Department of Computer Science Southern Illinois University Carbondale
-
Upload
shay-cannon -
Category
Documents
-
view
24 -
download
4
description
Transcript of Department of Computer Science Southern Illinois University Carbondale
Wireless & Network Security 1© Kemal Akkaya
Department of Computer ScienceSouthern Illinois University Carbondale
CS591 – Wireless & Network Security
Lecture 1: Communication BasicsDr. Kemal Akkaya
E-mail: [email protected]
Wireless & Network Security 2© Kemal Akkaya
Transmission Fundamentals How to relay information?
Electromagnetic Signals TV, Radio, Internet etc.
Signal A function of time Has 3 components:
Amplitude (A) : Signal strength Frequency (f) : # of cycles Phase ( ) : Relative position
Sine wave for the signal s(t) = A sin(2 f t + )
Either analog or digital Wavelength (λ):
Distance occupied by 1 cycle λ = c*T = c / f
Analog: No breaks in the signal
Digital: Signal intensity is discrete
1 CycleWavelength
Amplitude
Wireless & Network Security 3© Kemal Akkaya
Effects on Signal Attenuation: Decrease in amplitude of
signal along transmission
Distortion: Interference of different
frequency components of a
signal
Noise: In the absence of signal,
there is random mixture of
frequencies on the channel
called channel noise
Error: When digital signals are
combined with noise, some
bits can be received in error
Wireless & Network Security 4© Kemal Akkaya
Signal/Bandwidth/Data Rate Signal may include many frequencies
Combination of sinusoids
Spectrum: Range of frequencies a signal contains The signal in the figure contains frequencies
between f and 3f
Bandwidth: Width of the spectrum is called bandwidth Bandwidth for the figure : 3f – f = 2f Increasing the bandwidth makes the wave
look like more square (i.e. digital signal) Hence, increasing the bandwidth helps to
reduce the distortion at the receiver side.
sin(2ft)+(1/3) sin(23ft)
Bandwidth = 7f – f = 6f
How much data can we communicate with a certain bandwidth?
Wireless & Network Security 5© Kemal Akkaya
Analog and Digital Data Transmission How analog and digital signals are transmitted?
Analog signals (continuous) can be propagated through Wire, twisted pair, coaxial cable, fiber optic cable and atmosphere
Digital signals (discrete) can only propagated through Wired medium – No wireless since it requires infinite frequencies How to propagate digital signals then?
Digital data can be represented as analog signals:What
does a Modem
do?
?
Wireless & Network Security 6© Kemal Akkaya
How to do that encoding? Modulation is the solution:
Modulate digital data so that an analog signal is generated Modem would be the classical example
Motivation:
When only analog transmission facilities are available, modulation is required to convert digital data into analog signals
How to do digital modulation? Operation in on or more of the 3 characteristics of a signal These are amplitude, frequency and phase
Three main techniques ASK: Amplitude Shift Keying – digital data over optical fiber FSK: Frequency Shift Keying – on LANs that use coaxial cable PSK: Phase Shift Keying – 802.11 Networks
Wireless & Network Security 7© Kemal Akkaya
ASK, FSK and PSK
Wireless & Network Security 8© Kemal Akkaya
Other digital modulation techniques Binary Frequency Shift Keying (BFSK)
Uses two different frequencies Multiple Frequency Shift Keying (MFSK)
More than two frequencies are used Gaussian Frequency Shift Keying (GFSK)
Two level shift from base frequency : Bluetooth uses this Binary Phase Shift Keying (BPSK)
Two phrases used to represent bits : In Satellite Systems Differential Phase Shift Keying (DPSK)
Phase shift with reference to previous bit Four-level (QPSK) and Multilevel Phase Shift Keying
Each element represents more than 1 bit Differential QPSK (DQPSK) is used in 802.11b networks
Quadrature Amplitude Modulation (QAM) Combination of ASK and PSK Two different signals sent simultaneously on the same carrier frequency Started to be used in Wireless Sensor Networks
Wireless & Network Security 9© Kemal Akkaya
AM and FM Example
AM FM
Wireless & Network Security 10© Kemal Akkaya
Digitization Converting analog data into digital signals
Digital data can then be transmitted using NRZ-L NRZ-L a way to transmit digital signals
Digital data can then be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec
Pulse Code Modulation (PCM) Delta Modulation (DM)
Rs = Bit rate = # bits/sample x # samples/second
Original signal
Sample value
Approximation
3 b
its /
sam
ple
Wireless & Network Security 11© Kemal Akkaya
Multiplexing Carrying multiple signals on a single medium
Capacity of transmission medium usually exceeds capacity required for transmission of a single signal
More efficient use of transmission medium: Combine multiple signals
Increased data rate provides cost efficiency Transmission and reception equipment
Analog multiplexing Frequency Division Multiplexing (FDM)
Digital Multiplexing Time Division Multiplexing (TDM)
Wireless & Network Security 12© Kemal Akkaya
FDM Example
Combining analog signals Takes advantage of the fact
that the useful bandwidth of the medium exceeds the required bandwidth of a given signal
Tra
nsm
issi
on
3 Channels1 Link
Multiplexer
Wireless & Network Security 13© Kemal Akkaya
TDM Example
Digital technique to combine data Takes advantage of the fact
that the achievable bit rate of the medium exceeds the required data rate of a digital signal
Wireless & Network Security 14© Kemal Akkaya
Transmission Media for Signals It is the physical path between transmitter and receiver
Guided media: Solid media such as copper, optical fiber etc. Unguided media: Atmosphere or outer space: Wireless Transmission
Here is the electromagnetic spectrum for telecommunications:
Wireless & Network Security 15© Kemal Akkaya
General Frequency Ranges Microwave frequency range
1 GHz to 40 GHz Used for satellite communications
Radio frequency range 3 KHz to 300 GHz Can be analog : TV, Radio Or digital: Cell phones, wireless networks
Medium Wave Radio
ElectricWaves
RadioWaves Infra-red
VisibleLight
UltraViolet X-Rays
GammaRays
CosmicRays
30 3 30300
Long Wave Radio
FM Radio
GSM
3G
Microwave Radio LinksTV
VLF LF MF HF VHF UHF SHF EHF
Radio SpectrumRadio Spectrum
kHz MHz GHz330300 300
DECT WiFiBluetoothTETRA
LMDS
3
“Sweetspot”
Infrared frequency range Roughly 3x1011 to 2x1014 Hz Useful in local point-to-point
multipoint applications within confined areas
Wireless & Network Security 16© Kemal Akkaya
Other7%
Defence30%
Emergency services
2%
Business radio5%
Cellular4%
Aeronautical and Maritime
14% Science1%
Broadcasting13%
Fixed / Satellite24%
Frequency Regulations Federal Communications Commission (FCC)
Charged with regulating interstate and international communications by radio, television, wire, satellite and cable
Prevent interferences between different devices
Current Allocation of the Radio Spectrum by frequency