Department of Computer Science Southern Illinois University Carbondale

1. 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

2. Transmission Fundamentals 1 Cycle Wavelength Amplitude Analog: No breaks in the signal Digital: Signal intensity is discrete • 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 Mobile & Wireless Computing 2

3. 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 Mobile & Wireless Computing 3

4. 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

5. Channel Capacity 1 0 T=1sec, f=1/1=1Hertz Data rate = 2 bits/sec C = Blog2(1+SNR) Theoretical upper bound C= 2 B logM Channel capacity Voltage levels Bandwidth • 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) • 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 Mobile & Wireless Computing 5

6. Analog and Digital Data Transmission What does a Modem do? • 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: 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 – 802.11 Networks Mobile & Wireless Computing 7

8. ASK, FSK and PSK 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 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

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) Mobile & Wireless Computing 10

11. AM and FM Example FM AM Mobile & Wireless Computing 11

12. Digitization 7D/2 5D/2 3D/2 D/2 -D/2 -3D/2 -5D/2 -7D/2 • 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 Approximation 3 bits / sample Rs = Bit rate = # bits/sample x # samples/second 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) Mobile & Wireless Computing 13

14. FDM Example 3 Channels 1 Link Transmission Multiplexer • Combining analog signals • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal 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 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: Mobile & Wireless Computing 16

17. General Frequency Ranges Electric Waves Radio Waves Visible Light Ultra Violet Gamma Rays Cosmic Rays Infra-red X-Rays Radio Spectrum “Sweetspot” 3G LMDS WiFiBluetooth DECT TETRA GSM FM Radio Medium Wave Radio Microwave Radio Links Long Wave Radio TV VLF LF MF HF VHF UHF SHF EHF 3 30 300 3 30 300 3 30 300 kHz MHz GHz • Infrared frequency range • Roughly 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas • 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 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 Current Allocation of the Radio Spectrum by frequency Mobile & Wireless Computing 18