EC8395 COMMUNICATION ENGINEERING UNIT II

Pulse Communication:
Low pass sampling theorem – Quantization – PAM – Line coding –
PCM, DPCM, DM, and ADPCM And ADM, Channel Vocoder -
Time Division Multiplexing, Frequency Division Multiplexing
UNIT II
PULSE AND DATA COMMUNICATION

Sampling process
Sampling is the reduction of a continuous-time signal to a discrete-time signal.
A continuous time signal can be completely represented in its samples and
recovered back if the sampling frequency is twice of the highest frequency content
of the signal. fs ≥ 2fm, Where fm is the highest frequency content
Sampling rate is the reciprocal of sampling period. Fs = 1/Ts.
Nyquist rate = 2fm and Nyquist Interval = 1/2fm . Where fm is the bandwidth
UNIT II PULSE MODULATION

• T= 10S
• W = 1/10
• Fs >= 1/5
• Fm = 50 Hz
• Fs >= 2*50 >= 100 Hz

Aliasing: Fs<2fm When a high frequency interferes with low frequency and
appears as low frequency
Effects of aliasing:
Interference
Data cannot be recovered
Ways to avoid aliasing
fs ≥ 2fm
Bandlimit the signal to fm
UNIT II PULSE MODULATION

M = sin500Πt/Πt
NR = 2fm
NI = 1/2fm
M = sin2Πft
500 Πt = 2Πft
Fm = 500/2 = 250
NR = 500 Hz
NI = 1/500 = 0.002 s

M = cos4000Πtcos1000Πt
Cos a cos b = ½(Cos(a-b)+cos(a+b))
M = ½(cos(3000Πt)+cos(5000Πt))
Fm1 = 1500 fm2 = 2500
Fm = 2500
NR = 5000 Hz
NI = 1/5000 S

Pulse Modulation
Process of changing a binary pulse signal to represent the information to be
transmitted
Bandwidth of the earlier communication techniques is high
But with pulse modulation, signals can be transmitted with short pulses with
low duty cycle. Duty cycle = Ton / (Ton+Toff)
The primary benefits are great noise tolerance and the ability to regenerate the
degraded signal
UNIT II

Pulse-amplitude modulation (PAM)
Pulse-width modulation (PWM)
Pulse-position modulation (PPM)
Pulse-code modulation (PCM)
UNIT II

Pulse-code modulation (PCM)
• Alex H. Reeves invented PCM in 1937
• easy to combine digitized voice and digital data into a single, high-speed
digital signal and propagate it over either metallic or optical fiber cables.
• PCM is the only digitally encoded modulation technique
• With PCM, the pulses are of fixed length and fixed amplitude
• PCM is a binary system where a pulse or lack of a pulse within a prescribed
time slot represents either a logic 1 or a logic 0 condition.
UNIT II

UNIT II

• The bandpass filter limits the frequency of the analog input signal to
the standard voice-band frequency range of 300 Hz to 3000 Hz
• The sample-and-hold circuit periodically samples the analog input
signal and converts those samples to a multilevel PAM signal.
• The analog-to-digital converter (ADC) converts the PAM samples to
parallel PCM codes
• Parallel-to-serial converter transforms parallel data to serial data
then outputted onto the transmission line as serial digital pulses
• The hold circuit converts the PAM signals back to its original analog
form
• An integrated circuit that performs the PCM encoding and decoding
functions is called a codec
UNIT II

PCM SAMPLING
UNIT II
Natural Sampling Flat Top Sampling

Natural Sampling
Tops of the sample pulses retain their natural shape during the sample interval
Difficult for an ADC to convert the sample to a PCM code
Requires frequency equalizers at the receiver
Flat Top Sampling
Accomplished in a sample-and-hold circuit
With flat-top sampling, the input voltage is sampled with a narrow pulse and
then held relatively constant until the next sample is taken.
Aperture error which is when the amplitude of the sampled signal changes
during the sample pulse time.
UNIT II

