154
Networking Fundamentals
ISM band
the band of radio
frequencies associ-
ated with indus-
trial, scientifi c, and
medical devices.
omni-directional
the ability of an
antenna to transmit
electromagnetic
signals in all
directions.
directional
the ability of an
antenna to transmit
electromagnetic
signals in a focused
or aimed direction.
by combining with it. The problem occurs if there is too much delay between
the original signal and the refl ected signal. When there is too much delay, the
two signals overlap causing the combined signal to be distorted. The difference
in time it takes the two signals to travel from the source to the destination
determines the level of distortion. The amount of time it takes to reach the
destination depends on the angle and distance of the refl ection.
There are many sources of radio interference that can affect communication
on an IEEE 802.11 wireless LAN. Industrial, scientifi c, and medical devices
occupy the frequency band allocated to the wireless network devices specifi ed
under the IEEE 802.11 standard. Wireless phones and microwave ovens are also
included in the same frequency ranges. The band of radio frequencies associated
with industrial, scientifi c, and medical devices is referred to as the ISM band,
Figure 4-6. The acronym ISM represents Industrial, Scientifi c, and Medical.
Antenna Styles
Two major classifi cations of antennae are associated with radio wave- and
microwave-based wireless networks: omni-directional and directional. These
classifi cations are based on an antenna’s ability to transmit electromagnetic signals.
Omni-directional is the transmission of electromagnetic signals in all directions.
Directional is the transmission of electromagnetic signals in a focused or aimed
direction. Antennae can be further broken down by their individual style of
construction such as omni, dipole, fl at panel, Yagi, and parabolic dish. Figure 4-7
shows each antenna style and the electromagnetic wave pattern it produces. Note
that the electromagnetic wave patterns are viewed from overhead.
Omni
The omni is a straight piece of wire. The wire is engineered to match the
exact length or a fraction of the frequency’s wavelength. For example, a frequency
of 2.4 GHz produces a wavelength of approximately 2.19″. An antenna 2.19″ in
length would match the wavelength exactly. Matching the antenna length to
the wavelength of the radio frequency ensures the best possible reception and
reduces the possibility of picking up interference.
The omni antenna is typically used for a wireless transmitter to broadcast
in a 360° pattern. This type of antenna is used for the source of a wireless
transmission. For example, a wireless Internet service provider would use an
omni antenna to facilitate broadcasting in all directions to better serve customers
throughout the area.
Figure 4-6.
The Industrial,
Scientific, and
Medical (ISM) band
is often a source of
wireless network
interference.
Industrial Scientific Medical
902–928 MHz 2.4–2.48 GHz
2.4–2.48 GHz
802.11b
802.11g
802.11n
5.15–5.825 GHz
802.11a
802.11n
ISM band
5.725–5.85 GHz