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Physical Layer

Wireless LANs June – September 2005. Physical Layer. ผศ. ดร . อนันต์ ผลเพิ่ม Asst. Prof. Anan Phonphoem, Ph.D. anan@cpe.ku.ac.th http://www.cpe.ku.ac.th/~anan Computer Engineering Department Kasetsart University, Bangkok, Thailand. Outline. Physical Layer Architecture

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Physical Layer

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  1. Wireless LANsJune – September 2005 Physical Layer ผศ. ดร. อนันต์ผลเพิ่ม Asst. Prof. Anan Phonphoem, Ph.D. anan@cpe.ku.ac.th http://www.cpe.ku.ac.th/~anan Computer Engineering Department Kasetsart University, Bangkok, Thailand

  2. Outline • Physical Layer Architecture • Physical Layer Operations • IEEE 802.11 Physical Layer • FHSS • DSSS • IR

  3. Physical Layer Architecture MAC Layer PHY SAP Physical Layer PLCP Sublayer PMD SAP PMD Sublayer

  4. PLCP Sublayer • Physical Layer Convergence Procedure • Communicate to MAC via primitives through Physical Layer Service Access Point (SAP) • Prepare PLCP protocol data unit (PPDU) (append fields to MPDU) • PPDU provides for asynchronous transfer of MPDU between stations

  5. PMD Sublayer • Physical Medium Dependent • Provide actual transmission and reception of Physical Layer entities via wireless medium • Interface directly to the medium • Provides modulation and demodulation of the transmission frame

  6. Physical Layer Operations • 3 State machines • Carrier Senses: determine the state of the medium • Transmit: send the data frame • Receive: receive the data frame

  7. Physical Layer Service Primitives

  8. MAC PMD PLCP Carrier Sense Function • Station is not in Tx • or Rx mode • Clear channel • assessment Check medium Carrier Sense info • Medium Idle PHY-CCA.indicate • Clear channel • assessment Check medium Carrier Sense info • Medium Busy PHY-CCA.indicate • Check preamble • monitor header Carrier Sense info • Try to synchronize

  9. MAC PLCP PMD … … PHY-DATA.request Pass Data PHY-DATA.confirm Transmit Function PHY-TXSTART.request Switch to TX mode PHY-TXSTART.confirm • Switch to TX mode • Sending preamble & header to antenna @ 1 Mbps PHY-DATA.request Pass Data PHY-DATA.confirm • Transmit data @ specified rate … PHY-TXEND.request Switch to RX mode PHY-TXEND.confirm • Switch to RX mode

  10. MAC PMD PLCP PHY-DATA.indication DATA … … PHY-DATA.indication DATA Receive Function • Clear channel • assessment found • media busy Carrier Sense info • Check preamble • monitor header • Check Power level • > 85 dBm PHY-RXSTART.indication • Check CRC • Set octet counter … • Final Octet PHY-RXEND.indication

  11. Multiple Antenna Diversities • Receive function will operate with • Single Antenna • Multiple Antennas • Signal Degradation Factors • Distance • Atmosphere • Barrier • Multiple-path propagation • Decrease the signal strength • Use multiple antennas (diversity) to improve the received signal

  12. IEEE 802.11 PHY Layer • FHSS Physical Layer • DSSS Physical Layer • Infrared (IR) Physical Layer

  13. FHSS Physical Layer • Low cost • Low power consumption • Most tolerant to noise • Low potential data rate • Medium range (< DSSS)

  14. FHSS Architecture • FHSS PLCP Sublayer • FHSS PMD Sublayer • Primitives

  15. PLCP Preamble PLCP Header 80 bits 16 bits 12 bits 4 bits 16 bits 0-4095 Octets SYNC Start Frame Delimiter PLW PSF Header Error Check Whitened PSDU Payload (MPDU) Reduce DC bias, scramble PLCP Signaling Field: data rate (1- 4.5 Mbps) 16-bit CRC 0000110010111101: define the beginning of a frame PSDU Length word 0 & 1 alternating : synchronization purpose FHSS PLCP frame PSDU=PLCP Service Data Unit

  16. FHSS PMD • Perform actual Tx/Rx of PPDU by hopping between channel (hopping sequence) • Provides FHSS modulation/demodulation

  17. FHSS PMD Service Primitives

  18. DSSS Physical Layer • High cost • High power consumption • High potential data rate • More range

  19. DSSS Architecture • DSSS PLCP Sublayer • DSSS PMD Sublayer • Primitives

  20. PLCP Preamble PLCP Header 8 bits 128 bits 16 bits 8 bits 8 bits 16 bits SYNC Start Frame Delimiter Signal Service Length Frame Check Sequence MPDU 16-bit CRC Reserved #microsec. To transmit the MPDU 1111001110100000: define the beginning of a frame Modulation type: data rate 0 & 1 alternating : synchronization purpose DSSS PLCP frame

  21. DSSS PMD • Perform actual Tx/Rx of PPDU • Provides DSSS modulation/demodulation

  22. DSSS PMD Service Primitives

  23. IR Physical Layer • Lowest cost • Highest tolerant to RF noise • Lowest range • Need ceiling • More secure • No frequency regulating • No product ? • IrDA: Infrared Data Association Standard

  24. IR Architecture • IR PLCP Sublayer • IR PMD Sublayer

  25. PLCP Preamble PLCP Header 57-73 slots 4 slots 3 slots 32 slots 16 slots 16 slots 0-2500 octets SYNC Start Frame Delimiter Data Rate DC Level Adjust. Length Frame Check Sequence MPDU 16-bit CRC Specified for 1 and 2 Mbps #microsec. To transmit the MPDU 1001: define the beginning of a frame Data rate Pulse alternating : synchronization purpose IR PLCP frame

  26. IR PMD • Mostly use diffused infrared • Perform actual Tx/Rx of PPDU, translate binary to infrared light • Provides IR modulation/demodulation

  27. IR PMD • Noise affects amplitude (not phase)  Pulse position reduces interference • Pulse position modulation :PPM • Vary position of pulse • For 1 Mbps  16 PPM • For 2 Mbps  4 PPM

  28. Pulse Position Modulation

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