[UFO Chicago] Wirelessness in Chicago

[The Archduke of Chicago and Subjugator of Michigan]

Just now Neil R. Ormos made 15 LEDs in my apartment flash with this:
> Nate Riffe wrote:
> [deleted: non-technical issues]
>
> > In particular, this network will not provide Internet
> > access for quite some time.  Legitimate Internet access
> > for a network such as this will not be cheap, which is one
> > the main goals.  When the network is sufficiently large
> > (and by large I mean in coverage and bandwidth, NOT in
> > users), we will have the means to negotiate for cheap
> > Internet access.  Until then, it is an infeasibility.
> 
> Why would coverage, as opposed to N of users, help you
> negotiate cheap internet access?  For that matter, unless
> you plan to charge users on a per-byte basis, how would
> higher bandwidth help you negotiate cheap internet access?

With wide coverage and significant bandwidth it may be possible to trade
data transit over our network for other services, like an Internet
connection.  I have no idea how much leverage this would get us, and
obviously ISPs have their own networks for data transit.  Most likely,
we'd offer the transit services in exchange for a monthly contribution
to the uplink fund.  Right now I'm sort of in "if you build it, they
will come" mode as far as public service goes.

> Unless the rules have changed since October 2001, that's not
> quite right.
> [summary: the directional bonus applies to directional antenna over
> 6dbi, not 30dbi.]

Sorry, my bad.

> 
> In addition, it's not a "gift" from the FCC either; it's
> simply a common-sense regulatory quid pro quo.  The point of
> the regulations is to minimize interference to other users,
> including licensed users.  Directional antennas concentrate
> radiated energy in the desired direction, which means that
> the radiated signal is less likely to interfere with users
> in any other direction, and in any case, the total radiated
> power is reduced with higher antenna gain.

This is most certainly the reason it's done, but compared to not
allowing the extra EIRP for directional antennas, it's a gift.

> 
> > So, knowing all that, how far can you make a signal go?
> > [ . . . ]
> 
> > [Deletia: List of materials and corresponding attenuations]
> 
> Note that the list might be useful in back-of-the-envelope,
> aggregate planning, but for any particular RF path, the
> actual attenuation is likely to be quite different than
> expected.  To the extent there are conductive surfaces
> involved in construciton, attenuation will depend, in large
> part, on the number, size, and geometry of apertures in the
> surface.  Diffraction should also be considered.  One thing
> that hasn't been mentioned is that older buildings often
> have plaster walls built on a metal lathe (mesh) substrate,
> which produces very high attenuation.  Newer buildings
> sometimes have an aluminum-foil-backed sheath under the
> exterior walls (it's either a vapor barrier or a radiant
> energy insulator--I forget which) that also causes high
> attenuation.
> 
> > [ . . . ]  Attenuation of a 2.4 Ghz signal through open
> > space follows the formula 100 + 20 * log(d), with d
> > measured in kilometers, so the free space loss over 200
> > meters will be about 67 db.
> 
> I believe that log() function referenced in the formula is
> the common logarithm (base 10), so free space path loss over
> 200 M would be about 86 dB, according to the formula.

Heh, yes.  I'm obviously estimating a lot of things here.  One of them
is the distance to Elliot's.  My initial gut estimate was 150 meters,
which would be 67 dB.  After I got up and walked over to the liquor store
for a soda (just past Elliot's) I thought maybe 200 m was a better
estimate, but forgot to recalculate free space loss.

> 
> I've seen this formula, too, but I'm sure it doesn't tell
> the whole story.  Hams have reported exceptional 1500 KM
> contacts over land and nearly 4000 KM contacts over water at
> 2.3 GHz, and that would correspond to -171 dB path loss.
> Hams can use higher power, bigger antennas, and better
> receivers, but that's still a lot of attentuation to make
> up.  This would suggest that not all propagation is
> consistent with the free-space model.

I have read various things about the reliability of this particular
formula, but it's the only estimator I've been able to find, and appears
to be the one that other community wireless projects are using for their
calculations.

> 
> > [ . . . ] There are some possible solutions, however.  We
> > could use higher gain antennas, but antennas with a gain
> > higher than 35 dB are expensive, difficult to build, and
> > VERY difficult to aim.  We could use amplifiers, but we
> > would soon be in violation of FCC regulations, since the
> > limit on ERP is 30dBm.
> 
> Actually, if the goal is "cheap", antennas with 27-30 dBi
> gain are likely to be at the practical upper limit.
> Commercial rectangular-cut-out paraboloid mesh reflector
> antennas (24 by 40 inches) in the 24 dBi range are $150-200.
> Commercial dish antennas (48 inch) in the 27 dBi range are
> $300-400.  You could probably make your own 27-30 dBi dish
> for $100-200 (maybe less if you're very handy and have
> access to good tools), but it would be large, and you'd have
> to figure out how mount it securely on a roof, etc.  You can
> build 15-18 dBi gain antennas, in a variety of
> configurations, for under $30.

My understanding is that if you aim a directional antenna into a dish,
you can get a combined effect, which could be quite large.  I could
easily be mistaken.

On the subject of antenna construction, if there are two or three other
people interested in building antennas, I'd like to have an antenna
shootout sometime this summer.

> 
> You could also try to buy better receivers, but that probably means
> you can't use cheap 802.11(b) adapters, as you'll either have to buy
> purpose-built equipment or wait for the performance of the cheap stuff
> to improve.  -85 dBm seems pretty lousy compared to the sensitivity of
> communications receivers.  I would think -100 to -110 would be
> achievable without great expense.  Too bad Bob Parnass has left the
> list, as he could comment on what's reasonable here.  Another issue
> is, IIRC, the receiver sensitivity is specified for 11 Mbps, but the
> system is operable over lower quality links at degraded speeds.  So
> even if it appears that the link is at the edge of the loss budget,
> you may still have useable service.

The best receive sensitivity that I know of is -94dBm and it's in
certain cards in the Cisco Aironet series, which are noticeably more
expensive than other cards.  It's true that 802.11b will work at slower
speeds (1, 2, and 5.5 Mb/s) and that slower speeds have lower associated
receive sensitivities.  I've never seen them specified, though.

> [deleted: antenna placement and cabling, summary: clear line of sight,
> minimum cable is better than big amplifiers and expensive antennas ]

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pub  1024D/05A058E0 2002-03-07 Nate Riffe (06-Mar-2002) <inkblot@movealong.org>
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