Sunday, 26 January 2014

Chapter 2 - Physical Layer Format

Physical Layer Operations

Each station in one of 3 states: waiting for data, transmitting data or listening for clear channel to transmit

Carrier Sense/Clear Channel Assessment

If station not receiving or transmitting, listening for beginning of signal to receive (carrier sense). If has data to send, needs to understand if channel unused so can send data (clear channel assessment.


If channel clear (using CS/CCA), station will transmit frame & return to receive mode. Cannot detect collisions like Ethernet, so must receive ACK to verify delivery.


If medium busy (via CS/CCA), station needs to  understand if due to frame. Transmitting station sends pre-amble (string of 1's and 0's) to alert & sync receiver. Preamble also include start of frame delimiter (SFD) to indicate beginning of frame. After pre-amble, length field indicates show long frame is. Receiver sends ACK when frame successfully received.

Physical Layer

Two sub-layers:

  • PLCP - Physical Layer Convergence procedure (upper layer)
    • Takes MPDU (called PSDU at this layer) and prepares for transmission
    • Creates PPDU
  • PMD - Physical Media Dependent (lower layer)
    • Modulates & transmits PPDU as bits

PLCP Service Data Unit (PSDU)

  • PSDU is the MAC layer MPDU, but different name at physical layer.

PLCP Protocol Data Unit (PPDU)

  • PLCP layer adds preamble & PHY header to PSDU (MPDU). Preamble provides sync between stations.

Physical Medium Dependant (PMD)

  • Responsible for transmitting and receiving PPDU at physical layer
  • modulates/demodulates binary data in to/from  RF signals


PLCP Protocol Data Unit

Three parts which combine to form PPDU:

  • PLCP Preamble
  • PLCP Header
  • PSDU

PLCP Preamble

  • Preamble is string of 1's and 0's to sync to incoming transmission
  • 802.11-2007 defines 3 preambles:
    • Long PPDU format
    • Short PPDU format
    • OFDM PLCP preamble
  • 802.11n ammendment defines 3 additional:
    • non-HT legacy PPDU
    • HT-mixed PPDU
    • HT-Greenfield PPDU

Long PLCP Preamble

  • 144 bit preamble
  • 128 bit sync field + 16 bit SFD (start of frame delimiter)
  • Sync between Tx & RX must occur before SFD field
  • SFD indicates PLCP header coming next
  • Long preamble & header sent using DBPSK 1mbps modulation
  • Modulation of PSDU not necessarily sent at same rate as preamble & header


Short PLCP Preamble

  • 72 bit preamble
  • 56 bit sync fielf + 16 bit SFD
  • Half the overhead of long preamble
  • Short preamble sent using DBPSK 1mbps, Header sent using DQPSK 2mbps (both fixed)
  • Modulation of PSDU not necessarily sent at same rate as preamble & header


OFDM PLCP Preamble

  • Also known as OFDM training structure
  • 10 short symbols, 2 long symbols 
  • No SFD, signal field of header follows preamble
  • Total training length 16uS
  • short training symbol: 12 sub-scarriers
  • long training symbol 53 sub-carriers


PLCP Header

  • PLCP Header for long & short headers both 48 bits long, made up of 4 fields:
    • Signal (8 bits) - indicates modulation method for PSDU
      • short header, PSDU may be 1, 2 5.5 or 11mbps
      • Long header, PSDU may be 2,5.5, 11 mbps
    • Service (8 bits) - bit 3 indicates modulation method used CCK/PBCC
    • Length (16 bits) - # of micro-secs required to transmit PSDU
    • CRC (16 bits) - protects jist the signal service & length fields
  • Clause 17  OFDM transmission, only have signal field, 24 bits long:
    • bits 0-3 indicate data rate (6 - 54 mbps)
    • bits 5-16 form PCLP length field
    • bit 17 parity bit
    • bits 18023 are signal tail (all zeros)
802.11n PPDUs

802.11n ammendment defines 3 additional:

  • non-HT legacy PPDU
    • legacy format
    • structured as clause 17 (OFDM) and clause 18 (ERP)
    • Preamble is 10 short & 2 long symbols
    • support mandatory for 802.11n radios
    • 20MHz support only
    • Image(5)
  • HT-mixed PPDU
    • Preamble contains (legacy) non-HT short & long training symbols (can be decoded by clause 17 & 19 radios)
    • Rest of non-legacy headers cannot be decoded by legacy radios, but enough to detect PPDU get carrier freq & timing. But, can be decoded by HT  devices so they can get freq & timing and detect PPDU
    • Most commonly used format as supports HT & legacy 802.11a/g OFDM radios
    • Mandatory to support
    • 20 or 40MHz channels
      • When use 40MHz, all broadcasts sent on 20MHz channel for interoperability with non-HT clients
    • Image(6)
  • HT-Greenfield PPDU
    • Pre-amble not compatible with legacy radios, only HT radios supported
    • 20 or 40MHz
    • Image(7)
  • Data field
    • data portion is the PSDU (same as MPDU from layer 2)
    • In all HT formats & clause 17 & 19 frames, Service field is pre-pended to data field
    • data field (PSDU) is scrambled to breal up long strings of 1 or 0
    • Image(8)

