chettinad college of engineering &...
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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