CS388: Wireless and Mobile Security --  Introduction Xiuzhen Cheng [email_address]
Mobile and  W ireless  S ervices –  Always Best Connected UMTS, DECT 2 Mbit/s UMTS, GSM 384 kbit/s LAN 100 Mbit/s, WLAN 54...
On the Road ad hoc UMTS, WLAN, DAB, GSM,  cdma2000, TETRA, ... Personal Travel Assistant, DAB, PDA, laptop,  GSM, UMTS, WL...
Home Networking Camcorder HDTV Game Game iPod High-quality speaker UWB WiFi WiFi WiFi Surveillance Surveillance Surveillan...
Last-Mile <ul><li>Many users still don’t have broadband </li></ul><ul><ul><li>End of 2002 </li></ul></ul><ul><ul><ul><li>W...
Disaster Recovery Network <ul><li>9/11, Tsunami, Hurricane Katrina, South Asian earthquake … </li></ul><ul><li>Wireless co...
Environmental Monitoring <ul><li>Micro-sensors, on-board processing, wireless interfaces feasible at very small scale--can...
Challenges in Wireless Networking Research
Challenge 1: Unreliable and Unpredictable Wireless Links Asymmetry vs. Power Reception v. Distance Standard Deviation v.  ...
Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul>S1 S2 R1 R2
Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals </li></ul>S1 S2 R1 R2 S...
Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals  </li></ul><ul><li>Expo...
Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals </li></ul><ul><li>Expos...
Challenge 3: Intermittent Connectivity <ul><li>Reasons for intermittent connectivity </li></ul><ul><ul><li>Mobility </li><...
Challenge 4: Limited Resources <ul><li>Limited battery power </li></ul><ul><li>Limited bandwidth </li></ul><ul><li>Limited...
Introduction to  Wireless Networking
Internet Protocol Stack <ul><li>Application:  supporting network applications </li></ul><ul><ul><li>FTP, SMTP, HTTP </li><...
Physical Layer
Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></u...
Overview of Wireless Transmissions bit stream receiver bit stream analog signal sender source decoding channel decoding de...
Signals <ul><li>Physical representation of data </li></ul><ul><li>Function of time and location </li></ul><ul><li>Classifi...
Signals (Cont.) <ul><li>Signal parameters of periodic signals:  </li></ul><ul><ul><li>period T, frequency f=1/T </li></ul>...
Fourier  Transform: Every Signal Can be Decomposed as a Collection of Harmonics 1 0 1 0 t t ideal periodic al   digital  s...
 
Why Not Send Digital Signal in Wireless Communications? <ul><li>Digital signals need </li></ul><ul><ul><li>infinite freque...
Frequencies for  C ommunication VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency  SHF = Super High F...
Frequency vs. Bandwidth <ul><li>Frequency is a specific location on the electromagnetic spectrum </li></ul><ul><li>Bandwid...
Bandwidth vs. Capacity <ul><li>Capacity is usually measured by Mega bits per second (Mbps) </li></ul><ul><li>Bandwidth for...
An example: IEEE 802.11b (WiFi) <ul><li>Operating center frequency: 2.4 GHz. </li></ul><ul><ul><li>There are 11 channels i...
Why Need A Wide Spectrum :  Shannon  Channel Capacity <ul><li>The maximum number of bits that can be transmitted per secon...
Signal, Noise, and Interference <ul><li>Signal (S) </li></ul><ul><li>Noise (N) </li></ul><ul><ul><li>Includes thermal nois...
Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></u...
Signal  P ropagation  R anges distance sender transmission detection interference <ul><li>Transmission range </li></ul><ul...
Signal  P ropagation <ul><li>Propagation in free space always like light (straight line) </li></ul><ul><li>Receiving power...
Path Loss <ul><li>Free space model </li></ul><ul><li>Two-ray ground reflection model </li></ul><ul><li>Log-normal shadowin...
Multipath  P ropagation <ul><li>Signal can take many different paths between sender and receiver due to reflection, scatte...
Fading <ul><li>Channel characteristics change over time and location  </li></ul><ul><ul><li>e.g., movement of sender, rece...
Typical Picture shadow fading Rayleigh fading path loss log (distance) Received  Signal  Power  (dB)
Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></u...
<ul><li>Goal: multiple use of a shared medium </li></ul><ul><li>Multiplexing in 4 dimensions </li></ul><ul><ul><li>space (...
