1 kyung hee university digital transmission. 2 kyung hee university 4 장 digital transmission 4.1...

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Kyung Hee University

Digital TransmissionDigital Transmission

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4 4 장 장 Digital TransmissionDigital Transmission

4.1 Line Coding

4.2 Block Coding

4.3 Sampling

4.4 Transmission Mode

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Line CodingLine Coding

Converts sequence of bits to a digital signal

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Characteristics of Line Coding

Signal level vs. data level

Pulse rate vs. bit rate

dc components

Self-synchronization

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Signal Level versus Data LevelSignal Level versus Data Level

Signal level – number of values allowed in a signal

Data level – number of values used to represent data

Three signal levels, two data levels

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Pulse Rate versus Bit RatePulse Rate versus Bit Rate

Pulse rate – number of pulses per second

Bit rate – number of bits per second

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Example 1>

A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

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

Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 2 = 1000 bps 2 = 1000 bps

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Example 2 >

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

We calculate the pulse rate and bit rate as follows

Pulse Rate = = 1000 pulses/sPulse Rate = = 1000 pulses/s

Bit Rate = PulseRate x logBit Rate = PulseRate x log22 L = 1000 x log L = 1000 x log22 4 = 2000 bps 4 = 2000 bps

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DC ComponentsDC Components Some systems (such as transformer) will not allow passage of dc

component

DC Component is just extra energy on the line, but useless

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Self-SynchronizationSelf-Synchronization

Receiver’s bit intervals must correspond exactly to the sende

r’s bit intervals

Self-synchronizing signal includes timing information

If the receiver’s clcok is out of synchronization, these alerting

points can reset the clock.

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Self-Synchronization Self-Synchronization - Lack of synchronization- Lack of synchronization

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Self-SynchronizationSelf-Synchronization

Example 3Example 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 Solution

At 1 Kbps:

1000 bits sent 1001 bits received1 extra bps

At 1 Mbps:

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

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Line Coding SchemeLine Coding Scheme

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Unipolar codingUnipolar coding

Unipolar encoding uses only one voltage level.

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Unipolar encoding disadvantages

dc component

no synchronization

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PolarPolar

Polar encoding uses two voltage levels (positive and Polar encoding uses two voltage levels (positive and

negative).negative).

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Variations of Polar EncodingsVariations of Polar Encodings

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Variations of Polar EncodingsVariations of Polar Encodings

In Nonreturn to Zero-level (NRZ-L) the level of the signal is dependenIn Nonreturn to Zero-level (NRZ-L) the level of the signal is dependen

t upon the state of the bit.t upon the state of the bit.

In Nonreturn to Zero-Invert (NRZ-I) the signal is inverted if a 1 is encIn Nonreturn to Zero-Invert (NRZ-I) the signal is inverted if a 1 is enc

ountered.ountered.

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NRZ-L (level) and NRZ-I (invert) encoding

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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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Manchester EncodingManchester Encoding

In Manchester encoding, the transition at the middle of the bit is used In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.for both synchronization and bit representation.

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Differential ManchesterDifferential Manchester

In differential Manchester encoding, the transition at the middle of the bIn differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by it is used only for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit.the inversion or noninversion at the beginning of the bit.

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BipolarBipolar

In bipolar encoding, we use three levels: positive, zero, In bipolar encoding, we use three levels: positive, zero,

and negative.and negative.

Bipolar AMI encodingBipolar AMI encoding

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Some Other SchemesSome Other Schemes

2B1Q (two binary, one quaternary)

MLT3 (Multiline Transmission, three level (MLT-3)

-1

-2

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4.2 Block Coding4.2 Block Coding

Steps in transmission

Step 1: Division

divide sequence of bits into groups of m bits

Step 2: Substitution

substitute an m-bit code for an n-bit group

Step 3: Line Coding

create the signal

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Block codingBlock coding

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Block codingBlock coding Substitution in block coding

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Block codingBlock coding 4B/5B encoding4B/5B encoding

no more than 1 leading 0

no more than 2 trailing 0s

normally encoded w/ NRZ-I (1 indicated by transition)

Data Code Data Code

0000 1111011110 1000 1001010010

0001 0100101001 1001 1001110011

0010 1010010100 1010 1011010110

0011 1010110101 1011 1011110111

0100 0101001010 1100 1101011010

0101 0101101011 1101 1101111011

0110 0111001110 1110 1110011100

0111 0111101111 1111 1110111101

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Rest of the 5 bit codes

Synchronization

Error correction

Example

J (start delimiter) : 11000

T (end delimiter) : 01101

S (set) : 11001

R (reset) : 00111

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8B/6T

substitute 8 bit group w/ 6 symbol code

each symbol is ternary (+1, 0, -1)

8 bit code = 28 = 256

6 bit ternary = 36 = 729

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4.3 Sampling4.3 Sampling

PAM : Pulse Amplitude Modulation

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SamplingSampling

Pulse amplitude modulation has some applications, but Pulse amplitude modulation has some applications, but

it is not used by itself in data communication. However, it is not used by itself in data communication. However,

it is the first step in another very popular conversion it is the first step in another very popular conversion

method called pulse code modulation.method called pulse code modulation.

•Term sampling means measuring the amplitude of the signal at equal intervals.

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SamplingSampling

Quantized PAM Signal

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Sampling Sampling

Quantizing by using sign and magnitude

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PCMPCM

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PCMPCM

From analog signal to PCM digital code

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Sampling Rate and Nyquist TheoremSampling Rate and Nyquist Theorem

According to the Nyquist theorem, the sampling rate musAccording to the Nyquist theorem, the sampling rate mus

t be at least 2 times the highest frequency.t be at least 2 times the highest frequency.

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Example 4Example 4

What sampling rate is needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)?

SolutionSolution

The sampling rate must be twice the highest frequency in the signal:

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

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Example 5Example 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?

SolutionSolution

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 not enough since 22 = 4. A 4-bit value is too much because 24 = 16.

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Example 6Example 6

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

SolutionSolution

The human voice normally contains frequencies from 0 to 4000 Hz. Sampling rate = 4000 x 2 = 8000 samples/sSampling rate = 4000 x 2 = 8000 samples/s

Bit rate = sampling rate x number of bits per sample Bit rate = sampling rate x number of bits per sample = 8000 x 8 = 64,000 bps = 64 Kbps= 8000 x 8 = 64,000 bps = 64 Kbps

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4.4 Transmission Mode4.4 Transmission Mode

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Parallel TransmissionParallel Transmission

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Serial TransmissionSerial Transmission

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Asynchronous TransmissionAsynchronous Transmission

In asynchronous transmission, we send 1 start bit (0) In asynchronous transmission, we send 1 start bit (0)

at the beginning and 1 or more stop bits (1s) at the end at the beginning and 1 or more stop bits (1s) at the end

of each byte. There may be a gap between each byte.of each byte. There may be a gap between each byte.

Asynchronous here means “asynchronous at the byte Asynchronous here means “asynchronous at the byte

level,” but the bits are still synchronized; their level,” but the bits are still synchronized; their

durations are the same.durations are the same.

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Asynchronous TransmissionAsynchronous Transmission

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Synchronous TransmissionSynchronous Transmission

In synchronous transmission, we send bits one after In synchronous transmission, we send bits one after

another without start/stop bits or gaps. It is the responsibility another without start/stop bits or gaps. It is the responsibility

of the receiver to group the bits.of the receiver to group the bits.

1 kyung hee university digital transmission. 2 kyung hee university 4 장 digital transmission 4.1...

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