CSCD 439/539 Wireless Networks and Security Lecture 5 Physical Layer, and 802.11 b,g,a Differences Fall 2007

Introduction Physical Layer in 802.11 Differences between 802.11 b,g,a Frequency ranges Speed Spread Spectrum Techniques DSSS Spread Spectrum, 802.11b

802.11 Bands 802.11 networks use microwave size wavelengths But, its really a radio technology Microwaves are radio waves with short wavelength Wavelength of AM radio is 1000 feet 802.11 devices, wavelength is 12 cm

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

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

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

Physical Layer 3 Layers proposed in 802.11 back in 1997 FH – Frequency Hopping Spread Spectrum DS – Direct Sequence Spread Spectrum IR – Infrared - Never developed – no products 3 Further Layers Developed 802.11a OFDM 802.11b High Rate DS or DSSS 802.11g Extended Rate or ERP Newest one 802.11n – MIMO PHY – High Throughput PHY

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

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 …

Radio Communications There are 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

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

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

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

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

Spread Spectrum 802.11 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 802.11 standard However, it was quickly bypassed by more sophisticated spread spectrum technologies We won’t cover it … link on CourseNotes page

Spread Spectrum 802.11 uses three different Spread Spectrum technologies 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 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

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 802.11 network But, as station gets further from Access Point, signal level drops and SNR will be lower

RF Propagation AP1 Received Signal Noise Distance

RF Propagation Signal Degradation When no obstacles, signal degradation can be calculated by following equation Depends on distance and frequency Path loss (dB) = 32.5 + 20 log F + log d where F = GHz , d = distance in meters Higher F leads to more path loss at equal distances Explains why 802.11a has a shorter range It operates in the 5 GHz frequency range

802.11 Signal Propagation Techniques

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

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

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

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

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

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

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

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

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

DSSS and HR/DSSS

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

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

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

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

DSSS Chipping Sequence Encoded Data Correlation Data Spreading Modulo 2 Subtract Modulo 2 add 01001000111 1 1 10110111000 10110111000 10110111000 Spreading Code Spreading Code

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

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

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

DSSS Encoding DS 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

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 802.11 meet regulatory requirements Recall The ratio (in dB) between the spread baseband and the original signal is processing gain

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 1.375 million code symbols/sec

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

HR/DSSS High Rate DSSS PHY Is actually a different form of the PHY layers Came up with a new “short” framing format that improves protocol efficiency and throughput Uses short headers cuts overhead by 14% Some other details are discussed for HR/DSSS involving transmission speeds See references in CourseNotes page

Finish Next time More about 802.11 a, g and OFDM Briefly talk about what you found for tools