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Ch 14. Wireless LANs

Ch 14. Wireless LANs. 14.1 IEEE 802.11. Specification for a wireless LAN Cover physical and data link layers Basic service sets (BSS) and extended service set (ESS). BSS. ESS. MAC Sublayer Architecture. MAC in IEEE 802.11 standard. Distributed Coordination Function (DCF).

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Ch 14. Wireless LANs

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  1. Ch 14. Wireless LANs

  2. 14.1 IEEE 802.11 • Specification for a wireless LAN • Cover physical and data link layers • Basic service sets (BSS) and extended service set (ESS) BSS ESS

  3. MAC Sublayer Architecture • MAC in IEEE 802.11 standard

  4. Distributed Coordination Function (DCF) • CSMA/CA – why not CSMA/CD? • A wireless station may not send data and receive collision signals at the same time • Collision may not be detected due to the hidden station problem (will discuss later) • Signal fading prevents a station of one end from hearing a collision at the other end • Control frames • Request-To-Send (RTS): sent by the sender when it wants send a frame • Clear-To-Send (CTS): sent by the receiver when it receives an RTS and can receive the data

  5. RTS/CTS Exchanges of DCF • Network Allocation Vector (NAV) • RTS/CTS includes the duration of data transmission time • Stations affected by the transmission create a NAV timer to wait the end of the transmission

  6. PCF • Point coordination function (PCF) • Centralized, contention-free “polling” access • Use a shorter inter-frame space (PIFS) • Useful for time-sensitive service

  7. Frame Format Duration Sequence number of the frame (RTS, CTS, ACK) Transmission type for addressing (See Fig. 14.9)

  8. Transmission Type for Addressing

  9. Control Frames • RTS/CTS and ACK

  10. Hidden Station Problem • Station C cannot hear the transmission of station B (to A), and vice versa

  11. Hidden Station Problem • Solved by RTS/CTS exchange

  12. Exposed Station Problem • Station C can send to station D, but it is exposed to the transmission of station A (to B)

  13. Exposed Station Problem • Cannot solved by RTS/CTS exchange Normally, C does not transmit an RTS here. The figure shows that even if C transmits an RTS, it does not solve the problem

  14. Physical Layers

  15. 14.2 Bluetooth • Bluetooth: a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on • Ad hoc network • Piconet and scatternet

  16. Bluetooth Layers (1) • Radio layer • 10m range • 1 Mbps data rate • 2.4GHz ISM band, FHSS, GFSK modulation

  17. Bluetooth Layers (2) Time for frequency hopping control • Baseband layer (MAC) • TDD-TDMA (Time Division Duplex TDMA) • Single-Secondary Communication • Multiple-Secondary Communication

  18. Homework • Exercise in Chapter 14 • 11 • 12

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