Wireless Networking. IEEE Regulations and Standards (802.11) Spread Spectrum Technologies Class 9

Transcription

1 Wireless Networking IEEE Regulations and Standards (802.11) Spread Spectrum Technologies Class 9

2 Spread Spectrum A communication technique that spread a narrow band communication signals over a wide range of frequencies More resistant to interception, interference and jamming.

3 Narrow Band Transmission A technology that uses enough of the frequency spectrum to carry the data signal. Video Audio CB Radio, 5 8 miles

4 Spread Spectrum A signal is qualified Spread spectrum when the bandwidth is wider than required to send the information

5 Spread Spectrum - Noise floor -S/N - TX b at 4 25 miles - TX b at 20 mw 500 m - Jamming - Narrow Band requires FCC license - Most widely used in transmisson

6 Inter Symbol Interference (ISI) Signalling rate 1 MHz, 1 millisecond Signalling rate 1 GHz, 1 microsecond

7 Spread Spectrum Technologies Frequency Hopping Spread Spectrum, FHSS Direct Sequence Spread Spectrum, DSSS Orthogonal Frequency Division Multiplexing, OFDM

8 FHSS How it works

9 FHSS How it works

10 Frequency Hopping Spread Spectrum (FHSS) Is a technique that spreads the data over an 83 MHz span FCC allows frequency hopping over an 83.5 MHz span. WLAN GHz to GHz IEEE allows WLAN GHz to GHz

11 FHSS How it works The carrier changes frequencies or hops according to a predefined pseudorandom sequence. The pseudorandom sequence is a list of frequencies that the carrier will hop to at specific intervals

12 FHSS Dwell time FHSS systems must transmit data on a specific frequency for a specific period of time. The specific period of time is called: the Dwell time Once the dwell time has expired, the system hops to a different frequency and transmits again for the same dwell time

13 FHSS Hop time When an FHSS system switches from Frequency A to frequency B, the system must re tune to the new frequency. Either the system switches to a different circuit pre- tuned to the new frequency Or re-tunes the operational circuit to the new frequency

14 FHSS Hop time (2) The process of changing frequency must be completed before data transmission may begin The required to retune the circuitry to a new frequency is called: Hop time Data transmissions are inhibited during the hop time

15 FHSS Hop time (3) Typical hop time; µs Typical dwell time; ms Hop time can be considered as Over head, since no data transmission is taking place.

16 FHSS Hop time & Dwell time Longer Dwell time = greater throughput Shorter Hop time = greater throughput

17 FHSS Dwell time limits FCC defines the maximum dwell time of a frequency of an FHSS at 400 ms 400 ms per carrier in any 30 second time period This prevents an FHSS from behaving like a narrow band system

18 FHSS Dwell time limits Max 400 ms per carrier in 30 seconds Example dwell time = 100 ms, 75 channels

19 FHSS Hop sequence and channels FHSS use a hopping sequence consisting of a series of frequencies The IEEE standard specifies 1 MHz channels. Each hopping sequence or pattern must contain at least 75 channels, maximum 79 From to GHz

20 FHSS Dwell sequence Beacon from AP Pre defined hopping sets

21 FHSS Co location (un synchronized) FHSS allows for multiple systems in the same basic service area by using different hopping sequences The likely hood of collision is low If collision does occur, it will be for only 1 dwell time period

22 IEEE FHSS standard IEEE is responsible to create standards of operations to remain within the regulations created by the FCC IEEE states: Frequency and to e used Hop sequence Dwell time Data rates

23 Direct Sequence Spread Spectrum, DSSS Due to its ease of implementation and higher data rates, DSSS is the most widely used technology DSSS is used in b to achieve data rate: 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps

24 DSSS channels See page 187

25 DSSS channels See page 188

26 DSSS channels See page 189

27 DSSS channel spacing Non overlapping channels are defined as: DSSS b HR-DSSS a OFDM g ERP-OFDM 30 MHz (6 channels) 25 MHz (5 channels) 20 MHz (4 channels) 25 MHz (5 channels)

28 DSSS channels See page 189

29 DSSS Encoding In DSSS the carrier frequency does not change. Instead redundant information is added prior to transmission and removed on reception Causing the use of more bandwidth

30 DSSS processing gain The process of adding redundant information is called processing gain XOR = pseudorandom number PN 11 bit code +/- Shorter code = less spreading Less corruption resistance Less over head, higher data rate PN code is known as Barker sequence IEEE has set the processing gain requirement at chips / 1 bit 11:1 ratio

31 DSSS processing gain Image from page 191

32 DSSS Phase Shift Keying The last step in preparing the signal for transmission is to integrate the chips in to the RF signal Known as Encoding

33 DSSS Phase Shift Keying Amplitude (X) Frequency Phase

34 Differential Phase Shift Keying (DPSK) DPSK is widely used in DPSK encodes information by advancing the phase of a signal by a certain number of degrees The number of degrees shifted represents the data

35 DPSK 180 o = 1 0 o = 0 Insert image from bottom of page 194

36 DPSK 180 o = 1 0 o = 0 Insert image from bottom of page 194

37 DBPSK 2 states such as 180 and 0 degrees Can represent 2 bits of data Δ Differential Binary Phase Shift Keying Used in and b for 1 Mbps

38 DQPSK Assume a 4 state system Shifts of : 0, 90, 180, 270 degrees Δ Differential Quaternary Phase Shift Keying Used in and b for 2 Mbps

39 DQPSK Each state can represent 4 unique two bit combinations: 11, 10, 01, 00

40 CCK DBPSK and DQPSK are used for 1 and 2 Mbps data rates To achieve data rates of of 5.5 and 11 Mbps Complementary Code keying, CCK 8 chips vs 11 chips at 11 Mbps rate

41 Encoding vs Modulation AM, FM, PSK = modulation Baker coding and CCK = encoding (1,0)

42 Orthogonal Frequency Division Multiplexing, OFDM Offers the highest data rates OFDM provides 54 Mbps in a and g

43 OFDM how it works (recent) Typically eliminates channel harmonics by precisely placing adjacent channels at frequencies that cancel out the harmonics between each other. Leaving only the fundamental signals Not used by consumer grade systems

44 OFDM how it works (recent) Requires the use of Digital Signal Processing, DSP Digitally calculates the RF waveform Divides up a single channel in many small sub carriers tx d in parrallel

45 Convolution coding Not part of OFDM Used in a and g A mathematical error correction technique that allows the reconstruction of sub carrier data in reference to other su carriers. Uses parity bit error correction Variable Tx bit / encoded bits

46 OFDM in a Used to transmit in the 5 GHz frequency band Three 5 GHz bands See page 199 and 200

47 OFDM in g Used to transmit in the 2.4 GHz frequency band See page 199 and 200