1. CSC 581 Communication Networks II Chapter 6c: Local Area Network (Wireless LAN – 802.11) Dr. Cheer-Sun Yang

2. 2 Wireless LAN - Physical Infrared –1Mbps and 2Mbps –Wavelength 850-950nm Direct sequence spread spectrum –2.4GHz ISM band –Up to 7 channels –Each 1Mbps or 2Mbps Frequency hopping spread spectrum –2.4GHz ISM band –1Mbps or 2Mbps Others under development

3. 3 Challenges Radio and infread transmission is susceptible to noise and interference. The strength of a radio transmission varies in time and in space and so coverage is inconsistent and unpredictable. Radio signals can be evedroped. Radio spectrum is limited. Radio spectrum has traditionally been regulated by government. It can be difficult to design products for a global market.

4. 4 Motivations Mobility is desirable in many cases since portable computers are ubiquitous. For example, a doctor or nurse in a hospital accessing up-to-date information on a patient may not be able to log off and on frequently. It is beneficial to provide wireless points so that portable devices can communicate with each other via a backbone network. A conference participants may need to create a temporary ad hoc LAN.

5. 5 Ad Hoc Network A single BSS can be used to form an ad hoc network. An ad hoc network consists of a group of stations within range of each other. Ad hoc networks are typically temporary in nature.

6. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 6 B D C A Figure 6.65

7. 7 Hidden Station Problem Why not using wireless Ethernet using CSMA/CD? –It is difficult to detect collision in a radio environment. –Radio environment is not as well controlled as a broadcast medium and transmissions from users in other LANs can interfere with the operation of CSMA/CD. –Hidden station: Between A and C, there could be another station B. The transmissions of A and C can collide at the intermediate station B. –CSMA/CA is a solution to the hidden station problem.

8. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 8 Data Frame A transmits data frame B A B C C transmits data frame and collides with A at B (a) (b) C senses medium, station A is hidden from C Data Frame C A Figure 6.64

9. 9 Wireless LANs IEEE 802.11 incorporates CSMA/CA as the MAC layer protocol. Basic service set (cell) –Set of stations using same MAC protocol –Competing to access shared medium –May be isolated –May connect to backbone via access point (bridge) Extended service set –Two or more BSS connected by distributed system –Appears as single logic LAN to LLC level

10. 10 Types of station No transition –Stationary or moves within direct communication range of single BSS BSS transition –Moves between BSS within single ESS ESS transition –From a BSS in one ESS to a BSS in another ESS –Disruption of service likely

11. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 11 A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to Internet portal BSS A BSS B Figure 6.66

12. 12 MAC Frame Structure and Addressing Frame Header MAC Header Frame Body CRC Checksum

13. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 13 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Frame Body CRC Protocol Version TypeSubtype To DS From DS More Frag Retry Pwr Mgt More Data WEPRsvd 22666260-23124 To DS From DS Address 1 Address 2 Address 3 Address 4 00 Destination Address Source Address BSSIDN/A 01 Destination Address BSSID Source Address N/A 10BSSID Source Address Destination Address N/A 11 Receiver Address Transmitter Address Destination Address Source Address Meaning Data frame from station to station within a BSS Data frame exiting the DS Data frame destined for the DS WDS frame being distributed from AP to AP 2 2 MAC Header (bytes) 411111111 Figure 6.67

14. 14 802.11 MAC Timing

15. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 15 Physical Distribution coordination function (CSMA-CA) PCF Contention-free service Contention service MAC Figure 6.68

16. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 16 Busy Medium DIFS PIFS SIFS Contention Window Next Frame Defer Access Wait for Reattempt Time Time Figure 6.69

17. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 17 RTS CTS Data frame A requests to send B C A A sends B B C C remains quiet B announces A ok to send (a) (b) (c) Figure 6.70

18. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 18 Data DIFS SIFS Defer access Wait for reattempt time ACK DIFS NAV Source Destination Other Figure 6.71

19. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 19 Data SIFS Defer access Ack DIFS NAV (RTS) Source Destination Other RTS DIFS SIFS CTS SIFS NAV (CTS) NAV (Data) Figure 6.72

20. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 20 CF End NAV PIFS B D1 + Poll SIFS U 1 + ACK D2+Ack +Poll SIFS U 2 + ACK SIFS Contention-Free Repetition Interval Contention Period CF_Max_Duration Reset NAV D1, D2 = frame sent by Point Coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = Beacon Frame TBTT Figure 6.73

21. 21 Physical Layer Physical Layer Convergence Procedure (PLCP) Physical Medium Dependent (PMD)

22. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 22 Physical Layer LLC Physical Layer Convergence Procedure Physical Medium Dependent LLC PDU MAC SDU MAC Layer MAC HDR CRC PLCP PRMBL PLCP HDR PLCP PDU Figure 6.74

23. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 23 Sync Start Frame Delimiter LengthSignalingCRCPayload data 80 bits1612416Variable length PLCP preamble PLCP header Figure 6.75

24. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 24 11 chip Barker sequence: +1 +1 +1+1 -1 -1-1 11 symbol times To transmit +1, send: +1 +1 +1+1 -1 -1 -1 11 symbol times To transmit -1, send: +1 -1 -1 -1 +1 +1+1 +1 11 symbol times Figure 6.76

25. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 25 Sync Start frame delimiter SignalLengthCRCPayload data 128 bits168 8 Variable length PLCP preamble PLCP header Service 16 Figure 6.77

26. Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 26 Sync Start frame delimiter Data rate LengthCRCPayload data 57-73 slots43 32 16Variable length PLCP preamble PLCP header DC level adjust 16 Figure 6.78

27. 27 Reading Assignment Section 6.6.4