CHETTINAD COLLEGE OF ENGINEERING & TECHNOLOGY

NH-67, TRICHY MAIN ROAD, PULIYUR, C.F. – 639 114, KARUR DT DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

COURSE MATERIAL

Subject Name: Analog and digital communication Class / SEM: B. E. (CSE) / III Subject Code : CS2204 Staff Name: Suganya. J

UNIT – IV

DATA COMMUNICATIONS Syllabus: Introduction, History of Data communications, standards organizations for data communication,

Data communication circuits, Data Communication codes, Error control, Error detection, Error

correction, Data Communication hardware, Serial and parallel interfaces, Data modems,

Asynchronous Modem, Synchronous modem, Low-speed, medium and high-speed modems,

Modem Control.

DATA COMMUNICATIONS AND NETWORKING Objectives:

To define data, data communication, data communication circuit and data

communication network.

To describe the evolution of data communications.

To define and explain the necessity of data communication standards and standard

organizations like ISO, ITU-T, IEEE, ANSI, EIA, TIA, IAB, IETF and IRTF.

Development:

Since early 1970s, the technological advances around the world have occurred at

phenomenal rate transforming telecommunication industry into a highly sophisticated

and extremely dynamic field.

Early telecommunication systems accommodate voice only.

VLSI chips and low cost microprocessors, computers and peripheral equipment increased the need for exchange of digital information. It is necessary to develop and implement higher capacity and faster rate of communication.

Introduction:

Data is the information stored in digital form. Data is the plural form, Single unit of data

is datum.

Data Communication is the process of transferring digital information (usually in

binary form) between two or more points.

Information is the knowledge or intelligence. The information that has been

processed, organized and stored is called as data.

The fundamental purpose of data communication circuit is to transfer the digital

information from one place to another.

Data communication is summarized as transmission, reception and processing of digital

information.

The original source information is in analog form (human voice or music) or in digital

form (binary coded numbers or alphanumeric codes).

If the source information is in analog form, it must be converted to digital form at

source and converted back to analog form at destination.

Network is a set of devices interconnected by media links sometimes called

nodes/stations.

Data Communication Networks are the systems of inter-related computers and

computer equipment. They are simple as Personal Computers connected to a printer,

two personal computers connected together through public telephone network or a

complex one comprised of one or more main frame computers and 100s, 1000s or even

millions of remote terminals, personal computers and workstations.

There is virtually no limit to capacity or size of data communication network.

In earlier days, a single computer did every computing needed. Today, a single

computer concept is replaced by networking concept where large number of separate

but interconnected computers shares their resources.

Uses of Data Communication Networks:

1. A system of networks is used to interconnect virtually all kinds of digital computing

equipment from Automatic Teller Machines (ATMs) to bank computers.

2. Personal Computers can be connected to information highways such as Internet.

3. Workstations can be connected to main frame computers using data

communication network.

4. Data communication network is also used for airline and hotel reservation systems.

5. It is used in mass media and news networks.

6. It is also used in mail delivery systems and the list of applications is virtually endless.

History of Data Communications:

Data communications began early in the form of smoke signals or tom-tom drums. It

did not involve electricity or any electronic apparatus. It is binary coded.

Earliest electrically coded information occurred in 1753, when a proposal was submitted

to Scottish Magazine suggested to run a communication line between villages

comprised of 26 parallel wires, each wire for one letter of alphabet.

Swiss inventor constructed a prototype of 26-wire system but current wire-making

technology proved the idea impractical.

In 1833, Carl Friedrich Gauss developed an unusual system based on 5x5 matrix representing 25

letters (I and J combined). His idea was to send messages over a single wire by deflecting a

needle to right/left between 1 and 5 times. Initial set of deflections indicated a row and second

set indicated a column. It takes as many as 10 deflections to carry a single character through a

system.

The first successful data communication system was invented by Samuel F. B. Morse in

1832. It is a Telegraph which uses binary coded electrical signals to transmit information. He

developed the first practical data communication code called Morse Code. In this code, dots

and dashes (Logic 1s and 0s) were transmitted across a wire using electromechanical

induction.

Various combinations of dots, dashes and pauses represented binary codes for letters,

numbers and punctuation marks. All codes did not contain the same number of dots and

dashes. Then, Morse’s system combined human intelligence with electronics as decoding depends

on hearing and reasoning capability of a person receiving the message.

