sept 18, 20021 wireless lan final presentation csuf, computer science dept. cynthia michele vincent...
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Sept 18, 2002 1
Wireless LANWireless LAN
Final PresentationCSUF, Computer Science
Dept.Cynthia Michele Vincent
CPSC 589 Section 1 Fall 02c_m_vincent@yahoo.com
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
Spread Spectrum TechnologySpread Spectrum Technology
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
Spread Spectrum TechnologySpread Spectrum Technology
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
Spread Spectrum TechnologySpread Spectrum Technology
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
Spread Spectrum TechnologySpread Spectrum Technology
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
Figure taken from ...
http://alpha.fdu.edu/~kanoksri/IEEE80211b.html
16
Media Access Control (Cont’d)Media Access Control (Cont’d)
Figure taken from ...
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
21
Basic Service SetBasic Service Set
Existing Network(Wired LAN)
Wireless LAN Network
ISP
Access Point
Backbone
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
Figure taken from ...
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)
Addr4 PayloadSeqCtrlAddr3Addr2Addr1 CRC
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
Addr4 PayloadSeqCtrlAddr3Addr2Addr1 CRC
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
Addr4 PayloadSeqCtrlAddr3Addr2Addr1 CRC
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.
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