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Physical layer

Physical layer. 802.11a 802.11b 802.11n. IEEE 802.11 Standard Regarding Layer 1. The 802.11 standard is written by IEEE committees IEEE splits the Physical Layer (Layer 1) functions into two sub-layers: Physical Layer Convergence Procedure PLCP Physical Medium Sub-layer PMD.

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Physical layer

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  1. Physical layer 802.11a 802.11b 802.11n

  2. IEEE 802.11 Standard Regarding Layer 1 • The 802.11 standard is written by IEEE committees • IEEE splits the Physical Layer (Layer 1) functions into two sub-layers: • Physical Layer Convergence Procedure PLCP • Physical Medium Sub-layer PMD

  3. Physical Layer L2 • Upper layer queries L2 • Upper layer passes PDU down MAC Protocol Data Unit MPDU Add physical layer PHY preamble for Synch Physical Layer Convergence Procedure PLCP Preamble MAC Protocol Data Unit MPDU IEEE Layer 1 Physical Medium Sub-layer Modulation Preamble MAC Protocol Data Unit MPDU P-PDU

  4. Physical Layer in 802.11 • The physical layer, in 802.11, adds a PLCP to the layer 2 MPDU. • The PLCP contains layer-specific framing parameters • The PLCP format depends on the different coding systems: DSSS, CCK, OFDM.

  5. Layer 1 Physical Layer in 802.11 • The PHY layer adds a preamble to the L2 frame (MPDU) • The PHY layer also adds trailing bits at the end of the MPDU Added by L1 MPDU passed down from L2 Added by L1

  6. IEEE 802.11a Physical layer

  7. Layer 1 Physical Layer for 802.11a • The PHY layer adds a preamble of 12 symbols. • The PHY layer also adds 1 special signal symbol. • The PHY layer also adds trailing bits at the end of the MPDU Added by L1 MPDU passed down from L2 Added by L1 Preamble 12 symbols Signal 1 Symbol

  8. Layer 1 Physical Layer for 802.11a • The Physical Layer Preamble is composed of “training sequences” of signals (called symbols). • The Physical Layer Preamble in OFDM is meant for the receivers to: • Lock on to the signal • Select the antenna (if multiple settings are available) • Synchronize parameters for coding and decoding Preamble 12 symbols Signal 1 Symbol

  9. Layer 1 Physical Layer for 802.11a • The Physical Layer Signal in OFDM has several fields: • Data bit rate: it is indicated with 4 bits • Length of the L2 frame: 12 bits • Other fields Preamble 12 symbols Signal 1 Symbol

  10. Layer 1 Physical Layer for 802.11a - OFDM • The Physical Layer Signal in 802.11a OFDM has several fields: • Data bit rate: it is indicated with 4 bits • There are four (4) data rate tiers: • 6 and 9 Mbps • 12 and 18 Mbps • 24 and 36 Mbps • 48 and 54 Mbps Preamble 12 symbols Signal 1 Symbol

  11. Layer 1 Physical Layer for 802.11 a - OFDM • The Data Bit Rate in OFDM: • Different tiers = different MODULATION • 6 and 9 Mbps ===========Binary Phase Shift Keying BPSK • 12 and 18 Mbps=========Quadrature Phase Shift Keying QPSK • 24 and 36 Mbps=========Quadrature Amplitude Modulation 16-QAM • 48 and 54 Mbps========= Quadrature Amplitude Modulation 64-QAM Preamble 12 symbols Signal 1 Symbol

  12. Layer 1 Physical Layer for 802.11 a - OFDM • OFDM (802.11a) uses the 5 GHz spectrum divided in channels. • Each channel is 20 MHz wide. • Sub-carriers are located every 0.3125 MHz in one 20 MHz channel. • These sub-carriers are orthogonal to each other. • That yields 52 “mini-channels” • These mini-channels are numbered from -26 to 26, skipping 0 • Four (4) sub-carriers are Test Pilots (-21,-7, 7, and 21) • The test pilots are for synchronization. • The remaining 48 “mini-channels” are used to transmit data

  13. Layer 1 Physical Layer for 802.11 a - OFDM • OFDM (802.11a) uses the 5 GHz spectrum divided in channels • Each channel is 20 MHz • 52 sub-carriers are set within this bandwidth • Sub-carriers are numbered from -26 to 26, skipping 0 • Sub-carriers are allocated every 0.3125 MHz • Four (4) sub-carriers are Test Pilots (-21,-7, 7, and 21) • 48 sub-carriers are used to transmit data Expanded View These are the 52 orthogonal sub-carriers inside one channel

  14. Layer 1 Physical Layer for 802.11 a - OFDM • OFDM (802.11a) uses the 5 GHz spectrum divided in channels • Each channel is 20 MHz • 52 sub-carriers are set within this bandwidth • Sub-carriers are numbered from -26 to 26, skipping 0 • Sub-carriers are allocated every 0.3125 MHz • Four (4) sub-carriers are Test Pilots (-21,-7, 7, and 21) • 48 sub-carriers are used to transmit data Expanded View These are the 52 orthogonal sub-carriers with the position of the four (4) test pilots. The other 48 sub-carriers are used to deliver data.

