Data Communication - e-PG Pathshala

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Electronic Science Electronic Communication

33. Data Communication

Module -30

Data Communication

1. Introduction

2. Basic Data Communication System

3. Various data codes and their representation

3.1 Baudot code

3.2 Morse code

3.3 ASCII code

3.4 EBCDIC code

4. Data Representation

5. Data Transmission Modes

5.1 The direction of the exchanges

5.2 The number of data bits sent simultaneously

5.3 Synchronization between the transmitter and receiver

6. Standard Organizations for Data Communication

7. Summary

Learning outcome –

After studying this module, you will be able to:

1. Understand the concept of data communication system

2. Learn the various data codes and their representation

3. Understand the various data transmission modes

4. Know about different types of data communication standard organizations

5. Apply the knowledge of encoders in different digital systems

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1. Introduction

Data communication refers to the exchange of data between two or more devices through

suitable transmission media either wired or wireless as shown in fig. 1. It permits the transfer of

binary or digital information between remote computers. The data communication circuits

comprise of electronic equipments that aids in the interconnection of digital computer

equipments.

Figure 1: Computer Network

Almost any type of data can be digitized. The effectiveness, low cost, reliability and high speed

of digital technology have made data communication more widespread and inevitable in today‟s

world.

One of the major applications of data communication is internet, linking billions of devices

worldwide and through which we have access to extensive range of information and resources at

our fingertips. One of the other applications is Electronic mail (e-mail) services that enable us to

send data from our personal computer (PC) to anyone anywhere in this world and receive

information from anyone across the globe.

2. Basic Data Communication System

The Data Communication System has basic five components as shown in fig. 2:

1. Message: It is the information data that is intended to be communicated. Information data

can be in the form of text, numbers, image, audio and video.

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2. Sender: The device that sends the data information is called the sender or transmitter. Sender

can be a computer, workstation, telephone handset, video camera, mobile phone and other

peripheral devices.

Figure 2 : Block diagram of Data Communication System

3. Transmission media: The physical path through which data is transmitted or sent from the

sender to the receiver. It is also known as communication channel. Transmission medium can

be wired like twisted-pair wire, co-axial cable, fiber optic cable, telephone line etc. or it can

be wireless over the free space using radio, microwave and infrared signals.

4. Receiver: The device that receives the data information from the sender is called the

receiver. It is also called Sink. A receiver may be a computer, workstation, telephone

handset, mobile phone, television set, printer, fax machine, and so on.

5. Protocol: Protocol is a set of rules that governs data communication. It refers to the

agreement between the communicating devices. It defines the procedures the devices will use

during the process of communication. Without a proper protocol, the devices may be

connected but they cannot communicate with each other. Numerous protocols are being used

to provide the networking capabilities specifying the data rate, flow control, data

segmentation and assembly, sequence control, error detection and control.

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3. Various data codes and their representation

Data is communicated between computers as sequences of binary digits or bits. A data code

refers to the way in which bits are grouped together to represent different symbols.

A sequence of bits is grouped to form a data character and an encoding scheme translates each

group of bits into a character. Hence a unique binary code for every possible character to be

communicated is generated and stored in the computer. The two communicating devices must

use the same code in order to communicate properly. There are a number of different codes as

shown in fig.3, but the most common code in use today is the ASCII code.

Figure 3: Simple block diagram of digital system

3.1 Baudot code

The Baudot code developed in the year 1875 was named after the pioneer in telegraph printing

Emile Baudot. It was used extensively in early teletype machines which was like a typewriter

and was used to send and receive coded signal over a communication link. Pressing a key on the

typewriter keyboard generates a unique code which is further transmitted to the receiving

machine that prints the corresponding character. It is recognized as the first fixed- length

character code for machines.

It is a five bit code representing 32 different characters. In order to accommodate 26 letters of the

alphabet, 10 numbers, and various punctuation marks, it uses two shift codes: letter and figure

shift codes. Two of the 32 combinations were used to select the shift codes. If the message is

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preceded by the letter shift code (11011), all of the following codes are interpreted as alphabet

letters. When preceded by the figure shift code (11111), all the following characters are

interpreted as numbers or punctuation marks.

