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: kemal@cs.siu.edu

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