Computer Networks 2.1 General Issues 2.2 Medium An Open Source - PDF document

9/29/2014 Content Computer Networks  2.1 General Issues  2.2 Medium An Open Source Approach  2.3 Information Coding and Baseband Transmission  2.4 Digital Modulation and Multiplexing Chapter 2: Physical Layer  2.5 Advanced Topics  2.6 Summary Ying-Dar Lin, Ren-Hung Hwang, Fred Baker Chapter 2: Physical Layer 1 Chapter 2: Physical Layer 2 Data and Signal: Analog or Digital 2.1 General Issues  Data  Digital data – discrete value of data for storage or  Data and Signal: Analog or Digital communication in computer networks  Transmission and Reception Flow  Analog data – continuous value of data such as sound or image  Transmission: Line Coding and Digital Modulation  Signal  Transmission Impairments  Digital signal – discrete-time signals containing digital information  Analog signal – continuous-time signals containing analog information Chapter 2: Physical Layer 3 Chapter 2: Physical Layer 4 Chapter 2: Physical Layer 1

9/29/2014 Periodic and Aperiodic Signals (1/4) Periodic and Aperiodic Signals (2/4)  Spectra of periodic analog signals: discrete  Spectra of aperiodic analog signals: continous f1=100 kHz f2=400 kHz periodic analog signal Amplitude Amplitude aperiodic analog signal Time Time Amplitude Amplitude f1 f2 Frequency Frequency 100k 400k Chapter 2: Physical Layer 5 Chapter 2: Physical Layer 6 Periodic and Aperiodic Signals (3/4) Periodic and Aperiodic Signals (4/4)  Spectra of periodic digital signals: discrete  Spectra of aperiodic digital signals: continuous (frequency pulse train, infinite) (infinite) periodic digital signal frequency = f kHz Amplitude Amplitude aperiodic digital signal ... Time Time Amplitude Amplitude frequency pulse train ... ... 0 Frequency f 2f 3f 4f 5f Frequency Chapter 2: Physical Layer 7 Chapter 2: Physical Layer 8 Chapter 2: Physical Layer 2

9/29/2014 Principle in Action: Nyquist Transmission and Reception Flows Theorem vs. Shannon Theorem  A digital communications system  Nyquist Theorem:  Nyquist sampling theorem f s ≧ 2 x f max  From Other Sources Interference  Maximum data rate for noiseless channel Message Channel Channel Baseband Bandpass Symbols Symbols Symbols Waveform Waveform & Noise 2 B log 2 L (B: bandwidth, L: # states to represent a symbol)  Information Source/Channel Transmit Multiplexing Line Coding Modulation Source Coding 2 x 3k x log 2 2 = 6 kbps Transmitted  Signal  Shannon Theorem: Channel Bit Stream Digital Signal  Maximum data rate for noisy channel Received Signal Information Source/Channel Demultiplexing Line Decoding Demodulation B log 2 (2(1+S/N)) (B: bandwidth, S: signal, N: noise) Sink Decoding Receive  3k x log 2 (2 x (1+1000)) = 32.9 kbps  To Other Destinations Chapter 2: Physical Layer 9 Chapter 2: Physical Layer 10 Baseband vs. Broadband Line Coding Synchronization, Baseline Wandering, and DC Components  Baseband transmission:  Synchronization  Digital waveforms traveling over a baseband channel without further conversion into analog waveform by  Calibrate the receiver’s clock for synchronizing bit modulation. intervals to the transmitter’s  Baseline Wandering (or Drift)  Broadband transmission:  Make a received signal harder to decode  Digital waveforms traveling over a broadband channel  DC components (or DC bias) with conversion into analog waveform by modulation.  A non-zero component around 0 Hz  Consume more power Chapter 2: Physical Layer 11 Chapter 2: Physical Layer 12 Chapter 2: Physical Layer 3