Sampling Rate fs ≥ 2fm
If fs is less than two times fm, an impairment called alias or foldover distortion occurs
Quantization - Quantization is the process of converting an infinite number of possibilities to a finite
number of conditions. quantization is the process of rounding off the amplitudes of flat-top samples to a
manageable number of levels
Folded Binary Code – Mirror Image
Quantization Interval
Overload distortion
Resolution - The resolution is the minimum voltage
other than 0 V that can be decoded by the DAC.
UNIT II

Dynamic range = Vmax/Vmin
Ratio between largest possible magnitude to the smallest possible magnitude
that can be decoded by the DAC
Coding Efficiency = (Nmin/ N) *100
UNIT II

PULSE COMMUNICATION
PAM
The amplitude of a carrier pulse is varied proportional to the amplitude of
message signal
PAM MODULATOR
Message signal is transmitted to LPF
LPF performs band limiting
Band limited signal is then sampled at the multiplier.
Multiplier samples with the help of pulse train generator. Balanced modulator is widely used as the
multiplier circuit.
Pulse train generator produces the pulse train
The multiplication of message signal and pulse train produces PAM
UNIT II
Message
Pulse
PAM

𝑓𝑠 ≥ 2𝑓𝑚
𝑇𝑠 ≤
1
2𝑓𝑚
𝜏 ≪ 𝑇𝑠 𝜏 ≪ 1/(2𝑓𝑚)
𝑓𝑚𝑎𝑥 =
1
2𝜏
𝐵𝑇 ≥ 𝑓𝑚𝑎𝑥
𝐵𝑇 ≥
1
2𝜏
𝐵𝑇 ≫
1
2
1
2𝑓𝑚
𝐵𝑇 ≫ 𝑓𝑚
UNIT II
Transmission Bandwidth
Advantages:
Can be easily generated and detected
Other forms can be easily generated
Disadvantages
Large Bandwidth needed
Noise Interference is high
High power Requirement

Pulse width modulation
In PWM system, the message signals are used to vary the duration of carrier
pulse. The message signal may vary either the trailing edge or leading edge
or both of the carrier pulses n order to accommodate the intelligence of
information system.
Width of pulse is proportional to the amplitude of the modulating signal. The
amplitude and position of the pulse remains unchanged.
PWM Modulator
UNIT II

Pulse Position Modulation
The position of a carrier pulse is altered in accordance with information
contained in sampled waveform
PPM Modulator
Saw-tooth generator generates saw-tooth signal of frequency which is applied to inverting input of
comparator
Modulating signal is applied to the non-inverting input of comparator
When the value of message signal is higher than value of saw-tooth ,then the output is high
When the value of message signal is lower than value of saw-tooth ,then the output is high.
PPM Demodulator
UNIT II

History of Data Communication
Data communication can be defined as two personal computers connected
through a Public Telecommunication Network (PTN). Point to point
communication is the link between two stations A and B ie., information is
transferred between a main frame computer and a remote computer
terminal.
A multipoint line configuration is one in which more than two specific devices
share a single link. Morse code is used to send messages. A key which
turned the carrier of a transmitter ON and OFF to produce the dots and
dashes. These dots and dashes were detected at the receiver and it is
converter back into letters and numbers makes the original message.
UNIT II

Data Communications History
1753 : communications line between villages comprised of 26 parallel wires, each wire for one letter of
the alphabet
1833: Carl Friedrich Gauss developed an unusual system based on a five-by-five matrix representing 25
letters. The idea was to send messages over
a single wire by deflecting a needle to the right or left between one and five times.
1832: First successful data communications system was invented by Samuel F. B. Morse called the
telegraph. Morse also developed the first practical data communications code, which he called the
Morse code. Dots and dashes are transmitted across a wire using electromechanical induction.
1844: First telegraph line was established between Baltimore and Washington, D.C., with the first
message conveyed over this system being “What hath God wrought!”
1849 : First slow-speed telegraph printer was invented.
1850 : Western Union Telegraph Company was formed in Rochester, New York, for the purpose of
carrying coded messages from one person to another.
1874 : Emile Baudot invented a telegraph multiplexer, which allowed signals from up to six different
telegraph machines to be transmitted simultaneously over a single wire.
1875: Alexander Graham Bell invented Telephone but Telegraph was the only means of
sending information across large spans of water until 1920.
1930’s : Konrad Zuis, a German engineer, demonstrated a computing machine.
1949: U.S. National Bureau of Standards developed the first all-electronic diode-based computer
capable of executing stored programs.
UNIT II

