23-01-2013, 10:53 AM
Wireless Fidelity
[attachment=48115]
INTRODUCTION
Wi-Fi, or Wireless Fidelity, is freedom: it allows you to connect to
the Internet from your couch at home, a bed in a hotel room or at a
conference room at work without wires. How? Wi-Fi is a wireless
technology like a cell phone. Wi-Fi enabled computers send and receives
data indoors and out: anywhere within the range of a base station. And the
best thing of all, it’s fast. In fact, it’s several times faster than the fastest
cable modem connection.
However, you only have true freedom to be connected anywhere if
your computer is configured with a Wi-Fi CERTIFIED radio (a PC Card or
similar device). Wi-Fi certification means that you will be able to connect
anywhere there are other Wi-Fi CERTIFIED products-whether you are at
home, the office or corporate campus, or in airports, hotels, coffee shops
and other public areas equipped with a Wi-Fi access available.
The Wi-Fi certified logo is your only assurance that the product has
met rigorous interoperability testing requirements to assure products from
different vendors will work together .The Wi-Fi CERTIFIED logo means
that it’s a “safe” buy.
OPERATION MODES
IEEE 802.11 defines two operating modes: Ad hoc mode and
Infrastructure mode.
AD HOC MODE: In ad hoc mode, also known as peer-to-peer mode,
wireless clients communicate directly with each other (without the use of a
wireless AP). Two or more wireless clients who communicate using ad hoc
mode form an Independent Basic Service Set (IBSS). Ad hoc mode is used
to connect wireless clients when a wireless AP is not present.
RADIO TECHNOLOGY
Wi-Fi network uses radio technology called IEEE 802.11b to
provide secure, fast, reliable, wireless connectivity. 11b defines the physical
layer and media access control (MAC) sublayer for communications across
a shared, wireless local area network (WLAN). At the physical layer, IEEE
802.11b operates at the radio frequency of 2.45 gigahertz (GHz) with a
maximum bit rate of 11 Mbps. It uses the direct sequence spread spectrum
(DSSS) transmission technique. At the MAC sublayer of the Data Link
layer, 802.11b uses the carrier sense multiple access with collision
avoidance (CSMA/CA) media access control (MAC) protocol
DIRECT SEQUENCE SPREAD SPECTRUM
Direct Sequence is the best known Spread Spectrum Technique. A
DSSS transmitter converts an incoming data stream into a symbol stream
where each symbol represents a group of one or more bits. Using a phase
varying modulation technique, DSSS transmitter modulates or multiplies
each symbol with a noise like code called ‘chip’ sequence. This is also
called processing gain. The multiplication operation in a DSSS transmitter
artificially increases the used bandwidth based on the length of chip
sequence.
CARRIER SENSE MULTIPLE ACCESS/COLLISION AVOIDANCE
The basic access method for 802.11 is the Distributed Coordination
Function (DCF) which uses Carrier Sense Multiple Access / Collision
Avoidance (CSMA / CA). This requires each station to listen for other
users. If the channel is idle, the station may transmit. However if it is busy,
each station waits until transmission stops, and then enters into a random
back off procedure. This prevents multiple stations from seizing the
medium immediately after completion of the preceding transmission.
OPERATION BASICS
When a wireless adapter is turned on, it begins to scan across the
wireless frequencies for wireless APs and other wireless clients in ad hoc
mode. Assuming that the wireless client is configured to operate in
infrastructure mode, the wireless adapter chooses a wireless AP with which
to connect. This selection is made automatically by using SSID and signal
strength and frame error rate information. Next, the wireless adapter
switches to the assigned channel of the selected wireless AP and negotiates
the use of a port. This is known as establishing an association.
If the signal strength of the wireless AP is too low, the error rate too
high, or if instructed by the operating system (in the case of Windows XP),
the wireless adapter scans for other wireless APs to determine whether a
different wireless AP can provide a stronger signal or lower error rate. If
such a wireless AP is located, the wireless adapter switches to the channel
of that wireless AP and negotiates the use of a port. This is known as
reassociation.
RANGE IN A Wi-Fi NETWORK
One of the factors that affect the range of a Wi-Fi network is the
distance of the client devices to your base station. In an open area with no
walls, furniture or interfering radio devices you may be able to get a range
of 500 feet or more from your base station to the Wi-Fi equipped computer.
In fact you could get a signal from up to a mile away depending on the
antennas you use and environmental conditions.
Many base stations can also act as relay stations for your network.
For example if you locate one Wi-Fi equipped computer 100 feet away
from your base station, another Wi-Fi computer 100 feet away in another
direction and then position your base station in the middle, you can create a
network with a range of 200 feet from the Wi-Fi computer to the other.