Space Multiplexing <ul><li>Assign each region a channel </li></ul><ul><li>Pros </li></ul><ul><ul><li>no dynamic coordinati...
Frequency Multiplexing <ul><li>Separation of the whole spectrum into smaller frequency bands </li></ul><ul><li>A channel g...
Time Multiplex <ul><li>A channel gets the whole spectrum for a certain amount of time </li></ul><ul><li>Pros: </li></ul><u...
Time and Frequency Multiplexing <ul><li>Combination of both methods </li></ul><ul><li>A channel gets a certain frequency b...
Code Multiplexing <ul><li>Each channel has a unique code </li></ul><ul><li>All channels use the same spectrum simultaneous...
Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></u...
Modulation I <ul><li>Digital modulation </li></ul><ul><ul><li>digital data is translated into an analog signal (baseband) ...
Modulation and Demodulation digital modulation digital data analog modulation radio carrier analog baseband signal 1011010...
Modulation Schemes <ul><li>Amplitude Modulation (AM) </li></ul><ul><li>Frequency Modulation (FM) </li></ul><ul><li>Phase M...
Digital  M odulation <ul><li>Modulation of digital signals known as  Shift Keying </li></ul><ul><li>Amplitude Shift Keying...
Digital  M odulation II <ul><li>Frequency Shift Keying (FSK): </li></ul><ul><ul><li>Pros: less susceptible to noise </li><...
Digital  M odulation III <ul><li>Phase Shift Keying (PSK): </li></ul><ul><ul><li>Pros:  </li></ul></ul><ul><ul><ul><li>Les...
Phase Shift Keying <ul><li>BPSK (Binary Phase Shift Keying): </li></ul><ul><ul><li>bit value 0: sine wave </li></ul></ul><...
Quadrature Amplitude Modulation <ul><li>Example: 16-QAM (4 bits = 1 symbol) </li></ul><ul><li>Symbols 0011 and 0001 have t...
Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></u...
Spread spectrum technology <ul><li>Problem of radio transmission: frequency dependent fading can wipe out narrow band sign...
DSSS  (Direct Sequence Spread Spectrum) <ul><li>XOR of the signal with pseudo-random number (chipping sequence) </li></ul>...
FHSS  (Frequency Hopping Spread Spectrum) <ul><li>Discrete changes of carrier frequency </li></ul><ul><ul><li>sequence of ...
FHSS: Example user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 0 1 t b 0 1 1 t f f 1 f 2 f 3 t t d f f 1 f 2 ...
Comparison between Slow Hopping and Fast Hopping <ul><li>Slow hopping </li></ul><ul><ul><li>Pros: cheaper  </li></ul></ul>...
Wireless Standards
Wireless technologies/standards <ul><li>802.11a </li></ul><ul><li>802.11b (Wi-Fi) </li></ul><ul><li>802.11g (Wi-Fi) </li><...
IEEE 802.11a/b/g (Wi-Fi)                            Less range than 802.11b Not as fast as other technologies Not as widel...
IEEE 802.16 (WiMAX) <ul><li>802.16d – A.K.A 802.16-2004 </li></ul><ul><ul><li>Intended for &quot;last mile&quot; connectiv...
IEEE 802.20 (MBWA) <ul><li>Mobile Broadband Wireless Access (MBWA) Working Group  </li></ul><ul><ul><li>1 Mbps </li></ul><...
IEEE 802.11i (WPA2) <ul><li>Provide improvements to WiFi security </li></ul><ul><li>Address security shortcomings in WEP <...
Evolution Data Only (EvDO) <ul><li>Available in Larger Metro Areas </li></ul><ul><ul><li>Offered by Sprint, Verizon, Other...
Elements of a wireless network <ul><li>wireless hosts </li></ul><ul><li>base station </li></ul><ul><li>wireless link </li>...
Elements of a wireless network <ul><li>Ad hoc mode </li></ul><ul><li>no base stations </li></ul><ul><li>nodes can only tra...
Why a wireless network is more subjected to attacks?
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  • Vehicles transmission of news, road condition, weather, music via DAB (Digital Audio Broadcasting) personal communication using GSM position via GPS local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance Emergencies early transmission of patient data to the hospital, current status, first diagnosis replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc. crisis, war, ... GPS, GSM, WLAN, satellite, Vehicle networks, emergency networks,
  • How to deploy, manage, operate such networks?
