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網路概論 Class 2-Physical Layer Data Encoding/Decoding

網路概論 Class 2-Physical Layer Data Encoding/Decoding. 授課老師 楊人順 2001/10/02-2001/10/03. Presentation Outlines. Data/Signal Transmission Data Encoding Types Encoding Methods for Digital Input and Transmission Group Discussion. Transmission Signal Types. Analog Signal

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網路概論 Class 2-Physical Layer Data Encoding/Decoding

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  1. 網路概論Class 2-Physical Layer Data Encoding/Decoding 授課老師 楊人順 2001/10/02-2001/10/03

  2. Presentation Outlines • Data/Signal Transmission • Data Encoding Types • Encoding Methods for Digital Input and Transmission • Group Discussion -2-

  3. Transmission Signal Types • Analog Signal • Represent data with continuously varying electromagnetic wave • Digital Signal • Represent data with sequence of voltage pulses electromagnetic wave • E.g., +12 V = 0, -12 V = 1 • E.g., Transition from Low to High voltage-level = 1, Transition from High to Low voltage-level = 0 -3-

  4. Communication Block Diagram 1 2 3 4 5 6 Output Data g or signal g’(t) Output Information m’ Input Information m Input Data g or signal g(t) Transmitted signal s(t) Received signal s’(t) Agent Input device Transmitter Transmission medium Receiver Output device Agent Destination system Source system Voice Telephone Binary Modem Voltage-Pulses Analog Signal CODEC Digital Data Digital Transmitter Analog Signal Twisted Pair Coaxial Cable Optical Fiber Terrestrial Microwave Satellite Microwave Digital Signal -4-

  5. Data Encoding Type • Digital data input, Digital Signal Transmission • Analog Signal input, Digital Signal Transmission • Pulse Code Modulation (PCM) • Digital data input, Analog Signal Transmission • ASK (幅移鍵控), FSK (頻移鍵控), PSK (相移鍵控) (pp.143 figure 5.7) • Analog Signal, Analog Signal Transmission (pp. 159 figure 5.17) • Amplitude (振幅) Modulation, AM • Frequency (頻率) Modulation, FM • Phase (相位) Modulation, PM • Quadrature Amplitude Modulation, QAM : AM + PM used in ADSL (asymmetric digital subscriber line) • ASK modulation • Two different signals simultaneously transmitted on the same carrier frequency by phase shift 90o • Digital or Analog input, Spread Spectrum Transmission (also a multiplexing technique) • Frequency Hopping • Direct Sequence -5-

  6. Pulse Code Modulation • Input Voice Data : less than 4000 Hz • Sample Rate : 8000 samples/s • Sampled Analog Voice Data • Digital Data • (8 bits represent a quantizing level) • Hence, • This system will have: • 28 = 256 quantizing levels • 8000 samples/s × 8 bits/sample = 64 kbps -6-

  7. Quantization Procedure An example of codewords please referred as pp.150 Figure 5.20 -7-

  8. Nonlinear PCM Encoding • The problem of linear PCM encoding, i.e., equal spacing for all quantizing levels, could cause lower amplitude values (小信號) are relatively more distorted (失真). • Solutions : • Nonlinear Coding • Companding (compressing-expanding) Both ,methods can improve Signal-to-Noise Ratio (SNR), SNR=Eb/N0. -8-

  9. Effect of Nonlinear Encoding -9-

  10. Companding -10-

  11. Digital Input, Digital Signal Transmission • Nonreturn to Zero : used in the digital magnetic recording • NRZ-L • NRZI • Multilevel Binary : used in ISDN for relatively low data rate transmission • Bipolar-AMI • Pseudoternary • Biphase : used in LAN • Manchester • Differential Manchester • Scrambling Technique : used in ISDN and other WAN • B8ZS • HDB3 • Multilevel Coding : high data rate network • 4B3T : access circuit in ISDN (WAN, because DC wander problem) • 8B6T : Fast Ethernet 100Base 4T (LAN, to reduce the clock rate in T-pair) • 2B1Q -11-

