網路概論Class 2-Physical Layer Data Encoding

/Decoding授課老師楊人順

2001/10/02-2001/10/03

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Presentation Outlines

• Data/Signal Transmission• Data Encoding Types• Encoding Methods for Digital Input and

Transmission• Group Discussion

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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

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Communication Block Diagram

Inputdevice Transmitter Transmission

medium Receiver Outputdevice

1 2 3 4 5 6

Voice Telephone Binary Modem Voltage-Pulses

Analog Signal CODECDigital Data Digital Transmitter

Analog Signal

Digital Signal

Agent Agent

Twisted PairCoaxial CableOptical FiberTerrestrial MicrowaveSatellite Microwave

Source system Destination system

InputInformation

m

Input Data g or signal

g(t)

Transmitted signals(t)

Received signals’(t)

Output Data g or signal

g’(t)

OutputInformation

m’

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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

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Pulse Code Modulation

(A)Input Voice Data : less than 4000 Hz(B)Sample Rate : 8000 samples/s(C)Sampled Analog Voice Data(D)Digital Data (8 bits represent a quantizing level)

Hence,

This system will have:

28 = 256 quantizing levels8000 samples/s × 8 bits/sample = 64 kbps

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Quantization Procedure

An example of codewords please referred as pp.150 Figure 5.20

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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.

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Effect of Nonlinear Encoding

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Companding

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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

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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

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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

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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 :

1. A long string of 0’s in the case of AMI or 1’s in the case of pseudoternary still present DC component

2. More bandwidth used than NRZ (3 voltage levels use log23 = 1.58 bits v.s. 2 voltage levels use 1 bit)

3. SIR is bigger than NRZ– Disadvantage 1 can be solved by scrambling techniques (B8ZS, HDB3)

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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)

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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

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Examples of Encoding Methods

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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)

Polarity of Preceding

Number of Bipolar Pulse (1’s) since Last Substitution

Odd Even

000 +00+

+ 000+ 00

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Examples of Encoding Methods (1)

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Examples of Encoding Methods (2)

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4B3T and 2BIQ• 4B3T (4 bits 3 ternary levels)

• Baud rate 3/4

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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

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Example of 4B3T and 2B1Q

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Theoretical Bit Error Rate

Group Discussion

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Homework

1. 網路協定為何要分層 ?2. 試簡述 OSI 7 layers 之每一層的主要工作為何 ?3. 請將 10100001100000000010 數位資料用 Bipo

lar 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？