  • Wireless communication capability not only makes our life more entertaining and convenient, but also can make a difference between life and death. Tsunami washed away significant parts of Indonesia, Thailand and India and killed nearly 120,000; Hurricane Katrina ravaged the United States Gulf Coast and killed thousands as it destroyed New Orleans; South Asian earthquake leveled entire villages in the Kashmir region of Pakistan, leaving millions homeless. Wireless communication is no longer a luxury, but could make a big difference between life and death.
  • Does multiple radio enable CSMA/CACD
  • Does multiple radio enable CSMA/CACD
  • Does multiple radio enable CSMA/CACD
  • Does multiple radio enable CSMA/CACD
  • Channel coding redundancy added to encoded signal to improve system performance (error rate) Modulation: convert discrete-time encoded data stream into continuous waveform
  • LF: submarine MF &amp; HF: radio (AM &amp; FM) VHF &amp; UHF: TV UHF mobile phoes, cordless phones, 3G SHF: microwave, wifi
  • These ranges are not circular in the real world.
  • If the size of an object is on the order of wavelength, the signal can be scattered.
  • L is the path loss
  • ISI limits the bandwidth of radio channels.
  • Fast fading: usually observed over distances of about half a wavelength. For VHF and UHF, a vehicle traveling at 30 miles per hour can pass through several fast fades in a second Slow fading: movement over distances large enough to produce gross variations in the overall path
  • W orldwide I nteroperability for M icrowave Acc ess
  • PPT

    1. 1. CS388: Wireless and Mobile Security -- Introduction Xiuzhen Cheng [email_address]
    2. 2. Mobile and W ireless S ervices – Always Best Connected UMTS, DECT 2 Mbit/s UMTS, GSM 384 kbit/s LAN 100 Mbit/s, WLAN 54 Mbit/s UMTS, GSM 115 kbit/s GSM 115 kbit/s, WLAN 11 Mbit/s GSM 53 kbit/s Bluetooth 500 kbit/s GSM/EDGE 384 kbit/s, WLAN 780 kbit/s LAN, WLAN 780 kbit/s
    3. 3. On the Road ad hoc UMTS, WLAN, DAB, GSM, cdma2000, TETRA, ... Personal Travel Assistant, DAB, PDA, laptop, GSM, UMTS, WLAN, Bluetooth, ...
    4. 4. Home Networking Camcorder HDTV Game Game iPod High-quality speaker UWB WiFi WiFi WiFi Surveillance Surveillance Surveillance WiFi WiFi GSM
    5. 5. Last-Mile <ul><li>Many users still don’t have broadband </li></ul><ul><ul><li>End of 2002 </li></ul></ul><ul><ul><ul><li>Worldwide: 46 million broadband subscribers </li></ul></ul></ul><ul><ul><ul><li>US: 17% household have broadband </li></ul></ul></ul><ul><ul><li>Reasons: out of service area; some consider expensive </li></ul></ul><ul><li>Broadband speed is still limited </li></ul><ul><ul><li>DSL: 1-3 Mbps download, and 100-400Kbps upload </li></ul></ul><ul><ul><li>Cable modem: depends on your neighbors </li></ul></ul><ul><ul><li>Insufficient for several applications (e.g., high-quality video streaming) </li></ul></ul>
    6. 6. Disaster Recovery Network <ul><li>9/11, Tsunami, Hurricane Katrina, South Asian earthquake … </li></ul><ul><li>Wireless communication capability can make a difference between life and death! </li></ul><ul><li>How to enable efficient, flexible, and resilient communication? </li></ul><ul><ul><li>Rapid deployment </li></ul></ul><ul><ul><li>Efficient resource and energy usage </li></ul></ul><ul><ul><li>Flexible: unicast, broadcast, multicast, anycast </li></ul></ul><ul><ul><li>Resilient: survive in unfavorable and untrusted environment </li></ul></ul>
    7. 7. Environmental Monitoring <ul><li>Micro-sensors, on-board processing, wireless interfaces feasible at very small scale--can monitor phenomena “up close” </li></ul><ul><li>Enables spatially and temporally dense environmental monitoring </li></ul><ul><li>Embedded Networked Sensing will reveal previously unobservable phenomena </li></ul>Contaminant Transport Ecosystems, Biocomplexity Marine Microorganisms Seismic Structure Response
    8. 8. Challenges in Wireless Networking Research
