13667 digital trnsmission

8/13/2019 13667 Digital Trnsmission

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DigitalTransmission


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Line Coding

Line Coding is the process of converting binary data, a sequence ofbits, to a digital signal. For example, data, text, number, graphical

image, audio and video that are stored in computer memory are allsequence of bits. Line coding converts a sequence of bits to a digitalsignal.


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4.1 Line Coding

Some Character istics

L ine Coding Schemes


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Line Coding Some Characteristics

A digital signal can have a limited number of values. However, onlysome of these values can be used to represent data; rest are used for

other purposes as we shall see shortly.Signal Levels: The number of values allowed in a particular signal.

Data Levels: The number of values used to represent data.

Signal Levels versus Data Levels


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Line Coding Some Characteristics Pulse Rate versus Bit Rate

Pulse Rate: It defines the number of pulses per second. A pulse is theminimum amount of time required to transmit a symbol.

Bit Rate: It defines the number of bits per second.

Relation between the two: If a pulse carries only 1 bit, the pulse rateand the bit rate are the same. If the pulse carries more than 1 bit, thenthe bit rate is greater than the pulse rate. So we have a formula to

calculate bit rate in relation with pulse rate:

Bit Rate = Pulse Rate X log2L

Where L is the number of data levels of the signal.


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Example 1

A signal has two data levels with a pulse duration of 1

ms. Calculate the pulse rate and bit rate.

Pulse Rate = 1/ 10-3= 1000 pulses/s

Bit Rate = Pulse Rate x log2L = 1000 x log22 = 1000 bps

Solution


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Example 2

A signal has four data levels with a pulse duration of 1

ms. Calculate the pulse rate and bit rate.

Pulse Rate = = 1000 pulses/s

Bit Rate = Pulse Rate x log2L = 1000 x log24 = 2000 bps

Solution


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Line Coding Some Characteristics Self-Synchronization

To correctly interpret the signals received from the sender, thereceiver's bit intervals must correspond exactly to the senders bitinterval. If the receiver clock is faster or slower, the bit intervals are

not matched and the receiver might interpret the signals differentlythan the sender intended. Look at figure below to visualize theproblem. The sender sends 10110001, while the receiver receives110111000011.


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Example 3

In a digital transmission, the receiver clock is 0.1 percent

faster than the sender clock. How many extra bits per

second does the receiver receive if the data rate is 1

Kbps? How many if the data rate is 1 Mbps?

Solution

At 1 Kbps:

1000 bits sent1001 bits received1 extra bps

At 1 Mbps:

1,000,000 bits sent1,001,000 bits received1000 extra bps


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A Self-Synchronizing digital signal includes timing information in thedata being transmitted. This can be achieved if there is the signal thatalerts the receiver to the beginning, middle, or end of pulse. If thereceivers clock is out of synchronization, these alerting points canreset the clock.


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Line Coding Line Coding Schemes


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Line Coding Line Coding Schemes Unipolar Encoding

The polarity of a pulse refers to whether it is positive or negative.

Unipolar encoding uses only one voltage level. It is named so because ituses only one polarity. This polarity is assigned to one of the two binarystates, usually the 1. the other state, usually the 0, is represented byzero voltage.

Problems:

1) DC component.

2) Lack of synchronization in case of data containing long sequenceof0s and 1s.


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Line Coding Line Coding Schemes Polar Encoding

Polar encoding uses two voltage levels, one positive and one negative.

Polar encoding is classified as follows:


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Non return to Zero (NRZ): In it, the value of the signal is always eitherpositive or negative. It is classified in two categories as follows:

1) In NRZ-L the level of the signal is dependent upon the state of thebit.

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


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Return to Zero (RZ): It uses three values: positive, negative, and zero.In it signal changes not between bits but during each bit. A one bitis represented by positive-to-zero transition in the halfway of bit

and a 0 bit by negative to-zero transition.

Disadvantage:

It requires two signal changes to encode 1 bit and therefore

occupies more bandwidth.


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In Manchester Encoding, The transition at the middle of the bit is usedfor both synchronization and bit representation.


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In Differential Manchester encoding, the transition at the middle of thebit is used only for synchronization. The bit representation isdefined by the inversion or non inversion at the beginning of the

bit.


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Line Coding Line Coding Schemes Bipolar Encoding

In bipolar encoding, we use three levels: positive, zero, and negative.

Bipolar alternate mark inversion (AMI): Binary 1 is represented by

alternate 1 inversions.


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Line Coding Line Coding Schemes 2B1Q

2B1Q (two binary, one quaternary): uses four voltage levels, eachrepresenting two bits.


