sept 18, 20021 wireless lan final presentation csuf, computer science dept. cynthia michele vincent...

Sept 18, 2002 1

Wireless LANWireless LAN

Final PresentationCSUF, Computer Science

Dept.Cynthia Michele Vincent

CPSC 589 Section 1 Fall [email protected]

1 minute

2

IntroductionIntroduction

Wireless LANs– Became popular only in 2000– Still in its infancy, but it’s

growing rapidly– Technologies available since

1940’s– Most popular wireless networking

products use Radio Frequency

3

ObjectivesObjectives

Discuss the different media that are used in Wireless LANs to transmit data

Demonstrate the techniques used to send radio signals in Wireless LANs

Explain the communication protocols or media access control

Present the different topologies or configurations

4

OutlineOutline

Electromagnetic Spectrum Radio vs. Infrared Spread Spectrum Media Access Control Different Configurations Distribution System Frame Format Wireless LAN Design Conclusion

5

Electromagnetic SpectrumElectromagnetic Spectrum

Radio Infrared UVMicrowave Gamma ray

f (Hz)

FM

Coax

Satellite

TV

AM Terrestrial microwave

Fiber optics

X ray

100

104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016

102 106 108 1010 1012 1014 1016 1018 1020 1022 1024104

Figure taken from ...

Larry Peterson, “Computer Networks: A Systems’ Approach”

6

MicrowaveMicrowave

10 Mbps – 15 Mbps 18 GHz Range Needs License from FCC Expensive Can penetrate through haze, light

rain and snow, clouds, or smoke Motorola’s ALTAIR Not commonly deployed

7

InfraredInfrared

No license required 3 kinds of infrared

– Directed•Remote control

– Diffused• Indoor Wireless LAN

– Focused•Building to Building Wireless LAN

8

Radio FrequencyRadio Frequency

Most popular media that can is used for data transmission over WLANs

No license required2.4 GHz1-2 Mbps (11 Mbps)Can be easily modulated

9

Radio Frequency Vs. InfraredRadio Frequency Vs. Infrared

RadioDiffused Infrared

Focused Infrared

Bandwidth LowHigh but shared

High

Reliability More Less Less

Distances Longer Shorter Much Longer

Security Less More Lesser

Power consumption High Low Higher

Sensitivity to atmosphere Low N/A High

Vulnerability to interference Yes No Yes

RequirementsSpread

SpectrumNo line of

sightNo line of

sight

10

Spread Spectrum TechnologySpread Spectrum Technology

Spread-Spectrum Technology– “it works by using mathematical

functions to diffuse signal power over a large range of frequencies” Gast, M.

– 2 kinds•FHSS (Frequency Hopping Spread

Spectrum)•DSSS (Direct Sequence Spread

Spectrum)

11


Frequency Hopping Spread Spectrum (FHSS)– “FH systems jump from one

frequency to another in a random pattern, transmitting a short burst ... “ Gast, M.

– Sender and receiver use the same•Pseudorandom algorithm•Seed

12


FHSS (Cont’d)

Hopping Pattern{1, 6, 4, 5}

0 1 2 3 4 5

Time Slot

1

2

3

4

5

6

7

8

Fre

qu

ency

Slo

t

13


Direct Sequence Spread Spectrum (DSSS)– “Direct Sequence systems spread

the power out over a wider frequency band using mathematical coding functions.” Gast, M.

– Sender and receiver use the same•Pseudorandom algorithm•Seed

14


DSSS (Cont’d)original bits

1010

n-bit1101001001111000

XOR0010001010001000

1111000011110000

1101001001111000

0010001010001000

random n-bitoriginal bits (padded)

result of XOR

Transmit this instead of the original bits

15

Media Access ControlMedia Access Control

Wired Networks– CSMA/CD (Carrier Sense

Media Access / Collision Detection)

• Wait until line is idle before transmitting

Wireless LANs– CSMA/CA (Carrier Sense Media

Access / Collision Avoidance)• Must request before transmitting


http://alpha.fdu.edu/~kanoksri/IEEE80211b.html

16

Media Access Control (Cont’d)Media Access Control (Cont’d)


http://alpha.fdu.edu/~kanoksri/IEEE80211b.html

In wireless LANs we cannot assume that stations hear one another.

When a station senses that the medium is free, it doesn’t necessarily mean it’s free around the receiver’s area.

17

Why can’t we use CSMA/CD in wireless LANs?

– Hidden Node Problem• Scenario: A and C sends to B• A is hidden to C and vice versa• Result: Collision

– Exposed Node Problem• Scenario: B sends to A• B is exposed to C• Result: C falsely concludes he can’t transmit

CSMA/CDCSMA/CD

A B C D

A B C D

18

CSMA/CACSMA/CA

Did we solve the problems by using CSMA/CA for Wireless LANs?