Sir Charles Wheatstone and Sir William Cooke invented the first telegraph in England

which required 6 different wires for a single telegraph line.

In 1840, Morse secured the American rights for telegraph.

In 1844, the first telegraph line was established between Baltimore and Washington. The

first message conveyed is “What hath God wrought!”.

The first slow-speed telegraph printer was invented in 1849, but it was not used until 1860.

Then the high-speed (15bps) printer was available.

In 1850, Western Union Telegraph Company was formed in Rochester, New York for

carrying coded messages from one person to another.

In 1874, Emile Baudot invented a telegraph multiplexer which allowed signals from up to 6

different telegraph machines to be transmitted simultaneously over a single wire.

In 1875, Telephone was invented by Alexander Graham Bell. Until 1899, only a very little

news evolved in telegraph. Again in 1899, Guglielmo Marconi sent radio telegraph

messages through a wireless medium.

Telegraph is an only means of sending information across large spans of water until 1920,

when first commercial radio stations carrying voice information were installed. But it was

not known when the first electrical computer was developed. Konraud Zuis, a German

engineer demonstrated a computer machine in late 1930s. At the time, Hitler tried to

conquer the rest of the world, so the project was dropped out. Bell Telephone

Laboratories developed the first special purpose computer in 1940 using electromechanical

relays for performing logical operations. J. Presper Eckert and John Mauchley at University

of Pennsylvania began modern-day computing when they developed ENIAC computer on

Feb. 14,1946.

In 1949, United States National Bureau of Standards developed the first all-electronic-diode-

based computer capable of executing stored programs. US census Bureau installed machine which is considered as the first commercially

produced American Computer.

In 1950s, the computer used punched cards for inputting information, printers for

outputting information and magnetic tape reels for permanently storing information. This

early computers process only one job at a time, called BATCH PROCESSING.

The first general purpose computer was an automatic-sequence-controlled calculator

developed by Harvard University and International Business Machines (IBMs) corporation.

UNIVAC computer was built in 1951 by Remington Rand corporation which was the first

mass produced electronic computer.

In 1960s, the batch processing system was replaced by on-line processing system with

terminals connected directly to computer through serial or parallel communication lines.

In 1970s, the microprocessor controlled microcomputers were developed and in 1980s, the

personal computers were essential in home and workplaces.

Then a number of main frame computers, small business computers, personal computers

and computer terminals were used to exchange digital information with each other

between the people. So the need for data communication circuits, networks and systems

increased exponentially.

After the invention of Telephone, American Telephone and Telegraph Company (AT & T)

provided long distance and local telephone service and data communication service

throughout United States. AT & T system is also called as ‘BELL System’ or ‘Ma Bell’.

During this time, Western Union Corporation provided the telegraph service.

Until 1968, AT & T operating tariff allowed only the equipment furnished by AT

&T to be connected to AT & T lines. In 1968, a landmark supreme court division

(Caterfone Decision) allowed non-BELL companies to interconnect to vast AT & T

communication networks. This decision started to interconnect industry, which led to

competitive data communication offering by a large number of independent companies.

In 1983, as a direct result of antitrust suit filed by Federal Government, AT & T agreed in a

court settlement to deny itself from operating companies that provide basic local telephone

service to various geographic regions of US. So the complexity of public telephone system

in US increased.

The recent development of data communication includes INTERNET, INTRANET and

WWW.

An infinite number of people from home makers to chief executive officers need to

communicate over a finite number of facilities. So the demand for higher capacity and

higher speed data communication system increase with no end. INTERNETS: Internet is the public data communication network used by millions of people all over

world to exchange business and personal information.

It began to evolve in 1969 at Advanced Research Projects Agency (ARPA). ARPANET was

formed in late 1970s to connect sites around US. From mid 1980s to April 30, 1995, National Science Foundation (NSF) funded a high speed

backbone called NSFNET. INTRANETS: Intranet is a private data communication network used by many companies to exchange

information among employees and resources.

It is used for security reasons or to satisfy specific connectivity requirements. The company intranets are generally connected to public internet through firewall. This

converts the intranet addressing system to public internet addressing system and provides

security functionality by filtering incoming and outgoing traffic based on addressing and

protocols.

WORLD WIDE WEB (WWW): WWW is a server based application which allows the subscribers to access services offered

by the web.

Browsers such as Netscape Communicator and Microsoft Internet Explorer are commonly

used for accessing data over World Wide Web.

