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Understanding Data Transmission and Communication Channels in Telecommunications

Explore the abstract view of data transmission, receiver communication, channel properties, and various transmission methods such as analog and digital. Learn the impact of noise, idealized channel characteristics, different encoding methods, modulation techniques, and the electromagnetic spectrum. Discover how channels are characterized in both time and frequency domains, the significance of error probability, and channel capacity calculation.

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Understanding Data Transmission and Communication Channels in Telecommunications

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  1. Abstract View of Data Transmission Transmitter Receiver Communication channel Communication Channel Properties: -- Bandwidth -- Transmission and Propagation Delay -- Jitter -- Loss/Error rates -- Buffering

  2. Analog vs. Digital Transmission Received Sent (a) Analog transmission: all details must be reproduced accurately • e.g. AM, FM, TV transmission (b) Digital transmission: only discrete levels need to be reproduced Received Sent • e.g digital telephone, CD Audio

  3. A Typical Communication Channel Transmission segment Destination Source Repeater Repeater

  4. An Analog Repeater Recovered signal + residual noise Attenuated & distorted signal + noise Amp. Equalizer Repeater

  5. A Digital Repeater Decision Circuit. & Signal Regenerator Amplifier Equalizer Timing Recovery

  6. d meters 0110101... communication channel 0110101...

  7. Characteristics of an Idealized Channel (a) Lowpass and idealized lowpass channel A(f) A(f) 1 f f 0 W 0 W (b) Maximum pulse transmission rate is 2W pulses/second (Nyquist rate) Channel t t

  8. Impact of Noise on Communication signal + noise signal noise High SNR t t t noise signal + noise signal Low SNR t t t Average Signal Power SNR = Average Noise Power SNR (dB) = 10 log10 SNR

  9. Channel Characterization -Frequency Domain Aincos 2ft Aoutcos (2ft + (f)) Channel t t Aout Ain A(f) =

  10. A(f)=1 1+42f2 Signal Amplitude Attentuation 1 f

  11. Signal Phase Modulation (f)=tan-1 2f 1/2 0 f -45o -90o

  12. 1 0 0 0 0 0 0 1 . . . . . . t A Pulse 1 ms

  13. Output of Low-pass Communication Channel

  14. Channel Characterization -Time Domain h(t) Channel t t 0 td

  15. Signaling a Pulse with Zero Inter-symbol Interference s(t) = sin(2Wt)/ 2Wt t T T T T TT T T T TT T TT

  16. Digital Baseband Signal and Baseband Tx. System 1 0 1 1 0 1 +A 2T 4T 5T T 3T 0 t -A r(t) Receiver Transmitter Filter Comm. Channel Receiver Filter Received signal

  17. (a) 3 separate pulses for sequence 110 t T T T T T T (b) Combined signal for sequence 110 t T T T T T T

  18. typical noise 4 signal levels 8 signal levels

  19. Signal levels -- Error Probability 0 2 4 6 8 /2 /2 = A/(M-1)  Channel Capacity = W log (1 +SNR)

  20. 0 1 0 1 1 1 1 0 0 Unipolar NRZ Polar NRZ NRZ-Inverted (Differential Encoding) Bipolar Encoding Manchester Encoding Differential Manchester Encoding

  21. Coding Methods -Properties • Unipolar NRZ - power = A^2/2 • Polar NRZ - power = A^2/4 • Bipolar encoding reduces the low-frequency spectrum • Timing Recovery is also easier, used in telephones • NRZ Inverted -- A transition means 1, no transition is 0 • Errors occur in pairs • Ethernet uses Manchester encoding • A transition from + to - is 1, - to + is 0 (in the middle) • Twice the pulse rate of binary coding • Differential Manchester encoding -used in Token rings • Every pulse has a transition in the middle • A transition at the beginning is 0, no transition is 1

  22. f f2 f1 0 fc Figure 3.27

  23. 1 0 1 1 0 1 6T 6T 6T 2T 2T 2T 4T 4T 4T 5T 5T 5T 3T 3T 3T T T T 0 0 0 Amplitude, Frequency and Phase Modulation Information +1 (a) Amplitude Shift Keying t -1 +1 (b) Frequency Shift Keying t -1 +1 (c) Phase Shift Keying t -1