Data Communications History
1951: The UNIVAC computer, built by Remington Rand Corporation, was the first mass-produced
electronic computer.
1960: batch-processing systems were replaced by on-line processing systems with terminals connected
directly to the computer through serial or parallel communications lines
1970: microprocessor-controlled microcomputers
1980: personal computers became an essential item in the home and workplace
1970 : The Internet began to evolve
Recent developments in data communications networking, such as the Internet, intranets, and the World
Wide Web (WWW), have created a virtual explosion in the data communications industry. A
seemingly infinite number of people, from homemaker to chief executive officer, now feel a need to
communicate over a finite number of facilities.
UNIT II

Standards for Data Communication
• Need to provide communications between dissimilar computer equipment and systems has also
increased
• A major issue facing the data communications industry today is worldwide compatibility. Major
areas of interest are software and programming language, electrical and cable interface, transmission
media, communications signal, and format compatibility.
• Wide number of hardware manufacturers
A standard is an object or procedure considered by an authority or by general consent as a basis of
comparison. Standards are authoritative principles or rules that imply a model or pattern for guidance
by comparison
• The guidelines outline procedures and equipment configurations that help ensure an orderly transfer
of information
• Standards are not laws
• Proprietary standards are generally manufactured and controlled by one company
• With open system standards, any company can produce compatible equipment or software; however,
often a royalty must be paid to the original company.
UNIT II

Standards Organizations for Data Communication
A consortium of organizations, governments, manufacturers, and users meet on a regular basis to ensure an orderly flow of
information within data communications networks and systems by establishing guidelines and standards.
• International Standards Organization (ISO) - The ISO creates the sets of rules and standards for graphics and document
exchange and provides models for equipment and system compatibility, quality enhancement, improved productivity, and
reduced costs
• International Telecommunications Union—Telecommunications Sector – develops the recommended sets of rules and
standards for telephone and data communications.
• Institute of Electrical and Electronics Engineers - Develop communications and information processing standards with the
underlying goal of advancing theory, creativity, and product quality in any field associated with electrical engineering
• American National Standards Institute - It serves as the national coordinating institution for voluntary standardization in
the United States
• Electronics Industry Association - developing the RS (recommended standard) series of standards for data and
telecommunications and also increasing public awareness and lobbing
• Telecommunications Industry Association - facilitates business development opportunities and a competitive marketplace
through market development, trade promotion, trade shows, domestic and international advocacy, and standards development
• Internet Architecture Board - accelerate the advancement of technologies that could possibly be useful
to the U.S. military
• Internet Engineering Task Force - evolution of the Internet architecture and the smooth operation of the Internet.
• Internet Research Task Force - promotes research of importance to the evolution of the future Internet by creating focused,
long-term and small research groups
UNIT II

DATA COMMUNICATIONS CIRCUITS
• provide a transmission path between locations and to transfer digital information from one station to
another using electronic circuits
• A station is simply an endpoint where subscribers gain access to the circuit. A station is sometimes
called a node, which is the location of computers, computer terminals, workstations, and other digital
computing equipment
UNIT II

SERIALAND PARALLEL DATA TRANSMISSION
• Parallel : all bits can be transmitted simultaneously during the time of a
single clock pulse
• Serial : In a single transmission line and, thus, only one bit can be
transmitted at a time. Consequently, it requires four clock pulses (4TC) to
transmit the entire four-bit code.
UNIT II