LETTER FIGURE BINARY LETTER FIGURE BINARY

A - 11000 Q 1 11101

B ? 10011 R 4 01010

C : 01110 S BEL 10100

D $ 10010 T 5 00001

E 3 10000 U 7 11100

F ! 10110 V ; 01111

G & 01011 W 2 11001

H # 00101 X / 10111

I 8 01100 Y 6 10101

J ' 11010 Z " 10001

K ( 11110 LETTER SHIFT 11011

L ) 01001 FIGURE SHIFT 11111

M . 00111 SPACE 00100

N , 00110 Line feed (LF) 01000

O 9 00011 Blank(null) Blank 00000

P 0 01101

Table 1: Baud Dot Code

For example, if we have 01001 preceded by letter shift code 11011, the character is interpreted as

alphabet L and the same binary bit combination when preceded by figure shift code 11111, is

interpreted as a right closing parenthesis.

As seen from the table 1, letter shift and figure shift code are represented by 11011 and 11111

respectively. In most data communications, baudot has been replaced by codes that can represent

more characters and symbols.

3.2 Morse code

In 1844, Samuel F. B. Morse successfully demonstrated an electrical telegraph system. The

transmitting end of the telegraph system sent text information in the form of electrical pulses

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along wires which controlled the electromagnet at the receiving end. Morse deve loped a special

code called Morse code to transmit and receive messages at high speeds up to 80 words per

minute.

Morse code is a series of dots and dashes representing alphabets, numbers and punctuation

marks. Dot and dash are usually represented as a short electrical pulse and a long electrical pulse

respectively. The telegraph key when depressed causes current flow in an electromagnetic coil

that attracts the armature and releases it quickly when the current stops, making clicks during

both the instances. When the armature is closed for a short duration, a dot is produced and when

closed for longer time, a dash is produced. The transmitter switches the carrier signal on and off

to produce dot and dashes.

Table 2: Morse code

Table 2 displays the Morse telegraph code. As we can see from the table, alphabet A is

represented by single dot and single dash, B is represented by single dash and three dots and so

on.

3.3 American standard code for Information Interchange (ASCII)

The ASCII code was developed by a committee of the American National Standards Institute

(ANSI) for binary data coding. ASCII code is a 7-bit code representing 128 alphanumeric

symbols with a distinctive code word. The least significant bit is designated bit 0 and the most

significant bit is designated as bit 1. The first three bits from MSB onwards indicate whether a

number, letter or character is being specified.

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As seen from table 3, the first 32 values (character code 0 to 31) are non-printing control

characters, such as NUL- null character, STX- start of text, ETX- end of text, LF -Line Feed,

DLE- Data link escape, US-Unit separator, CR- Carriage return, File separator(FS),Group

Separator(GS) CAN-cancel, SUB-substitute ,ESC- escape etc . Mostly they are used to control

peripherals such as printers.Codes 32-127 are common for all the different variations of the

ASCII table, they are called printable characters, representing letters, digits, punctuation marks,

and a few miscellaneous symbols. Almost every character can be found on the computer

keyboard.