  12. Evaluating the Encoding Methods • Signal Spectrum (訊號頻寬) and DC Component (直流部分) • Clocking (時脈) • Error Detection (錯誤檢測) • Signal Interference and Noise Immunity (信號干擾和雜訊污染) • Cost and Complexity -12-

  13. Nonreturn to Zero • NRZ-L • 0 : high level • 1 : low level • NRZI (differential encoding) • 0 : no transition at beginning of interval • 1 : transition at beginning of interval • Advantage and disadvantage • A : the easiest way to engineer, efficient use of bandwidth • D : DC component, lack of synchronization capacity -13-

  14. Multilevel Binary • Bipolar-AMI (Alternate Mark inversion) • 0 : no line signal • 1 : positive or negative level, alternating for successive 1’s • Pseudoternary • 0 : positive or negative level alternating for successive 0’s • 1 : no line signal • Advantage and disadvantage • A : provide a degree of synchronization • D : • A long string of 0’s in the case of AMI or 1’s in the case of pseudoternary still present DC component • More bandwidth used than NRZ (3 voltage levels use log23= 1.58 bits v.s. 2 voltage levels use 1 bit) • SIR is bigger than NRZ • Disadvantage 1 can be solved by scrambling techniques (B8ZS, HDB3) -14-

  15. Biphase • Manchester • 0 : transition from high to low in middle of interval • 1 : transition from low to high in middle of interval • Differential Manchester • Clocking : always a transition in middle of interval • 0 : transition at beginning of interval • 1 : no transition at beginning of interval • Advantage • Synchronization • NO DC component • Error Detection : the absence of an excepted transition can be used to detect error • Remove Noise : noise on the line would have to invert both the signal before and after the excepted transition to cause an undetected error (so noise can ease to detect) -15-

  16. Biphase (con’t) • Disadvantage : the maximum modulation rate among encoding methods, twice than NRZ • Usage • IEEE 802.3 CSMA/CD bus LAN standard uses Manchester encoding • MIL-STD-1553B, which is a shielded twist-pair bus LAN designed for high-noise environments, uses Difference Manchester encoding • IEEE 802.5 token ring LAN standard uses Difference Manchester encoding -16-

  17. Examples of Encoding Methods -17-

  18. Scrambling Technique • Bipolar with 8-zeros substitution (B8ZS) used in North America • Same as bipolar AMI except of replacing eight 0’s to • 000+0+, if the last pulse preceding this octet was positive • 000+0+, if the last pulse preceding this octet was negative • High-density bipolar-3 zero (HDB3) used in Europe and Japan • Same as bipolar AMI except of replacing four 0’s with sequence containing one or two pulse(s) -18-

  19. Examples of Encoding Methods (1) -19-

  20. Examples of Encoding Methods (2) -20-

  21. 4B3T and 2BIQ • 4B3T (4 bits 3 ternary levels) • Baud rate 3/4 -21-

  22. 4B3T and 2BIQ (con’t) • 2B1Q (2 bits 1 quaternary levels) • Each 2-bit input sequence transmitted in one 4-level pulse (+3, +1, -1, -3) • The first bit (1 = +, 0 = ) • The second bit (1 = 1, 0 = 3) • Baud rate 1/2 -22-

  23. Example of 4B3T and 2B1Q -23-

  24. Theoretical Bit Error Rate -24-

  25. Group Discussion

  26. Homework • 網路協定為何要分層? • 試簡述OSI 7 layers之每一層的主要工作為何? • 請將10100001100000000010數位資料用Bipolar AMI編碼(繪出波型圖)並試述其缺點；又已知有兩種Scrambling Techniques可以解決上述缺點，請用這兩種技術將前列數位資料編碼(繪出波型圖)，並描述他們的優缺點及實際應用在那些網路架構中？ -26-

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