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Introduction to Wireless Networking

Introduction to Wireless Networking. Module-5 Physical Layer Access Methods and Spread Spectrum CCRI ENGINEERING AND TECHNOLOGY Jerry Bernardini. REFERENCES. CWTS Certified Wireless Technology Specialist Official Study Guide , Chapter-5

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Introduction to Wireless Networking

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  1. Introduction to Wireless Networking Module-5 Physical Layer Access Methods and Spread Spectrum CCRI ENGINEERING AND TECHNOLOGY Jerry Bernardini CCRI Engineering and Technology Jbernardini 1

  2. REFERENCES • CWTS Certified Wireless Technology Specialist Official Study Guide , Chapter-5 • CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-6 • The California Regional Consortium for Engineering Advances in Technological Education (CREATE) project • Spread Spectrum Scene http://www.sss-mag.com/primer.html CCRI Engineering and Technology Jbernardini 2

  3. Chapter Objectives • Define concepts which make up the functionality of RF and spread spectrum technology • Define and differentiate between the following physical layer (PHY) wireless technologies CCRI Engineering and Technology Jbernardini 3

  4. IEEE 802.3 CSMA/CD vs. IEEE 802.11 CSMA/CA • CSMA/CD is for wired collision handling • CSMA/CA is for wireless collision handling • CSMA = Carrier Sense Multiple Access • CD = Collision Detection • CA = Collision Avoidance • Why do collisions occur? • Answer = Two or more stations transmit at the same time • Why is it important to detect or avoid collisions? • Answer = Because there is data loss and retransmission is necessary • Wired networks are designed for the transmitting station to detect most collisions • Many collisions will not be detected by Wireless networks – therefore avoid collisions CCRI Engineering and Technology Jbernardini 4

  5. IEEE 802.11 Collision Handling CSMA/CA • In CSMA/CA a Wireless node that wants to transmit performs the following sequence: • Listen on the desired channel. • If channel is idle (no active transmitters) it sends a packet. • If channel is busy the node waits random time until transmission stops and then waits an additional time period. • If the channel is still idle at the end of the time period the node transmits its packet otherwise it repeats the process defined in 3 above until it gets a free channel. • Additional support mechanisms such as ACK, RTS/CTS can be used but increase overhead noticeably. CCRI Engineering and Technology Jbernardini 5

  6. CSMA/CA and ACK • CSMA/CA also reduces collisions via explicit frame acknowledgment • Acknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intact • If ACK not returned, transmission error assumed • CSMA/CA does not eliminate collisions and does not solve hidden node problem CCRI Engineering and Technology Jbernardini 6

  7. Two Kinds of Carrier Sensing Mechanisms • Physical Carrier Sense • Uses Clear Channel Assessment (CCA) • Is the RF energy on the channel above a threshold? • If CCA>threshold --->wait for CCA< threshold before transmitting • Checks received signal strength using RSSI • RF energy from a hidden node could be missed • Virtual Carrier Sense • Uses the Network Allocation Vector (NAV) in each station • NAV is a timer that determines if station can contend for RF medium • NAV >0 --->wait for count down to NAV=0 • NAV=0 --->use CCA to check for RF energy on medium • IF NAV=0 and CCA > threshold --->station resets NAV>0 and waits CCRI Engineering and Technology Jbernardini 7

  8. Network Access Methods • Reserving Time for Data Transmission Using Distributed Coordination Function (DCF) • Employs a contention period for devices competing to send data on the wireless network CCRI Engineering and Technology Jbernardini 8

  9. CSMA/CA Request to Send/Clear to Send • Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem • Significant overhead upon the WLAN with transmission of RTS and CTS frames • Especially with short data packets • RTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTS CCRI Engineering and Technology Jbernardini 9

  10. IEEE 802.11 -Half Duplex Communication • Effects of Half Duplex on Wireless Throughput • Half Duplex: two way communication that occurs in only one direction at a time • Effective halves the max bit rate CCRI Engineering and Technology Jbernardini 10

  11. Telecommunication Channel • Channel - a path along which information in the form of an electrical signal passes. • Usually a range of contiguous frequencies involved in supporting information transmission Center Channel Frequency Amplitude Bandwidth Frequency Channel CCRI Engineering and Technology Jbernardini 11

  12. RF Bands for Wireless Networks • ISM- Industrial Scientific and Medical – Three Bands • 900 MHz band • 2.4 GHz band • 5 GHz Band • UNII- Unlicensed National Information Infrastructure • 5 GHz band • UNII-1 (Lower) • UNII-2 (middle) • UNII2 Extended • UNII-3 (Upper) CCRI Engineering and Technology Jbernardini 12

  13. DSSS USA Channel Allocation • 14 Channels available • 11 Channels in the United States Amplitude Channels 1 2 3 4 5 6 7 8 9 10 11 Freq. 2.401 GHz 2.473 GHz CCRI Engineering and Technology Jbernardini 13

