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CSC 581 Communication Networks II

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

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CSC 581 Communication Networks II

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

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

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

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

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

  13. Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Frame Body CRC Protocol Version Type Subtype To DS From DS More Frag Retry Pwr Mgt More Data WEP Rsvd To DS 0 1 0 1 0 From DS 1 1 0 BSSID Destination Address Receiver Address Destination Address Address 1 Transmitter Address Source Address Source Address Address 2 BSSID BSSID Destination Address Destination Address Source Address Address 3 Address 4 N/A N/A Source Address N/A 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 MAC Header (bytes) 2 2 6 6 6 2 6 0-2312 4 2 2 4 1 1 1 1 1 1 1 1 Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks Figure 6.67

  14. 802.11 MAC Timing

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

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

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

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

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

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

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

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

  23. 80 bits 16 12 4 16 Variable length Sync Start Frame Delimiter Length Signaling CRC Payload data PLCP header PLCP preamble Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks Figure 6.75

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

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

  26. 57-73 slots 4 3 16 Variable length 32 16 DC level adjust Sync Start frame delimiter Data rate Length CRC Payload data PLCP header PLCP preamble Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks Figure 6.78

  27. Reading Assignment • Section 6.6.4

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