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

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CSCD 439/539Wireless Networks and Security

Lecture 5Physical Layer, and 802.11 b,g,a

Differences

Fall 2007

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Introduction

• Physical Layer in 802.11

• Differences between 802.11 b,g,a– Frequency ranges– Speed

• Spread Spectrum Techniques

• DSSS Spread Spectrum, 802.11b

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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

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Frequency BandISM: Industry, Science, Medicine

unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz

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IEEE 802.11 Frequency Band

and 802.11b/g 802.11a

Wavelength

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802.11b/g Channels

2400 – 2483 Each channel spaced 5 MHz apart

Only non-overlapping channels are 1, 6 and 11

Channel Width = 22 MHz Channels – 12 – 14, not sanctioned by FCC

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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

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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

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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 …

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• 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

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• 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

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• 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

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• 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

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• Trivia Question– Who patented spread spectrum transmission

and when was it patented?

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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

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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

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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

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RF Propagation

AP1

Distance

Received Signal

Noise

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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 rangeIt operates in the 5 GHz frequency range

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802.11 Signal Propagation Techniques

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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

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• Typical applications include– Satellite-positioning systems (GPS)– 3G mobile telecommunications– W-LAN (IEEE802.11a, IEEE802.11b,

IEE802.11g)– Bluetooth

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• 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

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• 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

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Spread Spectrum Techniques

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

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• 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

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Same code must be known in advance at both ends of the transmission channel


Codes are what DSSS uses … talk about next

Spreading de-Spreading

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• 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

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• Can see results of interference attempts, interferer signals are not recovered

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DSSS and HR/DSSS

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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

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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

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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

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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

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DSSS Chipping Sequence

Data SpreadingEncoded Data Correlation

1

0

1

0

Modulo 2

add

Spreading Code

10110111000

10110111000

01001000111Modulo 2

Subtract

10110111000

Spreading Code

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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

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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

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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

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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

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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

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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

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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

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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

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Finish

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

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

Documents

fcc slide

b slide

wavelength slide

power of signal noise

radio waves

frequency band ism

narrow frequency band

rg spectrum