connectivity allowed an increase in productivity by allowing
the sharing of printers, servers, and software. Traditional
networked systems require that the workstation remains stationary
permitting moves only within the limits of the media and office
area. The introduction of wireless technology removes these
restraints and brings true portability to the computing world.
Administrators often consider wireless when installing a new
network or when upgrading an existing network. A simple wireless
network could be working just a few minutes after the workstations
are turned on. Connectivity to the Internet is provided through
a wired connection, router, cable or DSL modem and a wireless
access point that acts as a hub for the wireless nodes. In a
residential or small office environment these devices may be
combined into a single unit.
An understanding of the regulations and standards that apply
to wireless technology will ensure that deployed networks will
be interoperable and in compliance. Just as in cabled networks,
IEEE is the prime issuer of standards for wireless networks.
The standards have been created within the framework of the
regulations created by the Federal Communications Commission
A wireless network may consist of as few as two devices. - The
nodes could simply be desktop workstations or notebook computers.
Equipped with wireless NICs, an Ďad hocí network could be established
which compares to a peer-to-peer wired network. Both devices
act as servers and clients in this environment.Wireles networks
face the dual problems of security and compatibility.Usually
a central hub is fixed called an access point(AP). APs are equipped
with antennae and provide wireless connectivity over a specified
area referred to as a cell. Depending on the structural composition
of the location in which the AP is installed and the size and
gain of the antennae, the size of the cell could greatly vary.
Most commonly, the range will be from 91.44 to 152.4 meters
(300 to 500 feet). To service larger areas, multiple access
points may be installed with a degree of overlap. The overlap
permits "roaming" between cells. When a client is
activated within the WLAN, it will start "listening"
for a compatible device with which to "associate".
This is referred to as "scanning". Scanning causes
a probe request to be sent from the wireless node seeking to
join the network. The probe request will contain the Service
Set Identifier (SSID) of the network it wishes to join. When
an AP with the same SSID is found, the AP will issue a probe
response. The authentication and association steps are completed.These
steps are necessary for security purposes. After establishing
connectivity to the WLAN, a node will pass frames through the
network. When a source node sends a frame, the receiving node
returns a positive acknowledgment (ACK). Since radio frequency
(RF) is a shared medium, collisions can occur just as they do
on wired shared medium. The major difference is that there is
no method by which the source node is able to detect that a
collision occurred. For that reason WLANs use Carrier Sense
Multiple Access/Collision Avoidance (CSMA/CA).
Computers send data signals electronically. Radio transmitters
convert these electrical signals to radio waves. Changing electric
currents in the antenna of a transmitter generates the radio
waves. These radio waves radiate out in straight lines from
the antenna. However, radio waves attenuate as they move out
from the transmitting antenna. In a WLAN, a radio signal measured
at a distance of just 10 meters (30 feet) from the transmitting
antenna would be only 1/100th of its original strength. Because
radio signals weaken as they travel away from the transmitter,
the receiver must also be equipped with an antenna. When radio
waves hit the antenna of a receiver, weak electric currents
are generated in that antenna. These electric currents, caused
by the received radio waves, are equal to the currents that
originally generated the radio waves in the antenna of the transmitter.
The receiver amplifies the strength of these weak electrical
signals.In a transmitter, the electrical (data) signals from
a computer or a LAN are not sent directly into the antenna of
the transmitter. Rather, these data signals are used to alter
a second, strong signal called the carrier signal.
process of altering the carrier signal that will enter the antenna
of the transmitter is called modulation. There are three basic
ways in which a radio carrier signal can be modulated. For example,
Amplitude Modulated (AM) radio stations modulate the height
(amplitude) of the carrier signal. Frequency Modulated (FM)
radio stations modulate the frequency of the carrier signal
as determined by the electrical signal from the microphone.
In WLANs, a third type of modulation called phase modulation
is used to superimpose the data signal onto the carrier signal
that is broadcast by the transmitter.In this type of modulation,
the data bits in the electrical signal change the phase of the
carrier signal. A receiver demodulates the carrier signal that
arrives from its antenna. The receiver interprets the phase
changes of the carrier signal and reconstructs from it the original
electrical data signal.
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