2.4GHz Communications

  • 2.4GHz ISM band 83.5MHz wide
  • 2.400GHz to 2.4835GHz
  • Band used for:
    • 802.11 (FHSS & DSSS clause 14 & 15)
    • 802.11b (HR-DSSS clause 18) 
    • 802.11g (ERP clause 19)
    • 802.11n (HT clause 20)
  • Band heavily used by many interfering devices (baby monitors, security cameras, microwaves etc.) 
  • 802.11-2007 allows for 14 channels across band, but varies by region & local regulatory body


2.4GHz Channels

  • Channels designated by centre freq
  • Each channel 22MHz wide, +/- 11Mhz around centre freq
  • Ch 1 spans 2.401 GHz to 2.423 GHz
  • Each ISM channel centre freq only 5MHz apart, so will overlap using 22MHz wide channels
  • Two channels must be be separated by 25MHz (5 channels) to avoid overlap
    • Channels 1,6,11 generally used as non-overlapping channels


  • DSSS, HR-DSSS & ERP all use same centre freqs, but require different channel widths
    • DSSS - 30MHz channel width, so 1,6,11 considered overlapping
    • HR-DSSS & ERP require 25MHz, channel width, so 1,6,11 not overlapping
    • Image(11)
  • In addition to centre carrier freq, side-band carrier freqs also generated.
    • Side bands are -11Mhz to -22Mhz from centre freq & +11MHz to +22MHz from centre freq 
    • Sideband levels must be at least 30dB below centre freq signal level
    • Any sidebands beyond -/+ 22MHz of centre freq must be at least 50dB below centre freq
  • APs even on non-overlapping channels must be at least 5 to 10 feet apart to mitigate effects of side-bands
    • Image(12)

5GHz Communications

  • 802.11a designated use of transmission for WLANs in 5GHz band
  • Known as UNII (Unlicensed National Information Infrastructure) band
  • Initially 3 band defined:
    • UNII-1 (lower) - 4 channels
    • UNII-2 (middle) - 4 channels
    • UNII-2 Extended - 11 channels
    • UNII-3 (upper) - 4 channels
  • All 3 bands are 100MHz wide
  • With 802.11h ammendment (TPC & DFS), UNII-2 Extended designated for use
    • 255MHz wide
  • UNII bands include radios that support following:
    • 802.11a (OFDM clause 17)
    • 802.11h (TPC & DFS)
    • 802.11n (HT clause 20)
  • 802.11-2007 allows for 23 channels, though this varies per region
  • UNII-1 (Lower Band)
    • 100 MHz wide
    • 5.150GHz to 5.250GHz
    • Indoor use, max power at intentional radiator 50mW (FCC)
      • IEEE specifies max of 40mW
  • UNII-2 (Middle Band)
    • 100MHz wide
    • 5.250Ghz to 5.350Ghz
    • Indoor or outdoor use, max power at IR 250mW (FCC)
      • IEE specifies max of 200mW
  • UNII-3 (Upper band)
    • 100MHz wide
    • 5.725GHz to 5.825 GHz
    • Typically outdoor use, indoors in some countries
      • band not used in Europe
      • max power 1000mW (FCC)
        • IEEE specifies max power at IR of 800mW
  • UNII-2 Extended
    • 255MHz wide
    • 5.470 to 5.725 GHz
    • indoor or outdoor use, max power 250mW (FCC)
      • IEEE specifies max 200mW
    • Equipment in band must comply with 802.11h (DFS & TPC) - protection for military & weather radar systems