DATA COMMUNICATIONS CIRCUIT ARRANGEMENTS
Circuit Configurations
1. Two-point configuration
involves only two locations or stations
2. Multipoint configuration
involves three or more stations.
Station can have one or more computers, computer terminals, or workstations.
Transmission Modes
1. Simplex - data transmission is unidirectional
2. Half duplex - data transmission is possible in both directions but not at the same time.
3. Full duplex - In the full-duplex (FDX) mode, transmissions are possible in both directions
simultaneously, but they must be between the same two stations
4. Full/full duplex - In the full/full duplex (F/FDX) mode, transmission is possible in both
directions at the same time but not between the same two stations
UNIT II

DATA COMMUNICATIONS CODES
Used to represent characters and symbols, such as letters, digits, and punctuation marks.
Data communications codes are called character codes, character sets, symbol codes, or character
languages.
Baudot Code
1. First fixed-length character code developed for machines
2. Five-bit Code
3. Primarily for low-speed teletype equipment, such as the TWX/Telex system and radio teletype
4. All characters are represented in binary and have the same number of bits
5. The latest version is recommended by the CCITT as the International Alphabet No. 2
UNIT II

DATA COMMUNICATIONS CODES
ASCII Code
1. 1963, 1965, 1967, 1977
2. 1977 version is recommended by the ITU as International Alphabet No. 5 (ANSI X3.4)
3. ASCII is a seven-bit fixed-length character set
4. Total 128 codes
5. MSB is used for error detection
6. 96 codes are graphic symbols
94 codes are printable and 2 codes (SPACE & DEL) are non printable
7. 32 codes control symbols (Col. 0 & 1) All are non printable
8. ASCII is probably the code most often used in data communications networks today.
EBCDIC Code
1. Eight-bit fixed length character set developed in 1962
2. 256 codes are possible
3. 139 codes are assigned characters. Unspecified codes can be assigned to specialized characters and
functions.
4. No parity bit for error checking
5. 96 codes are graphic symbols
UNIT II

ERROR CONTROL
1. Transmission errors
2. Data communications errors
Single Bit Error- Only one bit – One character
Multi-bit Error - More than one non consecutive bits – More one than character or even
only one character
Burst Error - More than one consecutive bits - More one than character or even only
one character
• The theoretical (mathematical) expectation of the rate at which errors will occur is called
probability of error ( 1/10000)
• Actual historical record of a system’s error performance is called bit error rate ( 1/10000)
• Error control can be divided into two general categories: error detection and error correction.
UNIT II

ERROR DETECTION
• Process of monitoring data transmission and determining when errors have
occurred
• Neither correct errors nor identify which bits are in error
• Redundancy checking - Duplicating each data unit
– vertical redundancy checking
– checksum
– longitudinal redundancy checking
– cyclic redundancy checking
UNIT II

Vertical Redundancy Checking
• character parity or parity
• each character has its own error-detection bit
• An n-character message would have n redundant parity bits
• Odd parity 000001101 000011100
• Even Parity 000001100 000011101
Checksum
• The characters within a message are combined together to produce an error-
checking character
• The checksum is appended to the end of the message
• The receiver replicates the combining operation and determines its own checksum.
• The receiver’s checksum is compared to the checksum appended to the message
Sender’s end Receiver’s end
Block 1: 11001100 Block 1 : 11001100
Block 2: 10101010 Block 2 : 10101010
Checksum: 01110111 Checksum: 01110111
UNIT II

Longitudinal redundancy checking
• Each bit position has a parity bit
Cyclic redundancy checking
• 99.999% of all transmission errors are detected
• 16 bits are used
UNIT II

Error Correction
Lost message - never arrives or unrecognizable
Damaged message - one or more transmission errors
Error Correction Methods
Retransmission
Receiver requests to transmit again
Often called as ARQ
Acknowledgments - indicate a successful transmission or an unsuccessful
transmission
Line turnarounds - retransmissions are sent in response to a negative
acknowledgment
Discrete ARQ - indicate the successful or unsuccessful reception of data.
Continuous ARQ - request the retransmission of a specific frame of data
Forward error correction
Redundant bits are added to the message before transmission
Eliminates the time wasted for retransmissions
UNIT II

