Wireless & Network Security Lecture 3: Radio Basics ...kemal/cs591/L3-RadioWireless.pdfWireless...

Wireless & Network Security 1© Kemal Akkaya

Department of Computer ScienceSouthern Illinois University Carbondale

Wireless & Network SecurityLecture 3: Radio Basics & Wireless

NetworksDr. Kemal AkkayaE-mail: [email protected]


Radio Basics (RF)Already seen how a radio signal looks like

SinusoidsCarrier waveInformation SignalSignal is modulated onto carrier waveCarrier has more bandwidth than the info itself

Radio WavesFrequency Range :3 KHz to 300 GHzEasy to generateCan travel long distancesCan penetrate buildings They are both used for indoor and outdoor communicationThey are omni-directional: can travel in all directionsThey can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas (like satellite dishes)

All signals converted to analogUnguided media allows analog transmission onlyAnalog Signal usage: TV, RadioDigital Signal usage: Cell Phone, Wireless Network

Can be transmitted through antennas


AntennasElectrical conductor

Transmits (radiates) electromagnetic waves into spaceReceives electromagnetic waves from spaceSame antenna can be used as both transmitter and receiver

Radiation Pattern of an AntennaThe graphical representation of radiation in all directions in the spaceWhat is the ideal radiation pattern?

Radiate equally in all directions in the spaceSun is the best exampleOmni directional radiation pattern

Real antennas are not isotropicDipoles

Half-wave dipoles (Hertz)Quarter-wave dipoles (Marconi)

Reflective parabolicIn satellite applications

Isotropic Radiatorλ/2

Half-wave dipole

http://www.amazon.com/exec/obidos/tg/detail/-/B00007KDVJ/104-4604240-8024759?v=glance&s=electronics&n=507846&me=ATVPDKIKX0DER&vi=pictures&img=14


Directional AntennasDirectional antennas are very common

Radiation pattern in a certain directionOften used for base stations in a cellular system

Beam width (half-power beam width)Measure of directivity of antennaAngle within which power radiated is at least half of what it is in the most preferred direction

Antenna gainPower output, in a particular direction, compared to that produced in any direction by a perfect omni-directional antenna (isotropic antenna)Measured in dBi :decibels relative to an isotropic radiatorA gain of 3dB means:

Antennas improves the signal upon the isotropic antenna in that direction by 3dB

Beam Width

Omni-directional

Directional

Antenna

Courtesy www.superpass.comside view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

directedantenna


Radio PropagationSignal Propagation Ranges

Transmission rangeCommunication possibleLow error rate

Detection rangeDetection of the signal possibleNo communication possible

Interference rangeSignal may not be detected Signal adds to the background noise

Wireless Propagation ModesHow a signal radiated from an antenna travels? There are three different routes:

Ground-wave propagationSky-wave propagationLine-of-sight propagation (LOS)

distance

sender

transmission

detection

interference

Courtesy Dr. Y. Richard Yang


Propagation ModesRadio signal behaves like light in free spaceGround Wave

Frequencies up to 2 MHzFollows contour of the earthExample: AM Radio

Sky WaveSignal reflected from ionosphere and earth’s surfaceCan travel thousands of kilometersFrequency: 2-30MHzAmateur Radio, Military Comm.

Line of SightTransmitting and receiving antennas must be within line of sightFrequency: More then 30MHzTV, satellite, optical comm.


Impairments in LOS TransmissionIn any system the signal received is different than the signal transmitted

Impairments that degrade analog signal qualityErrors in digital signal

Most common impairments in general (wired, wireless)Attenuation and Attenuation distortionFree Space LossNoiseThermal noiseAtmospheric absorptionMulti-path propagationFadingRefractionReflection, Scattering, Shadowing, Diffraction

Receiving power of the signal depends on these factorsA signal may arrive at a receiver Multi-path and fading!!

many different timesmany different directionsdue to vector addition

ReinforceCancel

signal strength differs from place to place


What is Fading?The main problem in wireless transmissionDefinition:

Time variation of received signal power caused by changes in thetransmission medium or path (s).In fixed environment is caused by atmospheric conditionsIn mobile environments creates more complex effects

Causes of fading:Free space lossMulti-path propagation

Reflection, scattering, diffraction, refraction

signal at sender

signal at receiver

LOS pulses Multipath pulses

Interference with other transmittersAtmospheric absorptionMobility

Fast fading, small fading



Free Space LossMain source of attenuation in Wireless Transmission

Any type of signal disperses with distance as signal is being spread over larger and larger areaCan be expressed in terms of decibels

