amplifiers that will go farther, but then you have to
supply power at the destination end. Use ethernet and PoE instead.

>Also, any idea of the bandwidth necessary and how to limit it to each
>client? (100 users, typical home use).

Look at the real bandwidth managers:
<http://www.etinc.com/index.php?page=bwmgr.htm>
and see what you might need. Also note that it's not enough to simply
configure a bandwidth manager and let the system free run. You gotta
monitor the traffic to look for abuse, changes, hackers, idiots, and
failures. You'll need to know the performance of each user and which
one's are hogging the system. Check out various traffic monitors.

In case it's not obvious, you're going to need EVERYTHING that a
typical wire line ISP uses, with the added enjoyment of an unreliable
means of delivery. If you want to do this for free, that only
eliminates the billing system. Everything else still has to be there.

>I bet Jeff has some ideas:)

Nope. I'm busy buying test equipment, updating up my lab, dealing
with the ultimate messy house and office, loafing, and playing hard to
find.

Well, ok... just one idea. Start small. Find a Linux based wireless
router and install it on your roof (using PoE) as an access point.
Deliver shared service to a few friends and neighbors. Keep it small
so that when you do something stupid, you don't have a torchlight
parade of irate neighbors banging on your castle door. Learn system
admin, bandwidth management, and abuse mitigation on a small scale.
Once you've made all the basic mistakes, grow the system so that you
can move on to making bigger mistakes.

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