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]
Wireless & Network Security 2© Kemal Akkaya
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
Wireless & Network Security 3© Kemal Akkaya
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
Wireless & Network Security 4© Kemal Akkaya
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
Wireless & Network Security 5© Kemal Akkaya
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
Wireless & Network Security 6© Kemal Akkaya
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.
Wireless & Network Security 7© Kemal Akkaya
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
Wireless & Network Security 8© Kemal Akkaya
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
Courtesy Dr. Y. Richard Yang
Wireless & Network Security 9© Kemal Akkaya
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
Wireless & Network Security 10© Kemal Akkaya
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
Courtesy Dr. Y. Richard Yang
Reflection, scattering, diffraction
Wireless & Network Security 11© Kemal Akkaya
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
Wireless & Network Security 12© Kemal Akkaya
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
Wireless & Network Security 13© Kemal Akkaya
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
Wireless & Network Security 14© Kemal Akkaya
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
Wireless & Network Security 15© Kemal Akkaya
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
Wireless & Network Security 16© Kemal Akkaya
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
Wireless & Network Security 17© Kemal Akkaya
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
Wireless & Network Security 19© Kemal Akkaya
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
Wireless & Network Security 20© Kemal Akkaya
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 Security 21© Kemal Akkaya
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!!
Wireless & Network Security 23© Kemal Akkaya
Applications: Home Networking
Courtesy Dr. Richard Yang, Yale
Wireless & Network Security 24© Kemal Akkaya
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
Wireless & Network Security 25© Kemal Akkaya
Application: Environmental Monitoring
Wireless Sensor Nodes
monitor an area of interest
Wireless & Network Security 26© Kemal Akkaya
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
Wireless & Network Security 27© Kemal Akkaya
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
Wireless & Network Security 28© Kemal Akkaya
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
Wireless & Network Security 29© Kemal Akkaya
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
Wireless & Network Security 30© Kemal Akkaya
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