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Wi-Fi
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Wi-Fi (Wireless-Fidelity) is a wirelesstechnology brand owned by the Wi-Fi Alliance
intended to improve the interoperability ofwireless local area network products basedon the IEEE 802.11 standards.
Common applications for Wi-Fi includeInternet, VOIP etc.,
Official Wi-Fi logo
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802.11 and Wi-FiIEEE 802.11 is a set of industry
standards for the operation of wirelessnetworks.
Wi-Fi (Wireless Fidelity) is amarketing term.
Synonymous with 802.11
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802.11: Several Flavors
1999:
802.11b(11/5 Mbits/s)
2001:
802.11a(54/32 Mbits/s)
2003:
802.11g(54/32 Mbits/s)
compatible
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Wi-Fi Access Points A Wi-Fi access point is a blackbox that
connects to the Internet, and also accepts
Wi-Fi connections. The area covered by one or several
interconnected access points is called hotspot.
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WiFi Network Topologies Ad Hoc Network
This is a peer to peer
network.
Infrastructure Network
This network allows to
access various wirednetworks.This requires abase station called
Access Point.
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802.11b The 802.11b standard defines a total of
14 frequency channels.
Channel represents a center frequency.Only 5 MHz separation between centerfrequencies of channels.
21 3 4 5 6 7 8 9 10 11
2.412 2.417 2.422 2.427 2.432 2.437 2.442 2.447 2.452 2.457 2.462
Channel
Center
Frequency
(GHz)
5 MHz
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Ideally, 802.11b supports wireless connectionsbetween an access point and a wireless device at
four possible data rates: 1 Mbps, 2 Mbps, 5.5 Mbps,and 11 Mbps.
Specifically, as terminal travels farther from its AP,the connection will remain intact but connectionspeed decreases (falls back).
11 Mbps
5.5 Mbps2 Mbps
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802.11a 802.11a specification operates at radio frequencies
between 5.15 and 5.825 GHz
The FCC has divided total 300 MHz in this band intothree distinct 100 MHz bands: low, middle, and high,each with different legal maximum power.
Band Channel Max Power
High band 5.725-5.825 GHz 9-12 1000 mW
Middle band 5.25-5.35 GHz 5-8 250 mW
Low band 5.15-5.25 GHz 1-4 50 mW
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Because of high power output, high band used forbuilding-to-building products. Lower two bands
suitable for in-building wireless products. In 802.11a, radio signals are generated using a
method called Orthogonal Frequency DivisionMultiplexing (OFDM).
802.11a (like 11b) has a rate fall back mechanism,i.e., as the distance between the transmitter andreceiver increases, the supported data ratedecreases.
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54 Mbps
48 Mbps
36 Mbps
24 Mbps
12 Mbps
802.11a
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802.11g 802.11g offers throughput of 802.11a with
backward compatibility of 802.11b
802.11g operates in 2.4 GHz band but itdelivers data rates from 6 Mbps to 54 Mbps.
802.11g also uses OFDM but supports
spread-spectrum capabilities if any onecomponent of the system has olderequipment, i.e., 802.11b equipment.
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Wireless Technologies
Up to 54Mbps
Up to 11 Mbps
Up to 54 Mbps
75Mbps, if range is 6 miles
25 to 75 feetindoors
Up to 300 feetindoors
Up to 150 feetindoors
35 miles
5GHz
2.4GHz
2.4GHz
10-66 GHz
802.11a
802.11b
802.11g
802.16 WiMax
SpeedRangeFrequency
Technology
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802.11 MAC Layer 802.11 makes use ofCSMA/CA (Carrier sense
multiple access with collision avoidance)
A CSMA/CD works as (in LAN) : A station desires totransmit, senses the medium, if the medium is busy,then the station will defer its transmission to a latertime, if the medium is sensed free then the station isallowed to transmit.
Two modes of operation are supported by CSMA/CA.
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In the first method, when a stationwants to transmit, it senses the
channel. If the channel is idle, it starts
transmitting else it waits until thechannel is idle.
The second method makes use ofvirtual carrier sensing.