  14. DSSS 3 Non-overlap Channels Amplitude Ch 1 Ch 6 Ch 11 (2.412 GHz) (2.437GHz) (2.462 GHz) Freq. 22 MHz 2.473 GHz 3MHz 2.401 GHz 2401 MHz 2423 MHz 2426 MHz CCRI Engineering and Technology Jbernardini 14

  15. 5 GHz Band and Channels CCRI Engineering and Technology Jbernardini 15

  16. Introduction to Spread Spectrum • Spread Spectrum – a telecommunications technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. Narrowband Amplitude Wideband Frequency CCRI Engineering and Technology Jbernardini 16

  17. Narrow Band and Spread Spectrum Communications • Narrowband Vs. Spread Spectrum Communication • Narrowband and Spread Spectrum are two examples of how devices can communicate using radio frequency CCRI Engineering and Technology Jbernardini 17

  18. 4-Types Spread Spectrum • Time Hopping, (THSS) • Frequency Hopping, (FHSS) • Direct Sequence Spread Spectrum, (DSSS) • Hybrid, DSSS/FHSS • Original IEEE 802.11 wireless LAN standard: • Frequency-hopping spread spectrum (FHSS) • Direct-sequence spread spectrum (DSSS) • High Rate/ Direct-Sequence Spread Spectrum (HR/DSSS) • DSSS and HR/DSSS Channels CCRI Engineering and Technology Jbernardini 18

  19. Uses of Spread Spectrum • Military - For low probability of interception of telecommunications. • Civil/Military - Range and positioning measurements. GPS – satellites. • Civil Cellular Telephony. • Civil Wireless Networks – 802.11 and Bluetooth. CCRI Engineering and Technology Jbernardini 19

  20. Frequency Hopping Patent • HedyLamarr and composer George Antheil, patent number 2,292,387 , circa 1942 • A Hollywood cocktail party with Navy officers present CCRI Engineering and Technology Jbernardini 20

  21. Frequency Hopping Spread Spectrum (FHSS) • FHSS - Acronym for frequency-hopping spread spectrum. 802.11,Bluetooth, & HomeRF. Amp. 1 3 2 4 Freq. Channel Wide Band Frequency Hop Sequence: 1, 3, 2, 4 CCRI Engineering and Technology Jbernardini 21

  22. Frequency Hopping Spread Spectrum – Simplistic View CCRI Engineering and Technology Jbernardini 22

  23. FHSS System Block Diagram Antenna FHSS Data Buffer Mixer Mod 1 3 2 4 Carrier Frequency Sequence Generator 1 3 2 4 Frequency Synthesizer CCRI Engineering and Technology Jbernardini 23

  24. Direct Sequence Spread Spectrum (DSSS) Amp. Signal 1 1 3 2 4 Freq. Channel DSSS Band CCRI Engineering and Technology Jbernardini 24

  25. DSSS System Block Diagram Carrier Frequency Antenna DSSS Mixer Carrier Generator Mod 11-bit Barker Code Pseudo – Noise Generator Data Buffer Encoder Chipping Code 10110111000 CCRI Engineering and Technology Jbernardini 25

  26. Comparing FHSS & DSSS CCRI Engineering and Technology Jbernardini 26

  27. Orthogonal Frequency Division Multiplexing (OFDM) • Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. • Orthogonal means to establish right angle relationships between frequencies • OFDM spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies and null out of channel sidebands f2 f3 f1 f4 CCRI Engineering and Technology Jbernardini 27

  28. OFDM Features • Used in IEEE 802.11a (OFDM) and IEEE 802.11g (ERP-OFDM) and IEEE 802.11n (HT-OFDM) amendments • Allows for much higher data rate transfers than DSSS and HR/DSSS • Up to 54 Mbps for OFDM, ERP-OFDM and 300-600 Mbps for HT-OFDM • OFDM functions in either the 2.4 GHz ISM or the 5 GHz UNII bands • The channel width is smaller than DSSS or HR/DSSS • The width of an OFDM channel is only 20 MHz compared to 22 MHz for DSSS CCRI Engineering and Technology Jbernardini 28

  29. Orthogonal Frequency Division Multiplexing (OFDM) • OFDM operates in either the 2.4 GHz ISM or the 5 GHz UNII bands • The width of an OFDM channel is only 20 MHz compared to 22 MHz for DSSS or HR/DSSS CCRI Engineering and Technology Jbernardini 29

  30. Multiple Input/Multiple Output (MIMO) Channels • MIMO networks can operate in both the 2.4 GHz ISM and 5 GHz UNII bands • Capable of either 20 or 40 MHz–wide channels • Wider channels mean more data can be transmitted over the RF medium simultaneously • In the 2.4 GHz ISM band, there is only one 40 MHz–wide channel without any adjacent-channel overlap CCRI Engineering and Technology Jbernardini 30

  31. Multiple Input/Multiple Output (MIMO) • Used by IEEE 802.11n devices • Wider channels mean more data can be transmitted over the RF medium simultaneously • In the 2.4 GHz ISM band, there is only one 40 MHz–wide channel without any adjacent-channel overlap CCRI Engineering and Technology Jbernardini 31