222

2

2

2 1*)4()4( df

Kfd

cdP

PLt

r ====ππ

λ

Here Pr is the mean received signal powerPt is the transmitted signal powerf is the frequency of the signald is the distance between transmitter and receiverIt is inversely proportional to d2

for free spaceCan be up to d4 for different environments

Environment Path Loss Exponent, n

Free space 2

Urban area cellular radio 2.7 to 3.5

Shadowed urban cellular radio 3 to 5

In building line-of-sight 1.6 to 1.8

Obstructed in building 4 to 6

Obstructed in factories 2 to 3


Multi-path Propagation Reflection:When signal encounters a surface that is large relative to the wavelength of the signal

Scattering:Occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

Diffraction:Occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

Refraction:Bending of radio waves as they propagate through the atmospherereflection scattering diffractionrefraction


Reflection, scattering, diffraction


Mobility effectsAs the user moves, signal paths may change

Distance to sender will changeObstacles will further away

Fast Fading As the mobile moves over small distances, the instantaneous received signal will fluctuate rapidly giving rise to small-scale fadingThe reason is that the signal is the sum of many contributors coming from different directions and since the phases of these signals are random, the sum behave like a noiseOccurs when receiver moves only about one half of the wavelength

Slow Fading As the mobile moves away from the transmitter over larger distances, the local average received signal will gradually decrease. This is called large-scale fadingAs the receiver covers distances larger than the wavelength


Slow and Fast Fading in an Urban Mobile Environment

14 16 18 20 22 24 26 28-70

-60

-50

-40

-30

Distance between transmitter and receiver (m)

Received Power (dBm)

Fast Fading

Slow Fading


Spread SpectrumMotivation:In radio transmission, sometimes narrow band signals can be wiped outReasons:

Fading due to multi path propagationInterference from other devices

To solve this problem:We can transmit at many different frequencies during transmissionSpread transmission over a wider bandwidth

Initial motivation:To prevent jamming and interference in military applicationsWhat is jamming?

To send noise like signals to distort information signalsNeed to know the frequency of the information signal

Used in every wireless networks todayEliminates interference and multi-path effectsMultiple transmitters can transmit at same frequency range

Actually FCC requires it for signals in ISM band having a certain transmission power


Frequency Hopping Spread SpectrumSimplest approach: FHSS

Discrete changes of carrier frequencyThere are multiple base frequencies (or channels)Transmitter randomly hops to one of those frequenciesReceiver should to the same thingThere is a shared spreading code between transmitter and receiver

Spreading code = Hopping Sequence


Types of FHSSTwo versions of FHSS

Fast Hopping: several frequencies per user bitSlow Hopping: several user bits per frequency

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time


Spread CodeHow to share the spread code?

Use predefined sequencesSequence 1 <1, 12, 23, 3, 5…>

Receiver listens a fixed frequency channel specifically for the sequence

IEEE 802.11 uses 96 1 MHz ChannelsDwell time is around 390ms

History of Spread SpectrumFirst invented by Hollywood Actress, Heidy Lamarr and George Antheil, an avant gard composer in 1940s

AdvantagesFrequency selective fading and interference limited to short period

Say you have 80 channels and 1 is knocked by interferenceThe other 79 are still free and ready to be used

Jammer must jumped each frequencySimple implementationUses only small portion of spectrum at any time


Direct Sequence Spread Spectrum (DSSS)Each bit in original signal is represented by multiple bits in the transmitted signal

Transmitted signal is code CHIPSpreading code spreads signal across a wider frequency band

Spread is in direct proportion to number of bits usedUses a pseudorandom bit sequence

One technique combines digital information stream with the spreading code bit stream using exclusive-OR (XOR)802.11 DS PHY uses Barker Sequence

11-to-1 Spreading Ratio: 1 bit 11 bitsDSSS is very resilient to interference

Many chips can be corrupted before the bits are lostMultiple users can share the bandwidth

By using different chipping sequencesNo need to allocate different frequencies


Example for DSSS


Mobile and Wireless Data NetworksExperiencing a tremendous growth over the last decade

Wide deployment of access infrastructureIn-door, out-door, MAN, WAN

Growth of Wireless DataMiniaturization of computing machinery : laptop PDA embedded sensorsIncreasing mobile work forceLuxury of tether less computingInformation on demand anywhere/anyplace

Some Facts:In 2005, more than 1/3rd of internet users had internet connectivity through a wireless enabled device (750 million users)!!! (Source: Intermarket group)In the year 2004 revenue from wireless data was $34B, and by the year 2010 the number of wireless data subscribers will hit 1B!!

