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802.11 FHSS PHY Specification. Contents. Frequency Hopping Specifications Physical Layer architecture Physical Layer Convergence Procedure (PLCP) Interactions between MAC, PLCP and PMD PHY procedures: Clear Channel Assessment Transmit Receive Management. MAC Layer. PHY Layer.

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  • Frequency Hopping Specifications

  • Physical Layer architecture

  • Physical Layer Convergence Procedure (PLCP)

  • Interactions between MAC, PLCP and PMD

  • PHY procedures:

    • Clear Channel Assessment

    • Transmit

    • Receive

  • Management


Where does fhss fit in the 802 11 architecture

MAC Layer

PHY Layer

2.4 GHz


1 Mbps

2 Mbps

2.4 GHz


1 Mbps

2 Mbps



1 Mbps

2 Mbps

2.4 GHz


5.5 Mbps

11 Mbps

5 GHz


6, 9, 12,

18, 24, 36,

48, 54 Mbps

IEEE 802.11b

IEEE 802.11a

IEEE 802.11

Where does FHSS fit in the 802.11 Architecture?


802 11 fhss phy overview
802.11 FHSS PHY Overview

  • 2.4 GHz ISM band

  • 1 Mbps data rate (GFSK modulation)

  • 2 Mbps data rate (4-level GFSK) (optional)

  • 79 nonoverlapped frequency channels in North America and Europe 23 nonoverlapped frequency channels in Japan

  • Center frequencies of adjacent channels are separated by 1 MHz

  • Up to 26 collocated networks can operate (N. America/Europe) with minimal interference


Channel frequencies
Channel Frequencies

  • In N. America, there are 79 channels

  • The center frequencies for adjacent channels are spaced 1MHz apart:


Hop sequences n america
Hop Sequences (N. America)

  • The 802.11 standard specifies a base sequence:

  • The channel number for the ith hop is given by [b(i) + 2]

  • Additional patterns are formed by adding k to each channel in the base sequence

    • i.e. the ith hop in sequence k is given by [b(i) + k] mod 79 + 2

  • Each sequence uses all 79 channels


Hop sequence example
Hop Sequence - example

  • The channel numbers in the base sequence are:

    • [b(i) + 2]

    • {2, 25, 64, 10, 45, 18, 27, … , 32, 48}

    • Center frequencies (in GHz) are:

      {2.402, 2.425, 2.464, 2.410, …}

  • The channel numbers in sequence 10 are :

    • {[b(i) + 10] mod 79} + 2

    • {12, 35, 74, 20, 55, 28, 37, … , 42, 58}

    • Center frequencies (in GHz) are

      {2.412, 2.435, 2.474, 2.420, …}


Hop sequence properties
Hop Sequence - Properties

  • The sequences are divided into 3 sets of 26 sequences.

  • Co-located networks using different sequences from the same set, will experience minimum collisions

  • Frequency hopping sequences are designed so that consecutive hops are at least 6 MHz apart



  • 802.11 FHSS PHY uses Frequency Shift Keying

  • Different values (0 or 1) are represented by different frequencies

  • In order to minimize (wasted) power, transmitted outside of the channel, transitions between frequencies are smoothed

  • The way 802.11 FHSS does this is known as Gaussian Frequency Shift Keying (GFSK)

  • This modulation gives a data rate of 1MB/s


Modulation for 2mb s
Modulation for 2Mb/s

  • 802.11 FHSS PHY specifies an optional2Mbps modulation scheme

  • This uses 4-level GFSK, where two data bits are ‘carried’ in each symbol :


Further fhss specifications
Further FHSS Specifications

  • Slot time (MAC layer) 50 s

  • SIFS time (MAC layer) 28 s

  • Hop time (i.e. time taken to change frequency) 224 s

  • Maximum dwell time (N. America) 400 ms

  • Maximum output power

    • 1000 mW USA

    • 100 mW Europe

    • 10 mW/MHz Japan

  • Minimum transmitted power 10 mW

  • Receiver sensitivity -80 dBm @ 0.03 FER (400 bytes)

  • Rx adjacent channel rejection > 30 dB @ 4 MHz (and 8 MHz) separation between channels


Protocol architecture and the physical layer convergence procedure plcp

Protocol Architecture and the Physical Layer Convergence Procedure (PLCP)


Phy layer architecture

PLCP Sublayer


PHY Layer

PMD Sublayer

PHY layer architecture

  • Physical Medium Dependent (PMD)sublayer: provides a mean to send and receive bits between stations through the wireless medium

  • Physical Layer Convergence Procedure (PLCP): maps the MAC PDU (MPDU) into a framing format suitable for transmission; this function simplifies the PHY service interface to the MAC services

