1. 1 CSCD 433/533 Wireless Networks and Security Lecture 8 Physical Layer, and 802.11 b,g,a,n,ac Differences Fall 2013

2. 2 Topics Differences between 802.11 b,g,a and n  Frequency ranges  Speed Spread Spectrum Techniques DSSS Spread Spectrum, 802.11b OFDM and 802.11n

3. 3 Introduction Today, discuss physical layer of 802.11 standard Many flavors and techniques that help to increase throughput via various techniques We will start with slowest and end with fastest

4. 4 Introduction General question we will address is how do we share bandwidth at physical wireless level? Look at wireless characteristics of signals and FCC regulations that govern sharing of unlicensed bands

5. 5 FCC Regulation In 1995, Federal Communications Commission allocated several bands of wireless spectrum for use without license FCC stipulated that use of spread spectrum technology would be required In 1990, IEEE began exploring a standard July 1999 the 802.11b standard was ratified

6. 5-6 Spread Spectrum Transmission –You are required by law to use spread spectrum transmission in unlicensed bands –Spread spectrum transmission reduces propagation problems Especially multipath interference –Spread spectrum transmission is NOT used for security in WLANs Although military does use spread spectrum transmission to make signals hard to detect This requires a different spread spectrum technology

7. 7 Frequency Band ISM: Industry, Science, Medicine unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz

8. 8 IEEE 802.11 Frequency Band and 802.11b/g 802.11a Wavelength

9. 9 802.11 Physical Channels The 802.11b standard defines 14 frequency channels in the 2.4GHz range  Only eleven are allowed for unlicensed use by the FCC in the US  Each channel uses "Direct Sequence Spread Spectrum" (DSSS) to spread data over channel that extends 11MHz on each side of center frequency  Channels overlap, but there are three out of 11 channels that don't

10. 10 802.11b/g Channels 2400 – 2483 Each channel spaced 5 MHz apart Only non-overlapping channels are 1, 6 and 11 Channel Width = 22 MHzChannels – 12 – 14, not sanctioned by FCC

11. Frequency Assignments Channel 1 2.412 GHz Channel 6 2.437 GHz Channel 11 2.462 GHz 25 MHz The Center frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. DSSS systems with overlapping channels in the same physical space would cause interference between systems.  Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc.  Channels 1, 6 and 11 are the only theoretically non-overlapping channels.

12. 12 Comparisons of 802.11 Physical Layer 3 Flavors of 802.11  802.11a  802.11b  802.11g Newest ones  802.11n and 802.11ac

13. 13 Radio Communications How do you transmit Radio Signals reliably?  Classic approach …. Confine information carrying signal to a narrow frequency band and pump as much power as possible into signal  Noise occurs as distortion in frequency band  Overcome noise Ensure power of signal > noise Recall, SNR = Signal to Noise Ratio » But, what if you include your own noise into the signal?

14. 14 Radio Communications Legal authority imposes rules on how RG spectrum is used FCC in US European Radiocommunications Office (ERO) European Telecommunications Standards Institute (ETSI) Ministry of Internal Communications (MIC) in Japan  Worldwide harmonization work done under International Telecommunications Union (ITU)  Must have license to transmit at given frequency except for certain bands …

15. 15 Radio Communications There are some unlicensed bands  802.11 Networks operate in bands which are license free, Industrial, Scientific and Medical (ISM)  Does require FCC oversight, requires manufacturer to file information with the FCC  Competing devices have been developed in 2.4 GHz range 802.11 products Bluetooth Cordless phones X10 – Protocol for home automation

16. 16 Radio Communications 2.4 GHz is Unlicensed but  Must obey FCC limitations on power, band use and purity of signal  No regulations specify coding or modulation  Thus, there is contention between devices  Solve the problems Stop using device, amplify its power or move it  Can’t rely on FCC to step in

17. 17 Radio Communications Given multiple devices compete in ISM bands, how do you reliably transmit data?  Spread Spectrum is one of the answers  Radio signals are sent with as much power as allowed over a narrow band of frequency Spread Spectrum  Used to transform radio for data  Uses math functions to diffuse signal over large range of frequencies  Makes transmissions look like noise to narrowband receiver

18. 18 Radio Communications Spread Spectrum continued  On receiver side, signal is transformed back to narrow-band and noise is removed  Spread spectrum is a requirement for unlicensed devices  Minimize interference between unlicensed devices, FCC imposes limitations on power of transmissions

19. 19 Radio Communications Trivia Question  Who patented spread spectrum transmission and when was it patented?

