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CSCD 433/533Wireless Networks and Security

Lecture 8Physical Layer, and 802.11 b,g,a,n,ac

Differences

Fall 2013

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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

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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

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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

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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

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Spread Spectrum Transmission

• 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

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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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802.11 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

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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

Frequency Assignments

Channel 12.412 GHz

Channel 62.437 GHz

Channel 112.462 GHz

25 MHz25 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.

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Comparisons of 802.11 Physical Layer

3 Flavors of 802.11 802.11a 802.11b 802.11g

Newest ones 802.11n and 802.11ac

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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 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?

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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 …

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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

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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

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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

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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

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Radio Communications

Trivia Question Who patented spread spectrum transmission

and when was it patented?

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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

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Spread Spectrum 802.11 uses three different Spread

Spectrum technologies1. 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

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Spread Spectrum

802.11 uses three different Spread Spectrum technologies2. 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

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802.11 and Spread Spectrum

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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

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

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Spread Spectrum Code Techniques

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

IEE802.11g) Bluetooth

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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

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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

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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 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

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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

General Model of Spread Spectrum System

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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

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Spread Spectrum Code Techniques

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 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

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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

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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 Chip is 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

Figure 6.33 DSSS example

Direct Sequence Spread Spectrum Example

Code Division Multiple Access (CDMA)

A multiplexing technique used with spread spectrumGiven a data signal rate DBreak each bit into k chips according to a fixed chipping code specific to each userResulting new channel has chip data rate kD chips per secondCan have multiple channels superimposed

CDMA Example

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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 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

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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

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OFDM

Orthogonal Frequency Division Multiplexing

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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

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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?

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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

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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

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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

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Traditional View FDM with Guards

Traditional view of signals carrying modulation

Receiver filter passband: one signal selected

Guards

Guard bands waste the spectrum

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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

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OFDM• OFDM offers higher data capacity in a given

spectrum while allowing a simpler system design

Others have zero power

Power

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OFDM

• Shows parallel nature of subcarriers

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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 …

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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

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Multipath Fading

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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

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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

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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

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OFDM vs. Single Channel

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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

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802.11a

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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

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ISM vs. U-NII

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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

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802.11a Channels

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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

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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)

Range and Data Rate

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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

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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 Keyinglike 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

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802.11g Rates, Transmission, Modulation

Data Rate Mbps Trans Type Modulation

54 OFDM 64 QAM

48 OFDM 64 QAM

11 DSSS QPSK1

6 OFDM BPSK

5.5 DSSS CCK

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802.11n … A miracle or …

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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

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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

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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

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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?

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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

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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

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MIMO Technology• Multiple independent streams are transmitted

simultaneously to increase the data rate

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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

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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

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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

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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

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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

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802.11 Comparison

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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 !!!

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End

Small Assignment 2a – Wireless Questions

due Monday, October 21