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Hack 80 Establishing Line of Sight

Use these methods to quickly tell if a long
distance wireless shot is possible.

Wireless
networks operate at microwave frequencies and, as such, work much
better when the client's antenna can see the
AP's antenna, with nothing but air between. Over
short distances (within a few hundred feet), wireless networks can
tolerate a few objects in the way. But as you try to push your signal
further, having clean Line Of Sight (LOS) is absolutely critical.

How can you tell if a given point has LOS to another point several
miles away? The short answer is that without actually having a look,
you can't. If the far end is more than a couple of
miles away, it becomes difficult to tell, even with a high-powered
telescope. But using these techniques, you can make an educated guess
as to what is at least within the realm of possibility.

Using a GPS to Log Prospective LAT/LONG/ALT

When visiting a potential node site,
it's a good idea to bring along an accurate GPS. It
can log not only the (more or less) precise
latitude and longitude,
but also the altitude of the site. After collecting points, you can
pull them into your topographical software and plot them. For
example, the Topo! package from National Geographic
(http://maps.nationalgeographic.com/topo/)
allows you to easily mark up a topographical map and analyze
arbitrary points and routes. Draw routes between any two points to
figure out how the land lies between them. This gives you an easy
visual reference for a given shot, as shown in the cross section
provided by Topo! in Figure 6-1.

Keep in mind that, although the Topo! software has surveyed
geological data, it won't have tree or building
information. You can get a general idea of how cluttered an area is,
but you won't really know until you try the shot.
Using the overhead view in conjunction with the cross section, you
can not only weed out the obvious negatives (as in Figure 6-2), but also find potential work-arounds.

Using the overhead view to locate key repeater points can be fun.
Find out where the good sites are, and try contacting the people at
those points. More often than not, people are willing to work with
local community groups to provide free access (particularly if they
don't have to do much besides provide electricity,
and can get free high-speed access besides.)

Plotting the Points on a 3D Map

A couple of software packages
from DeLorme (Topo USA and 3-D
TopoQuads, available at http://www.delorme.com/) have the ability to
create 3D renderings of a topological region,
complete with data markers and labels (Figure 6-3).
While it's a cool feature and is rather compelling
in presentations, it has limited practical value beyond helping to
visualize the surrounding terrain.

Generally speaking, the more data points you collect, the more
impressive your visual presentation will be.

Once your points are plotted on a map, you can very quickly determine
which sites are worth developing. If you can't get
direct line of sight to a place you'd like, take a
look at the surrounding geography and see whether you can find
another way. If you can't go through,
you'll have to go over or around. Software topo maps
can make finding the "bank shots"
much easier.

Remember that these tools will show you the

lay of the land but don't
provide data on ground clutter, such as trees and buildings. The
tools aren't perfect, but they can give you an idea
of what you have to work with when building a large wireless
network.