20. 20 Hedy Lamarr Austrian actress Hedy Lamarr became a pioneer in the field of wireless communications following her emigration to the United States With co-inventor George Anthiel, developed a "Secret Communications System" to help combat the Nazis in World War II By manipulating radio frequencies at irregular intervals between transmission and reception, the invention formed an unbreakable code to prevent classified messages from being intercepted by enemy personnel Patented in 1941

21. 21 Spread Spectrum 802.11 uses three different Spread Spectrum technologies 1. FH – Frequency Hopping (FHSS) Jumps from one frequency to another in random pattern  Transmits a short burst at each subchannel 2 Mbps FH or FHSS is the original spread spectrum technology developed in 1997 with the 802.11 standard However, it was quickly bypassed by more sophisticated spread spectrum technologies We won’t cover it, not enough time FHSS is covered in, http://www.cs.clemson.edu/~westall/851/spread-spectrum.pdf

22. 22 Spread Spectrum 802.11 uses three different Spread Spectrum technologies 2. DS or DSSS Direct Sequence Took over from FHSS and allowed for faster throughput Used in 802.11b Spreads out signal over a wider path Uses frequency coding functions 3. OFDM – Orthogonal Frequency Division Multiplexing Divides channel into several subchannels and encode a portion of signal across each subchannel in parallel 802.11a and 802.11g uses this technology Allows for even faster throughput than DSSS

23. 23 802.11 and Spread Spectrum

24. 24 Spread Spectrum Code Techniques Spread-spectrum is a signal propagation technique  Employs several methods Decrease potential interference to other receivers  Generally makes use of noise-like signal structure to spread normally narrowband information signal over a relatively wideband (radio) band of frequencies  Receiver correlates (matches) received signals to retrieve original information signal

25. Spread Spectrum Defined Spread spectrum is a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive. Spread spectrum is characterized by:  wide bandwidth and  low power Jamming and interference have less effect on Spread spectrum because it  Resembles noise  Hard to detect  Hard to intercept

26. 26 Spread Spectrum Code Techniques Typical applications include  Satellite-positioning systems (GPS)  3G mobile telecommunications  W-LAN (IEEE802.11a, IEEE802.11b, IEE802.11g)  Bluetooth

27. 27 Spread Spectrum Code Techniques Three characteristics of Spread Spectrum techniques 1. Signal occupies bandwidth much greater than that which is necessary to send the information - Many benefits, immunity to interference, jamming and multi-user access … talk about this later 2. Bandwidth is spread by means of code independent of data - Independence of code distinguishes this from standard modulation schemes in which data modulation will always spread spectrum somewhat 3. Receiver synchronizes to code to recover the data - Use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time

28. 28 Spread Spectrum (SS) Code Techniques Transmitted signal takes up more bandwidth than information signal that is being modulated  Name 'spread spectrum' comes from fact that carrier signals occur over full bandwidth (spectrum) of a device's transmitting frequency  Military has used Spread Spectrum for many years They worry about signal interception and jamming  SS signals hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times information bandwidth

29. 29 Spread Spectrum Techniques In a spread-spectrum system, signals spread across wide bandwidth, making them difficult to intercept and demodulate

30. 30 Spread Spectrum Code Techniques Spread Spectrum signals use “fast codes”  These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes  Called "Pseudo" because they are not truly random distributed noise  Will look at an example of this later

31. 31 Same code must be known in advance at both ends of the transmission channel Spread Spectrum Code Techniques Codes are what DSSS uses … talk about next Spreading de-Spreading

32. General Model of Spread Spectrum System

33. 33 Spread Spectrum Code Techniques Real advantage of SS –If Intentional or un-intentional interference and jamming ----> Signal rejected … does not contain SS key Only desired signal, which has key, will be seen at receiver when despreading operation Practically can ignore all other signals if it does not include key used in despreading operation  Allows different SS communications to be active simultaneously in same band Each will have their own PN code

34. 34 Spread Spectrum Code Techniques Can see results of interference attempts, interferer signals are not recovered

