Wireless LANs - LAN Wiring - page 336

The two site survey methods are what we will call “casual” and “full.” In both
situations, the building layout is considered, probable bridge locations plotted, and
the extent of wireless coverage is detailed.
The casual site survey is done with actual WLAN links. A wireless bridge is tem-
porarily set up in each likely bridge location and the antenna that will be used is tem-
porarily mounted at or near the expected permanent location. For example, if you
plan to mount the bridges above the ceiling tile in a hallway, place the antenna in the
approximate planned location below the ceiling grid and drape the coax down to the
bridge. Remember that it is the antenna that transmits the signal into space, and the
bridge location does not matter. If you plan to use the internal antennas on the
bridge's transceiver card, then you really should try to get the bridge up near the ceil-
ing. Try mounting the bridge on an 8-ft board (such as a 1 8 ft) and propping

the board up against the wall to simulate mounting the bridge on the wall.
Now, use a laptop that is equipped with an appropriate W-NIC to test the sig-
nal strength in the desired locations with the expected range of the bridge. The client
software drivers for the W-NIC generally have a test mode that displays the signal
strength at each end of the link. You need to know how well the bridge is receiving
the remote signal, as well as how well you are receiving the bridge's signal.
Remember that walls, ceilings, and even floors are important in relation to the
wireless devices. So, place the laptop on a desk where it will be used to test the link.
If you doubt this phenomenon, simply go to a distance from the bridge where the
signal becomes weak, and place your hand over the W-NIC's antenna. The signal
will drop to unusable in an instant.
The full site survey is performed with sophisticated field-strength measuring
equipment. A test signal is sent to a test antenna in the planned wireless bridge loca-
tion and the receive signal strength is measured at various locations through the
desired coverage area. The test antenna is moved as necessary to achieve adequate
signal strength at every desired point. The path loss can be determined from the
measurements, and appropriate corrective actions taken. Typical actions that
increase signal strength are to use gain antennas, use secondary antennas, adjust
antenna (and possibly bridge) locations, and add additional bridge locations.
It is amazing what a tiny adjustment to a bridge antenna placement can do for
the received signal strength at these frequencies. For example, in a location where
metallic shielding of sheet metal or foil-covered ductwork is a factor, a mere 6- to
12-in change in antenna location can cause a 10- to 12-dB increase in signal
strength, measured 50 ft away!
It sometimes helps to pretend that you can sense the LAN signals. Try to imag-
ine that the radio waves are light from a bulb. The farther away you go, the dim-
mer the light, and the less the signal. If you move the light behind an opaque object,

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