[attachment=48115]
INTRODUCTION
Wi-Fi, or Wireless Fidelity, is freedom: it allows you to connect to
the Internet from your couch at home, a bed in a hotel room or at a
conference room at work without wires. How? Wi-Fi is a wireless
technology like a cell phone. Wi-Fi enabled computers send and receives
data indoors and out: anywhere within the range of a base station. And the
best thing of all, it’s fast. In fact, it’s several times faster than the fastest
cable modem connection.
However, you only have true freedom to be connected anywhere if
your computer is configured with a Wi-Fi CERTIFIED radio (a PC Card or
similar device). Wi-Fi certification means that you will be able to connect
anywhere there are other Wi-Fi CERTIFIED products-whether you are at
home, the office or corporate campus, or in airports, hotels, coffee shops
and other public areas equipped with a Wi-Fi access available.
The Wi-Fi certified logo is your only assurance that the product has
met rigorous interoperability testing requirements to assure products from
different vendors will work together .The Wi-Fi CERTIFIED logo means
that it’s a “safe” buy.
OPERATION MODES
IEEE 802.11 defines two operating modes: Ad hoc mode and
Infrastructure mode.
AD HOC MODE: In ad hoc mode, also known as peer-to-peer mode,
wireless clients communicate directly with each other (without the use of a
wireless AP). Two or more wireless clients who communicate using ad hoc
mode form an Independent Basic Service Set (IBSS). Ad hoc mode is used
to connect wireless clients when a wireless AP is not present.
RADIO TECHNOLOGY
Wi-Fi network uses radio technology called IEEE 802.11b to
provide secure, fast, reliable, wireless connectivity. 11b defines the physical
layer and media access control (MAC) sublayer for communications across
a shared, wireless local area network (WLAN). At the physical layer, IEEE
802.11b operates at the radio frequency of 2.45 gigahertz (GHz) with a
maximum bit rate of 11 Mbps. It uses the direct sequence spread spectrum
(DSSS) transmission technique. At the MAC sublayer of the Data Link
layer, 802.11b uses the carrier sense multiple access with collision
avoidance (CSMA/CA) media access control (MAC) protocol
DIRECT SEQUENCE SPREAD SPECTRUM
Direct Sequence is the best known Spread Spectrum Technique. A
DSSS transmitter converts an incoming data stream into a symbol stream
where each symbol represents a group of one or more bits. Using a phase
varying modulation technique, DSSS transmitter modulates or multiplies
each symbol with a noise like code called ‘chip’ sequence. This is also
called processing gain. The multiplication operation in a DSSS transmitter
artificially increases the used bandwidth based on the length of chip
sequence.
CARRIER SENSE MULTIPLE ACCESS/COLLISION AVOIDANCE
The basic access method for 802.11 is the Distributed Coordination
Function (DCF) which uses Carrier Sense Multiple Access / Collision
Avoidance (CSMA / CA). This requires each station to listen for other
users. If the channel is idle, the station may transmit. However if it is busy,
each station waits until transmission stops, and then enters into a random
back off procedure. This prevents multiple stations from seizing the
medium immediately after completion of the preceding transmission.
OPERATION BASICS
When a wireless adapter is turned on, it begins to scan across the
wireless frequencies for wireless APs and other wireless clients in ad hoc
mode. Assuming that the wireless client is configured to operate in
infrastructure mode, the wireless adapter chooses a wireless AP with which
to connect. This selection is made automatically by using SSID and signal
strength and frame error rate information. Next, the wireless adapter
switches to the assigned channel of the selected wireless AP and negotiates
the use of a port. This is known as establishing an association.
If the signal strength of the wireless AP is too low, the error rate too
high, or if instructed by the operating system (in the case of Windows XP),
the wireless adapter scans for other wireless APs to determine whether a
different wireless AP can provide a stronger signal or lower error rate. If
such a wireless AP is located, the wireless adapter switches to the channel
of that wireless AP and negotiates the use of a port. This is known as
reassociation.
RANGE IN A Wi-Fi NETWORK
One of the factors that affect the range of a Wi-Fi network is the
distance of the client devices to your base station. In an open area with no
walls, furniture or interfering radio devices you may be able to get a range
of 500 feet or more from your base station to the Wi-Fi equipped computer.
In fact you could get a signal from up to a mile away depending on the
antennas you use and environmental conditions.
Many base stations can also act as relay stations for your network.
For example if you locate one Wi-Fi equipped computer 100 feet away
from your base station, another Wi-Fi computer 100 feet away in another
direction and then position your base station in the middle, you can create a
network with a range of 200 feet from the Wi-Fi computer to the other.