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Virtual Carrier Sense
RTS
CTS
Data
ACK
Source
Access Point
A station willing to transmit sends a short control packet called RTS(Request to Send) which includes source, destination and duration of thefollowing transaction. Then the Access Point will send a response controlpacket, CTS (Clear To Send) which consists the same information.
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Access
Point Laptop
RTS
CTS
Data
ACK
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How WiFi Works? When a station wants to join the Cell, it can
know the synchronization information from
the Access Point in two ways Passive scanning waits to receive
Beacon Frames from the AP
Active scanning transmits the Proberequest to find the AP and waiting for theProbe response from AP
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WLAN DiscoveryOpen WLANs
Access Point continuously transmits
Beacon Frames containing networksSSID.
Closed WLANs Beacon frames do not contain SSID and/or
beacons frames are turned off.
SSID
RESPONSE:SSID
PROBEREQUEST(SSID)
Passive Scanning
Active Scanning
PROBEREQUEST
(ANY)
RESPONSE:SSID
PROBEREQUEST
(ANY)
OPEN
CLOSED
PROBEREQUEST
(ANY)
PROBEREQUEST
(ANY)
{
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Wi-Fi Privacy Problem: Your radio transmissions can be
picked up by anyone.
Access PointAccess PointLaptopLaptop(Wireless PCCard)(Wireless PCCard)
Solution: Encryption
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Wi-Fi privacy is provided by using somekeys called as
Wired EquivalentPrivacy (WEP)
Wi-Fi ProtectedAccess (WPA)
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Wi-Fi Privacy: Observations
38%:WEP
62%:Completelyopen
Source: http://www.worldwidewardrive.org/wwwdstats.html
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Services The services provided by 802.11 are divided in to two
categories as
Distribution Services Station Services
Distribution services relate to managing cellmembership and interacting with stations outside
the cell.In contrast, the station services relate to activity
with in a single cell.
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Distribution Services Association: This service is used by mobile stations
to connect themselves to the base stations.
Disassociation: Either station or the base stationmay use this service before shutting down or leaving.
Reassociation: This process of moving a stationfrom old base station to new base station.
Distribution: This service determines how to route
frames sent to base station. Integration: If a frame needs to be send through a
non-802.11 network with different format, thisservice handles the format translation.
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Station Services There are 4 station services which are intracell.They
are used after association has taken place and are asfollows.
Authentication:
Because wireless communication can easily be sentor received by unauthorized stations, a station mustauthenticate itself before it is permitted to send data.
After a mobile station has been associated by thebase station, the base station sends a challengeframe to see if the mobile knows the secret key(WEP)
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The mobile is supposed to encrypt the challengeframe with the key and send back to the basestation.
If the result is correct then the mobile is enrolled inthe cell.
Authentication Frame
Encrypted Text (using WEP key)
Authentication Frame (Accept/Reject)
Decrypt text using correct WEPkey
Device
NIC
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Deauthentication: When a previouslyauthenticated station wants to leave the network, itis deauthenticated.
Privacy: For information sent over a wireless LAN tobe kept confidential, it must be encrypted.Thisservice manages encryption and decryption.
Data delivery: Finally, data transmission is what itis all about, so this service handles this function.
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ExamplesVirtual Guide inShopping Mall
Virtual Guide in Shopping Mall is an applicationwhich guide the a customer who comes to a big
shopping mall.
As soon as he enters into shopping mall a virtualcharacter comes on his wireless terminal and assistyou through out your shopping
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ExamplesFriendly Job Fair
Friendly Job Fair is a WiFibased application which
allow the job seekers variousinformation such as whichcompanies are participationin job fair what are theirrequirements and what isthe selection process for
various job levels.This alsoprovide you in which stallswhich companies are their.
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Patient MonitoringSystem onTips
This is an application which allows the doctors to
medicate the patients without the help of nurses bygetting their details instantly through their wirelessterminals. A doctor no needs to depend on nurses forthe patient condition. All the details of the patientsare maintained by the server which can be accessedby wireless terminals such as PDA.