35. 35 DSSS and HR/DSSS

36. 36 DSSS DSSS is a spread spectrum technique  Modulation is altering carrier wave in order to transmit a data signal (text, voice, audio, video, etc.)  Phase-modulates a sine wave pseudorandomly Continuous string of pseudonoise (PN) code symbols called "chips“ Each of which has a much shorter duration than an information bit Each information bit is modulated by a sequence of much faster chips

37. 37 DSSS Why this works...  To a narrowband receiver, transmitted signal looks like noise  Original signal can be recovered through correlation that reverses the process  The ratio (in dB) between the spread baseband and the original signal is called processing gain  Typical SS processing gains run from 10dB to 60dB

38. 38 DSSS How DSSS works  Apply something called a “chipping” sequence to the data stream  Chip is a binary digit  But, spread-spectrum developers make distinction to separate encoding of data from the data itself Talk about data is bits Talk about encoding is chips or chipping sequence

39. 39 DSSS Chipping sequence  Also called Pseudorandom Noise Codes (PNC)  Must run at a higher rate than underlying data At left, is a data bit 0 or 1 For each bit, chip sequence is used Chip is an 11 bit code combined with a data bit to produce an 11 bit code This gets transmitted to receiver

40. 40 DSSS Chipping Sequence DataSpreading Encoded Data Correlation 1010 1010 Modulo 2 add Spreading Code 10110111000 01001000111 Modulo 2 Subtract 10110111000 Spreading Code

41. Figure 6.33 DSSS example

42. Direct Sequence Spread Spectrum Example

43. Code Division Multiple Access (CDMA) A multiplexing technique used with spread spectrum Given a data signal rate D Break each bit into k chips according to a fixed chipping code specific to each user Resulting new channel has chip data rate kD chips per second Can have multiple channels superimposed

44. CDMA Example

45. 45 DSSS Chipping stream –Two costs to increased chipping ratio 1. Direct cost of more expensive RF components that operate at higher frequencies 2. Amount of bandwidth required

46. 46 DSSS Encoding DSSS  802.11 originally adopted an 11-bit Barker word  Each bit encoded using entire Barker word or chipping sequence  Key attribute of Barker words Have good autocorrelation properties  High signal recovery possible when signal distorted by noise Correlation function operates over wide range of environments and is tolerant of propagation delay

47. 47 DSSS Encoding DSSS  Why 11 bits? Regulatory authorities require a 10 dB processing gain in DSSS systems Using an 11 bit spreading code for each bit let 802.11 meet regulatory requirements Recall  The ratio (in dB) between the spread baseband and the original signal is processing gain

48. 48 OFDM Orthogonal Frequency Division Multiplexing

49. 49 Intro to OFDM 802.11a and 802.11g based on OFDM –Orthogonal Frequency Division Multiplexing Revolutionized Wi-Fi and other cellular products by allowing faster throughput and more robustness OFDM makes highly efficient use of available spectrum

50. 50 OFDM Based on FDM Recall … –Frequency division multiplexing (FDM) is technology that transmits multiple signals simultaneously over single transmission path, such as cable or wireless system –Each signal travels within its own unique frequency range (carrier) – What do you recall about the efficiency of this technique?

51. 51 FDM Comment –FDM transmissions are least efficient since each analog channel can only be used one user at a time Each User has their own channel

52. 52 OFDM based on FDM OFDM, data divided among large number of closely spaced carriers –"frequency division multiplex" part of name –Entire bandwidth is filled from single source of data –Instead of transmitting data serially, data is transferred in parallel –Divided among multiple subcarriers –Only small amount of data is carried on each carrier

53. 53 OFDM An OFDM signal consists of –Several closely spaced modulated carriers –When modulation of any form - voice, data, etc. is applied to a carrier Sidebands spread out on either side A receiver must be able to receive whole signal to be able to demodulate data So, when signals are transmitted close to one another they typically spaced with guard band between them

54. 54 Traditional View FDM with Guards Traditional view of signals carrying modulation Receiver filter passband: one signal selected Guards Guard bands waste the spectrum

55. 55 OFDM Making Subcarriers Mathematically Orthogonal –Breakthrough for OFDM –Enables OFDM receivers to separate subcarriers via Fast Fourier Transform (FFT) Eliminates guard bands OFDM subcarriers can overlap to make full use of spectrum Peak of each subcarrier spectrum, power in all other subcarriers is zero

