Wi-fi

Wi-Fi

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Wi-Fi (Wireless-Fidelity) is a wireless technology brand owned by the Wi-Fi Alliance intended to improve the interoperability of wireless local area network products based on the IEEE 802.11 standards. Common applications for Wi-Fi include Internet, VOIP etc., Official Wi-Fi logo 2

802.11 and Wi-Fi 

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“IEEE 802.11” is a set of industry standards for the operation of wireless networks. Wi-Fi (“Wireless Fidelity”) is a marketing term. 

Synonymous with 802.11

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802.11: Several Flavors

1999:

2001:

2003:

802.11b

802.11a

802.11g

(11/5 Mbits/s)

(54/32 Mbits/s)

(54/32 Mbits/s)

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Wi-Fi “Access Points” 

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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 “hot spot”.

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WiFi Network Topologies 

Ad Hoc Network This is a peer to peer network.

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Infrastructure Network This network allows to access various wired networks. This requires a base station called Access Point.

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802.11b 

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Channel Center Frequency (GHz)

The 802.11b standard defines a total of 14 frequency channels. Channel represents a center frequency. Only 5 MHz separation between center frequencies of channels. 5 MHz 1 2.412

2

3

4

2.417

2.422

2.427

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6

7

2.432

2.437

2.442

8 2.447

9 2.452

10

11

2.457

2.462

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Ideally, 802.11b supports wireless connections between 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 connection speed decreases (falls back). 2 Mbps 5.5 Mbps 11 Mbps 8

802.11a 

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802.11a specification operates at radio frequencies between 5.15 and 5.825 GHz The FCC has divided total 300 MHz in this band into three distinct 100 MHz bands: low, middle, and high, each with different legal maximum power.

High band

Band 5.725-5.825 GHz

Channel 9-12

Max Power 1000 mW

Middle band 5.25-5.35 GHz

5-8

250 mW

Low band

1-4

50 mW

5.15-5.25 GHz

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Because of high power output, high band used for building-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 Division Multiplexing (OFDM). 802.11a (like 11b) has a rate fall back mechanism, i.e., as the distance between the transmitter and receiver increases, the supported data rate decreases.

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802.11a 12 Mbps 24 Mbps 36 Mbps 48 Mbps 54 Mbps

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802.11g 

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802.11g offers throughput of 802.11a with backward compatibility of 802.11b 802.11g operates in 2.4 GHz band but it delivers data rates from 6 Mbps to 54 Mbps. 802.11g also uses OFDM but supports spreadspectrum capabilities if any one component of the system has older equipment, i.e., 802.11b equipment.

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Wireless Technologies Technolog y

Freque ncy

Range

Speed

802.11a

5GHz

25 to 75 feet indoors

Up to 54Mbps

802.11b

2.4GHz

Up to 300 feet indoors

Up to 11 Mbps

802.11g

2.4GHz

Up to 150 feet indoors

Up to 54 Mbps

802.16 WiMax

10-66 GHz

35 miles

75Mbps, if range is <4 miles 50Mbps, if range is 4-6 miles range 17Mbps, if is >6 miles 13

802.11 MAC Layer 

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802.11 makes use of CSMA/CA (Carrier sense multiple access with collision avoidance) A CSMA/CD works as (in LAN) : A station desires to transmit, senses the medium, if the medium is busy, then the station will defer its transmission to a later time, if the medium is sensed free then the station is allowed to transmit. Two modes of operation are supported by CSMA/CA.

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In the first method, when a station wants to transmit, it senses the channel. If the channel is idle, it starts transmitting else it waits until the channel is idle. The second method makes use of virtual carrier sensing. 15

Virtual Carrier Sense A station willing to transmit sends a short control packet called RTS (Request to Send) which includes source, destination and duration of the following transaction. Then the Access Point will send a response control packet, CTS (Clear To Send) which consists the same information.

Source

Access Point

RTS

Data

CTS

ACK

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Access Point

Laptop RTS CTS

Data ACK 17

How WiFi Works? 

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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 Probe request to find the AP and waiting for the Probe response from AP

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WLAN Discovery 

“Open” WLANs

RESPONS  SSID E: SSID OPEN

RESPONS E: SSID

Active Scanning

“Closed” WLANs 

CLOSE D

Access Point continuously transmits Passive “Beacon Frames” containing Scanning network’s SSID.

{

PROBE REQUEST (ANY) PROBE REQUEST SSID (SSID)

Beacon frames do not contain and/or beacons frames are turned off.19

Wi-Fi Privacy 

Problem: Your radio transmissions can be picked up by anyone. Laptop (Wireless PC Card)

Access Point

Solution:

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Wi-Fi privacy is provided by using some keys called as Wired Equivalent Privacy (WEP) Wi-Fi Protected Access (WPA)

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Wi-Fi Privacy: Observations

38%: WEP 62%: Completely open

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 cell membership 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 

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Association: This service is used by mobile stations to connect themselves to the base stations. Disassociation: Either station or the base station may use this service before shutting down or leaving. Reassociation: This process of moving a station from 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 non802.11 network with different format, this service handles the format translation.

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Station Services 

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There are 4 station services which are intracell. They are used after association has taken place and are as follows. Authentication: Because wireless communication can easily be sent or received by unauthorized stations, a station must authenticate itself before it is permitted to send data. After a mobile station has been associated by the base station, the base station sends a challenge frame to see if the mobile knows the secret key (WEP) 25

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The mobile is supposed to encrypt the challenge frame with the key and send back to the base station. If the result is correct then the mobile is enrolled in the cell.

Authentication Frame

Device NIC

Encrypted Text (using WEP key) Decrypt text using correct WEP key

Authentication Frame (Accept/Reject)

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Deauthentication: When a previously authenticated station wants to leave the network, it is deauthenticated.

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Privacy: For information sent over a wireless LAN to be kept confidential, it must be encrypted. This service manages encryption and decryption.

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Data delivery: Finally, data transmission is what it is all about, so this service handles this function. 27

Examples Virtual Guide in Shopping Mall 

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Virtual Guide in Shopping Mall is an application which guide the a customer who comes to a big shopping mall. As soon as he enter’s into shopping mall a virtual character comes on his wireless terminal and assist you through out your shopping

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Examples Friendly Job Fair 

Friendly Job Fair is a WiFi based application which allow the job seekers various information such as which companies are participation in job fair what are their requirements and what is the selection process for various job levels. This also provide you in which stalls which companies are their.

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Patient Monitoring System on Tips 

This is an application which allows the doctor’s to medicate the patients without the help of nurses by getting their details instantly through their wireless terminals. A doctor no needs to depend on nurses for the patient condition. All the details of the patients are maintained by the server which can be accessed by wireless terminals such as PDA.

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