Standard Organizations for Data Communications:

Several organizations, Governments, manufacturers and users meet on a regular basis

to establish guidelines and standards. Standard Organizations generate, control and

administer the standards.

The competing companies form a joint committee to create a compromised standard

acceptable to everyone.

In North America, the standards and recommendations were published for data,

telecommunications and networking industries.

Figure 5.1 shows the various standards.

International Standards Organizations (ISO):

ISO was created in 1946.

ISO is the international organization for standardization on a wide range of

subjects.

It is voluntary, non-treaty organization whose membership is comprised mainly of

members from the standard committees of various Governments throughout the

world. ISO creates a set of rules and standards for graphics and document

exchange. It provides models for equipment and system compatibility, quality

enhancement, improved productivity and reduced costs. ISO is responsible for

approving and co-ordinating the work of other standards organizations. The

member body of ISO from US is American National Standards Institute (ANSI).

International Telecommunication Union – Telecommunication Services:

ITU-T formerly called Comite Consultatif de Telegraphie et Telephonie (CCITT) is one of

the 4 permanent parts of ITU based in Geneva, Switzerland.

The membership is comprised of Government authorities and representatives from

many countries.

ITU-T is now the standard organization for UN and develops a recommended set of rules

and standards for telephone and data communications.

ITU-T has developed three sets of specifications:

1. V – Series for modem interfacing and data transmission over telephone lines.

2. X – Series for data transmission over public digital networks, e-mail and

directory services.

3. I and Q – Series for its Integrated Services Digital Network (ISDN) and its

extension Broadband ISDN (sometimes called Information Superhighway).

ITU-T is separated into 14 study groups to prepare recommendations on,

1. Network and service operation.

2. Tariff and accounting principles.

3. Telecommunication management network and network maintenance.

4. Protection against electromagnetic environment effects.

5. Outside plant.

6. Data networks and open system communications.

7. Characteristics of telematic systems.

8. Television and sound transmission.

9. Language and general software aspects for telecommunication systems.

10. Signaling requirements and protocols.

11. End-to-end transmission performance of networks and terminals.

12. General network aspects.

13. Transport networks, systems and equipments.

14. Multimedia services and systems.

Institute of Electrical and Electronic Engineers (IEEE): IEEE is an international professional organization founded in US. It is comprised of

electronics, computer and communications engineers.

Currently, it is the world’s largest professional society with 200,000 members.

IEEE works closely with ANSI to develop communication and information

processing standards with the goal of advancing theory, creativity and product

quality in any field associated with Electrical Engineering.

American National Standards Institute (ANSI):

ANSI is the official standard agency for US and US voting representative for ISO.

ANSI is a completely private, non-profit organization comprised of equipment

manufacturers and users of data processing equipments and services.

It has no affiliations with Federal Government of US, but serves as National

Coordinating Institution for voluntary standardization in US.

It is comprised of people from professional societies, industry associations,

governmental and regulatory bodies and consumer groups as members.

Electronics Industry Association (EIA):

EIA is a non-profit US trade association that establishes and recommends industrial

standards.

Its activities include standards development, increasing public awareness and

lobbying.

It is responsible for developing Recommended Standards (RS) Series of standards

for data and telecommunications.

Telecommunications Industry Association (TIA):

TIA is the leading trade association in communication and IT industry. It facilitates

the business development opportunities and competitive market place through

market development trade position, trade shows, domestic and international

advocacy and standards development.

It represents manufacturers of communication and IT products and service

providers for global market place through its core competencies.

It also facilitates the convergence of new communication networks while working

for a competitive and innovative market environment.

Internet Architecture Board (IAB):

In 1957, Advanced Research Projects Agency (ARPA), the research arm of

Department of Defense was created in response to Soviet Union’s launching of

Sputnik.

The original purpose of ARPA was to accelerate the advancement of technologies

that could possibly be useful to US military.

When ARPANET was initiated in late 1970s, ARPA formed a committee to oversee

it.

In 1983, the name of committee was changed to ‘Internet Activities Board’ (IAB)

later changed to Internet Architecture Board.

IAB responsibilities are:

1. Overseas architecture protocols and procedures used by internet.

2. Manages processes used to create internet standards and serves as an appeal

board for complaints of improper execution of standardization processes.

3. Responsible for administration of various internet assigned numbers.

4. Acts as representative for Internet society in relation with other

organizations concerned with standards and other technical and

organizational issues relevant to worldwide internet.

