lecture04- digital and analog transmission

8/19/2019 Lecture04- Digital and Analog Transmission

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By Dr Mir Yasir Uma

Assistant Professor, MCS, NUS

Computer &

Communication Netw

EE357


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Lecture

Digital and AnalogTransmission


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Digital Transmission• Digital Signals

• Line Coding Characteristics & Schemes

• Block Coding

• Sampling / Pulse Code Modulation

Outline for the Lecture


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Analog Transmission• Aspects of Digital-to-Analog Conversion

• Amplitude Shift Keying

• Frequency Shift Keying

• Phase Shift Keying

• Quadrature Amplitude Modulation• Analog Modulation

Outline for the Lecture


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•Methods to transmit data digitally

oLine coding

oBlock coding

o Sampling

Digital Transmission


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Digital Signals• Digital – have a limited number of defined values

•

Use binary (0s and 1s) to encode information• Less affected by interference (noise); fewer errors


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Line Coding• Process of converting binary data to a digital signal


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Line Coding Characteristic

•

Signal Level versus Data Level

• Pulse Rate versus Bit Rate

• DC Components

• Self-Synchronization


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Signal Level versus Data Lev• Signal level – number of different values allowed in a signal

•

Data level –

number of symbols used to represent data


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Signal Level versus Data Lev

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Pulse Rate versus Bit Rate• Pulse rate – defines number of pulses per second

o Pulse – minimum amount of time required to transmit a symbol

• Bit rate – defines number of bits per second

When L is the number of data level of the signal

Bit rate = Pulse rate × log 2 L


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DC Components

•

Residual direct-current (dc) components or zero frequencies are uo Some systems do not allow passage of a dc component (such as a transformer); may distort t

create output errors

o DC component is extra energy residing on the line and is useless

• When the voltage level in a digital signal is constant for a while, t

creates very low frequencies, called DC components , that presen

for a system that cannot pass low frequencies


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Self-Synchronization• Digital signal includes timing information in the data being transm

prevent misinterpretation

Lack of synchronization


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Line Coding Schemes


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Unipolar• Simplest method; inexpensive Provides a background/base t

developed techniques

• Uses only one voltage level Almost obsolete• Polarity (+ or -) is usually assigned to binary 1;a 0 is represented by

voltage


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Unipolar

•

Potential problems:o DC component

o Lack of synchronization


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Polar

•

Uses two voltage levels, one positive and one negative• Alleviates DC component

• Variationso Nonreturn to zero (NRZ)

o Return to zero (RZ)

o Manchester

o Differential Manchester


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Nonreturn to Zero (NRZ)• Value of signal is always positive or negative

• NRZ-L (NRZ-Level)o Signal level depends on bit represented; positive usually means 0, negative usually means 1

o Problem : synchronization of long streams of 0s or 1s

• NRZ-I (NRZ-Invert)o Inversion of voltage represents a 1 bit

o 0 bit represented by no change

o Allows for synchronization


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NRZ-L and NRZ-I Encoding


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Return to Zero (RZ)

•

In NRZ-I, long strings of 0s may still be a problem• May include synchronization as part of the signal for both 1s and

• How?o Must include a signal change during each bit

o Uses three values: positive, negative, and zero

o 1 bit represented by positive-to-zero

o 0 bit represented by negative-to-zero


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RZ Encoding


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Manchester

•

Uses an inversion at the middle of each bit interval for both syncand bit representation

• Negative-to-positive represents binary 1

• Positive-to-negative represents binary 0

• Achieves same level of synchronization with only two levels of a


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Manchester Encoding


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Differential Manchester

• Inversion at middle of bit interval is used for synchronization

• Presence or absence of additional transition at beginning of interthe bit

• Transition means binary 0; no transition means 1

• Requires two signal changes to represent binary 0 but only one to


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Differential Manchester


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Bipolar Encoding• Uses three voltage levels: positive, negative, and zero

• Zero level represents binary 0; 1s are represented with alternating negative voltages, even when the 1 bits are not consecutive

• Two schemeso Alternate mark inversion (AMI)

o Bipolar n-zero substitution (BnZS)


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Bipolar AMI• Neutral, zero voltage represents binary 0

• Binary 1s represented by alternating positive and negative voltage


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Other Schemes• 2B1Q (two binary, one quaternary) uses four voltage levels

o One pulse can represent 2 bits; more efficient


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Other Schemes• MLT-3 (multi-line transmission, three level) – similar to NRZ-I usin

levels of signals; signal transitions occur at beginning of 1 bit, no tr

beginning of 0


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Block Coding

• Coding method to ensure synchronization and detection of error

• Three steps: division, substitution, and line coding


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Steps in Transformation

Step 1

Step 3


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Transformation Steps

• Step 1: bit stream is divided into groups of m bits

• Step 2: substitute an m-bit code for an n-bit groupo Codes with no more than three consecutive 0s or 1s are used to achieve synchronization

o Since only a subset of blocks are used, if one or more bits are changed and an invalid code is can easily detect the error

