802 11 fhss phy specification
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802.11 FHSS PHY Specification

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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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contents
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

2

where does fhss fit in the 802 11 architecture

MAC Layer

PHY Layer

2.4 GHz

FHSS

1 Mbps

2 Mbps

2.4 GHz

DSSS

1 Mbps

2 Mbps

Infrared

IR

1 Mbps

2 Mbps

2.4 GHz

DSSS

5.5 Mbps

11 Mbps

5 GHz

OFDM

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?

3

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

5

channel frequencies
Channel Frequencies
  • In N. America, there are 79 channels
  • The center frequencies for adjacent channels are spaced 1MHz apart:

8

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

9

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, …}

10

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

11

modulation
Modulation
  • 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

12

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 :

13

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

14

phy layer architecture

PLCP Sublayer

PLME

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

16

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

17

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!

18

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

Synchronization

(80 bits)

Start Frame Delimiter

(16 bits)

PSF

(4 bits)

CRC

(16 bits)

PLW

(12 bits)

19

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

20

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

21

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

22

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)

27

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

28

phy layer functions

PHY layer functions

Clear Channel Assessment

Transmit

Receive

30

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

32

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

34

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

37

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