  15. Layer 1 Physical Layer for 802.11 a - OFDM • PHY for OFDM uses: • 48 channels with 250 KHz bandwidth • A CONVOLUTIONAL CODE for Error Correction. This adds bit redundancy. • 1/2, ¾, or 2/3 of all the bits are data bits, the rest are for error detection • The Data Bit Rate in OFDM: • Calculation = Convolutional Code x 48 Channels x 250 KHz x Coded bits

  16. Example • An IEEE 802.11a radio has the following parameters: • Spread Spectrum is OFDM (11a) • Convolution Code 2/3 • QAM modulation which constellation is shown in a QAM Analyzer screen below: • What is the data bit rate?

  17. Example • An IEEE 802.11a radio has the following parameters: • Spread Spectrum is OFDM (11a) • Convolution Code 2/3 • QAM modulation which constellation is shown in a QAM Analyzer screen below: • What is the data bit rate? • The QAM constellation shows 64 states, so this is a 64-QAM modulator, then: • 2bits = 64 states • 2(6bits) = 64 states • Bits = 6 bits per symbol

  18. Example • An IEEE 802.11a radio has the following parameters: • Spread Spectrum is OFDM (11a) • Convolution Code 2/3 • QAM modulation which constellation is shown in a QAM Analyzer screen below: • What is the data bit rate? • The QAM constellation shows 64 states, so this is a 64-QAM modulator, then each state represents: • 2bits = 64 states • 2(6bits) = 64 states • Bits = 6 bits per symbol

  19. IEEE 802.11b Physical layer

  20. Layer 1 Physical Layer for 802.11b • The PHY layer adds a preamble and a PLCP • The PHY layer also adds trailing bits at the end of the MPDU Added by L1 MPDU passed down from L2 Added by L1 Preamble Long (128) and Short (56) Start of Frame Delimiter (16 bits) 1 Mbps DBPSK

  21. PHY 802.11b Data Bit Rates • Four (4) data bit rates are supported

  22. PHY 802.11b Lower Rates • High Rate Direct Sequence Spread Spectrum HR-DSSS • Barker Code of 11 chips per bit • The Chipping Rate is 11 Mega chips per second • So 1 logical bit needs 11 chips (Barker) to be transmitted Barker Code word for logical 0 1 0 0 0 0 1 1 1 0 1 1

  23. PHY 802.11b Higher Rates • Complementary Code Keying CCK for higher rates • CCK codes are orthogonal • Each CCK code has 8 bits • 11 / 8 = 1.375 million code symbols per second

  24. IEEE 802.11g Physical layer

  25. Layer 1 Physical Layer for 802.11g • The PHY layer adds a preamble and a PLCP • The PHY layer also adds trailing bits at the end of the MPDU Added by L1 MPDU passed down from L2 Added by L1 Preamble Long (128) and Short (56) Start of Frame Delimiter (16 bits)

  26. Layer 1 Physical Layer for 802.11g • The preamble can be modulated on a single-carrier to be compatible in a mixed environment with 802.11b • The preamble can be modulated on a multiple carrier (in OFDM system) when it is 802.11g only mode • When 802.11g only mode is used the implementation of OFDM is similar to the one described in 802.11a Preamble Long (128) and Short (56) Start of Frame Delimiter (16 bits)

  27. Allocation of 2.4 GHz band channels to 802.11g • Each channel in the 2.4 GHz band has a 22 MHz bandwidth • There are 52 sub-carriers defined in one 22 MHz channel • Each sub-carrier is separated by 0.3125 MHz • 48 sub-carriers are used for data transmission • 4 sub-carriers are pilots

  28. 802.11 Physical Layer • The data to be transmitted is previously passed thru the following processes: • Spread Spectrum: the signals are bandwidth spread using DSSS, CCK, and OFDM. The system FHSS is normally not implemented by 802.11 devices. • Modulation: the data that results after the coding is modulated using systems such as DBPSK, DQPSK, QAM and others.

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