1 CSCD 433/533 Wireless Networks and Security Lecture 8 Physical Layer, and b,g,a Differences Fall 2012

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

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

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

5 FCC Regulation In 1995, Federal Communications Commission allocated several bands of wireless spectrum for use without a license The FCC stipulated that the use of spread spectrum technology would be required In 1990, the IEEE began exploring a standard In 1997 the standard was ratified and is now obsolete July 1999 the b standard was ratified

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 the military does use spread spectrum transmission to make signals hard to detect This requires a different spread spectrum technology

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

8 IEEE Frequency Band and b/g a Wavelength

Physical Channels The b 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 the data over the channel that extends 11MHz on each side of the center frequency  The channels overlap, but there are three out of 11 channels that don't

b/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 Comparisons of Physical Layer 3 Flavors of  a OFDM  b High Rate DS or DSSS  g Extended Rate or ERP Newest one  n – MIMO PHY – High Throughput PHY

12 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 is naturally occurring distortion in frequency band  Overcome noise Ensure power of signal > noise

13 Radio Communications Legal authority must impose 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 …

14 Radio Communications There are unlicensed bands  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 products Bluetooth Cordless phones X10 – Protocol for home automation

15 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

16 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

17 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

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

19 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

20 Spread Spectrum uses three different Spread Spectrum technologies  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 standard However, it was quickly bypassed by more sophisticated spread spectrum technologies We won’t cover it, not enough time FHSS is covered in,

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

22 RF Propagation As radio signals travel through space, they degrade over distance  Performance determined by signal to noise ratio (SNR) Says how strong is my signal compared to noise?  Degradation of signal will limit signal to noise ratio of receiver  Noise floor stays the same over network  But, as station gets further from Access Point, signal level drops and SNR will be lower

23 RF Propagation AP1 Distance Received Signal Noise

24 RF Propagation Signal Degradation  When no obstacles, signal degradation can be calculated by following equation Depends on distance and frequency Path loss (dB) = log F + log d where F = GHz, d = distance in meters Higher F leads to more path loss at equal distances Explains why a has a shorter range It operates in the 5 GHz frequency range

Signal Propagation Techniques

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

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

28 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

29 Spread Spectrum 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 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

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

31 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

32 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

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

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

35 DSSS and HR/DSSS

36 DSSS DSSS is a spread spectrum technique  Modulation scheme used to transmit signal over wider frequency bandwidth  Modulation is the altering of carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) from one location to another via a discrete channel  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 DSSS DSSS Techniques  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 It is the ratio by which unwanted signals or interference can be suppressed relative to the desired signal when both share the same frequency channel  Typical SS processing gains run from 10dB to 60dB

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 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 Originally, the chip was an 11 bit code combined with a data bit to produce an 11 bit code This gets transmitted to receiver

40 DSSS Chipping Sequence DataSpreading Encoded Data Correlation Modulo 2 add Spreading Code Modulo 2 Subtract Spreading Code

41 DSSS Chipping stream  Receiver uses correlation recovers bits by looking at each 11 bit segment of stream  Compares it to chipping sequence which is static If it matches, bit is a zero If it doesn’t match, bit is a one  Result of using a high chip-to-bit signal if signal is spread out over a wider bandwidth

42 DSSS Chipping stream  DS system is concerned with Spreading Ratio Number of chips used to transmit a single bit Higher spreading ratios improve ability to recover transmitted signal  Because, also, spreading out noise over a larger area  Ratio of noise to actual spread and data is less Doubling spreading ratio requires doubling chipping rate and doubles required bandwidth too

43 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

44 DSSS Encoding DS  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

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

46 DSSS Complementary Code Keying (CCK)  Different modulation scheme used to encode more bits per code word  In 1999, CCK was adopted to replace the Barker code in wireless digital networks  CCK divides chip stream up into 8-bit code symbols so underlying transmission based on series of million code symbols/sec

47 DSSS Complementary Code Keying (CCK)  Based on mathematical transforms allow use of 8-bit sequences to encode 4 or 8 bits per code word  Helped increase data throughput to 5.5 Mbps or 11 Mbps  CCK selected over competing modulation techniques as it utilized same bandwidth and could utilize same header as pre-existing 1 and 2 Mbit/s wireless networks Guarantee interoperability

48 Intro to a a was approved in September 1999, two years after standard approved –Operates in 5 GHz unlicensed national information infrastructure (UNII) band –Spectrum is divided into three “domains,” each having restrictions imposed on the maximum allowed output power First 100 MHz in the lower frequency portion is restricted to a maximum power output of 50 mW Second 100 MHz has a higher 250 mW maximum Third 100 MHz, which is mainly intended for outdoor applications, has a maximum of 1.0 W power output