    9. 9. Challenge 1: Unreliable and Unpredictable Wireless Links Asymmetry vs. Power Reception v. Distance Standard Deviation v. Reception rate * Cerpa, Busek et. al What Robert Poor (Ember) calls “The good, the bad and the ugly” <ul><li>Wireless links are less reliable </li></ul><ul><li>They may vary over time and space </li></ul>
    10. 10. Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul>S1 S2 R1 R2
    11. 11. Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals </li></ul>S1 S2 R1 R2 S1 R1 S2
    12. 12. Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals </li></ul><ul><li>Exposed terminal </li></ul>S1 S2 R1 R1 S1 R1 R2 R1 S1 S2 R2
    13. 13. Challenge 2: Open Wireless Medium <ul><li>Wireless interference </li></ul><ul><li>Hidden terminals </li></ul><ul><li>Exposed terminal </li></ul><ul><li>Wireless security </li></ul><ul><ul><li>Eavesdropping, Denial of service, Jamming… </li></ul></ul>S1 S2 R1 R1 S1 R1 S2 R1 S1 S2 R2
    14. 14. Challenge 3: Intermittent Connectivity <ul><li>Reasons for intermittent connectivity </li></ul><ul><ul><li>Mobility </li></ul></ul><ul><ul><li>Environmental changes </li></ul></ul><ul><li>Existing networking protocols assume always-on networks </li></ul><ul><li>Under intermittent connected networks </li></ul><ul><ul><li>Routing, TCP, and applications all break </li></ul></ul><ul><li>Need a new paradigm to support communication under such environments </li></ul>
    15. 15. Challenge 4: Limited Resources <ul><li>Limited battery power </li></ul><ul><li>Limited bandwidth </li></ul><ul><li>Limited processing and storage power </li></ul>Sensors, embedded controllers <ul><li>Mobile phones </li></ul><ul><li>voice, data </li></ul><ul><li>simple graphical displays </li></ul><ul><li>GSM </li></ul><ul><li>PDA </li></ul><ul><li>data </li></ul><ul><li>simpler graphical displays </li></ul><ul><li>802.11 </li></ul><ul><li>Laptop </li></ul><ul><li>fully functional </li></ul><ul><li>standard applications </li></ul><ul><li>battery; 802.11 </li></ul>
    16. 16. Introduction to Wireless Networking
    17. 17. Internet Protocol Stack <ul><li>Application: supporting network applications </li></ul><ul><ul><li>FTP, SMTP, HTTP </li></ul></ul><ul><li>Transport: data transfer between processes </li></ul><ul><ul><li>TCP, UDP </li></ul></ul><ul><li>Network: routing of datagrams from source to destination </li></ul><ul><ul><li>IP, routing protocols </li></ul></ul><ul><li>Link: data transfer between neighboring network elements </li></ul><ul><ul><li>Ethernet, WiFi </li></ul></ul><ul><li>Physical: bits “on the wire” </li></ul><ul><ul><li>Coaxial cable, optical fibers, radios </li></ul></ul>application transport network link physical
    18. 18. Physical Layer
    19. 19. Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Spread Spectrum </li></ul>
    20. 20. Overview of Wireless Transmissions bit stream receiver bit stream analog signal sender source decoding channel decoding demodulation source coding channel coding modulation
    21. 21. Signals <ul><li>Physical representation of data </li></ul><ul><li>Function of time and location </li></ul><ul><li>Classification </li></ul><ul><ul><li>continuous time/discrete time </li></ul></ul><ul><ul><li>continuous values/discrete values </li></ul></ul><ul><ul><li>analog signal = continuous time and continuous values </li></ul></ul><ul><ul><li>digital signal = discrete time and discrete values </li></ul></ul>
    22. 22. Signals (Cont.) <ul><li>Signal parameters of periodic signals: </li></ul><ul><ul><li>period T, frequency f=1/T </li></ul></ul><ul><ul><li>amplitude A </li></ul></ul><ul><ul><li>phase shift  </li></ul></ul><ul><ul><li>sine wave as a special periodic signal for a carrier: s(t) = A t sin(2  f t t +  t ) </li></ul></ul>1 0 t
    23. 23. Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics 1 0 1 0 t t ideal periodic al digital signal decomposition The more harmonics used, the smaller the approximation error.