  24. 1 0 1 1 0 1 +A (c) Modulated Signal Yi(t) 6T 2T 4T 5T 3T T 0 -A 6T 2T 4T 5T 3T T 0 (a) Information +A (b) Baseband Signal Xi(t) t 6T 2T 4T 5T T 3T 0 -A t +2A (d) 2Yi(t) cos(2fct) t -2A

  25. Modulator and Demodulator (a) Modulate cos(2fct) by multiplying it by Akfor (k-1)T < t <kT: x Ak Yi(t) = Akcos(2fct) cos(2fct) (b) Demodulate (recover) Akby multiplying by 2cos(2fct) and lowpass filtering: Lowpass Filter with cutoff W Hz x Yi(t) = Akcos(2fct) Xi(t) 2cos(2fct) 2Akcos2(2fct) = Ak {1 + cos(2fct)}

  26. x Ak Yi(t) = Akcos(2fc t) cos(2fc t) + Y(t) x Bk Yq(t) = Bksin(2fc t) sin(2fc t) QAM Modulator Modulatecos(2fct)and sin (2fct)bymultiplying them by Akand Bk respectively for (k-1)T < t <kT:

  27. QAM Demodulator Lowpass Filter with cutoff W/2 Hz x Y(t) Ak 2cos(2fc t) 2cos2(2fct)+2Bkcos(2fct)sin(2fct) = Ak {1 + cos(4fct)}+Bk{0 + sin(4fct)} Lowpass Filter with cutoff W/2 Hz x Bk 2sin(2fc t) 2Bk sin2(2fct)+2Akcos(2fct)sin(2fct) = Bk{1 - cos(4fct)}+Ak {0 + sin(4fct)}

  28. 2-D signal Bk Ak 4 “levels”/ pulse 2 bits / pulse 2W bits per second Signal Constellations Bk 2-D signal Ak 16 “levels”/ pulse 4 bits / pulse 4W bits per second

  29. Bk Bk Ak Ak 16 “levels”/ pulse 4 bits / pulse 4W bits per second Other Signal Constellations 4 “levels”/ pulse 2 bits / pulse 2W bits per second

  30. Electromagnetic Spectrum Frequency (Hz) 106 108 1010 1012 1014 1016 1018 1020 1022 1024 102 104 power & telephone broadcast radio microwave radio gamma rays infrared light visible light ultraviolet light x rays 106 104 102 10 10-2 10-4 10-6 10-8 10-10 10-12 10-14 Wavelength (meters)

  31. Twisted Pair - Attentuation vs. Frequency 26 gauge 30 24 gauge 27 24 22 gauge 21 18 Attenuation (dB/mi) 19 gauge 15 12 9 6 3 f (kHz) 100 1000 1 10 Figure 3.37

  32. Center conductor Dielectric material Braided outer conductor Outer cover Coaxial Cable

  33. Coaxial Cable Attentuation vs. Frequency 35 0.7/2.9 mm 30 25 1.2/4.4 mm Attenuation (dB/km) 20 15 2.6/9.5 mm 10 5 0.01 0.1 10 100 f (MHz) 1.0

  34. Head end Cable TV Distribution Tree Unidirectional amplifier

  35. Upstream fiber Fiber Fiber Head end Fiber node Fiber node Downstream fiber Coaxial distribution plant Bidirectional Split-Band Amplifier Hybrid Fiber-Coaxial System

  36. Downstream 54 MHz 500 MHz Downstream Upstream 42 MHz 5 MHz 54 MHz 500 MHz (a) Current allocation Proposed downstream (b) Proposed hybrid fiber-coaxial allocation 750 MHz 550 MHz

  37. light cladding jacket core c (a) Geometry of optical fiber (b) Reflection in optical fiber

  38. (a) Multimode fiber: multiple rays follow different paths reflected path direct path (b) Single mode: only direct path propagates in fiber

  39. Electrical signal Electrical signal Optical fiber Modulator Receiver Optical source

  40. Frequency (Hz) 106 1012 105 108 107 104 1011 109 1010 FM radio & TV Wireless cable AM radio Cellular & PCS satellite & terrestrial microwave LF MF HF VHF UHF SHF EHF 10-1 1 102 10-3 10-2 101 104 103 Wavelength (meters) Figure 3.48

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