49 Intro to a a –Offered an alternative to the overcrowded band 2.4 GHz, 5GHz –The 5GHz ISM bandwidth is not continuous –There are two areas 5.15GHz GHz and 5.725GHz Ghz –More details about a later …

50 Intro to OFDM a and g 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 the available spectrum –This characteristic will be important in coming years as wireless networks dominate especially in enterprise environments

51 OFDM Based on FDM Recall … –Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system –Each signal travels within its own unique frequency range (carrier)

52 FDM Comment: –FDM Access transmissions are the least efficient networks since each analog channel can only be used one user at a time Each User has their own channel

53 OFDM based on FDM In OFDM, data divided among large number of closely spaced carriers –The "frequency division multiplex" part of the name –The entire bandwidth is filled from a single source of data –Instead of transmitting data serially, data is transferred in a parallel –Divided among multiple subcarriers –Only a small amount of the data is carried on each carrier, which besides the obvious benefit of being parallel –Provides benefits related to the radio nature of wireless

54 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 the whole signal to be able to demodulate the data So, when signals are transmitted close to one another they must be spaced with a guard band between them

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

56 OFDM Making the subcarriers mathematically orthogonal –Breakthrough for OFDM –Enables OFDM receivers to separate subcarriers via an Fast Fourier Transform Eliminate the guard bands OFDM subcarriers can overlap to make full use of the spectrum Peak of each subcarrier spectrum, power in all the other subcarriers is zero

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

58 OFDM Shows parallel nature of subcarriers

59 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 …

60 Multipath Fading The mobile or indoor 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 –Even worse in urban areas, the line-of-sight is often blocked by obstacles, and collection of differently delayed waves is received by a mobile antenna –These reflected waves interfere with direct wave, causes significant degradation link performance –Reason is that waves arrive at slightly different times, so they are out of phase with original wave Will randomly boost or cancel out parts of the signal

61 Multipath Fading

62 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

63 Benefits of OFDM Longer the guard intervals - more robust system to multipath effects –But during guard interval, system gets no use from the 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

64 Benefits of OFDM OFDM meets this challenge by dividing transmissions among multiple subcarriers. –Same guard interval can then be applied to each subcarrier, while the symbol transmission time is multiplied by the number of subcarriers –With a, there are 52 channels, so the system has 52 times the transmission capacity compared to single channel

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

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

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

68 Trying to Use a a products started shipping in 2001 –Lagged b products slow availability of the 5 GHz components needed to implement products a was not widely adopted because b was already widely adopted Because of a's disadvantages, poor initial product implementations, making its range even shorter, and because of regulations –Manufacturers of a equipment responded to lack of market success by improving the implementations –Plus making technology that can use more than one standard. –There are dual-band, or dual-mode or tri-mode cards that can automatically handle a and b, or a, b and g, as available – Similarly, there are mobile adapters and access points which can support all these standards simultaneously

69 Comparing a and b –The throughput of a is 2 to 4.5 times better than b up to a certain range … Example: At 225 ft, a averages yielded 5.2 Mbps compared to 1.6 Mbps for b Next slide shows this as a graph

70 Throughput Range Performance Averaged throughput performance for 1500 byte packets: a thoughputs always better by 2 to 4.5 times up to 225 ft.

g June 2003, a third modulation standard ratified –802.11g –Works in 2.4 GHz band (like b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like a –802.11g hardware will work with b hardware –Older networks, b node significantly reduces the speed of an g network

g The modulation scheme used in g –OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and reverts to CCK, like b for 5.5 and 11 Mbit/s –DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s Even though g operates in same frequency band as b –Achieve higher data rates because of its similarities to a The maximum range of g devices is slightly greater than that of b devices –Range in which a client can achieve full (54 Mbit/s) data rate speed is much shorter than that of b

73 Beyond a and b, g Despite its major acceptance, g suffers from same interference as b in the already crowded 2.4 GHz range

n … A miracle or …

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

n Features n utilizes larger number of antennas The number of antennas relates to the 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

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

n Features Wireless solutions based on n standard employ 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 n solutions to achieve an approximate fivefold performance increase over a/b/g networks

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

80 MIMO n 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 –Provide greater data throughput

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

82 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 a/g client communicating with a conventional access point might drop from 54 Mbps to 48 Mbps or 36 Mbps –Same client communicating with a MIMO access point may be able to continue operating at 54 Mbps

83 Channel Bonding Most straightforward way to increase capacity of a network is to increase the 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 the channel bandwidth

84 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

85 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

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

Comparison