Digital (kỹ THUẬT TRUYỀN số LIỆU SLIDE)

Digital Transmission


Outline
 Line coding
 Encoding considerations
 DC components in signals
 Synchronization
 Various line coding methods

2


Line Coding
 Process of converting binary data to digital signal

3


Signal Levels vs. Data Levels
 Number of signal levels
 Number of different voltage levels allowed in a signal

 Number of data levels
 Number of voltage levels that actually represent data values

4


Signal vs. Data Elements

5


Pulse Rate vs. Bit Rate

+3

00

11

01

10

01

10

11

00

+1

t
-1
-3

One pulse

(one signal element)

BitRate = PulseRate × b = PulseRate × log2L
BitRate = PulseRate × b = PulseRate × log2L

b – number of bits per pulse
L – number of different signal elements

Bit rate → Bits per second
Bit rate → Bits per second
Pulse rate → Baud (pulses or signals per second)
Pulse rate → Baud (pulses or signals per second)

6


Pulse Rate vs. Bit Rate
 Example: In Manchester Encoding, if the bit rate is
10 Mbps, what is the pulse rate?

0

1

0

0

1

1

0

1

t

One bit

One pulse
(one signal element)

7


Encoding Considerations
 Signal spectrum
 Lack of DC components
 Lack of high frequency components

 Clocking/synchronization
 Error detection
 Noise immunity
 Cost and complexity

8


DC Components

 DC components in signals are not desirable
 Cannot pass thru certain devices
 Leave extra (useless) energy on the line

0

1

0

0

1

1

0

1

Signal with
t

0

1

0

0

1

1

0

DC component

1

Signal without
t

DC component

9


Synchronization
 To correctly decode a signal, receiver and sender must agree on
bit interval
0

1

0

0

1

1

0

1

Sender sends:
01001101
t

0

1

0

0

0

1

1

0

1

1

Receiver sees:
0100011011
t

10


Providing Synchronization
 Separate clock wire
data

Sender

Receiver

clock

 Self-synchronization
0

1

0

0

1

1

0

1

t

11


Line Coding Methods
 Unipolar
 Uses only one voltage level (one side of time axis)

 Polar
 Uses two voltage levels (negative and positive)
 E.g., NRZ, RZ, Manchester, Differential Manchester

 Bipolar
 Uses three voltage levels (+, 0, and –) for data bits

 Multilevel

12


Unipolar
 Simplest form of digital encoding
Rarely used

 Only one polarity of voltage is used
 E.g., polarity assigned to 1

0

1

0

0

1

1

0

0

t

13


Polar Encoding
 Two voltage levels (+,-) represent data bits
 Most popular four
 Nonreturn-to-Zero (NRZ)
 Return-to-Zero (RZ)

 Manchester
 Differential Manchester

14


NRZ Encoding
 Nonreturn to Zero
 NRZ-L (NRZ-Level):
Signal level depends on bit value

0

1

0

0

1

1

1

0

t

1

0
0
1
1
1
0
 NRZ-I0(NRZ-Invert):
Signal is inverted if 1 is encountered

N = Bit rate
Save = Average signal rate

t

15


RZ Encoding
 Return to Zero
 Uses three voltage levels: +, - and 0, but only + and - represent data
bits

 Half way thru each bit, signal returns to zero

0

1

0

0

1

1

0

0

?

t

16


Manchester Encoding
 Uses an inversion at the middle of each bit
 For bit representation
 For synchronization

0

1

0

0

1

1

0

=0

1

t
=1

17


Differential Manchester Encoding
 The inversion on the middle of each bit is only for
synchronization

 Transition at the beginning of each bit tells the value

0

1

0

0

1

1

0

1

t

18


Bipolar Encoding
 Bipolar encoding uses three voltage levels: +, - and 0
 Each of all three levels represents a bit
 E.g., Bipolar AMI (Alternate Mark Inversion)
 0V always represents binary 0
 Binary 1s are represented by alternating + and -

0

1

0

0

1

1

0

1

t

19


BnZS Schemes
 BnZS – Bipolar n-zero substitution
 Based on Bipolar AMI
 n consecutive zeros are substituted with some +/- levels
 provides synchronization during long sequence of 0s

 E.g., B8ZS

1

1

0

0

0

0

0

0

0

0

1

1

0

0

0

0

0

1

0

Bipolar
AMI

t
0

0

0

V

B

0

V

B

B8ZS

V – Bipolar violation
B – Valid bipolar signal

20


Other Schemes
 mBnL
 m data elements are substituted with n signal elements
 E.g., 2B1Q (two binary, 1 quaternary)

+3

00

11

01

10

01

10

11

00

Bit sequence

Voltage level

00

-3

01

-1

10

+3

11

+1

+1

t
-1
-3

21


Multilevel: 8B6T
 Eight binary, six ternary

22


Block Coding
 Improves the performance of line coding
 Provides
 Synchronization
 Error detection

Division

…01011010001…

Line

Substitution

:

Coding

0010

10110

1101

01011

0001

01010

:

t

:

:

23


4B/5B Encoding Table

Data

Code

Data

Code

Data

Code

0000

11110

1000

10010

Q (Quiet)

00000

0001

01001

1001

10011

I (Idle)

11111

0010

10100

1010

10110

H (Halt)

00100

0011

10101

1011

10111

J (start delimiter)

11000

0100

01010

1100

11010

K (start delimiter)

10001

0101

01011

1101

11011

T (end delimiter)

01101

0110

01110

1110

11100

S (Set)

11001

0111

01111

1111

11101

R (Reset)

00111

24


Analog to Digital Conversion
 Pulse Amplitude Modulation (PAM)
 Converts an analog signal into a series of pulses by sampling

PAM

Analog signal

PAM signal
(Sampled analog data)

25