5. Acts as a source of advice and guidance to the board of trustees and officers

of internet society concerning technical, architectural, procedural and

policy matters pertaining to Internet and its establishing technologies.

Internet Engineering Task Force (IETF):

IETF is a large international community of network designers, operators, vendors

and researchers concerned with evolution of internet architecture and smooth

operation of internet.

Internet Research Task Force (IRTF):

IRTF promotes research of importance to evolution of future internet by creating

focused, long-term and small research groups working on internet protocols,

applications, architecture and technology.

Data Communication Circuits:

The purpose of data communication is to provide a transmission path between

locations and to transfer digital information from one station to another using

electronic circuit.

Station is an end point where subscribers gain access to the circuit. Station is

sometimes called Node, which is the location of computers, computer terminals,

workstations and other digital computing equipment.

Many types of data communication circuits exist since many data communication

equipment are available.

It uses electronic communication equipment and facilitates to interconnect digital

computer equipment.

The common facilities are physical means of interconnecting stations within a data

communication system and include any type of physical transmission media or

wireless radio system.

The common facilities are provided to data communication users through public

telephone networks (PTN), public data networks (PDN) and a multitude of private

data communication systems.

The fundamental components of circuits are source of digital information,

transmitter, transmission medium, receiver and destination for digital information.

Figure 5.2 shows the transmission in only one direction but bidirectional

transmission is also possible by providing a duplicate set of circuit components in

opposite direction.

1. Source: The source generates the data. It can be a main frame

computer, personal computer, work station or virtually any other piece of digital equipment. It provides a means for humans to enter data into

the system.

2. Transmitter: The source data is in a form suitable to propagate through the

transmission medium. For e.g., digital signals can not be propagated

through a wireless radio system without being converted to analog first. The

transmitter encodes the source information and converts into a different

form allowing to be efficiently propagated through transmission medium.

Transmitter acts as an interface between source equipment and transmission

medium.

3. Transmission Medium: The transmission medium carries the encoded

signals from the transmitter to receiver. The different types of transmission

media such as free space radio transmission (wireless transmission such as

terrestrial microwave, satellite radio and cellular telephone) and physical

facilities such as metallic and optical fiber cables. The transmission path is

comprised of several different types of transmission facilities.

4. Receiver: The receiver converts the encoded signals received from the

transmission medium back to their original form or the form used in

destination equipment. It acts as an interface between the transmission

medium and destination equipment.

5. Destination: The destination can be a main frame computer, personal

computer, workstation or virtually any other piece of digital equipment,

same as source.

Data Communication Codes:

The fundamental concepts of data communication networks include data

communication code, error control and character synchronization.

The fundamental hardwares include various pieces of computer and networking

equipment such as line control units, serial interfaces and data communication

modems.

Data communication codes are used to represent characters and symbols such as

letters, digits and punctuation marks.

Data communication codes are also called character codes, character sets, symbol

codes or character languages.

Baudot Code:

Baudot Code, sometimes called Telex Code was the first fixed length character

code developed for machines but not for people.

It was developed by a French Postal Engineer named Thomas Murray in 1875. It was

named as Baudot after Emile Baudot, an earlier pioneer in telegraph printing.

It is sometimes called fixed length block code since it is a fixed length source code.

All the characters are represented in binary and have the same number of symbols.

It is a 5-bit character code, used primarily for low speed teletype equipment such as

TWX / Telex system and Radio Teletype (RTTY).

The latest version of Baudot code is recommended by CCITT as International

Alphabet No. 2. Table 5.1 shows some of the Baudot codes.

ASCII Code:

To standardize data communication codes in 1963, US adopted Bell System Model-

33 type code as United States of American Standard Code for Information

Interchange (USASCII) better known as ASCII – 63.

ASCII (as-key) has progressed through 1965, 1967 and 1977 versions. With 1977

version recommended by ITU as International Alphabet No. 5 in US, as ANSI

standard X 3.4 – 1986 (R 1997) and by ISO as ISO-14962 (1997).

ASCII is the standard character set for source coding the alphanumeric character set

that the human can understand but the computers don’t.

ASCII is a 7-bit fixed-length character set. With ASCII code, the LSB is designated as

b0 and MSB is designated as b7.

The character codes don’t represent weighted binary numbers and therefore all bits

are equally significant. Bit b7 is not a part of ASCII code but generally reserved for

an error detection bit called parity bit.