Hamming code
• Hamming code will correct only single-bit errors
• It cannot identify errors that occur in the Hamming bits
• The combination of the data bits and the Hamming bits is called the
Hamming code
• Sender and the receiver must agree on the placement of hamming bits
• Number of Hamming bits 2n ≥ m+n+1
UNIT II

Data Communications Hardware
a transmitter (source), a transmission path (data channel), and a receiver
(destination)
UNIT II

Data Terminal Equipment
• Generates, transmits, receives, or interprets data messages
• DTE is where information originates or terminates
• Data terminal equipment includes the concept of terminals, clients,
hosts, and servers
Data Communications Equipment
• Interfaces data terminal equipment to a transmission channel
• The output of a DTE can be digital or analog
• DCE is a signal conversion device
UNIT II

Serial Interface
The serial interface coordinates the flow of data, control signals and timing
information between the DTE and the DCE
• The RS-232 specifications identify the mechanical, electrical, functional,
and procedural descriptions for the interface between DTEs and DCEs.
• 25 Pins male connector, female connector
UNIT II

1. Determine the odd and even parity bits for the ASCII character R = 52
L = 421 R=8421 1010010
P1010010
Odd parity = 01010010
Even parity = 101010010
2. Determine the VRCs and LRC for the following ASCII-encoded message:
MOON. Use odd parity for the VRCs and even parity for the LRC.
ASCII Code for M = 4D O =4F O =4F N = 4E
M = 1001101 O = 1001111 O = 1001111 N = 1001110
VRC
M = 1 O = 0 O = 0 N = 1
MOON = 110011010 1001111 010011111 1001110
LRC
B0 = 1 B1 = 1 B2 = 0 B3 = 0 B4 = 0 B5 = 0 B6 = 0 B7 = 0
MOON = 00000011 1001101 1001111 1001111 1001110
UNIT II

Determine the BCS for the following data and CRC generating polynomials:
Data G(x) = x7 + x5 + x4 + x2 + x1 + x0
= 1011011101001
CRC P(x) = x5 + x4 + x1 + x0
= 110011
UNIT II

For a 12-bit data string of 101100010010, determine the number of Hamming
bits required, arbitrarily place the Hamming bits into the data string,
determine the logic condition of each Hamming bit, assume an arbitrary
single-bit transmission error, and prove that the Hamming code will
successfully detect the error
UNIT II

For a PCM system with a maximum audio input frequency of 4 kHz, determine
the minimum sample rate and the alias frequency produced if a 5-kHz
audio signal were allowed to enter the sample-and hold circuit.
Sampling Theorem:
fs ≥ 2fa
fs ≥ 8 kHz
Aliasing Frequency : 3KHz
For the PCM coding scheme, determine the quantized voltage, quantization
error (Qe), and PCM code for the analog sample voltage of +1.07 V.
Quantized voltage = Sampled Voltage / Resolution
= +1.07/1 = +1.07
Qe = Sampled Voltage – Quantized level
= +1.07 - 1 = 0.07
PCM code for 1 is 101.

For a PCM system with the following parameters, determine (a) minimum
sample rate, (b) minimum number of bits used in the PCM code, (c)
resolution, and (d) quantization error
• Maximum analog input frequency 4 kHz
• Maximum decoded voltage at the receiver +-2.55 V
• Minimum dynamic range 46 dB
minimum sample rate is fs≥ 2fa
≥ 2(4 kHz)
≥ 8 kHz
2n =DR+ 1
Dynamic Range:
46 dB = 20 log(Vmax/Vmin)
Vmax/Vmin = A.log(2.3)
= 199.5
2n = 199.5+1 = 200.5
n= 7.63=8
Total number of bits = 8+1 = 9

Resolution = Vmax/(2n-1)
= 2.55/(28-1)
= 2.55/(256-1)
= 2.55/255
=0.01
Qe = Resolution/2 = 0.005
Coding Efficiency = (8.63/9)*100 =95.88 %

EC8395 COMMUNICATION ENGINEERING UNIT II

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EC8395 COMMUNICATION ENGINEERING UNIT II