• Step 3: line encoding scheme is then used to create the signal


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Common Block Codes

• 4B/5B – every 4 bits of data is encoded into a 5-bit code; NRZ-1 isused for line coding

• 8B/10B – group of 8 bits of data is substituted by a 10-bit code

• 8B/6T – each 8-bit group is substituted with a six-symbol code; u bandwidth since three signal levels may be used


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Figure 4.16 Substitution in block coding

4B/5B encoding


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4B/5B encoding

Data Code Data Code

0000 11110 1000 10010

0001 01001 1001 10011

0010 10100 1010 10110

0011 10101 1011 10111

0100 01010 1100 11010

0101 01011 1101 11011

0110 01110 1110 11100

0111 01111 1111 11101


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4B/5B encoding (Continued)

Data Code

Q (Quiet)00000

I (Idle) 11111

H (Halt) 00100

J (start delimiter) 11000

K (start delimiter) 10001

T (end delimiter) 01101

S (Set) 11001

R (Reset) 00111


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Example of 8B/6T encoding


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Sampling

• Analog data must often be converted to digital format (ex: long-d

services, audio)

• Basic Approach is Pulse Code Modulation

• Before that we must know Sampling

• Sampling is process of obtaining amplitudes of a signal at regula


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Pulse amplitude modulation has some applic

but i t is not used by itself in data communic

However, it is the f irst step in another very po

conversion method called

pulse code modulation.

Note:


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Pulse Coded Modulation (PC• First quantizes PAM pulses; an integral value in a specific range to

instances is assigned

• Each value is then translated to its 7-bit binary equivalent• Binary digits are transformed into a digital signal using line codin

Figure 4 19 Quantized PAM signal


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Figure 4.19 Quantized PAM signal

Fi 4 20 Q ti i b i i d it d


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Figure 4.20 Quantizing by using sign and magnitude


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Digitization of an Analog Sig


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Sampling Rate: Nyquist Theo

• Accuracy of digital reproduction of a signal depends on number

• Nyquist theorem: number of samples needed to adequately repreanalog signal is equal to twice the highest frequency of the origin


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According to the Nyquist theorem, the sampli

must be at least 2 times the highest f reque

Note:


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Digital-to-analog conversion


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Types of digital-to-analog convers


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Bit rate is the number of bits per second. Baud rate is thesignal

elements per second.

In the analog transmission of digital data, the baud rate

or equal to the bit rate.

Note


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Binary amplitude shift keying


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Binary frequency shift keying (BF


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Binary phase shift keying (BPSK

1

0

0 1


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• But phase shift keying is more stable than either amplitude

•shift keying or frequency shift keying.

• So we can create systems that use more than two phase

• angles. What about a system that has 4 phase angles?

• QPSK (Quadrature Phase Shift Keying)

• Phase shifts occur on the 45, 135, 225, and 315 degrees. / 0, 90, 180, 270

QPSK

QPSK


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QPSK

QPSK


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QPSK

C f ll i di


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Concept of a constellation diagra


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Quadrature amplitude modulation is a combinaASK and PSK.

Note


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QAM


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• Combination of ASK and PSK so that a maximum contrast betwe

signal unit (bit, dibit, tribit, and so on) is achieved• x number of variations in phase and y variations in amplitude

• Number of phase shifts is always larger than number of amplitudto amplitude susceptibility to noise

• QAM is therefore less susceptible to noise than ASK

•Same bandwidth is required for ASK and PSK

QAM

Th 4 QAM d 8 QAM t ll


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The 4-QAM and 8-QAM constella

Time domain for an 8 QAM sig


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Time domain for an 8-QAM sig

16 QAM constellation


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Grey Code Mapping


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Grey Code Mapping



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Types of analog-to-analog modula


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Types of analog-to-analog modula

Amplitude Modulation


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Amplitude Modulation


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Phase modulation


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Phase modulation

Conclusion


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DATA SIGNAL APPROACH

ANALOG DIGITAL ENCODING

DIGITAL DIGITAL ENCODING

ANALOG ANALOG MODULATION

DIGITAL ANALOG MODULATION

Conclusion

DEPENDS ON THE SITUATION AND THE BW REQUIREMENTS, WHICH TO

Credits


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CreditsData Communications and Networking, 3rd edition by Behrouz A. Forouzan. McGraw Hill Pub

lecture04- digital and analog transmission

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