  32. MIMOThroughput and Features • Allows for data rates up to 600 Mbps • Current data rates up to 450 Mbps • More throughput, reliable, predictable • Lower latency for mobile communications • More consistent coverage and throughput for mobile applications • MIMO networks can operate in both the 2.4 GHz ISM and 5 GHz UNII bands • Capable of either 20 MHz or 40 MHz–wide channels CCRI Engineering and Technology Jbernardini 32

  33. IEEE 802.11n Features • Uses three modes of OFDM • 20MHz and 40 MHz bands • Data rates up to 600 Mbps • Non-HT mode • OFDM • Backward compatibility to a, b, g • HT mixed mode • Supports OFDM and ERP-OFDM • Greenfield mode • Only ERP-OFDM • Highest data rates • Channel Bonding CCRI Engineering and Technology Jbernardini 33

  34. Co-Location WLAN systems • Co-location of IEEE 802.11b HR/DSSS and IEEE 802.11a/g/n OFDM Systems CCRI Engineering and Technology Jbernardini 34

  35. Co-Location WLAN systems • Adjacent-channel and Co-channel Interference • Adjacent-channel and co-channel interference • WLAN/WPAN Coexistence • IEEE 802.11 wireless LANs can be affected when co-located with WPAN devices CCRI Engineering and Technology Jbernardini 35

  36. Encoding and Modulation • Encoding - To change or translate one bit stream into another. • Modulation – Appling information on a carrier signal by varying one or more of the signal's basic characteristics - frequency, amplitude and phase. DBPSK (Differential Binary Phase Shift Keying) DQPSK (Differential Quaternary PSK) CCRI Engineering and Technology Jbernardini 36

  37. Modulation • Carrier signal is a continuous electrical signal • Carries no information • Three types of modulations enable carrier signals to carry information • Height of signal • Frequency of signal • Relative starting point • Modulation can be done on analog or digital transmissions CCRI Engineering and Technology Jbernardini 37

  38. Analog vs. Digital Transmissions Analog Signal = A signal that has continuously varying voltages, frequencies, or phases. All amplitude values are present from minimum to maximum signal levels. Digital Signal = A signal in which information is carried in a limited number of different discrete states or levels; High/Low, One/Zero, 1/0 CCRI Engineering and Technology Jbernardini 38

  39. Analog and Digital Modulation • Analog Transmission use analog carrier signals and analog modulation. • Digital Transmission use analog carrier signals and digital modulation. • Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium • On originating end, converts distinct digital signals into continuous analog signal for transmission • On receiving end, reverse process performed • WLANs use digital modulation of analog signals (carrier signal) CCRI Engineering and Technology Jbernardini 39

  40. Frequency and Period CCRI Engineering and Technology Jbernardini 40

  41. Analog Modulation • Amplitude: Height of carrier wave • Amplitude modulation (AM): Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit • Frequency modulation (FM): Changes number of waves representing one cycle • Number of waves to represent 1 bit more than number of waves to represent 0 bit • Phase modulation (PM): Changes starting point of cycle • When bits change from 1 to 0 bit or vice versa CCRI Engineering and Technology Jbernardini 41

  42. Analog Modulation Amplitude modulation (AM) – Carrier frequency varies in amplitude Frequency modulation (FM) – Carrier frequency varies in frequency Phase modulation (PM) –Carrier varies in phase CCRI Engineering and Technology Jbernardini 42

  43. Digital Modulation • Advantages over analog modulation: • Better use of bandwidth • Requires less power • Better handling of interference from other signals • Error-correcting techniques more compatible with other digital systems • Unlike analog modulation, changes occur in discrete steps using binary signals • Uses same three basic types of modulation as Amplitude shift keying (ASK) CCRI Engineering and Technology Jbernardini 43

  44. Frequency vs. Phase Shift Key Modulation Frequency shift keying (FSK) Phase shift keying (PSK) CCRI Engineering and Technology Jbernardini 44

  45. Throughput vs. Data Rate • Data Rate = Total Data Rate through system • Throughput = Data Payload Rate • Data Rate = Data Payload Rate + Overhead • Overhead = Coding + Modulation+ Bandwidth + Hardware + Software + Retransmission(errors) 5 Mbps Throughput 11 Mbps Data Rate 5 Mbps Throughput CCRI Engineering and Technology Jbernardini 45

  46. Analog vs. Digital Bandwidth • Analog Bandwidth – Frequency in Khz,Mhz (1 Mhz) • Digital Bandwidth – bits per second (11 Mbps) • Wireless Bandwidth – Frequency Space made available to network devices (22 Mhz) Digital Bandwidth (Average Bit Rate) Bandwidth Amplitude Frequency CCRI Engineering and Technology Jbernardini 46

  47. Quadrature phase shift keying (QPSK) CCRI Engineering and Technology Jbernardini 47

  48. 16-QAM Modulation CCRI Engineering and Technology Jbernardini 48

  49. 64-QAM - 64-level Quadrature Amplitude Modulation CCRI Engineering and Technology Jbernardini 49

  50. Spread Spectrum Comparisons CCRI Engineering and Technology Jbernardini 50

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