[UFO Chicago] Wirelessness in Chicago

[Nate Riffe]

I had a meeting at Coffee Chicago at Broadway and Berwyn earlier tonight
with David Clayton and two other north side gentlemen regarding
community wireless networking in Chicago.  If you'll recall, David
Clayton was the guy at the last meeting who sat across from Pete.

We talked about various ins and outs of community wireless networking,
including technicals, the regulatory environment, possible
organizational structures, and politicization.  Most significantly,
however we simply discussed how get started with this thing.  And in
that arena I think we may have found what is necessary to move forward.

The long-term goal of this network is cheap, ubiquitous, and wireless
Internet access throughout the city.  And to that end, there are certain
short-term realities that are unavoidable barring a major contribution
of resources.  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.

In the meantime, what good is it?  Consider the fact that UFO generates
quite a bit of data traffic.  A lot of data goes in and out of flynn in
particular that just ends up somewhere else in the city.  Internalizing
that data flow onto our own high-speed network makes that more efficient
and opens up other possibilities, such as remote backups, UFO Radio (all
xy003, all the time ;> ), and low-lag Quake/Counterstrike/whatever
matches (to name a few).

On the subject of technology, the issues are still what they were last
summer.  802.11b (WiFi) networks operate in the 2.4 Ghz unlicensed band
(also called the 2.4 Ghz ISM band, where ISM stands for
Industrial/Scientific/Medical), and that band has certain physical
and regulatory limitations.

Water reflects signals in this band, which means things like vegetation,
weather, and large, thick flocks of pigeons will adversely affect
network performance and even network availability.  Walls, windows, and
furniture will also reduce a signal's strength.  The best way to work
within these limitations is to connect the endpoints of point-to-point
links via clear line-of-sight paths using highly directional antennas.
That way, the only factor is free space loss, which is the natural
fading of a signal the further you are from its source.

The FCC regulations for the 2.4 Ghz ISM band make very low allowances
for signal power.  The rules themselves are not especially easy to read,
so here are some important definitions and a quick summary of the regs:

  Radiator - The FCC calls any equipment that generates any sort of
    signal a 'radiator'.  For our purposes, a radiator is comprised of
    the 802.11b transceiver, the antenna, any cabling and electronics
    (such as amplifiers) between them, and their configuration.

  watts (w) and millwatts (mw) - This is simply a unit of power in
    physics.  1 watt is 1000 milliwatts.
  
  decibels (dB), and decibels relative to 1 milliwatt (dBm) - To
    understand decibels, it helps to first understand bels.  Bels
    measure the difference between two different power levels on a
    logarithmic scale.  That may sound complicated, but it's not.  Say
    you've got two signals and one is ten times stronger than the other.
    That's 1 bel.  If the one is one hundred times stronger than the
    other, that's 2 bels, and so on.  10 decibels is 1 bel.  Decibels
    relative to 1 milliwatt (dBm) is used to measure the strength of one
    signal against a standard, and the standard is 1 milliwatt.  A 10 mw
    signal, since it is ten times stronger than 1 milliwatt, is 10 dBm.
    A 100 milliwatt signal is 20 dBm.  And a 1000 milliwatt (1 watt)
    signal is 30 dBm.

  Input Power - This is the strength of the signal going into an
    antenna.  It is measured in watts or dBm.

  Omnidirectional, Directional, and Isotropic Antennas - There are a lot
    of different antenna designs.  The main difference between them is
    the "shape" of the signal they radiate.  Some antennas radiate all
    of their input power in one direction and some radiate it all around
    in a pancake or donut shaped pattern.  Antennas that radiate a
    signal all around are called omnidirectional antennas.  Antennas
    that shoot it all one way are called directional.  There is also a
    theoretical antenna which is not possible to build that radiates
    power equally in all directions in a spherical pattern, and it is
    called an isotropic antenna.

  Lobes and Nulls - When real antennas (in other words, not isotropic
    antennas) radiate a signal, the input power is concentrated into
    certain areas when it is radiated and other areas receive less
    signal power (in the shapes discussed above).  These areas where
    power is concentrated are called lobes, and the areas where power is
    sparse are called nulls.  The lobe with the highest concentration of
    power is called an antenna's main lobe, and is the one that most
    people are interested in.

