Information Technology Reference
In-Depth Information
these areas. A third bridge is placed at the back of the modular offices to provide
coverage there, as well as to nearby offices and the storeroom. A large room (which
could be an auditorium, a cafeteria, or a meeting room) has walls that obstruct the
wireless signal, so bridge 4 is placed there.
In some cases, you can use bridges with multiple transceivers and antennas to
cover additional areas. For example, some of the manufacturers offer dual-trans-
ceiver bridges, so one antenna can be placed in an interior location and the other
placed outside the building. In the example floor plan, it might be possible to elim-
inate bridge 4 by using a dual NIC bridge in location 3, with one antenna inside the
large room. In addition, an external antenna could be used either for outside wire-
less coverage or to bridge to another building's LAN.
The exact number of wireless devices that can be covered from a single
bridge/antenna depends totally on the conditions and the desired coverage area. An
upper limit is about 200 devices, but you will want to limit the number of devices
in the same way you would with a normal Ethernet segment. Most network man-
agers like to keep this number well under 100 stations for a 10 Mbps Ethernet seg-
ment, depending on usage and applications.
In a crowded environment, with limited coverage because of building walls or
other obstructions, you will probably want to limit the number of wireless worksta-
tions to no more than 50. Actually, this limit is not that low, as it may be difficult
to get 50 wireless workstations within the 200- to 300-ft indoor range of the wire-
less bridge.
Using Range Extender Antennas
Gain antennas may effectively extend the range of a wireless system (see Fig. 13.2).
Antennas are rated in dB over isotropic, and the higher the number, the greater the
amplitude of the transmitted or received signal. Decibels are a logarithmic power
ratio. A good rule of thumb is that a 3 dB increase represents a doubling of the power.
Unfortunately, the available captured power decreases as the square of the distance
from the transmitter, so it takes about a 6 dB increase to effectively double the range.
Nevertheless, gain antennas are a very effective means of increasing the usable
coverage area of a wireless bridge. A cautious approach is to use gain antennas for
every bridge, and add a gain antenna for specific workstations that suffer the slow
data rates that go along with a marginal signal.
It is important to recognize that the effect of the extender antenna is bidirec-
tional. That is, you increase both the received signal and the effective transmitted
signal. With a gain antenna, you really cannot lose.
