Wireless LAN

1. CPSC 589 Section 1 Fall 02 c_m_vincent@yahoo.com Wireless LAN Final Presentation CSUF, Computer Science Dept. Cynthia Michele Vincent 1 minute

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

5. Electromagnetic Spectrum Figure taken from ... Larry Peterson, “Computer Networks: A Systems’ Approach”

6. Microwave • 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. Infrared • No license required • 3 kinds of infrared • Directed • Remote control • Diffused • Indoor Wireless LAN • Focused • Building to Building Wireless LAN

8. Radio Frequency • Most popular media that can is used for data transmission over WLANs • No license required • 2.4 GHz • 1-2 Mbps (11 Mbps) • Can be easily modulated

9. Radio Frequency Vs. Infrared

10. Spread 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 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 Technology • FHSS (Cont’d) Hopping Pattern {1, 6, 4, 5}

13. Spread 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 Technology • DSSS (Cont’d) original bits (padded) random n-bit result of XOR Transmit this instead of the original bits

15. Media 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) • 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. Figure taken from ... http://alpha.fdu.edu/~kanoksri/IEEE80211b.html

17. CSMA/CD • 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 A B C D A B C D

18. CSMA/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 Configurations • Ad-hoc • No access point • 2 or more nodes • Basic Service Set • Exactly 1 access point + at least 1 node • Extended Service Set • Multiple Basic Service Sets

20. Ad-hoc Configuration Wireless LAN Network

21. Basic Service Set Existing Network (Wired LAN) ISP Backbone Access Point Wireless LAN Network

22. Extended Service Sets ISP AP1 AP3 User3 AP2 User1 Note: “AP” = Access Point

23. Distribution System DISTRIBUTION SYSTEM AP1 AP2 AP3 User1 User3 • No Interoperability among APs • 802.11 defines only scanning and roaming

24. Frame Format bits More More Sub- From Power WEP Version Type ToDS Retry Order DS Frag Mngt Data Type bits 2 2 4 1 1 1 1 1 1 1 1 Figure taken from ... Larry Peterson, “Computer Networks: A Systems’ Approach”

25. Frame Format (Cont’d) • 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. Frame Format (Cont’d) Addr1 = AddrOfDestinationNode Addr2 = AddrOfSourceNode AP2 AP1 Source Destination If( ToDS & FromDS == 1) If( ToDS & FromDS == 0) Addr1 = AddrOfDestinationNode Addr2 = AccPtOfDestinationNode Addr3 = AccPtOfSourceNode Addr4 = AddrOfSourceNode

27. WLAN WLAN LAN Professors Students Admin Port 1 Port 2 Port 3 Port 1 Port 2 Port 3 Port 1 Port 2 Port 3 VLAN1 VLAN2 VLAN3 Wireless LAN Design Note: VLAN = Virtual LAN

28. Wireless LAN Software Design Encrypt It App Encrypted

29. Conclusion • Wireless LANs • Continuing to develop • Interoperability between access points • Signaling standard • Improve security and access • Technologies and ideas are out there!

30. References [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 Radiosand 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.