CS441 – Mobile & Wireless Computing Communication Basics Department of Computer Science Southern Illinois University Carbondale CS441 – Mobile & Wireless Computing Communication Basics Dr. Kemal Akkaya E-mail: kemal@cs.siu.edu Mobile & Wireless Computing 1

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 Cycle Wavelength Amplitude Mobile & Wireless Computing 2

Effects on Signal Attenuation: Distortion: Noise: Error: 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 Mobile & Wireless Computing 3

Signal/Bandwidth/Data Rate sin(2ft)+(1/3) sin(23ft) 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. Bandwidth = 7f – f = 6f How much data can we communicate with a certain bandwidth? Mobile & Wireless Computing 4

Channel Capacity Channel Capacity: 1 Noise: Nyquist Bandwidth: Noise, attenuation, distortion etc. limit the data rate that can be achieved in a channel. The maximum rate at which data can be transmitted over a given communication path is called Channel capacity Noise: Should be minimized to get more data rate Nyquist Bandwidth: Assumes an error free channel (no noise) In case of noise: Shannon Capacity Formula SNR : Signal to Noise Ratio : Signal Power / Noise Power (often represented in decibels) Sets upper bound on achievable data rate 1 T=1sec, f=1/1=1Hertz Data rate = 2 bits/sec C= 2 B logM Channel capacity Bandwidth Voltage levels C = Blog2(1+SNR) Theoretical upper bound Mobile & Wireless Computing 5

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? Modem converts a series of binary pulses into an analog signal by modulating a carrier frequency. They use voice spectrum to represent digital data and hence can use the telephone lines. At the other end, the analog signal is converted to digital signal again and received. Mobile & Wireless Computing 6

How to do that encoding? Modulation is the solution: Motivation: 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 Mobile & Wireless Computing 7

ASK, FSK and PSK Mobile & Wireless Computing 8

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 Mobile & Wireless Computing 9

Modulation of Analog Signals Digital Modulation had a motivation What was that? What is analog modulation and the idea behind it? Sometimes a higher frequency may be needed for transmission Modulation will help to provide frequency division multiplexing 3 types of analog modulation Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) Mobile & Wireless Computing 10

AM and FM Example FM AM Mobile & Wireless Computing 11

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) Original signal Sample value D/2 3D/2 5D/2 7D/2 -D/2 -3D/2 -5D/2 -7D/2 Approximation 3 bits / sample Rs = Bit rate = # bits/sample x # samples/second Mobile & Wireless Computing 12

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) Mobile & Wireless Computing 13

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 Transmission 3 Channels 1 Link Multiplexer Mobile & Wireless Computing 14

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 Mobile & Wireless Computing 15

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: Mobile & Wireless Computing 16

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 Infrared frequency range Roughly 3x1011 to 2x1014 Hz Useful in local point-to-point multipoint applications within confined areas Medium Wave Radio Electric Waves Radio Infra-red Visible Light Ultra Violet X-Rays Gamma Rays Cosmic 30 3 300 Long Wave Radio FM Radio GSM 3G Microwave Radio Links TV VLF LF MF HF VHF UHF SHF EHF Radio Spectrum kHz MHz GHz DECT WiFi Bluetooth TETRA LMDS “Sweetspot” Mobile & Wireless Computing 17

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 Mobile & Wireless Computing 18