msis 4523 ch4.digital transmission

8/3/2019 MSIS 4523 Ch4.Digital Transmission

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Data Communications SystemsCh 4: Digital Transmission

JinKyu Lee, Ph.D.

[email protected]

Include the course code (MSIS4523) in every email subject!!

Layer1


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A Categorization of Transmission Techniques

Digital

Transmission

Analog

Transmission

Data Transmission

Line

Coding

Digital Source

Conversion

Analog Source

Conversion

Block

CodingPCM DM

ASK

Digital Source

Conversion

Analog Source

Conversion

FSK

AM FM

PSKQAM

PM

Ch.4

Ch.5


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Topics

Digital-to-Digital Conversion Transmission Modes Parallel vs. Serial

Sync vs. Async

Line coding Schemes

Block coding Schemes

Analog-to-Digital Conversion Sampling, Quantization, Encoding

PCM, Delta Modulation

Nyquists theorem

Baud Rate (Signal Rate) vs. Bit Rate


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Data and Signals, Conversion Techniques


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Data and Signal Conversions Examples

Let us transmit the message Sam, what time is themeeting with accounting? Hannah.This message first leaves Hannahs workstation and

travels across a local area network


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Converting Digital Data into Digital SignalsDigital Encoding Compared

NRZ-L

NRZI

Manchester

DifferentialManchester

Bipolar-AMI


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Transmission Modes - Parallel Transmission


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Transmission Modes - Serial Transmission


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Transmission Modes - Synchronous

In synchronous transmission, bits are sent one after

another without start/stop bits or gaps. Doesnt mean

signal never stop. Bits are sent by block (hundreds of

bytes) by block (e.g., Ethernet frames)

Receiver must pickup individual bits from the stream

of bits.

In synchronous transmission, bits are sent one after

another without start/stop bits or gaps. Doesnt mean

signal never stop. Bits are sent by block (hundreds of

bytes) by block (e.g., Ethernet frames)Receiver must pickup individual bits from the stream

of bits.


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Line Coding Schemes

Direct data-signal conversion


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Unipolar Encoding

Unipolar encoding uses only one voltage levelUnipolar encoding uses only one voltage level


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Polar Encoding Schemes

Non return to zero-level (NRZ-L)

Non return to zero inverted (NRZI)

Bipolar -AMI

Pseudoternary

Manchester

Differential Manchester

B8ZS

HDB3 Polar encoding uses twovoltage levels (positive

and negative)

Polar encoding uses twovoltage levels (positive

and negative)


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Return to Zero (RZ) Encoding

A good encoded digital signal must contain a

provision for synchronization

A good encoded digital signal must contain a

provision for synchronization


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NRZ-L and NRZ-I encoding

In NRZ-L the level of the signal is dependent upon the state of

the bit. In NRZ-I the signal is inverted if a 1 is encountered

In NRZ-L the level of the signal is dependent upon the state of

the bit. In NRZ-I the signal is inverted if a 1 is encountered


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The Synchronization Problem

How does the receiver know when the bit period

begins and ends?

Digital communications requires that the receiver

know when to sample the signal

This requires a mechanism to synchronize thetransmitter and receiver

Solutions: Bi-phase signaling

Substitution for line coding

Block coding


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Biphase Alternatives to NRZ

Require at least one transition per bit time, and may even

have two

Advantages Synchronization due to predictable transitions

Error detection based on absence of a transition

However, modulation rate is greater, so bandwidth

requirements are higher

Known as Manchestercoding and Differential

Manchestercoding


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Manchester Encoding

In Manchester encoding, the transition at the middle

of the bit is used for both synchronization and bitrepresentation.

IEEE 802.3 Ethernet uses Manchester encoding

In Manchester encoding, the transition at the middle

of the bit is used for both synchronization and bitrepresentation.

IEEE 802.3 Ethernet uses Manchester encoding


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Differential Manchester Encoding

In differential Manchester encoding, the transition atthe middle of the bit is used only for synchronization.

The bit representation is defined by the inversion or

non-inversion at the beginning of the bit

In differential Manchester encoding, the transition atthe middle of the bit is used only for synchronization.

The bit representation is defined by the inversion or

non-inversion at the beginning of the bit

Differential Encoding Data represented by changes rather than levels Receivers can more reliably detect transitions rather

than specific levels

In complex transmission layouts it is also easy to

lose sense of polarity


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Substitution Techniques

Use substitution to replace sequences that would produce

constant voltage Goal is to provide enough transitions to allow

synchronization Substitution must be recognized by receiver and replaced with

original

Ideally the same length as original

Advantages: No DC component

Eliminates long sequences of zero level line signal

No reduction in data rate Can provide error detection capability


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Bipolar With 8 Zeros Substitution (B8ZS)

Based on bipolar-AMI In Bipolar-AMI, signal patterns of +0+ or -0- should never happen

under normal condition.