ASCII BINARY ASCII BINARY ASCII BINARY ASCII BINARY

NUL 00000000 SP 00100000 @ 01000000 ` 01100000

SOH 00000001 ! 00100001 A 01000001 a 01100001

STX 00000010 " 00100010 B 01000010 b 01100010

ETX 00000011 # 00100011 C 01000011 c 01100011

EOT 00000100 $ 00100100 D 01000100 d 01100100

ENQ 00000101 % 00100101 E 01000101 e 01100101

ACK 00000110 & 00100110 F 01000110 f 01100110

BEL 00000111 ' 00100111 G 01000111 g 01100111

BS 00001000 ( 00101000 H 01001000 h 01101000

HT 00001001 ) 00101001 I 01001001 i 01101001

LF 00001010 * 00101010 J 01001010 j 01101010

VT 00001011 + 00101011 K 01001011 k 01101011

FF 00001100 , 00101100 L 01001111 l 01101100

CR 00001101 - 00101101 M 01001101 m 01101110

SO 00001110 . 00101110 N 01001110 o 01101111

SI 00001111 / 00101111 O 01001111 p 01110000

DLE 00010000 0 00110000 P 01010000 q 01110001

DC1 00010001 1 00110001 Q 01010001 r 01110011

DC2 00010010 2 00110010 R 01010010 s 01110100

DC3 00010011 3 00110011 S 01010011 t 01110101

DC4 00010100 4 00110100 T 01010100 u 01110110

NAK 00010101 5 00110101 U 01010101 v 01110111

SYN 00010110 6 00110110 V 01010110 w 01101101

ETB 00010111 7 00110111 W 01010111 x 01111000

CAN 00011000 8 00111000 X 01011000 y 01111001

EM 00011001 9 00111001 Y 01011001 z 01111010

SUB 00011010 : 00111010 Z 01011010 { 01111011

ESC 00011011 ; 00111011 [ 01011011 | 01111100

FS 00011100 < 00111100 \ 01011100 } 01111101

GS 00011101 = 00111101 ] 01011101 ~ 01111110

RS 00011110 > 00111110 ^ 01011110 DEL 01111111

US 00011111 ? 00111111 - 01011111

Table 3: ASCII Code

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The advantage of ASCII code is its ability to represent both upper and lower case letters of the

alphabet. For a 8- bit data, the 8th bit is not part of ASCII code but is reserved for parity bit

required in error detection scheme.

3.4 EBCDIC (Extended Binary Coded Decimal Interchange Code)

It is an eight-bit alphanumeric code developed by International Business Machines (IBM). It was

used in IBM computers and IBM compatible equipments. As seen from the fig.4, there are four

main blocks in the EBCDIC code page: 0000 0000 to 0011 1111 is reserved for control

characters; 0100 0000 to 0111 1111 are for punctuation; 1000 0000 to 1011 1111 for lowercase

characters and 1100 0000 to 1111 1111 for uppercase characters and numbers. EBCDIC allows a

representation of maximum of 256 characters. EBCDIC has a wider range of control characters

than ASCII.

The major disadvantage of this code is that parity checking for error detection cannot be used on

an 8 bit system. Most other computers use ASCII codes.

SYMBOL EBCDIC SYMBOL EBCDIC SYMBOL EBCDIC SYMBOL EBCDIC

NUL 00000000 RES 00010100 ESC 00100111 SP 01000000

SOH 00000001 NL 00010101 SM 00101010 - 01001011

STX 00000010 BS 00010110 CU2 00101011 < 01001100

ETX 00000011 C 00010111 ENQ 00101101 : :

PF 00000100 CAN 00011000 ACK 00101110 “ 01111111

HT 00000101 EM 00011001 BEL 00101110 a 10000001

LC 00000110 CC 00011010 SYN 00110010 b 10000010

DEL 00000111 CU1 00011011 PN 00110100 : :

SMM 00001010 IFS 00011100 RS 00110101 z 10101001

VT 00001011 IGS 00011101 UC 00110110 A 11000001

FF 00001100 IRS 00011110 EOT 00110111 B 11000010

CR 00001101 IUS 00011111 CU3 00111011 : :

SO 00001110 DS 00100000 DC4 00111100 Z 11101001

SI 00001111 SOS 00100001 NAK 00111101 0 11110000

DLE 00010000 FS 00100010 SUB 00111111 1 11110001

DC1 00010001 BYP 00100100

: :

DC2 00010010 LF 00100101

9 11111001

TM 00010011 ETB 00100110

Table 4: EBCDIC Code

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4. Data Representation:

Data can be represented in various forms as shown in fig-4 such as:

1. Audio: It refers to the transmission of sound or music. Audio signal is a continuous signal

representing sound as an electrical voltage with a frequency range of roughly 20 to 20,000 Hz.

Audio signals may be directly synthesized, or may be originating at a transducer such as

a microphone, musical instrument pickup or tape head.

