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Technical information
Wi-Fi: How it Works
Wi-Fi networks use radio technologies called IEEE
802.11 to provide secure, reliable, fast wireless connectivity. A
typical Wi-Fi setup contains one or more
Access Points (APs) and one or more clients. An AP broadcasts
its SSID
(Service Set Identifier, "Network name") via packets that are called
beacons, which are usually broadcast every 100 ms. The beacons are
transmitted at 1 Mbit/s, and are of relatively short duration and
therefore do not have a significant effect on performance. Since 1
Mbit/s is the lowest rate of Wi-Fi it assures that the client that
receives the beacon can communicate at at least 1 Mbit/s. Based on
the settings (e.g. the SSID), the client may decide whether to
connect to an AP. If two APs of the same SSID are in range of the
client, the client firmware might use
signal strength to decide with which of the two APs to make a
connection.
The Wi-Fi standard leaves connection criteria and
roaming totally open to the client. This is a strength of Wi-Fi, but
also means that one wireless adapter may perform substantially
better than another. Since Wi-Fi transmits in the air, it has the
same properties as a non-switched wired Ethernet network, and
therefore collisions can occur. Unlike a wired Ethernet, and like
most packet radios, Wi-Fi cannot do collision detection, and instead
uses an acknowledgment packet for every data packet sent. If no
acknowledgement is received within a certain time a retransmission
occurs. Also, a medium reservation protocol can be used when
excessive collisions are experienced or expected (RequestToSend/ClearToSend
used for
Collision Avoidance or CA) in an attempt to try to avoid
collisions.
A Wi-Fi network can be used to connect computers
to each other to the internet and to wired networks (which use IEEE
802.3 or Ethernet). Wi-Fi networks operate in the unlicensed 2.4
(802.11b/g) and 5 GHz (802.11a/h) radio bands, with an 11 Mbps
(802.11b) or 54 Mbps (802.11a or g) data rate or with products that
contain both bands (dual band). They can provide real world
performance similar to the basic 10BaseT wired Ethernet networks.
Channels
Except for 802.11a/h, which operates at 5GHz,
Wi-Fi devices historically primarily use the spectrum in 2.4GHz,
which is standardized and
unlicensed by international agreement, although the exact
frequency allocations and maximum permitted power vary slightly in
different parts of the world. Channel numbers, however, are
standardized by frequency throughout the world, so authorized
frequencies can be identified by channel numbers. The 2.4 GHz band
is also used by microwave ovens, cordless phones, baby monitors and
Bluetooth devices.
The maximum number of available channels for
Wi-Fi enabled devices are:
Advantages of Wi-Fi
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Allows LANs to be deployed without cabling
for client devices, typically reducing the costs of network
deployment and expansion. Spaces where cables cannot be run,
such as outdoor areas and historical buildings, can host
wireless LANs..
-
Built into most modern laptops, getting a
laptop without a built in WiFi has become an exception.
-
Wi-Fi chipset pricing continues to come down,
making Wi-Fi a very economical networking option and driving
inclusion of Wi-Fi in an ever-widening array of devices.
-
Wi-Fi products are widely available in the
market. Different competitive brands of access points and client
network interfaces are inter-operable at a basic level of
service. Products designated as Wi-Fi CERTIFIED by the Wi-Fi
Alliance are backwards inter-operable.
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Wi-Fi is a global set of standards. Unlike
cellular carriers, the same Wi-Fi client works in different
countries around the world.
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Widely available in more than 250,000 public
hot spots and tens of millions of homes and corporate and
university campuses worldwide.
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As of 2007,
WPA is not easily cracked if strong passwords are used and
WPA2
encryption has no known weaknesses.
-
New protocols for Quality of Service (WMM)
and power saving mechanisms (WMM Power Save) make Wi-Fi even
more suitable for latency-sensitive applications (such as voice
and video) and small Form-Factor
Disadvantages of Wi-Fi
-
Spectrum assignments and operational
limitations are not consistent worldwide; most of Europe allows
for an additional 2 channels beyond those permitted in the US
(1-13 vs 1-11); Japan has one more on top of that (1-14) - and
some countries, like Spain, prohibit use of the lower-numbered
channels. Furthermore some countries, such as Italy, used to
require a 'general authorization' for any Wi-Fi used outside an
operator's own premises, or require something akin to an
operator registration.[citation
needed]
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Equivalent isotropically radiated power (EIRP) in the EU is
limited to 20
dBm
(0.1 W).
-
Power consumption is fairly high compared to
some other low bandwidth standards (Zigbee and Bluetooth),
making battery life a concern.
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The most common wireless encryption standard,
Wired Equivalent Privacy or WEP, has been shown to be easily
breakable even when correctly configured.
Wi-Fi Protected Access (WPA and WPA2) which began shipping
in 2003 aims to solve this problem and is now available on most
products.
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Wi-Fi
Access Pointss typically default to an open (encryption-free)
mode. Novice users benefit from a zero configuration device that
works out of the box but without security enabled providing open
wireless access to their LAN. To turn security on requires the
user to configure the device, usually via a software GUI.
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Many 2.4 GHz
802.11b and
802.11g Access points default to the same channel on initial
start up, contributing to congestion on certain channels. To
change the channel of operation for an access point requires the
user to configure the device.
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Wi-Fi networks have limited range. A typical
Wi-Fi home router using
802.11bb or
802.11g with a stock antenna might have a range of 45 m (150
ft) indoors and 90 m (300 ft) outdoors. Range also varies with
frequency band. Wi-Fi in the 2.4 GHz frequency block has
slightly better range than Wi-Fi in the 5 GHz frequency block.
Outdoor range with improved antennas can be several kilometres
or more with line-of-sight.
-
Wi-Fi pollution, of an excessive number of an
access point with other access points in the area, especially on
the same or neighboring channel, can prevent access and
interfere with the use of other access points by others caused
by overlapping channels in the 802.11g/b spectrum as well as
with decreased
signal-to-noise ratio (SNR) between access points. This can
be a problem in high-density areas such as large apartment
complexes or office buildings with many Wi-Fi access points.
Additionally, other devices use the 2.4 GHz band: microwave
ovens, cordless phones, baby monitors, security cameras, and
Bluetooth devices can cause significant additional interference.
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It is also an issue when municipalities[3]
or other large entities such as universities seek to provide
large area coverage. Everyone is considered equal for the base
standard without 802.11e/WMM when they use the band. This
openness is also important to the success and widespread use of
2.4 GHz Wi-Fi, but makes it unsuitable for "must have" public
service functions or where reliability is required. Users
sometimes suffer network "frustrations" or a total network
breakdown if gaming because a neighbour microwaves some pop
corn.
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Interoperability issues between brands or
proprietary deviations from the standard can disrupt connections
or lower throughput speeds on other user's devices that are
within range. And, Wi-Fi devices do not presently pick channels
to avoid interference.
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Wi-Fi networks that are open (unencrypted)
can be monitored and used to read and copy data (including
personal information) transmitted over the network unless
another security method is used to secure the data like a VPN or
a secure web page.
Confusion with piggybackingg
Legality
Ethical considerations
Wireless network security
Overview
Uses
Technical Information
Devices
Social Concerns
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