If octet of all zeros and last voltage pulse preceding was

positive (+ 00000000 ): encode as + 000+-0-+

If octet of all zeros and last voltage pulse preceding wasnegative (- 00000000 ): encode as - 000-+0+-

Causes two violations of AMI code

Unlikely to occur as a result of noise

Receiver detects and interprets as octet of all zeros


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Indirect data-signal conversion

Block Coding Schemes


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Substitution in Block Coding


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Converts four bits of data into five-bit quantitiesNo five-bit code has more than 2 consecutive zeroes

Five-bit code is then transmitted using an NRZ-I encoded signal

Converting Digital Data into Digital Signals -4B/5B Digital Encoding Scheme


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4B/5B Encoding Table

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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Analog-to-Digital Conversion

Encoding digital data is easysimply represent 0s and 1s

with different voltage levels on the carrier wave

Not so easy with continuous data like music or voice

Have to convert the continues data (analog) to discrete

data (digital)

Do this with Pulse Code Modulation (PCM)


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Sampling

Analog voice data must be translated into a series ofbinary digits before they can be transmitted across digitaltransmission facilities

To convert analog data into a digital signal, there are twobasic techniques: Pulse code modulation (PCM) (used by telephone systems)

With PCM, the amplitude of the sound wave is sampled atregular intervals and translated into a binary number

Delta modulation

The difference between the original analog signal and the

translated digital signal is called quantizing error


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Quantization Process

Quantization process assigns a bit value

corresponding to the amplitude of each sampled

sound wave.

Similar concept to pixelization 8-bit range allows for 256 possible sample levels

If 128 levels, then each sample is 7 bits (2 ^ 7 =

128)


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Pulse Code Modulation (PCM)

64K PCM uses a sampling rate of 8000 samples

per second

Each sample is an 8 bit sample resulting in a

digital rate of 64,000 bps (8 x 8000)


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Analog waveform is sampled at specific intervalsSnapshots are converted to binary values

Pulse Code Modulation (PCM) - Sampling

Pulse

Amplitude

Modulation

(PAM) value


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Pulse Code Modulation (PCM) - Pulse Amplitude Modulation (PAM)


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Because the human voice has a fairly narrow

bandwidth

Telephone systems digitize voice into either 128 levels or

256 levels

Called quantization levels

More bits means greater detail, fewer bits means lessdetail

Pulse Code Modulation (PCM) - Quantization


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+1 +1 +1

+0 +0+0

+0

+0+0

+0+0

+0

-0

-1

-0

Pulse Code Modulation (PCM) - Quantizing of PAM Signals


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Converting Analog data to PCM Digital Signal The whole Picture!


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An analog waveform is tracked using a binary 1 torepresent a rise in voltage and a 0 to represent a drop

Delta Modulation


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The more snapshots taken in the same amount of time,or the more quantization levels, the better theresolution

Sampling Frequency, Quantization Level,

and the Quality of Reconstructed Data


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Binary values are later converted to an analog signalWaveform similar to original results

Sampling Frequency, Quantization Level,

and the Quality of Reconstructed Data (cont.)


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Nyquists Theorem Sampling Rate

According to Nyquists theorem, the sampling rate must be at

least 2 times the highest frequency

According to Nyquists theorem, the sampling rate must be at

least 2 times the highest frequency

=

==


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What sampling rate is needed for a signal with a

bandwidth of 10,000 Hz (1000 to 11,000 Hz)?

The sampling rate must be twice the highest frequency in

the signal:

Sampling rate = 2 x (11,000) = 22,000 samples/s

SolutionSolution

Example 15

Nyquists theorem: If a signal is sampled at regular intervalsof time and at a rate higher than twice the significant signal

frequency, the samples contain all the information of the

original signal

Nyquists theorem: If a signal is sampled at regular intervalsof time and at a rate higher than twice the significant signal

frequency, the samples contain all the information of the

original signal


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Baud Rate: The number of times a signal changes

value per second = Signal Rate.Manchester schemes: Baud rate is twice the bps

Two signal changes per bit

Baud Rate (baud/s) vs. Bit Rate (bps)


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An analog signal carries 4 bits in each signal unit. If 1000

signal units are sent per second, find the baud rate and

the bit rate.

Baud rate = 1000 bauds per second (baud/s)

Bit rate = 1000 x 4 = 4000 bps

Baud Rate vs. Bit Rate Example1

SolutionSolution

Bit rate is the number of bits per second. Baud

rate is the number of signal units per second.

Bit rate is the number of bits per second. Baud

rate is the number of signal units per second.


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The bit rate of a signal is 3000. If each signal unit carries

6 bits, what is the baud rate?

Baud rate = 3000 / 6 = 500 baud/s

Baud Rate vs. Bit Rate Example 2

SolutionSolution

msis 4523 ch4.digital transmission

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