    24. 25. Why Not Send Digital Signal in Wireless Communications? <ul><li>Digital signals need </li></ul><ul><ul><li>infinite frequencies for perfect transmissions </li></ul></ul><ul><ul><li>however, we have limited frequencies in wireless communications </li></ul></ul>
    25. 26. Frequencies for C ommunication VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length:  = c/f , wave length  , speed of light c  3x10 8 m/s, frequency f 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100  m 3 THz 1  m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable twisted pair
    26. 27. Frequency vs. Bandwidth <ul><li>Frequency is a specific location on the electromagnetic spectrum </li></ul><ul><li>Bandwidth is the range between two frequencies </li></ul><ul><ul><li>Bandwidth is measured in Hertz </li></ul></ul><ul><ul><li>A cellular operator may transmit signals between 824-849 MHz, for a total bandwidth of 25 MHz </li></ul></ul>
    27. 28. Bandwidth vs. Capacity <ul><li>Capacity is usually measured by Mega bits per second (Mbps) </li></ul><ul><li>Bandwidth for a particular service is fixed, but the number of calls and the rate of data transmission is not (capacity) </li></ul>
    28. 29. An example: IEEE 802.11b (WiFi) <ul><li>Operating center frequency: 2.4 GHz. </li></ul><ul><ul><li>There are 11 channels in 802.11b. Starting from 2.412 GHz to 2.462 GHz. </li></ul></ul><ul><li>Spectrum: 2.412 GHz ~ 2.462 GHz </li></ul><ul><li>Bandwidth: 40 MHz. </li></ul><ul><li>Capacity: 1, 2, 5.5, and 11Mbps. Typical data rate is about 6.5Mbps. </li></ul>
    29. 30. Why Need A Wide Spectrum : Shannon Channel Capacity <ul><li>The maximum number of bits that can be transmitted per second by a physical channel is: </li></ul><ul><li>where W is the frequency range that the media allows to pass through, S/N is the signal noise ratio </li></ul>
    30. 31. Signal, Noise, and Interference <ul><li>Signal (S) </li></ul><ul><li>Noise (N) </li></ul><ul><ul><li>Includes thermal noise and background radiation </li></ul></ul><ul><ul><li>Often modeled as additive white Gaussian noise </li></ul></ul><ul><li>Interference (I) </li></ul><ul><ul><li>Signals from other transmitting sources </li></ul></ul><ul><li>SINR = S/(N+I) (sometimes also denoted as SNR) </li></ul>
    31. 32. Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Spread Spectrum </li></ul>
    32. 33. Signal P ropagation R anges distance sender transmission detection interference <ul><li>Transmission range </li></ul><ul><ul><li>communication possible </li></ul></ul><ul><ul><li>low error rate </li></ul></ul><ul><li>Detection range </li></ul><ul><ul><li>detection of the signal possible </li></ul></ul><ul><ul><li>no communication possible </li></ul></ul><ul><li>Interference range </li></ul><ul><ul><li>signal may not be detected </li></ul></ul><ul><ul><li>signal adds to the background noise </li></ul></ul>
    33. 34. Signal P ropagation <ul><li>Propagation in free space always like light (straight line) </li></ul><ul><li>Receiving power proportional to 1/d² (d = distance between sender and receiver) </li></ul><ul><li>Receiving power additionally influenced by </li></ul><ul><ul><li>Shadow loss by obstructions </li></ul></ul><ul><ul><li>reflection at large obstacles </li></ul></ul><ul><ul><li>refraction depending on the density of a medium </li></ul></ul><ul><ul><li>scattering at small obstacles </li></ul></ul><ul><ul><li>diffraction at edges </li></ul></ul><ul><ul><li>fading (frequency dependent) </li></ul></ul>reflection scattering diffraction shadowing refraction
    34. 35. Path Loss <ul><li>Free space model </li></ul><ul><li>Two-ray ground reflection model </li></ul><ul><li>Log-normal shadowing </li></ul><ul><li>Indoor model </li></ul><ul><li>P = 1 mW at d0=1m, what’s Pr at d=2m? </li></ul>
