CS441 Mobile & Wireless Computing Communication Basics

Transcription

1 Department of Computer Science Southern Illinois University Carbondale CS441 Mobile & Wireless Computing Communication Basics Dr. Kemal Akkaya Kemal Akkaya Mobile & Wireless Computing 1

2 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 1 Cycle Wavelength Amplitude Analog: No breaks in the signal Digital: Signal intensity is discrete Kemal Akkaya Mobile & Wireless Computing 2

3 Attenuation: Decrease in amplitude of signal along transmission Distortion: Interference of different signal Noise: frequency components of a 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 Effects on Signal Kemal Akkaya Mobile & Wireless Computing 3

4 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(2 ft)+(1/3) sin(2 3ft) Bandwidth = 7f f = 6f How much data can we communicate with a certain bandwidth? Kemal Akkaya Mobile & Wireless Computing 4

5 Channel Capacity Channel Capacity: 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) C= 2 B logm Channel capacity Bandwidth Voltage levels In case of noise: Shannon Capacity Formula Theoretical upper bound C = Blog 2 (1+SNR) SNR : Signal to Noise Ratio : Signal Power / Noise Power (often represented in decibels) Sets upper bound on achievable data rate Kemal Akkaya Mobile & Wireless Computing T=1sec, f=1/1=1hertz Data rate = 2 bits/sec SNR signal. power 10log10 db noise. power

6 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? Kemal Akkaya Mobile & Wireless Computing 6

7 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 Networks Kemal Akkaya Mobile & Wireless Computing 7

8 ASK, FSK and PSK Kemal Akkaya Mobile & Wireless Computing 8

9 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 b 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 Kemal Akkaya Mobile & Wireless Computing 9

10 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) Kemal Akkaya Mobile & Wireless Computing 10

11 AM and FM Example AM FM Kemal Akkaya Mobile & Wireless Computing 11

12 3 bits / sample 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 7D/2 5D/2 3D/2 D/2 -D/2-3D/2-5D/2-7D/2 R s = Bit rate = # bits/sample x # samples/second Approximation Kemal Akkaya Mobile & Wireless Computing 12

13 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) Kemal Akkaya Mobile & Wireless Computing 13

14 Transmission 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 3 Channels 1 Link Multiplexer Kemal Akkaya Mobile & Wireless Computing 14

15 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 Kemal Akkaya Mobile & Wireless Computing 15

16 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: Kemal Akkaya Mobile & Wireless Computing 16

17 General Frequency Ranges Microwave frequency range Infrared frequency range 1 GHz to 40 GHz Roughly 3x1011 to 2x1014 Hz Used for satellite communications Useful in local point-to-point Radio frequency range multipoint applications within confined areas 3 KHz to 300 GHz Can be analog : TV, Radio Or digital: Cell phones, wireless networks Electric Waves Radio Waves Infra-red Radio Spectrum Visible Light Ultra Violet X-Rays Gamma Rays Cosmic Rays Long Wave Radio Medium Wave Radio Sweetspot 3G DECT WiFi LMDS TETRA GSM Bluetooth FM Microwave Radio TV Radio Links VLF LF MF HF VHF UHF SHF EHF khz MHz GHz Kemal Akkaya Mobile & Wireless Computing 17

18 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 Business radio 5% Cellular 4% Other 7% Aeronautical and Maritime 14% Science 1% Emergency services 2% Broadcasting 13% Defence 30% Fixed / Satellite 24% Current Allocation of the Radio Spectrum by frequency Kemal Akkaya Mobile & Wireless Computing 18