Bits are referred by their order than by their position, b0 is the zeroth order bit, b1 is

the first order bit, b7 is the seventh order bit. With serial data transmission, the first

transmitted bit is generally LSB. So the lower order bit b0 is transmitted first.

ASCII code is mostly used in data communication network today. The 1977 version

of ASCII code with odd parity is shown in Table 5.2.

EBCDIC Code:

Extended Binary Coded Decimal Interchange Code (EBCDIC) is an 8-bit fixed

length character set developed in 1962 by International Business Machines

Corporation (IBM).

This code is used exclusively with IBM main frame computers and peripheral

equipment.

With 8 bits, 256 (28) codes are possible though 139 out of 256 are assigned

characters. Unspecified codes can be assigned to specialized characters and

functions.

The name ‘Binary Coded Decimal (BCD)’ was selected because second

hexadecimal character for all letters and digit codes contains only hex values from 0

to 9, which have same binary sequence as BCD codes.

BAR Codes:

Bar codes are omnipresent black and white striped stickers; seem to appear virtually

on every consumer item in any part of the world. It was developed in early 1970s but

not used extensively until mid 1980s.

It consists of a series of vertical black bars separated by vertical white bars (spaces).

The width of bars and spaces along with their reflective abilities represent binary 1s

and 0s and combination of bits identifies specific items.

It contains information regarding cost, inventory management and control, security

access, shipping and receiving, production counting, document and order

processing, automatic billing.

There are several standard bar code formats. The format selection depends on the

types of data being stored, how the data stored, system performance and which

format is most popular with business and industry.

The bar codes are classified as: Discrete Code, Continuous Code and 2D code.

Discrete Code: The discrete bar code has spaces or gaps between characters. Each

character within the bar code is independent of every other character. E.g.: Code-

39.

Continuous Code: The continuous bar code does not include spaces between

characters. E.g.: Universal Product Code (UPC).

2D Code: The two dimensional bar code stores data in two dimensions in contrast

with a conventional linear bar code, which stores data along only one axis. They

have larger storage capacity than one dimensional bar codes (typically 1 Kilobyte or

more per data symbol).

Code – 39:

Code – 39 is one of the most popular bar codes developed in 1974. It is also called as

Code 3 of 9 or 3 of 9 Code.

It uses alphanumeric codes similar to ASCII code. It consists of 36 unique codes

representing 10 digits and 26 uppercase letters. There are 7 additional codes used

for special characters and an exclusive start/stop character used as an asterisk (*).

It is ideally suited for making labels such as badges. Each code-39 character contains 9

vertical elements (5 bars and 4 spaces). The logical condition 1 or 0 of each element is

encoded in the width of the bar or space (width modulation).

A wide element whether space or a bar represents logic 1 and a narrow element

represents logic 0.

3 of 9 elements in each code-39 character must be logic 1s and rest must be logic 0s.

If 3 logic 1s, then 2 must be bars and one must be space.

Each character begins and ends with a black bar with alternating white bars in

between.

Since code-39 is a discrete code, all characters are separated with an inter-character

gap, usually one character wide.

The character asterisk (*) at the beginning and at the end of the bar code are start

and stop characters.

Universal Product Code (UPC):

Grocery industry developed UPC in early 1970s to identify their products. National

Association of Food Chains officially adopted UPC in 1974. Today UPC codes are

found on virtually every grocery item.

UPC is a continuous code since there are no inter-character spaces.

As shown in Figure 5.4 (b), each UPC label contains a 12-digit number. Two long

bars on outermost left and right hand sides of the label are called start guard pattern

and stop guard pattern respectively.

Start and stop guard pattern consists of 101 sequence used to frame 12-digit UPC

number.

The left and right halves of label are separated by a center guard pattern, which

consists of two long bars in the center of the label.

The two long bars are separated with a space between them and have space on both

sides of bars. UPC center guard pattern is 01010. The 1st 6 digits of UPC codes are

encoded on the left half of the label and last 6 digits of UPC codes are encoded on right

half of the label. There are 2 binary codes for each character.

When a character appears in one of 1st 6 digits of code, it uses left hand code and

when a character appears in last 6 digits of code, it uses right hand code. Right hand

code is the complement of left hand code.