    35. 36. Multipath P ropagation <ul><li>Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction </li></ul><ul><li>Time dispersion: signal is dispersed over time </li></ul><ul><li> interference with “neighbor” symbols, Inter Symbol Interference (ISI) </li></ul><ul><li>The signal reaches a receiver directly and phase shifted </li></ul><ul><li> distorted signal based on the phases of different parts </li></ul>signal at sender signal at receiver LOS pulses multipath pulses LOS: Line Of Sight
    36. 37. Fading <ul><li>Channel characteristics change over time and location </li></ul><ul><ul><li>e.g., movement of sender, receiver and/or scatters </li></ul></ul><ul><li> quick changes in the power received (short term/fast fading) </li></ul><ul><li>Additional changes in </li></ul><ul><ul><li>distance to sender </li></ul></ul><ul><ul><li>obstacles further away </li></ul></ul><ul><li> slow changes in the average power received (long term/slow fading) </li></ul>short term fading long term fading t power
    37. 38. Typical Picture shadow fading Rayleigh fading path loss log (distance) Received Signal Power (dB)
    38. 39. Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Spread Spectrum </li></ul>
    39. 40. <ul><li>Goal: multiple use of a shared medium </li></ul><ul><li>Multiplexing in 4 dimensions </li></ul><ul><ul><li>space (s i ) </li></ul></ul><ul><ul><li>time (t) </li></ul></ul><ul><ul><li>frequency (f) </li></ul></ul><ul><ul><li>code (c) </li></ul></ul><ul><li>Important: guard spaces needed! </li></ul>Multiplexing
    40. 41. Space Multiplexing <ul><li>Assign each region a channel </li></ul><ul><li>Pros </li></ul><ul><ul><li>no dynamic coordination necessary </li></ul></ul><ul><ul><li>works also for analog signals </li></ul></ul><ul><li>Cons </li></ul><ul><ul><li>Inefficient resource utilization </li></ul></ul>s 2 s 3 s 1 f t c k 2 k 3 k 4 k 5 k 6 k 1 f t c f t c channels k i
    41. 42. Frequency Multiplexing <ul><li>Separation of the whole spectrum into smaller frequency bands </li></ul><ul><li>A channel gets a certain band of the spectrum for the whole time </li></ul><ul><li>Pros: </li></ul><ul><ul><li>no dynamic coordination necessary </li></ul></ul><ul><ul><li>works also for analog signals </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>waste of bandwidth if the traffic is distributed unevenly </li></ul></ul><ul><ul><li>Inflexible </li></ul></ul><ul><ul><li>guard spaces </li></ul></ul>k 2 k 3 k 4 k 5 k 6 k 1 f t c
    42. 43. Time Multiplex <ul><li>A channel gets the whole spectrum for a certain amount of time </li></ul><ul><li>Pros: </li></ul><ul><ul><li>only one carrier in the medium at any time </li></ul></ul><ul><ul><li>throughput high even for many users </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>precise synchronization necessary </li></ul></ul>f t c k 2 k 3 k 4 k 5 k 6 k 1
    43. 44. Time and Frequency Multiplexing <ul><li>Combination of both methods </li></ul><ul><li>A channel gets a certain frequency band for a certain amount of time (e.g., GSM) </li></ul><ul><li>Pros: </li></ul><ul><ul><li>better protection against tapping </li></ul></ul><ul><ul><li>protection against frequency selective interference </li></ul></ul><ul><ul><li>higher data rates compared to code multiplex </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>precise coordination required </li></ul></ul>f t c k 2 k 3 k 4 k 5 k 6 k 1
    44. 45. Code Multiplexing <ul><li>Each channel has a unique code </li></ul><ul><li>All channels use the same spectrum simultaneously </li></ul><ul><li>Pros: </li></ul><ul><ul><li>bandwidth efficient </li></ul></ul><ul><ul><li>no coordination and synchronization necessary </li></ul></ul><ul><ul><li>good protection against interference and tapping </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>lower user data rates </li></ul></ul><ul><ul><li>more complex signal regeneration </li></ul></ul><ul><li>Implemented using spread spectrum technology </li></ul>k 2 k 3 k 4 k 5 k 6 k 1 f t c
    45. 46. Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Spread Spectrum </li></ul>