What is Mobile and Wireless Computing?Distributed systems with portable computers and wireless communicationsUser can access data anytime, anywhere


BuzzwordsMobile Computing

Distributed systems with mobile usersIn-door/Out-doorVehicle/human speed

Nomadic ComputingSimilar to Mobile computing

Focuses more on in-door communications

Pervasive Computing : Ubiquitous ComputingMay add some user interface integrationSome says : AI + Mobile Computing stuff

Applications:Military

Border control, target tracking, intrusion detection etc.Civil

Habitat monitoring, search and rescue, meeting rooms etc.


Wireless Network TypesSatellite networks

Iridium (66 satellites)Qualcomm’s Globalstar (48 satellites)

Wireless WANs/MANsCDPD (Cellular Digital Packet Data )GPRS (General Packet Radio Service)

Wireless LANsIEEE 802.11 : SIU’s LAWN,

Wireless PANse.g. Infrared: Bluetooth

Mobile Ad-hoc networkse.g. Emergency relief, military

Sensor networkse.g. Environmental sensing-MICA motes

Wireless WAN Generations:1G (Past)

AMPS, TACS: No data 2G (Past/Present)

IS-136, GSM: <10Kbps circuit switched data

2.5G (Past/Present)GSM-GPRS, GPRS-136: <100Kbps packet switched

3G (Present/Immediate Future)

IMT-2000: <2Mbps packet switched

4G (Future)20-40 Mbps!!


Examples

PicoNet

Bluetooth

802.11 / WiFi

Wireless LAN


Applications: Home Networking

Courtesy Dr. Richard Yang, Yale


Applications: Outdoor Networking

ad ho

cUMTS, WLAN,DAB, GSM, cdma2000, TETRA, ...

Personal Travel Assistant,PDA, laptop, GSM, UMTS, WLAN, Bluetooth, ...

Courtesy Dr. Richard Yang, Yale


Application: Environmental Monitoring

Wireless Sensor Nodes

monitor an area of interest


Challenges of Wireless Computing1) Wireless Communication

Implications of using wireless communication for mobile computingThe differences between wireless and wired media

2) MobilityConsequences of mobility on mobile application and system design

3) Poor Resources due to PortabilityPressures that portability places in the design of mobile end-systems


1) Wireless CommunicationLimited Transmission Range

10m-500m Limited Bandwidth

Wireless networks deliver lower bandwidth than wired networks

1 Mbps Infrared communication11 Mbps wireless local radio communications (shared), IEEE 802.11b9.6 Kbps for wide-area wireless communication

Wired networks10-100 Mbps for Ethernet100 Mbps for FDDI155 Mbps for ATM1 Gbps for Gigabit Ethernet

DisconnectionsNetwork partitionsStall all applications

Uncertainty of PerformanceVariance of bit errorsVariance of delaysVariance of bandwidth

SecurityEasy to intrude in the wireless network

Heterogeneous devices and network connections

Wired links Same characteristics

Outdoor: RadioIndoor: InfraredRural Areas: Satellite


Heterogeneous Devices

PPerformanceerformance/Weight/Power Consumption/Weight/Power Consumption

Sensors,embeddedcontrollers

Mobile phones• voice, data• simple graphical displays• GSM

PDA• data• simpler graphical displays• 802.11

Laptop• fully functional• standard applications• battery; 802.11

Desktop• fully functional• standard applications• unlimited power supp• Gbps Ethernet

http://www.palminfocenter.com/ss.asp?f=sgh_i500_o_lg.jpg

http://www.amazon.com/exec/obidos/ASIN/B0000DALPW/ref=nosim/nowelectronic-20


2) MobilityAbility to change locations while connected to the network

A mobile computer can change its serverDNS server, print server, etc.

Dynamic EnvironmentNetwork Access Point Changes

Address changes: IP addressNetwork Performance Changes

Bandwidth, delay, error rate etc.Available resources change

Depends on the network it connected toData consistency changes

Writing/Reading to/from mobile databasesSecurity changes

Endpoint authentication harder


3) Poor ResourcesMobile devices are fundamentally different from stationary machines such as desktop computers

Must be designed with variety of constraints in mind, such as size and power consumption – properties much like a wristwatchThey should also be portable

Portability Constraints IncludeLow power consumption

You would not want to carry a battery that is bigger than your computer! Increased risk of data loss

Physical damageUnauthorized accessLoss and Theft

Small user-interfacesRequires a different windowing scheme Buttons versus Recognition

Limited on-board storage, memory, CPU etcPhysical restrictions, power constraints

Wireless & Network Security Lecture 3: Radio Basics ...kemal/cs591/L3-RadioWireless.pdfWireless...

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