5GHz Channels

  • Centres of outermost channels must be 30MHz from band edge: UNII-1 & UNII-2
  • Centres of outermost channels must be 20MHz from band edge: UNII-3
  • UNII-1,2,3:
    • 4 non-overlapping channels each
    • centre freqs 20MHz apart
  • UNII-2e:
    • 11 non-overlapping channels
    • centre freqs 20MHz apart
  • USA also allows use of ISM channel 165 to allow 24 channels in total on band
  • Image(14)
  • In OFDM spectrum mask, sideband freqs do no drop of very quickly, so are slightly overlapping
    • IEEE considers 20MHz separation non-overlapping for clause 17 (OFDM)
    • Number of channels in band allows physical separation of channels to avoid adjacent channel interference
    • Image(15)
Adjacent, Non-adjacent & Overlapping Channels
  • Channel width requirements for non-overlap:
    • DSSS: 30MHz
    • HR-DSSS & ERP: 25MHz
    • 5GHz OFDM: 20 MHz
  • Adjacent channel: first channel with non-overlapping freq
  • Image(16)

Clause 14 FHSS PHY

  • Frequency Hopping Spread Spectrum
  • In original 802.11 std: 1 & 2Mbps
  • In North America used 2.402Ghz to 2.480Ghz
  • Mechanism: hop to freq, tx data using small freq carrier, then after dwell time, hop to new freq & keep repeating
  • Uses pre-defined hopping sequence to sync Tx & Rx - number of hops in sequence varies between countries
  • IEEE mandates each hop 1MHz in size
  • Hopping sequence delivered to client via beacon mgt frame
  • Dwell time: amount of time system transmits of freq before hopping
  • Dwell times typically 100 to 200mS - shorter dwell time reduces throughput as hopping more often, less time to tx data
  • IEEE specifies hop seq at least 75 freqs, 1MHz wide
  • Hop time: time to shift from one freq to another: typically 200 to 300uS
  • Modulation: Gaussian FSK (GFSK)
    • Two level GFSK - 2GFSK: 2 freq represent 1 or 0
    • Four level GFSK - 4GFSK: 4 freq represent 2 bits (00,01,10,11)

Clause 15 DSSS PHY

  • Direct Sequence Spread Spectrum
  • 1Mbps or 2Mbps on ISM 2.4GHz
  • Data spread across range of freqs that make up channel
  • Data encoding:
    • To mitigate natural corruption of wireless data signals, multiple bits used to represent each data bit - allows recovery of data if  bits corrupted
    • Addition of additional redundant info known as processing gain
    • Each bit of data to be sent converted in to a number of 'chips' (bits)
    • data bit XOR'ed with pseudo-random number to create 'Barker Code' - 11 bit chips
    • Up to 9 chips can be corrupted & allow recovery of original data
  • Modulation:
    • Once data encoded, converted to RF using modulation method
    • DBPSK - Differential Binary Shift Keying - 2 phase shifts of carrier represent 1 & 0 (chips)
    • DQPSK - Differential Quadrature Shift Keying - 4 phase shifts of carrier represent two bits (chips)
    • Image(17)

Clause 17 OFDM PHY

  • OFDM - Orthogonal Frequency Division Multiplexing
  • Not strictly spread spectrum technology, but uses low tx power & more b/width than is req to tx data
  • Use 52 closely spaced carriers - freq width of each sub-carrier 312.5 Khz
  • Lower data rate per sub-carrier, but aggregate throughput higher
  • More resistant to multipath doe to lower inter-symbol interference
  • Carrier freqs chosen so that harmonics tend to overlap & cancel unwanted signals
  • 52 sub -carriers numbered -26 to +26 - 48 tx data, 4 used as pilot carriers (reference sigs)
  • Image(18)
  • Convolution Coding
    • Provides forward error correction so that more resistant to narrow-band interference 
    • Ratio of bits used to encode data vs actual data bits varies - lower ration, less resistance to noise, but higher data rate
    • Image(19)

Clause 18 HR-DSSS PHY

  • Clause 18 devices defined in 802.11b
  • 5.5 & 11 Mbps speeds know as HR-DSSS (High rate DSSS)
  • 802.11b devices can transmit at DSSS (1 & 2 Mbps) and HR-DSSS speeds, but not FHSS, not backward compatable with Clause 14 devices
  • HR-DSSS speeds provides by CCK (Complemetary Code Keying) for encoding instead of Barker coding - 8 chip pseudo random chipping code used
  • Modulation
    • Uses same modulation as Clause 15 devices, but CCK provides higher speeds
    • Image(20)

Clause 19 ERP PHY

  • 802.11g devices on 2.4GHz band
  • 2 mandatory PHYs:
    • ERP-OFDM - provides 6,9,12,18,24,36,48,54 Mbps rates
      • only 6,12,24 Mbps mandatory
    • ERP-DSSS/CCK - provides 1,2,5.5 & 11 Mbps for backwards compatibility with 802.11 & 802.11b
  • Protection mechanism required to prevent 802.11b (HR-DSSS) or 802.11 (DSSS) transmitting at same time as ERP stations


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