Understanding "line
of sight" and "near line of sight" as it applies to range
Wireless
networking
(WiFi)
at 2.4
GHz is
a two-way
system.
Each
device must be
capable
of both
sending
and receiving
a signal
equal
distances. Think of
it as
two people,
a substantial
distance
apart,
throwing
a ball
to each
other.
Person
A has to be strong enough to throw the ball, that substantial distance,
to person B. Person B also has to be strong enough to throw the ball
back, that substantial distance, to person A. If neither Person A nor
B can throw the ball that distance they will not have much of a throwing
game.
Access points and bridges (as shipped from the manufacturer) have an
advantage over laptop/desktop cards because they have a higher output
power and therefore have the ability to send a signal further then most laptop/desktop
cards. When a higher-gain antenna is installed on a desktop card the output power
of that device is now increased closer to the output level of the access point
or bridge therefore equaling the two devices. In some cases, the antennas
of both the access point/bridge and the desktop/laptop card may need
to be replaced if the distance you are attempting to achieve is greater
than the capabilities of the access point/bridge when using the (factory)
antennas that came with your card.
Please
note that
extending the range of your wireless 802.11b wireless device
is only a piece of the whole puzzle. At 2.4GHz (the frequency that 802.11b
WiFi operates at), Line-Of-Sight can be an important factor.
Please see below regarding Line-Of-Sight.
Understanding
The Importance Of Unobstructed Line-Of-Sight With WiFi
802.11b at 2.4GHz (equipped
with and using a vertically (or linear)-polarized antenna) requires unobstructed
visual Line-Of-Sight (LoS).
Unobstructed Line-Of-Sight means just that; there should not be trees, terrain,
buildings, or structures between your two (antenna) points. Basically both
antennas should physically see each other in an external outdoor bridge.
The radio waves at this low frequency will not penetrate metal, steel, concrete,
cement, stone, brick, etc. very well, if at all. For interior applications,
dry wall, sheet rock, and wood could be a problem with basic factory antennas.
Surrounding the visual Line-Of-Sight is
the Fresnel zone (image 1). Any obstructions that come into the Fresnel zone,
although not obstructing the visual Line-Of-Sight, may also slow down, hinder
and effect your signal. The radio waves may deflect off of those obstructions.
This
is called Near Line-Of-Sight (nLoS,
image 2). Although you may see a slight signal with nLoS situations, your data
transfer rate may decrease. You may find you are incapable of accessing the
Internet.
An obstruction that cuts
across the visual Line-Of-Sight and prohibits an optical visual between the
two antennas in your bridge is considered Non-Line-Of-Sight (NLoS,
image 3). Any signal, in this case, will be minimal or non-existent.
You may find in your bridge
application, that the two antennas can visually see each other through spaces
and breaks in an obstructing tree or tree line. Please note that tree
branches that cross the visual Line-Of-Sight will
move with the wind. This movement will disrupt and have an effect on
a vertically-polarized WiFi signal. Keep in mind, if you are conducting
your site survey during late Fall and Winter months, those trees will
fill in come Spring and Summer. Additionally, weather, RF interferences,
and other site variables can have an effect on your signal too.