– Hidden Node Problem• Scenario: A and C sends to B• C hears CTS, so silences• Result: Collision Avoided

– Exposed Node Problem• Scenario: B sends to A• C doesn’t hear CTS, so transmits to D• Result: C doesn’t falsely conclude he can’t transmit to D

A B C D

A B C D

19

Different ConfigurationsDifferent Configurations

Ad-hoc– No access point– 2 or more nodes

Basic Service Set– Exactly 1 access point + at least

1 nodeExtended Service Set

– Multiple Basic Service Sets

20

Ad-hoc ConfigurationAd-hoc Configuration

Wireless LAN Network

21

Basic Service SetBasic Service Set

Existing Network(Wired LAN)

Wireless LAN Network

ISP

Access Point

Backbone

22

Extended Service SetsExtended Service Sets

ISP

AP2

AP1 AP3

Note:“AP” = Access Point

User1

User3

23

Distribution SystemDistribution System

AP2AP1 AP3

DISTRIBUTION SYSTEM

User1 User3

No Interoperability among APs

802.11 defines only scanning and roaming

24

Frame FormatFrame Format

Addr4 PayloadSeqCtrlAddr3Addr2Addr1 CRC

0– 18,4964816 32484848

Duration

16

Control

16


Larry Peterson, “Computer Networks: A Systems’ Approach”

Version Type Sub-Type

ToDS FromDS

MoreFrag

Retry PowerMngt

MoreData

WEP Order

2 2 4 1 1 1 1 1 1 1 1bits

bits

25

Frame Format (Cont’d)Frame Format (Cont’d)


0– 18,4964816 32484848

Duration

16

Control

16

Duration – the time that the medium is expected to remain busy for the transmission currently in progress

SeqCtrl -- sequence number to recognize packet duplications

Payload -- data that is transmitted or received

CRC (Cyclic Redundancy Check)

26


0– 18,4964816 32484848

Duration

16

Control

16

Frame Format (Cont’d)Frame Format (Cont’d)

Addr1 = AddrOfDestinationNode

Addr2 = AddrOfSourceNode

AP2

Destination

AP1

Source

Addr1 = AddrOfDestinationNode

Addr2 = AccPtOfDestinationNode

Addr3 = AccPtOfSourceNode

Addr4 = AddrOfSourceNode

If( ToDS & FromDS == 1)If( ToDS & FromDS == 0)

27

Wireless LAN DesignWireless LAN Design

Note: VLAN = Virtual LAN

Students Professors

VLAN1 VLAN2 VLAN3

AdminWLAN WLANLAN

Port 1

Port 2Port 3

Port 1Port 2Port 3

Port 1Port 2Port 3

28

Wireless LAN Software DesignWireless LAN Software Design

EncryptIt App


0– 18,4964816 32484848

Duration

16

Control

16

Encrypted

29

ConclusionConclusion

Wireless LANs– Continuing to develop– Interoperability between access

points – Signaling standard– Improve security and access– Technologies and ideas are out

there!

30

ReferencesReferences[1] A. Tanenbaum, “Computer Networks”, Prentice Hall, 3rd Edition, pp. 262-265, 94-101,

March 1996.[2] J. Larocca and R. Larocca, “802.11 Demystified”, Mc-Graw Hill, pp. 3-48, June 2002 [3] C. Smith and D. Collins, “3G Wireless Networks”, McGraw-Hill, pp. 135-140, Sept 2001.[4] L. Peterson and B. Davie, “Computer Networks: A Systems Approach”, Morgan

Kaufmann Publishers, 2nd Edition, pp. 136-142, October 1999.[5] M. Gast, “802.11 Wireless Networks”, O’Reilly, 1st Edition, pp 293-328, April 2002[6] P. Gralla, “How Wireless Works”, Que, pp. 10-22, Nov. 2001[7] R. Flickenger, “Building Wireless Community Networks”, O’Reilly and Associates, pp.

31, Nov 2001 [8] J. Bhola, “Wireless LANS Demystified”, McGraw-Hill, pp. 25, 29-31, 46-47, 49-50, 52,

54, 2002.[9] “Spread Spectrum Radios and RF Safety” – A White Paper on

http://64.4.16.250/cgi-bin/linkrd?_lang=EN&lah=4d9af06ec5de065e32b4fa7c55622bcd&lat=1031946487&hm___action=http%3a%2f%2fwww%2ecisco%2ecom%2fwarp%2fpublic%2fcc%2fpd%2fwitc%2fao340ap%2fprodlit%2findex%2eshtml Cisco Systems, August 2002.

[10] M. Foster, “Wireless Local Area Networking: An Introduction” – An article on http://www4.tomshardware.com/network/01q3/010822/index.html , August 2001.

[11] “What is a Wireless LAN?” – A White Paper on http://www.proxim.com/learn/library/whitepapers/wp2001-06-what.html, Proxim Company, February 2001.

[12] “Infrared LAN Technology” – An article on http://www.jtap.ac.uk/reports/htm/jtap-014-5.html, James Dearden.

[13] “"High Rate" Wireless Local Area Networks” – An article on http://alpha.fdu.edu/~kanoksri/IEEE80211b.html, Kanoksri Sarinnapakorn, March 2001.

sept 18, 20021 wireless lan final presentation csuf, computer science dept. cynthia michele vincent...

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