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Basics of Data Communications

• Discrete-time Communication vs Continuous-time Communication

Is human communication continuous-time or discrete-time ?

Voice vs. words

Any piece of information can be represented in discrete-time signals !

What is Data Communications ?

•The exchange of digital information between two devices using an electronic transmission system.•Using Binary number system to represent digital information

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Basics of Data CommunicationsQuestion: How to send the word “Yes” from Bob to Alice?

Solution-1: Bob writes the letters “Y”, “e”, “s” on a piece of paper.(a 26-symbol alphabet is utilised in this solution)

Solution-2: Bob nodes his head down once, and shakes his head to say “No”. (a 2-symbol alphabet is utilised in this solution)

Question: Is it a coincidence that “Yes” and “No” are very short words in most human languages?

Question: How to send the word “Yes” from computer-1 to computer-2?

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Character Codes

Much of the data communication discipline deals with representing information in terms of 0’s and 1’s.

(Not as easy as it sounds !)

Fact: Computers are digital logic systems that operate on a 2-symbol alphabet : {0, 1}. – Binary Number System

There are many ways to represent characters and these different representation s are known as Character Codes.

Mostly formed by ‘Zero’ and ‘One’

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Morse Code: A variable-lenght character code

A .- B -... C -.-. D -.. E .

F ..-. G --. H .... I ..

J .--- K -.- L .-.. M --

N -. O --- P .--. Q --.- R .-. S ... T -

U ..- V ...- W .-- X -..- Y -.-- Z --..

0 ----- 1 .---- 2 ..--- 3 ...-- 4 ....- 5 ..... 6 -.... 7 --... 8 ---.. 9 ----.

Fullstop .-.-.- Comma --..-- Query ..--..

“Yes”: -.-- | . | …

Examples:silence interval to separate characters

Character Codes

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10111 Q 1

01010 R 4

00101 S '

10000 T 5

00111 U 7

11110 V ;

10011 W 2

11101 X /

10101 Y 6

10001 Z "

01000 CR CR

00010 LF LF

00100 SP SP

11111 LTRS LTRS

11011 FIGS FIGS

00000 (unused)

Baudot Code: A fixed-length character code

54321 LTRS FIGS

____________________________

00011 A -

11001 B ?

01110 C :

01001 D $

00001 E 3

01101 F !

11010 G &

10100 H #

00110 I 8

01011 J BELL

01111 K (

10010 L )

11100 M .

01100 N ,

11000 O 9 10110 P 0

Character CodesLetter code: switch to letters

Figure code: switch to figures

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Extended Binary Coded Decimal Interchange Code (EBCDIC): A fixed-length, 8-bit, character code.

Char EBCDIC Char EBCDIC Char EBCDIC

A 1100 0001 P 1101 0111 4 1111 0100

B 1100 0010 Q 1101 1000 5 1111 0101

C 1100 0011 R 1101 1001 6 1111 0110

D 1100 0100 S 1110 0010 7 1111 0111

E 1100 0101 T 1110 0011 8 1111 1000

F 1100 0110 U 1110 0100 9 1111 1001

G 1100 0111 V 1110 0101 blank ...

H 1100 1000 W 1110 0110 . ...

I 1100 1001 X 1110 0111 ( ...

J 1101 0001 Y 1110 1000 + ...

K 1101 0010 Z 1110 1001 $ ...

L 1101 0011 0 1111 0000 * ...

M 1101 0100 1 1111 0001 ) ...

N 1101 0101 2 1111 0010 - ...

O 1101 0110 3 1111 0011 / ...

• 256 different combinations of 1’s and 0’s

• does not utilise all possible combinations

• only a subset is displayed here

• no parity

Character Codes

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Character CodesAmerican National Standard Code for Information Exchange (ASCII): A fixed-length, 7-bit, character code.