  • Physical Layer Management Entity (PLME): performs management of the local PHY functions


Need for the plcp 1
Need for the PLCP (1)

  • PHY layer (PMD) is only concerned with transmitting bits

  • MAC layer is only concerned with MAC frames

  • Need function between PHY and MAC to achieve the following:

    • Allow receiver to acquire bit-synchronization (since PHY is not synchronous)

    • Indicate to the receiver the start and end of a MAC frame

    • Indicate the data rate (modulation) used

    • Carry out functions which are PMD-dependent but which act only on certain parts of the frame (e.g. scrambling)

  • Physical Layer Convergence Procedure does the above


Need for the plcp 2
Need for the PLCP (2)

  • The PLCP makes the MAC-PHY interface independent of the Physical Layer used

    • i.e. the same MAC can be used for many different Physical layer protocols

    • This makes the MAC layer much more simple!


Plcp frame format

FHSS PLCP preamble(12 octets)

FHSS PLCP header(4 octets)

Whitened PSDU

PLCP Frame Format

PLW: PSDU Length Word PSDU: PLCP Service Data Unit

PSF: PLCP Signaling Field CRC: Cyclic Redundancy Check

1 Mbps transmission

1 (or 2 Mbps) transmission


(80 bits)

Start Frame Delimiter

(16 bits)


(4 bits)


(16 bits)


(12 bits)


Fields in the plcp frame 1
Fields in the PLCP Frame (1)

  • Synchronization: 80-bit field containing an alternating 01 pattern; used for signal detection, antenna selection, frequency offset compensation and synchronization

  • Start Frame Delimiter (SFD): 0000 1100 1011 1101; used for frame timing

  • PSDU Length Word (PLW): The number of octets contained in the PSDU (up to 4095 octets); used for the end of frame detection

  • CRC: CCITT CRC-16 FCS; used to protect PLW and PSF


Fields in the plcp frame 2
Fields in the PLCP Frame (2)

  • PLCP Signaling Field (PSF): Data rate indication of the whitened PSDU

  • 1 Mbps is the only mandatory data rate up to now; the first bit is reserved for future use (set to 0) and the three other bits follow this table


Psdu whitening
PSDU ‘Whitening’

  • Data whitener: It uses a 127-bit frame-synchronous scrambler followed by a 32/33 bias-suppression encoding

  • This is done to:

    • Randomize the data

    • Minimize the data DC bias

    • Minimize maximum run lengths


Interactions between mac plcp and pmd

Interactions between MAC, PLCP and PMD


Management functions
Management Functions

  • The MAC Layer Management Entity(MLME) and PHY Layer Management Entity (PLME) contain attributes, processes, etc. necessary for the operation of the MAC and PHY layers respectively

  • Attributes are contained in a conceptual database called the Management Information Base(MIB)


Management example
Management - example

  • The MLME is responsible for the Frequency Hopping time synchronization, which ensures that all stations are hopping at the same time

  • The MLME will update the PLME using PLME-SET primitives to update the current hop set, current hop pattern, and current index in the MIB

  • The PLCP acts immediately on these updates (e.g. changing frequency)

  • Note: Unlike the MAC, the MLME is not entirely PHY-independent


Phy layer functions

PHY layer functions

Clear Channel Assessment




Clear channel assessment
Clear Channel Assessment

  • As we will see, the MAC protocol is based on a Carrier Sense mechanism

  • In order to support this, the PHY layer must provide an indication to the MAC layer of the state of the channel

  • Clear Channel Assessment (CCA) is the PHY-layer process which provides idle/busy information to the MAC layer


Plcp transmit procedure 1
PLCP Transmit Procedure (1)

  • PLCP switches PMD to transmit mode after receiving PHY.TXSTART.request with the total number of octets and the data rate from the MAC layer

  • PMD responds by sending the preamble at the antenna within 20 s

  • Transmitter sends preamble and header at 1 Mbps and then changes to the data rate specified in the request to send the PSDU

  • After completion of the transmission PLCP sends a PHY-TXEND.confirm to MAC

  • PHY shuts off the transmitter and switched PMD to receive mode


Plcp receive procedure 1
PLCP Receive procedure (1)

  • PMD will indicate a busy medium when it senses a signal having a power level of at least -85 dBm

  • When CCA discovers a busy medium and the valid preamble of an incoming frame, PLCP monitors the frame header

  • If PLCP finds the header is error-free, it will send a PHY-RXSTART.indicate to MAC along with the information in the header

  • PLCP sets a counter to keep track of the number of octets received so that PLCP will know the end of the frame

  • As PLCP receives data, it sends PHY-DATA.indicate to MAC

  • PLCP sends a PHY-RXEND.indicate to MAC when its counter indicates the end of the frame