    46. 47. Modulation I <ul><li>Digital modulation </li></ul><ul><ul><li>digital data is translated into an analog signal (baseband) </li></ul></ul><ul><ul><li>differences in spectral efficiency, power efficiency, robustness </li></ul></ul><ul><li>Analog modulation </li></ul><ul><ul><li>shifts center frequency of baseband signal up to the radio carrier </li></ul></ul><ul><ul><li>Reasons </li></ul></ul><ul><ul><ul><li>Antenna size is on the order of signal’s wavelength </li></ul></ul></ul><ul><ul><ul><li>More bandwidth available at higher carrier frequency </li></ul></ul></ul><ul><ul><ul><li>Medium characteristics: path loss, shadowing, reflection, scattering, diffraction depend on the signal’s wavelength </li></ul></ul></ul>
    47. 48. Modulation and Demodulation digital modulation digital data analog modulation radio carrier analog baseband signal 101101001 radio transmitter synchronization decision digital data analog demodulation radio carrier analog baseband signal 101101001 radio receiver
    48. 49. Modulation Schemes <ul><li>Amplitude Modulation (AM) </li></ul><ul><li>Frequency Modulation (FM) </li></ul><ul><li>Phase Modulation (PM) </li></ul>
    49. 50. Digital M odulation <ul><li>Modulation of digital signals known as Shift Keying </li></ul><ul><li>Amplitude Shift Keying (ASK): </li></ul><ul><ul><li>Pros: simple </li></ul></ul><ul><ul><li>Cons: susceptible to noise </li></ul></ul><ul><ul><li>Example: optical system, IFR </li></ul></ul>1 0 1 t
    50. 51. Digital M odulation II <ul><li>Frequency Shift Keying (FSK): </li></ul><ul><ul><li>Pros: less susceptible to noise </li></ul></ul><ul><ul><li>Cons: requires larger bandwidth </li></ul></ul>1 0 1 t 1 0 1
    51. 52. Digital M odulation III <ul><li>Phase Shift Keying (PSK): </li></ul><ul><ul><li>Pros: </li></ul></ul><ul><ul><ul><li>Less susceptible to noise </li></ul></ul></ul><ul><ul><ul><li>Bandwidth efficient </li></ul></ul></ul><ul><ul><li>Cons: </li></ul></ul><ul><ul><ul><li>Require synchronization in frequency and phase  complicates receivers and transmitter </li></ul></ul></ul>t
    52. 53. Phase Shift Keying <ul><li>BPSK (Binary Phase Shift Keying): </li></ul><ul><ul><li>bit value 0: sine wave </li></ul></ul><ul><ul><li>bit value 1: inverted sine wave </li></ul></ul><ul><ul><li>very simple PSK </li></ul></ul><ul><ul><li>low spectral efficiency </li></ul></ul><ul><ul><li>robust, used in satellite systems </li></ul></ul>Q I 0 1 11 10 00 01 Q I 11 01 10 00 A t <ul><li>QPSK (Quadrature Phase Shift Keying): </li></ul><ul><ul><li>2 bits coded as one symbol </li></ul></ul><ul><ul><li>needs less bandwidth compared to BPSK </li></ul></ul><ul><ul><li>symbol determines shift of sine wave </li></ul></ul><ul><ul><li>Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK </li></ul></ul>
    53. 54. Quadrature Amplitude Modulation <ul><li>Example: 16-QAM (4 bits = 1 symbol) </li></ul><ul><li>Symbols 0011 and 0001 have the same phase φ , but different amplitude a . 0000 and 1000 have same amplitude but different phase </li></ul><ul><li>Used in Modem </li></ul><ul><li>Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation </li></ul><ul><li>It is possible to code n bits using one symbol </li></ul><ul><ul><li>2 n discrete levels </li></ul></ul><ul><li>bit error rate increases with n </li></ul>0000 0001 0011 1000 Q I 0010 φ a
    54. 55. Physical Layer Outline <ul><li>Signal </li></ul><ul><li>Frequency allocation </li></ul><ul><li>Signal propagation </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Spread Spectrum </li></ul>
    55. 56. Spread spectrum technology <ul><li>Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference </li></ul><ul><li>Solution: spread the narrow band signal into a broad band signal using a special code </li></ul><ul><li>Side effects: </li></ul><ul><ul><li>coexistence of several signals without dynamic coordination </li></ul></ul><ul><ul><li>tap-proof </li></ul></ul><ul><li>Alternatives: Direct Sequence, Frequency Hopping </li></ul>detection at receiver interference spread signal signal spread interference f f power power