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Block Coding

Block Coding: It was introduced to improve the performance of linecoding. Some extra bits are include to:

-Ensure synchronization

-Detect errors


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4.2 Block Coding

Steps in Transformation

Some Common Block Codes

C i


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Block Coding Steps in Transformation Step 1 : Division

In this step, the sequence of bits is divided into groups of mbits.

Bl k C di


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Block Coding Steps in Transformation Step 2 : Substitution

In this step, we substitute an m-bit code for an n-bit group, wherenm. Therefore we can map some of the n-bit groups to the m-bitgroups and some of the n-bit groups remains unused. We choose onlythose n-bit codes that help us in synchronization and error detection.

Bl k C di


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Block Coding Steps in Transformation Step 3 : Line Coding

Now we can use one of the line coding schemes. Figure below showswhole of the process.

Bl k C di


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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

Block Coding Steps in Transformation 4B/5B encoding

Bl k C di


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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

Block Coding Steps in Transformation 4B/5B encoding


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4.3 Sampling

Pulse Amplitude Modulation

Pulse Code ModulationSampling Rate: Nyquist Theorem

How Many Bits per Sample?

Bit Rate

S li


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Sampling

If you note carefully, Line and block coding can be used to convertbinary data to a digital signal. What if data is analog, such as audio orvideo?

The solution is sampling. The term sampling means measuring theamplitude of the signal at equal intervals.

S li P l A lit d M d l ti (PAM)


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Sampling

It uses a technique called sample and hold. At a given moment, thesignal level is read, then held briefly. The sampled value occurs onlyinstantaneously in the actual waveform, but is generalized over a still

short but measurable period in the PAM result.

Pulse Amplitude Modulation (PAM)

Problem:

PAM converts waveform to a series of pulses, which are still analog.

S li P l C d M d l ti (PCM)


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Sampling Pulse Code Modulation (PCM)PCM modifies the pulses created by PAM to create a completely digital signal. To doso, PCM first quantizes the PAM pulses.

Quantization is a method of assigning integral values in a specific range to sampled

instances.

S li P l C d M d l ti (PCM)


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Sampling Pulse Code Modulation (PCM)

After that, it is quantized values are converted to binary data with signsas follows:

The binary data is then converted to digital signal by using one of theline coding or block coding technique.

Sampling P l C d M d l ti (PCM)


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Sampling Pulse Code Modulation (PCM)

Let us see the whole process:

Sampling S li R t N i t Th


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Sampling Sampling Rate: Nyquist Theorem

The accuracy of any digital reproduction of an analog signal depends onthe number of samples taken. Question is how many samples aresufficient?

According to Nyquist theorem, to ensure the accurate reproduction ofan original analog signal using PAM, the sampling rate must be at leasttwice the highest frequency of the originally signal.

Sampling S li R t N i t Th


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Note that we can always change a

band-pass signal to a low-pass signalbefore sampling to reduce sampling

rate.

Note:




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Example 4

What sampling rate is needed for a signal with a

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

Solution

The sampling rate must be twice the highest frequency in

the signal:

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


Sampling How many bits per sample


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Example 5

A signal is sampled. Each sample requires at least 12

levels of precision (+0 to +5 and -0 to -5). How many bits

should be sent for each sample?

Solution

We need 4 bits; 1 bit for the sign and 3 bits for the value.

A 3-bit value can represent 23= 8 levels (000 to 111),

which is more than what we need. A 2-bit value is notenough since 22= 4. A 4-bit value is too much because 24

= 16.




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Example 6

We want to digitize the human voice. What is the bit rate,assuming 8 bits per sample?

Solution

The human voice normally contains frequencies from 0

to 4000 Hz.

Sampling rate = 4000 x 2 = 8000 samples/s

Bit rate = sampling rate x number of bits per sample

= 8000 x 8 = 64,000 bps = 64 Kbps



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4.4 Transmission Mode

Parallel Transmission

Serial Transmission

Transmission Mode


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Transmission Mode

Transmission Mode Parallel Transmission


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Transmission Mode Parallel Transmission

Transmission Mode Serial Transmission


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Transmission Mode Serial Transmission

Transmission Mode Serial Transmission Asynchronous transmission


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I n asynchronous transmission, we

send 1 start bit (0) at the beginningand 1 or more stop bits (1s) at the end

of each byte. There may be a gap

between each byte.

Note:




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Asynchronous here means

asynchronous at the byte level, butthe bits are sti l l synchronized; their

durations are the same.

Note:




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Transmission Mode Serial Transmission Synchronous transmission


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I n synchronous transmission,

we send bits one after another withoutstar t/stop bits or gaps.

I t is the responsibil i ty of the receiver to

group the bits.

Note:

Transmission Mode Serial Transmission Synchronous transmission


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