  Gain - Due to the lobes and nulls in a real antenna's radiation
    pattern, there are measurable differences between the signal
    strength from any real antenna and the theoretical isotropic
    antenna.  Gain is the difference in signal strength between the main
    lobe of an antenna's radiation pattern and the signal strength in an
    isotropic antenna's radiation pattern.  It is measured in decibels.
    I should also note that signals that originate in the main lobe and
    are picked up by the antenna are also subject to this gain.

  Effective Radiated Power (ERP) - This is how strong the actual
    radiated signal is, including antenna gain.  It is measured in watts
    or dBm.

  Receive Sensitivity - This is the weakest signal that a receiver is
    able to receive.  It is measured in watts or dBm.  (Note that the
    FCC doesn't care one lick about receive sensitivity, so the lower
    the better!)

FCC regulations make these restrictions on the 2.4 Ghz ISM band.  The
effective radiated power (ERP) of an omnidirectional 2.4 Ghz radiator
must not exceed 1 watt (30 dBm).  The effective radiated power of a
directional radiator must not exceed 1 watt for any antenna with up to
30 dB of gain, and for every 1 dB above 30 dB of gain, the maximum
allowable ERP is raised by 3 dB (this is actually a very nice gift from
the FCC).

So, knowing all that, how far can you make a signal go?  First, let's
specify a radiator.  A standard Linksys base station like the one I
brought to a UFO meeting last summer puts a 100 mw input signal into a 6
db omnidirectional antenna.  100 mw is 100 times the strength of 1 mw,
so that's a 20 dBm signal, and after the antenna gain, the ERP will be
26 dBm.  I haven't been able to find an actual specification anywhere,
but the Linksys base station probably has a receive sensitivity of about
-75 to -85 dBm.  Also, here are some of the things that might get in the
way of a signal and how much they reduce it's power (signal reduction is
called attenuation):

> From:
> http://www.intersil.com/design/prism/papers/symposum.pdf
> 2.4 GHz Signal Attenuation:
> Window Brick Wall    2dB
> Metal Frame Glass Wall into Building 6dB
> Metal Door in Office 6dB
> Cinder Block Wall 4 dB
> Metal Door in Brick Wall 12.4dB
> Brick Wall next to Metal Door 3dB
(this data was found at
http://lists.nycwireless.net/pipermail/nycwireless/2001q3/000358.html)

Trees and shrubs are about 5 db per meter.

So, from my apartment to Elliot Shank's condo, there are approximately
20 to 30 walls, most of which are brick or cinder block, possibly some
doors and windows.  So the attenuation due to building-stuff is about
100 db (+/- about 20 db, let's say).  There is also about 200 meters of
space, a few trees, and occassionally people walking around their
apartments, so tack on another 40 or so decibels for the trees and
people.  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.  The total attenuation
from point to point is around 200 db.  The ERP (26 dBm) plus the
receiving antenna's gain (6 db) minus all the attenuation (200 db) is
-170 dBm.  Since -170 dBm is less than the receive sensitivity (-75 to
-85 dBm), the link will not work.  In fact, a -170 dBm signal is 1
billionth the strength of a -80 dBm signal (the difference between a
-80dBm and a -170dBm signal is 90dB.  90dB is 9B, which means a
difference in strength of 10^9, or 1 billion).

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.  We could put the antennas on our roofs, which would
reduce the number of obstructions.  If we could prop them up so that
there was nothing but open space between the antennas, then the
obstruction would be free space loss (67 dB), but with all the cable
between the base stations and their antannas, cable loss (with varies
depending on the quality of the cable) is also a major factor.

-- 
--< ((\))< >----< inkblot@movealong.org >----< http://www.movealong.org/ >--
pub  1024D/05A058E0 2002-03-07 Nate Riffe (06-Mar-2002) <inkblot@movealong.org>
     Key fingerprint = 0DAC F5CB D182 3165 D757  C466 CD42 12A8 05A0 58E0