Char 7 bit ASCII Char 7 bit ASCII Char 7 bit ASCII

A 100 0001 a 110 0001 0 011 0000

B 100 0010 b 110 0010 1 011 0001

C 100 0011 c 110 0011 2 011 0010

D 100 0100 d 110 0100 3 011 0011

E 100 0101 e 110 0101 4 011 0100

F 100 0110 f 110 0110 5 011 0101

G 100 0111 g 110 0111 6 011 0110

H 100 1000 h 110 1000 7 011 0111

I 100 1001 i 110 1001 8 011 1000

J 100 1010 j 110 1010 9 011 1001

K 100 1011 k 110 1011 blank 010 0000

L 100 1100 l 110 1100 . 010 1110

M 100 1101 m 110 1101 ( 010 1000

N 100 1110 n 110 1110 + 010 1011

O 100 1111 o 110 1111 $ 010 0100

P 101 0000 p 111 0000 * 010 1010

Q 101 0001 q 111 0001 ) 010 1001

R 101 0010 r 111 0010 - 010 1101

S 101 0011 s 111 0011 / 010 1111

T 101 0100 t 111 0100 , 010 1100

U 101 0101 u 111 0101 = 011 1101

V 101 0110 v 111 0110 RETURN 000 1101

W 101 0111 w 111 0111 LNFEED 000 1010

• 128 different combinations of 1’s and 0’s

• only a subset is displayed here

• can be used with a one-bit parity check 1+7=8 bits =1 byte

• there is also extended-ASCII that uses 8 bits instead of 7 bits.

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1 byte = 8 bits

bit-8 = parity check bit

bit-1 bit-7

bit-1 to 7 = data bit

previous byte

…

Character Codes

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Parity CheckOdd-Parity = number of bit ‘1’ must be odd

bit-1 bit-7

1 1 1 1 0 1

bit-8

0 0

bit-7

0 1 0 1 0 1

bit-8

0 0

or

bit-1

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Parity CheckEven-Parity = number of bit ‘1’ must be even

bit-1 bit-7

1 1 0 1 0 1

bit-8

0 0

bit-7

0 1 0 1 0 1

bit-8

1 0

or

bit-1

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WCB/McGraw-Hill

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Parallel Transmission

WCB/McGraw-Hill

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Serial Transmission

WCB/McGraw-Hill

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Parallel vs. Serial Transmission

1 0 0 0 0 0 1Parallel Transmission

1 0 0 0 0 0 1

Serial Transmission

• Faster • Expensive• Suitable for short distances

• Slower • Cheaper• Better for long distance

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Synchronous vs. Asynchronous Transmission

Synchronous Transmission

However the block must be preceded by SYNC bytes or a separate clock line must be provided !

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Synchronous vs. Asynchronous Transmission

Asynchronous Transmission

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_100%

.

number of data bits onlyEfficiency

Total no of bits sent

Synchronous vs. Asynchronous Transmission

Total bits sent = data-bits + control-bits

Transmission or Throughput

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If a Synchronous transmission involves sending 480 bits plus an overhead of 20 control bits for synchronisation, what is the efficiency of the transmission?

If 480 characters are sent using Asynchronous transmission, what is the transmission efficiency?

Example

Synchronous transmission:

Efficiency = 480/(480+20) = 96%

Asynchronous transmission:

Efficiency = 80% (if 8 bits is used for data bits)

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Transmission Modes: Simplex, Half-Duplex, Full-Duplex

Simplex Transmission

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Transmission Modes: Simplex, Half-Duplex, Full-Duplex

Half-Duplex Transmission

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Transmission Modes: Simplex, Half-Duplex, Full-Duplex

Full-Duplex Transmission

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Tutorial 2 What is a major difference between Morse code and ASCII code? How many characters can Baudot code represent? How many characters can ASCII code represent? What is a parity bit? What is parallel transmission? When do we use parallel transmission? 

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What is serial transmission? When do we use serial transmission? What is asynchrorous transmission? What is synchrorous transmission? Why asynchronous transmission always has 80% efficiency? If a synchronous transmission involves sending 1000 characters and an overhead of 10 characters are needed for handshake, what is the efficiency of the transmission? 

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What is simplex communication? Give examples.  What is half duplex communication? Give examples.  What is full duplex communication? Give examples.