56. 56 OFDM OFDM offers higher data capacity in a given spectrum while allowing a simpler system design Others have zero power Power

57. 57 OFDM Shows parallel nature of subcarriers

58. 58 Benefits of OFDM Radio signals are imperfect –General challenges of RF signals include Signal-to-noise ratio Self-interference (intersymbol interference or ISI) Fading owing to multipath effects –Same signal arrives at a receiver via different paths –Briefly look at multipath fading …

59. 59 Multipath Fading Indoor and Outdoor radio channel is characterized by multipath reception –Sent signal contains not only a direct line-of-sight radio wave, but also a large number of reflected radio waves –Outdoors line-of-sight often blocked by obstacles, and collection of differently delayed waves received by mobile antenna –These reflected waves interfere with direct wave, causes significant degradation link performance - Waves arrive at slightly different times, so they are out of phase with original wave Randomly boosts or cancels out parts of signal

60. 60 Multipath Fading

61. 61 Benefits of OFDM Main way to prevent Intersymbol Interference errors –Transmit a short block of data (a symbol) –Wait until all the multipath echoes fade before sending another symbol –Waiting time often referred to as guard interval

62. 62 Benefits of OFDM Longer guard intervals - more robust system to multipath effects –But during guard interval, system gets no use from available spectrum –Longer the wait, the lower the effective channel capacity Some guard interval is necessary for any wireless system –Goal is to minimize that interval and maximize symbol transmission time

63. 63 Benefits of OFDM OFDM meets this challenge by Dividing transmissions among multiple subcarriers –Symbol transmission time is multiplied by number of subcarriers –For example: With 802.11a, there are 52 channels, so the system has 52 times transmission capacity compared to single channel

64. 64 OFDM vs. Single Channel

65. 65 Benefits of OFDM Using multiple subcarriers also makes OFDM systems more robust to fading –Fading typically decreases received signal strength at particular frequencies, so problem affects only a few of the subcarriers at any given time and … –Error-correcting codes provide redundant information that enables OFDM receivers to restore information lost in these few erroneous subcarriers

66. 66 802.11a

67. 67 Intro to 802.11a 802.11a was approved in September 1999, two years after 802.11 standard approved –Operates in 5 GHz Unlicensed National Information Infrastructure (UNII) band –Spectrum is divided into three “domains,” –Each has restrictions imposed on maximum allowed output power

68. 68 ISM vs. U-NII

69. 69 802.11a OFDM 802.11a specifies 8 non-overlapping 20 MHz channels in lower two bands –Each divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz bandwidth each 4 non-overlapping 20 MHz channels are specified in upper band Receiver processes 52 individual bit streams, reconstructing original high-rate data stream –Four complex modulation methods are employed, depending on data rate that can be supported by conditions between transmitter and receiver –Include BPSK, QPSK, 16-QAM, and 64-QAM

70. 70 802.11a Channels

71. 71 Trying to Use 802.11a Advantage –Since 2.4 GHz band is heavily used, using 5 GHz band gives 802.11a advantage of less interference Disadvantage –However, high carrier frequency also brings disadvantages –It restricts use of 802.11a to almost line of sight, necessitating use of more access points –It also means that 802.11a cannot penetrate as far as 802.11b since it is absorbed more readily, other things (such as power) being equal

72. 72 802.11b vs 802.11a Path Loss Free Space Path Loss in dB for 2.4 and 5 GHz Spectrums Distance (miles) 2.4 GHz 5 GHz 0.5 98.36 104.56 5 104.38 110.58 1.5 107.91 114.10 3 113.93 120.12 4 116.42 122.62 5 118.36 124.56 10 124.38 130.58 Loss = 32.4 X 20Log(MHz) X 20Log(distance)

73. Range and Data Rate

74. 74 802.11g June 2003, a third modulation standard ratified –802.11g –Works in 2.4 GHz band (like 802.11b) –Maximum data rate of 54 Mbit/s –802.11g hardware works with 802.11b hardware –Older networks, 802.11b node significantly reduces the speed of an 802.11g network

75. 75 802.11g Modulation schemes used in 802.11g –OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and –Reverts to CCK – Complimentary Code Keying like 802.11b for 5.5 and 11 Mbit/s –DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s Even though 802.11g operates in same frequency band as 802.11b –Achieve higher data rates because it uses OFDM and better modulation