2. Video: This word originates from latin meaning “ I see”. Video refers to video recording and

transmission of image or movie. Video when captured by the TV camera is produced as a

continuous entity. It is a discrete entity when is in the form of series of images processed in rapid

succession creating the illusion of motion.

3. Images: Image comprises of matrix of picture elements known as p ixels. Each pixel is a tiny

dot of colour which collectively creates any image. Greater number of pixels on the screen yields

higher resolution enabling finer detail representation. A colour is typically represented by three

component intensities such as red, green and blue (RGB). Some combination of these three

colour components is represented by a pixel.

4. Numbers : Normally we write numbers using digits 0 to 9. This is called decimal number

system with a base of 10. However, any integer can be easily represented by a sequence of 0's

and 1's known as binary numbers. Numbers are represented using binary number system. ASCII

is not used to represent numbers.

Figure 4: Various Data forms

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5. Text: Text can be represented easily by assigning a unique numeric value for each symbol

used in the text. For example, the widely used ASCII code (American Standard Code for

Information Interchange) defines 128 different symbols (all the characters found on a standard

keyboard, plus a few extra), and assigns to each a unique numeric code between 0 and 127.

When you save a file as "plain text", it is stored using ASCII. The code value for any character

can be converted to base 2, so any written message made up of ASCII characters can be

converted to a string of 0's and 1's.

5. Data Transmission Modes

The term Data Transmission indicates the movement of the bits over a transmission medium

connecting the two communicating devices. Transmission on a communication channel between

two machines can occur in several different ways.

The transmission is characterised by:

1. The direction of the exchanges

2. The number of data bits sent simultaneously

3. Synchronization between the transmitter and receiver

5.1 Data Transmission Modes based on Direction of Exchange

As shown in fig-5, based on the direction of exchange, data transmission can be classified into

Simplex, Half Duplex and Full Duplex.

Figure 5: Data transmission modes based on direction of exchange

Data Transmission Modes

Simplex Half Duplex Full Duplex

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5.1.1 Simplex

In simplex communication networks, communication can take place in one direction only.

Connected to such a circuit are either a send only or receive only device. The transmitter and the

receiver operate on the same frequency. Simplex transmission generally involves dedicated

circuits. It is not possible to send back error or control signals to the transmit end through

Simplex channels. As shown the fig-6, A transmits and B receives. The direction of data signal is

only in one direction that is, from A to B.

Figure 6: Simplex Communication

This way of Communication can be also called as unidirectional or one-way Communication.

Simplex circuits are analogous to escalators, doorbells, fire alarms and security systems.

Some examples of Simplex communication are shown in fig-7. A Communication between

a computer and a keyboard involves simplex transmission. The CPU never needs to send

characters to the keyboard but the keyboard always send characters to the CPU.

Other simplex device is the Monitor that can only accept output. A television broadcast is

another example of simplex transmission.

Other example of simplex transmission is loudspeaker system. An announcer talks into a

microphone and his/her voice is sent through an amplifier and then to all the speakers. Signal

travels in only one direction from microphone to speaker.

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Figure 7: Examples of Simplex Communication

5.1.2 Half Duplex

A half duplex system can transmit data in both directions, but only in one direction at a time. In

other words, half duplex mode supports two-way traffic but in only one direction at a time. Both

the connected devices can transmit and receive but not simultaneously. When one device is

sending the other can only receive and vice-versa. As seen in the figure 8, when A is in transmit

mode, B is in receive mode and vice versa. It is possible to perform error detection and request

the sender to retransmit information that arrived corrupted. Two existing stations alternately (not

simultaneously) send signals to each other on the same frequency.

Figure 8: Half Duplex Communication

This type of connection makes it possible to have bidirectional communications using the full

capacity of the line.

The only advantage of half duplex is that the single connection is cheaper than the double

connection.

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One of the examples of half duplex communication as shown in fig- 9, is a walkie-talkie system.

It can only send or receive a transmission at any given time. It cannot do both at the same time.

A walkie-talkie requires only a single frequency for bidirectional communication.