If 2nd and 9th digits of a 12 digit code UPC are both 4s, digit is encoded as a 0100011

in position 2 and as 1011100 in position 9. UPC for 12 digit code 012345543210 is

0001101 0011001 0010011 0111101 0100011 1011100

0 1 2 3 4 5

0110001 1001110 1000010 1101100 1100110 1110010

5 4 3 2 1 0

The 1st left hand digit in UPC code is called UPC number system characters that

identify how the UPC symbol is used. For e.g., UPC number system 5 is used with a

coupon for item. Other 5 left hand characters are data characters and first 5 right

hand characters are data characters, 6th right hand character is the check character,

which is used for detection.

The decimal value of number system character is always printed to the left of UPC label

and in most UPC labels; the decimal value of check character is printed on right of UPC

label.

The width of bars and spaces does not correspond to logic 1s and 0s. Digits 0 through 9 are encoded into a combination of 2 variable width bars and 2 variable

width spaces that occupy equivalent of 7 bit positions.

Figure 5.4 (c) shows the variable width code for UPC character 4 when used in one of 1st 6

digit positions of code and when use din one of last 6 digit positions of code.

Single bar represents logic 1 and single space represents logic 0. The bar and space width

ranges from 1 to 4 bits. The left hand character is comprised of 1 bit space, 1 bit bar, 3 bits

space and 2 bit bar. The right hand character is 1 bit bar, 1 bit space, 3 bit bar and 2 bit

space.

Problem: Determine the UPC label structure for digit 0.

Solution:

For digit 0, the left hand character is 0001101 and right hand character is 1110010.

Error Control:

A data communication circuit can be short as feet or as long as several 1000 miles and

transmission medium can be as simple as a pair of wires or as complex as a microwave,

satellite or optical fiber communication systems.

Error will occur and so it is necessary to develop and implement error control

procedures.

Transmission errors are caused by electrical interference from natural sources such as

lightning as well as from man-made sources such as motors, generators, power lines

and fluorescent lights.

Data communication errors can be classified as single bit, multiple bits or burst. Single

bit errors occur when only one bit within a given data string is in error. Multiple bit

error occurs when two or more non-consecutive bits within a given data string are in

error. Burst errors occur when two or more consecutive bits within a data string are in

error.

Burst errors affect one or more characters within a message.

The error performance is the rate in which the errors occur and is described as an

expected or an empirical value. The theoretical expectation of the rate at which errors

will occur is called the probability of error P [e].

The actual historical records of systems error performance is called bit error rate (BER).

For e.g., if a system has P[e] of 10-5, one bit error for every 100,000 bits transported

through the system. If a system has a BER of 10-5, one bit error for every 100,000 bits

transported.

BER is measured and compared with P[e] to evaluate system performance.

Error control is divided into Error detection and Error correction.

Error Detection:

Error detection is a process of monitoring data transmission and determining when

errors have occurred.

Error detection techniques neither correct errors nor identify which bits are in error.

They indicate only when an error has occurred.

Error detection is not to prevent errors from occurring but to prevent undetected

errors from occurring.

The techniques used are Redundancy Checking

i. Vertical Redundancy Checking

ii. Checksum

iii. Longitudinal Redundancy Checking

iv. Cyclic Redundancy Checking Redundancy Checking:

Duplicating each data unit for detecting errors is a form of error detection called

redundancy.

Redundancy is an effective but costly means of detecting errors with long messages.

It is efficient to add bits to data units that check for transmission errors.

Adding bits only for detecting errors is called redundancy checking.

Types of Redundancy Checking:

(a) Vertical Redundancy Checking: It is the simplest error detection scheme, generally referred as character parity or simply

parity.

With character parity, each character has its own error detection bit called parity bit. Since parity bit is not actually part of character but considered as redundant. n – character

message has n-parity bits. The number of error detection bits is proportional to the length

of the message.

With character parity, a single parity bit is added to each character to force total number of

logic 1s in character, including parity bit either odd number (odd parity) or even number

(even parity).

For e.g., the ASCII code for letter ‘C’ is 43H or P1000011 binary, where P is the parity bit.

The 3 logic 1s without counting parity bit. Odd Parity : P=0 and Even Parity: P=1. Advantage: Simplicity.

Disadvantage:

1. When even number bits are received in error, parity checker won’t detect them since

when logic condition of an even number of bits is changed, parity remains the same.

2. Over a long time, theoretically, parity detect only 50% of transmission errors (Equally

probable even or odd number of bits in error).

Problem: Determine the even and odd parity bit for character R.

The ASCII code of R is 52 H. Binary value of 52H is 1010010. With parity bit, value is

P1010010, which consists of 3 ones (odd number of ones). For odd parity, P = 0, value is 0 1010010

and for even parity, P = 1, value is 1 1010010.