9/29/2014 Digital Modulation Transmission Impairments Amplitude, Frequency, Phase, and Code  Attenuation  Gradual loss in intensity of flux such as radio waves  Use analog signals, characterized by  Fading : A time varying deviation of attenuation when a amplitude, frequency, phase, or code, to modulated waveform traveling over a certain medium represent a bit stream.  Multipath fading: caused by multipath propagation  A bit stream is modulated by a carrier signal  Shadow fading: shadowed by obstacles into a bandpass signal (with its bandwidth  Distortion: commonly occurs to composite signals centered at the carrier frequency).  Different phase shifts may distort the shape of composite signals  Interference: usually adds unwanted signals to the desired signal, such as co-channel interference (CCI, or crosstalk), inter- symbol interference (ISI), inter-carrier interference (ICI)  Noise: a random fluctuation of an analog signal, such as electronic, thermal, induced, impulse, quantization noises. Chapter 2: Physical Layer 13 Chapter 2: Physical Layer 14 Historical Evolution: Software 2.2 Medium Defined Radio  A functional model of a software radio communications system Source Channel Set Set Network Analog/Digital  Wired Medium IF Baseband Protected Clear Source Waveform Waveform Bitsteam Bitsteam Bitsteam  Wireless Medium Service RF/ IF Information & Source Channel Modem Security Network Coding Processing Access Support RF Waveform Channel Coding/Decoding Joint Control (Radio Node) Multiple Personalities (Software Object) Load/Execute Host Processors Chapter 2: Physical Layer 15 Chapter 2: Physical Layer 16 Chapter 2: Physical Layer 4

9/29/2014 Wired Medium: Twisted Pair (1/2) Wired Medium: Twisted Pair (2/2)  Two copper conductor twisted together to Specifications of common twisted pair cables. prevent electromagnetic interference. Specifications Description  Shielded twisted pairs, STP Metal shield conductor Category 1/2 For traditional phone lines. Not specified in TIA/EIA. Category 3 Transmission characteristics specified up to 16 MHz Plastic cover Insulator Category 4 Transmission characteristics specified up to 20 MHz  Unshielded twisted pairs, UTP. Category 5(e) Transmission characteristics specified up to 100 MHz conductor Category 6(a) Transmission characteristics specified up to 250 MHz (Cat-6) and 500 MHz (Cat-6a) Category 7 Transmission characteristics specified up to 600 MHz Plastic cover Insulator Chapter 2: Physical Layer 17 Chapter 2: Physical Layer 18 Wired Medium: Coaxial Cable Wired Medium: Optical Fiber (1/3)  Coaxial Cable  Optical Fiber  An inner conductor surrounded by an insulating layer,  Refraction of light and total internal reflection a braided outer conductor, another insulating layer, perpenticular and a plastic jacket. Braided Inner outer conductor conductor q 2 air n 2 refractive index: water q q n 1 refractive index: Plastic jacket Insulator Insulator q 1 total internal reflection q c Chapter 2: Physical Layer 19 Chapter 2: Physical Layer 20 Chapter 2: Physical Layer 5

9/29/2014 Wired Medium: Optical Fiber (2/3) Wired Medium: Optical Fiber (3/3)  Single-mode:  Optical Fiber: a thin glass or plastic core is surrounded A fiber with a very thin core allowing only one mode of light to  by a cladding glass with a different density. be carried.  Multi-mode: A fiber carries more than one mode of light  Cladding core different modes (Glass) cladding multi-mode fiber Jacket Core (Plastic cover) (Glass or Plastic) single-mode fiber core Chapter 2: Physical Layer 21 Chapter 2: Physical Layer 22 Wireless Medium 2.3 Information Coding and Baseband Transmission  Propagation Methods  Three types – ground, sky, and line-of-sight propagation  Transmission Waves:  Source and Channel Coding  Radio, Microwave, Infrared waves  Line Coding  Mobility  Mostly use microwave Chapter 2: Physical Layer 23 Chapter 2: Physical Layer 24 Chapter 2: Physical Layer 6

9/29/2014 Source Coding Channel Coding  To form efficient descriptions of information  Used to protect digital data through a noisy sources so the required storage or bandwidth transmission medium or stored in an resources can be reduced imperfect storage medium.  Some applications:  The performance is limited by Shannon’s Theorem  Image compression  Audio compression  Speech compression Chapter 2: Physical Layer 25 Chapter 2: Physical Layer 26 Line Coding and Signal-to-Data Ratio Line Coding and Signal-to-Data Ratio (1/2) (2/2)  Line Coding: applying a pulse modulation to a  A simplified line coding process binary symbol and generating a pulse-code 1 0 1 modulation (PCM) waveform sdr=2 sdr > 1 Digital Transmission 1 0 1 0 sdr=1 sdr = 1  PCM waveforms are known as line codes. 1 1 0 1 1 1 sdr=1/2 sdr < 1  Signal-to-Data Ratio (sdr): digital signal 1 0 1 0 1 0 1 0 Line Coding Line Coding Channel  a ratio of the number of signal elements to the Encoder Decoder digital data digital data number of data elements Chapter 2: Physical Layer 27 Chapter 2: Physical Layer 28 Chapter 2: Physical Layer 7