Page 116 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes As shown in the figure 7.3, after the fat AP is enabled with user isolation, clients 1 through 4
cannot access each other directly, or learn one another’s MAC and IP addresses.

7.2.5 Wireless LAN Technology

There are many technologies that can be used to design a wireless LAN solution. Some of them
are discussed below.

z z Narrowband Technology: In radio, narrowband describes a channel in which the bandwidth
of the message does not significantly exceed the channel’s coherence bandwidth.
z In the study of wired channels, narrowband implies that the channel over consideration
is sufficiently narrow that its frequency response can be considered flat. The message
bandwidth will therefore be less than the coherence bandwidth of the channel. This is no
channel has perfectly flat fading, but the analysis of many aspects of wireless systems is
greatly simplified if flat fading can be assumed.
z Narrowband can also be used with the audio spectrum to describe sounds which occupy
a narrow range of frequencies. In telephony, narrowband is usually considered to cover
frequencies 300–3400 Hz.

z z Spread Spectrum Technology: In telecommunication and radio communication, spread-
spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic,
or acoustic signal) generated with a particular bandwidth is deliberately spread in the
frequency domain, resulting in a signal with a wider bandwidth. These techniques are used
for a variety of reasons, including the establishment of secure communications, increasing
resistance to natural interference, noise and jamming, to prevent detection, and to limit
power flux density (e.g. in satellite downlinks).

z z Frequency-Hopping Spread Spectrum Technology: Frequency hopping is one of two
basic modulation techniques used in spread spectrum signal transmission. It is the
repeated switching of frequencies during radio transmission, often to minimize the
effectiveness of “electronic warfare” - that is, the unauthorized interception or jamming of
telecommunications. It also is known as frequency- hopping code division multiple access
(FH-CDMA).
z Spread spectrum modulation techniques have become more common in recent years.
Spread spectrum enables a signal to be transmitted across a frequency band that is much
wider than the minimum bandwidth required by the information signal. The transmitter
“spreads” the energy, originally concentrated in narrowband, across a number of frequency
band channels on a wider electromagnetic spectrum. Benefits include improved privacy,
decreased narrowband interference, and increased signal capacity.

z z Direct-Sequence Spread Spectrum Technology: In telecommunications, direct-sequence
spread spectrum (DSSS) is a modulation technique. As with other spread spectrum
technologies, the transmitted signal takes up more bandwidth than the information signal
that is being modulated. The name ‘spread spectrum’ comes from the fact that the carrier
signals occur over the full bandwidth (spectrum) of a device’s transmitting frequency.

z z Infrared Technology: infrared radiation is the region of the electromagnetic spectrum
between microwaves and visible light. In infrared communication an LED transmits
the infrared signal as bursts of non-visible light. At the receiving end a photodiode or
photoreceptor detects and captures the light pulses, which are then processed to retrieve
the information they contain. Some common applications of infrared technology are listed
below.

110 LOVELY PROFESSIONAL UNIVERSITY