Communication between the computer and line printer is half duplex. Printers send messages to

the computer. The printer cannot send these messages while the computer is sending characters

but when the computer stops sending characters, then the printer can send messages back.

Figure 9: Examples of Half Duplex Communication

5.1.3 Full Duplex

A full duplex system can transmit data simultaneously in both directions on transmission path.

Both the connected devices can transmit and receive at the same time. Therefore it represents

truly bi-directional system. Here as shown in figure 10, A and B can simultaneously send and

receive data.

Figure 10: Full Duplex Communication

The communication link may contain two separate transmission paths one for sending and

another for receiving.

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Most of data networks are using duplex transmission using different channels in the same

medium connecting the transmitter and receiver. Each of the channels is half duplex, but together

it makes full duplex. This type of transmission can also be called bidirectional transmission. The

use of full duplex increases the data transmission rate. Modern network interface cards are

configured for full duplex support by default. In full duplex transmission, the channel capacity is

shared by both communicating devices at all times.

Best examples of full duplex communication are the Telephone networks as shown in fig-11.

When two persons talk on telephone line, both can listen and speak simultaneously.

Figure 11: Examples of Full Duplex Communication

5.2 Data transmission modes based on the number of data bits sent simultaneously

The transmission mode can be characterized by the number of elementary units of information

(bits) that can be simultaneously translated by the communications channel.

Based on this, the two types of transmission modes are parallel transmission and serial

transmission as shown in fig-12.

Figure 12 : Transmission modes based on the number of simultaneous data bits


Parallel Serial

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5.2.1 Parallel data transmission

In parallel transmission, all bits the data are transferred simultaneously. Transmission of parallel

data is extremely fast, can be of the order of ns, since there is a simultaneous transfer of all the

data bits. The transmission speed is limited to the speed of the logic circuits involved in the data

transfer.

Parallel data transmission is not practical for long distance communications as multiple wires

cost more than a single wire and also suffers signal attenuation. In fact, computers never process

a single bit at a time; generally they are able to process several and for this reason the basic

connections on a computer are parallel connections.

Figure 13 : Parallel Transmission

As shown in the fig-13, a register is loaded with n bit binary word that needs to be transmitted.

The register contains one flip-flop for each bit. All the flip-flop outputs are connected to

transmission lines that carry the data bits to the receiver. The receiver also has n bit storage

register. In parallel data communication you need one wire for transmission of each data bit.

Therefore there is a necessity of multi-wire cable. When a clock pulse is applied to the flip-flops

of the register, the bits of the word are transmitted simultaneously during the time period of a

single clock pulse.

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5.2.2 Serial transmission

In serial transmission, data bits are sent sequentially that is, one after the other on the same

channel (wire). It reduces costs for wire but also slows the speed of transmission. In computers,

digital data transmission between circuits is in parallel format. Therefore parallel to serial

conversion devices are required at interface between the transmitter and the single transmission

line. Also there is a requirement of serial to parallel conversion between the single transmission

line and the receiver. Such data conversions are made possible by shift registers.

Figure 14: Serial Transmission

As shown in fig-14, when a clock pulse is applied to the sequentially cascaded flip- flops of the

shift register, the bits of the word are shifted from one flip-flop to the next. At the end of n clock

pulses, all the n data bit of the word will be transmitted. The transmitted serial word over the

communication link is then received by the serial in-parallel out shift register at the receiving

end. Further the parallel data outputs from the shift register is transferred to the computer

circuits.

5.3 Data transmission modes based on the synchronization between receiver and

transmitter

Based on the synchronization between the receiver and transmitter, the two types of

transmission modes are asynchronous transmission and synchronous transmission as shown in

fig-15.

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Figure 15 : Transmission Modes based on the Rx and Tx Synchronization

5.3.1 Asynchronous data transmission

In Asynchronous data transmission as shown in fig-16, each data word representing single

character , has a “Start bit” before the byte and “Stop bit” at the end of the byte for Start/Stop

synchronisation or identifying the beginning and ending of the word.ie binary 1 which is

referred to as a mark. When there is no information being transmitted, communication line is at

high state.