    56. 57. DSSS (Direct Sequence Spread Spectrum) <ul><li>XOR of the signal with pseudo-random number (chipping sequence) </li></ul><ul><ul><li>generate a signal with a wider range of frequency: spread spectrum </li></ul></ul>user data chipping sequence resulting signal 0 1 0 1 1 0 1 0 1 0 1 0 0 1 1 1 XOR 0 1 1 0 0 1 0 1 1 0 1 0 0 1 = t b t c t b : bit period t c : chip period
    57. 58. FHSS (Frequency Hopping Spread Spectrum) <ul><li>Discrete changes of carrier frequency </li></ul><ul><ul><li>sequence of frequency changes determined via pseudo random number sequence </li></ul></ul><ul><li>Two versions </li></ul><ul><ul><li>Fast Hopping: several frequencies per user bit </li></ul></ul><ul><ul><li>Slow Hopping: several user bits per frequency </li></ul></ul><ul><li>Advantages </li></ul><ul><ul><li>frequency selective fading and interference limited to short period </li></ul></ul><ul><ul><li>simple implementation </li></ul></ul><ul><ul><li>uses only small portion of spectrum at any time </li></ul></ul>
    58. 59. FHSS: Example user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 0 1 t b 0 1 1 t f f 1 f 2 f 3 t t d f f 1 f 2 f 3 t t d t b : bit period t d : dwell time
    59. 60. Comparison between Slow Hopping and Fast Hopping <ul><li>Slow hopping </li></ul><ul><ul><li>Pros: cheaper </li></ul></ul><ul><ul><li>Cons: less immune to narrowband interference </li></ul></ul><ul><li>Fast hopping </li></ul><ul><ul><li>Pros: more immune to narrowband interference </li></ul></ul><ul><ul><li>Cons: tight synchronization  increased complexity </li></ul></ul>
    60. 61. Wireless Standards
    61. 62. Wireless technologies/standards <ul><li>802.11a </li></ul><ul><li>802.11b (Wi-Fi) </li></ul><ul><li>802.11g (Wi-Fi) </li></ul><ul><li>802.11i (Security) </li></ul><ul><li>802.16 2004, e & f (WiMAX) </li></ul><ul><li>Bluetooth (802.15) </li></ul><ul><li>1G: CDPD (Cellular Digital Packet Data) </li></ul><ul><li>2G: GSM (Global System for Mobile Communications) GPRS (General Packet Radio Service) </li></ul><ul><li>3G: CDMA2000, WCDMA </li></ul><ul><li>EvDO (Evolution Data Only) </li></ul>
    62. 63. IEEE 802.11a/b/g (Wi-Fi)                            Less range than 802.11b Not as fast as other technologies Not as widely implemented, shorter range Faster than 802.11b and better range than 802.11a Best over-all coverage range Less interference, more bandwidth 54 Mbps 11 Mbps 54 Mbps 2.4 GHz 2.4 GHz 5 GHz 802.11g 802.11b 802.11a
    63. 64. IEEE 802.16 (WiMAX) <ul><li>802.16d – A.K.A 802.16-2004 </li></ul><ul><ul><li>Intended for &quot;last mile&quot; connectivity at high data rates. </li></ul></ul><ul><ul><li>Point-to-multipoint only implementation </li></ul></ul><ul><li>802.16e – Adds mobility </li></ul><ul><ul><li>approved in December 2005. </li></ul></ul>
    64. 65. IEEE 802.20 (MBWA) <ul><li>Mobile Broadband Wireless Access (MBWA) Working Group </li></ul><ul><ul><li>1 Mbps </li></ul></ul><ul><ul><li>Mobile speeds of 100mph </li></ul></ul><ul><ul><li>Could compete with 3G cellular </li></ul></ul><ul><ul><li>Licensed band use only </li></ul></ul>
    65. 66. IEEE 802.11i (WPA2) <ul><li>Provide improvements to WiFi security </li></ul><ul><li>Address security shortcomings in WEP </li></ul><ul><li>Add user authentication </li></ul>
    66. 67. Evolution Data Only (EvDO) <ul><li>Available in Larger Metro Areas </li></ul><ul><ul><li>Offered by Sprint, Verizon, Other </li></ul></ul><ul><ul><li>700Mbps </li></ul></ul><ul><li>Supports Streaming Video </li></ul>
    67. 68. Elements of a wireless network <ul><li>wireless hosts </li></ul><ul><li>base station </li></ul><ul><li>wireless link </li></ul><ul><li>Network infrastructure </li></ul>network infrastructure
    68. 69. Elements of a wireless network <ul><li>Ad hoc mode </li></ul><ul><li>no base stations </li></ul><ul><li>nodes can only transmit to other nodes within link coverage </li></ul><ul><li>nodes organize themselves into a network: route among themselves </li></ul>
    69. 70. Why a wireless network is more subjected to attacks?

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