76. 76 802.11g Rates, Transmission, Modulation Data Rate Mbps Trans Type Modulation 54OFDM64 QAM 48OFDM64 QAM 11DSSSQPSK1 6OFDMBPSK 5.5DSSSCCK

77. 77 802.11n … A miracle or …

78. 78 802.11n Introduction 802.11n is long anticipated update to WiFi standards 802.11a/b/g –4x increase in throughput –Improvement in range –802.11n ratified by IEEE 2009

79. 79 802.11n Features 802.11n utilizes larger number of antennas Number of antennas relates to number of simultaneous streams –Two receivers and two transmitters (2x2) or four receivers and four transmitters (4x4) –The standards requirement is a 2x2 with a maximum two streams, but allows 4x4

80. 80 802.11n Features 802.11n standard operates in 2.4-GHz, the 5- GHz radio band, or both – more flexibility –Backward compatibility with preexisting 802.11a/b/g deployment –Majority of devices and access points deployed are dual-band Operates in both 2.4-GHz and 5-GHz frequencies

81. 81 802.11n Features Wireless solutions based on 802.11n standard use several techniques to improve throughput, reliability, and predictability of wireless Three primary innovations are –Multiple Input Multiple Output (MIMO) technology –Channel bonding (40MHz Channels) –Packet aggregation Techniques allow 802.11n solutions to achieve fivefold performance increase over 802.11a/b/g networks How does this work? Anyone?

82. 82 MIMO 802.11n builds on previous standards by adding multiple-input multiple-output (MIMO) –MIMO uses multiple transmitter and receiver antennas to improve system performance –MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end

83. 83 MIMO 802.11n access points and clients transmit two or more spatial streams Use multiple receive antennas and advanced signal processing to recover multiple transmitted data streams –MIMO-enabled access points use spatial multiplexing to transmit different bits of a message over separate antennas –Provides greater data throughput

84. 84 MIMO Technology Multiple independent streams are transmitted simultaneously to increase the data rate

85. 85 MIMO Performance gain is result of MIMO smart antenna technology –Allows wireless access points to receive signals more reliably over greater distances than with standard diversity antennas –Example, distance from access point at which an 802.11a/g client communicating with a conventional access point might drop from 54 Mbps to 48 Mbps or 36 Mbps –Same client communicating with MIMO access point may be able to continue operating at 54 Mbps

86. 86 Channel Bonding Most straightforward way to increase capacity of network is to increase operating bandwidth –However, conventional wireless technologies limited to transmit over one of several 20-MHz channels –802.11n networks employ technique called channel bonding to combine two adjacent 20-MHz channels into a single 40-MHz channel –Technique more than doubles channel bandwidth

87. 87 Channel Bonding –Channel bonding most effective in 5-GHz frequency given greater number of available channels 2.4-GHz frequency has only 3 non-overlapping 20- MHz channels Thus, bonding two 20-MHz channels uses two thirds of total frequency capacity –So, IEEE has rules on when a device can operate in 40MHz channels in 2.4GHz space to ensure optimal performance –5 GHz has larger number of channels available for bonding

88. 88 Packet Aggregation In conventional wireless transmission methods –Amount of channel access overhead required to transmit each packet is fixed, regardless of the size of the packet itself –As data rates increase, time required to transmit each packet shrinks –Overhead cost remains same

89. 89 Packet Aggregation 802.11n technologies increase efficiency by aggregating multiple data packets into a single transmission frame 802.11n networks can send multiple data packets with fixed overhead cost of just a single frame Packet aggregation is more beneficial for certain types of applications such as file transfers –Real-time applications (e.g. voice) don’t benefit from packet aggregation because its packets would need to be interspersed at regular intervals –And combining packets into larger payload would introduce unnecessary latency

90. 90 802.11 Comparison

91. 91 Summary From 1999 until 2013 … 14 years amazing changes in wireless LAN technology From 5.5 Mbps to 300 + Mbps and beyond How? Parallelism of data streams Increased number of antennas Resolving interference through math and multiplexing Cramming more data within limited frequencies Better modulation techniques Future – More of the same !!!

92. 92 End Small Assignment 2a – Wireless Questions due Monday, October 21