Figure 16 : Asynchronous Transmission

Start bit is always 1 bit duration and is always equal to binary „0‟ referred to as a space. The

transition from mark to space indicates the data word beginning and aids in receiver

synchronization. Stop bit may be 1 or 2 bits duration and is always equal to binary „1‟ or mark

indicating the end of word.

The start and stop bits ensure the transmitter and receiver synchronization. It is extremely

reliable communication mode but bit slow due to significant overhead caused by the addition of

start and stop bits per character transmission and is inefficient when large blocks and volume of

data needs to be transmitted.


Asynchronous Synchronous

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5.3.2 Synchronous data transmission

In Synchronous data transmission shown in fig-17, there are no start/stop bits. Continuous block

of data of multi-words are transmitted. Synchronization between the transmitter and receiver is

maintained by means of group of synchronization bits both at the beginning and ending of the

data block.

Figure 17 : Synchronous Transmission

Each of these data blocks may contain hundreds and thousands of characters. These blocks are

encapsulated with Header & Trailer along with Flags. As shown in the figure, two bit

synchronization characters indicate the start of data transmission. The end of the data block is

specified by another special code end of text (ETX). ETX is usually followed by one or more

error detection. Since the number of synchronizations bits used in synchronous transmission is

much less as compared to the number of start and stop bits used in asynchronous transmission

per block, it is much faster than asynchronous transmission.

6. Standard Organizations for Data Communication

An association of organizations, governments, manufacturers and users form the standards

organizations .They are responsible for developing, coordinating and maintaining the standards.

All the data communications equipment manufacturers and users should comply with these

standards. The major standards organizations for data communication are shown in figure 18.

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Figure 18 : Standard Organizations

7. Summary

We this lesson, we have learnt the following:

Data communication is the exchange of binary data between two or more devices

through suitable transmission media either wired or wireless. It permits the transfer of

binary or digital information between remote computers.

There are major five components of a data communication system: message, sender,

transmission medium, receiver and protocol.

A data code refers to the way in which bits are grouped together to represent different

symbols. There are a number of different codes like Baudot code, Morse code etc.,

but the most common code in use today is the 7 bit American Standard Code for

Information Interchange (ASCII) code.

Another popular code is an eight-bit alphanumeric code developed by International

Business Machines (IBM) known as EBCDIC (Extended Binary Coded Decimal

Interchange Code).

Data can be represented in various forms such as text, numbers, image, audio and

video.

Data Transmission indicating the movement of the bits over a transmission medium

connecting the two communicating devices can be characterised: based on the

direction of the exchanges into simplex, half duplex and full duplex; based on the

International Standard

Organization (ISO)

International Telecommunications

Union-Telecommunication

Sector (ITU-T)

Institute of Electrical and

Electronics Engineers

(IEEE)

American National

Standards Institute (ANSI)

Electronics Industry

Association (EIA)

Internet Engineering Task Force

(IETF)

Internet Research Task Force

(IRTF)

Telecommunications Industry Association

(TIA)

Internet Architecture Board (IAB)

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number of data bits sent simultaneously into parallel and serial transmission; based on

the synchronization between the transmitter and receiver into asynchronous and

synchronous transmission.

In serial transmission, data bits are sent sequentially that is, one after the other on the

same transmission channel where as in parallel transmission, the data bits are

transferred simultaneously. Transmission of parallel data is extremely fast than serial

transmission but it requires multiple wires of the channel. Hence it is costly and not

used for long distance communication.

In asynchronous transmission, use of start and stop bits per character transmission

makes it inefficient when large blocks and volume of data needs to be transmitted.

In synchronous data transmission, there are no start/stop bits. Continuous block of

data of multi-words are transmitted making it more faster than asynchronous

transmission.

An association of organizations, governments, manufacturers and users form the

standards organizations responsible for developing, coordinating and maintaining the

standards. Some of the standard organization for data communication are

International Standard Organization (ISO), Internet Engineering Task Force (IETF) ,

Institute of Electrical and Electronics Engineers (IEEE), American National

Standards Institute (ANSI) etc.

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Data Communication - e-PG Pathshala