The other forms of parity are:

(i) Marking Parity where the parity bit is always 1.

(ii) No parity where parity bit is not sent or checked.

(iii) Ignored Parity where the parity bit is always 0 or ignored.

Marking parity is useful only when errors occur in a large number of bits. Ignored parity

allows receivers that are incapable of checking parity to communicate with devices that use

parity.

(b) Checksum:

It is a simple form of redundancy error checking. Each character has a numerical value

assigned to it.

The characters within a message are combined together to produce an error checking

character which is simple as arithmetic sum of numerical values of all character in a

message.

The checksum is appended to the message.

The receiver replicates the combining operation and determines its own checksum.

Then the receiver’s checksum is compared with the checksum appended to the

message. If both are same, then there is no transmission error and if different, a

transmission error has definitely occurred.

c) Longitudinal Redundancy Checking:

It is a redundancy error detection scheme that uses parity to determine if a transmission

error has occurred within a message or not. It is sometimes called as message parity.

With LRC, each bit position has a parity bit. The bit b0 from each character in the

message is XORed with b0 from all other character from the message.

Similarly, b1, b2, … are XORed with respective bits from all character in the message.

The result of XORing the character codes that make up the message in VRC is XORing

of bits within a single character.

Even parity is generally used with LRC and odd parity is used with VRC.

LRC bits are computed in transmitter while data are being sent and then appended to

the end of the message as a redundant character.

In the receiver, LRC is recomputed from the data and is compared to LRC appended to

the message. If two LRC characters are same, no transmission error occurred. If both

are different, one/more transmission errors occurred.

The group of characters that compromise a message is called block or frame of data.

The bit sequence for LRC is called block check sequence (BCS) or frame check

sequence (FCS).

With LRC, all messages have same number of error detection character, used to

send long messages.

LRC detects between 95 and 98% of all transmission errors.

LRC can’t detect transmission errors when even number of character has an error in

same bit position. For e.g., If b4 in an even number of character is in error, LRC is

still valid even though multiple transmission errors have occurred.

Problem: Determine VRC and LRC for the following ASCII encoded message: THE CAT. Use odd

parity for VRCs and even parity for LRCs. Solution:

Character T H E Sp C A T LRC

Hex 54 48 45 20 43 41 54

ASCII Code b0 0 0 1 0 1 1 0 1

b1 0 0 0 0 1 0 0 1

b2 1 0 1 0 0 0 1 1

b3 0 1 0 0 0 0 0 1

b4 1 0 0 0 0 0 1 0

b5 0 0 0 1 0 0 0 1

b6 1 1 1 0 1 1 1 0

Parity Bit (VRC) b7 0 1 0 0 0 1 0 0

For T, ASCII code is 54H which is (1010100)2. VRC is odd parity and LRC is even

parity. LRC = 101111 = 2FH = “/” in ASCII i.e., “THE CAT/.”

d) Cyclic Redundancy Checking:

The reliable redundancy checking technique for error detection is conventional

coding scheme called CRC. Approximately 99.999% of all transmission errors are

detected. In US, most common CRC code is CRC – 16.

16 bits are used for block check sequence. The entire data stream is treated as a

long continuous binary number.

Since BCS is separated from the message but transported within same

transmission, CRC is considered as a systematic code.

Cyclic block codes are often written as (n, k) cyclic codes where n is the bit

length of transmitter and k is the bit length of message. The length of SCC in

bits is BCC = n – k.

CRC – 16 block check character is the remainder of a binary division process.

The data message polynomial G (x) is divided by unique generator polynomial

function P (x), the quotient is discarded and remainder truncated to 16 bits and

appended to message as BCS.

The generator polynomial must be a prime number.

2 MARKS

1. Define data communication code.

2. What is SYN character?

3. Define the start and stop bits.

4. What is clock slippage?

5. Difference between under and over slippage?

6. Compare and contrast synchronous and asynchronous data format.

7. List some of the data communication codes.

8. Difference between discrete and continuous ARQ.

DETAIL

1) Explain different data communication codes.

2) Explain different standards organizations for data communication

3) Explain Error detection and correction techniques in detail

4) Explain synchronous and asynchronous data communication in detail

5) Explain Low-speed, medium-speed and high-speed modems

Problems On:

1. CRC, VRC, LEC

2. Conversion codes