Data communications and Computer Networks

UNIT-II - Analog to Digital Conversion & Transmission modes

UNIT 2 - PHYSICAL LAYER

Physical layer functionalities - Analog to digital conversion

using PCM, Transmission Modes: simplex, half duplex and
full duplex - Transmission Media - Copper – Fiber – Optical –
Radio (wireless), Switching: Introduction, Circuit Switched
Networks and Packet switching.
 ANALOG-TO-DIGITAL CONVERSION
A digital signal is superior to an analog signal because it is
more robust to noise and can easily be recovered, corrected
and amplified. For this reason, the tendency today is to
change an analog signal to digital data. In this section we
describe two techniques, pulse code modulation and delta
modulation.
Topics discussed in this section:
Pulse Code Modulation (PCM)
Delta Modulation (DM)
PULSE AMPLITUDE MODULATION
 The sampling process is sometimes referred to
as pulse amplitude modulation(PAM).
 This technique takes an analog signal, samples
it, and generates a series of pulses based on
the results of sampling.
 PULSE CODE MODULATION - PCM
The most common technique to change an analog signal to digital data
(digitization) is called pulse code modulation (PCM).

PCM consists of three steps to digitize an analog signal:

1. Sampling
2. Quantization
3. Binary encoding
Before we sample, we have to filter the signal to limit the maximum frequency of
the signal as it affects the sampling rate.
Filtering should ensure that we do not distort the signal, ie remove high
frequency components that affect the signal shape.
FIGURE 4.21 COMPONENTS OF PCM ENCODER
 SAMPLING
Analog signal is sampled every Ts secs. Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
There are 3 sampling methods:
Ideal - pulses from the analog signal are sampled
Natural - a high-speed
switch is turned on for only the small period of time when the sampling occurs, The result
is a sequence of samples that retains the shape of the analog signal.
Flattop - sample and hold, like natural but with

The process is referred to as pulse amplitude modulation PAM and the outcome
is a signal with analog (non integer) values
 FIGURE 4.22 THREE DIFFERENT SAMPLING METHODS FOR PCM

4.8
 NOTE
According to the Nyquist theorem, the
sampling rate must be
at least 2 times the highest frequency
contained in the signal.

4.9
NYQUIST SAMPLING RATE FOR LOW-PASS AND BANDPASS SIGNALS

If the analog signal is low-pass, the bandwidth and the

highest frequency are the same value.
If the analog signal is bandpass, the bandwidth value is lower than the value of the
maximum frequency
FIGURE 4.24 RECOVERY OF A SAMPLED SINE WAVE FOR
DIFFERENT SAMPLING RATES

4.11
FIGURE 4.25 SAMPLING OF A CLOCK WITH ONLY ONE HAND
 QUANTIZATION
 Sampling results in a series of pulses of varying amplitude values
ranging between two limits: a min and a max.
 The amplitude values are infinite between the two limits.
 We need to map the infinite amplitude values onto a finite set of known
values.
 This is achieved by dividing the distance between min and max into L
zones, each of height
 QUANTIZATION LEVELS
 The midpoint of each zone is assigned a value from 0 to L-1
(resulting in L values)
 Each sample falling in a zone is then approximated to the
value of the midpoint.

4.14
QUANTIZATION AND ENCODING OF A SAMPLED SIGNAL

4.15
 QUANTIZATION ERROR
 When a signal is quantized, we introduce an error - the coded
signal is an approximation of the actual amplitude value.
 The difference between actual and coded value (midpoint)
is referred to as the quantization error.

 The more zones, the smaller which results in smaller errors.

BUT, the more zones the more bits required to encode the

samples -> higher bit rate

4.16
 QUANTIZATION ERROR AND SNQR
Signals with lower amplitude values will suffer more from quantization error as
the error range: /2, is fixed for all signal levels.
Non linear quantization is used to alleviate this problem. Goal is to keep
SNQR fixed for all sample values.
Two approaches:

The quantization levels follow a logarithmic curve. Smaller ’s at lower

amplitudes and larger ’s at higher amplitudes.
Companding: The sample values are compressed at the sender into
logarithmic zones, and then expanded at the receiver. The zones are
fixed in height.
4.17
BIT RATE AND BANDWIDTH REQUIREMENTS OF PCM
 The bit rate of a PCM signal can be calculated form the number
of bits per sample x the sampling rate
 Bit rate = nb x fs

 The bandwidth required to transmit this signal depends on the
type of line encoding used. Refer to previous section for

discussion and formulas.
 A digitized signal will always need more bandwidth than the
original analog signal. Price we pay for robustness and other
features of digital transmission.

4.18
PCM DECODER
To recover an analog signal from a digitized signal we follow the
following steps:

 We use a hold circuit that holds the amplitude value of a

pulse till the next pulse arrives.
 We pass this signal through a low pass filter with a cutoff
frequency that is equal to the highest frequency in the pre-
sampled signal.
The higher the value of L, the less distorted a signal is recovered.
FIGURE 4.27 COMPONENTS OF A PCM DECODER

4.20
 DELTA MODULATION
 This scheme sends only the difference between pulses, if the
pulse at time tn+1 is higher in amplitude value than the pulse at
time tn, then a single bit, say a “1”, is used to indicate the
positive value.
If the pulse is lower in value, resulting in a negative value, a
 “0” is used.

 This scheme works well for small changes in signal values

between samples.

If changes in amplitude are large, this will result in large
errors.
4.21
FIGURE 4.28 THE PROCESS OF DELTA MODULATION

4.22
FIGURE 4.29 DELTA MODULATION COMPONENTS

4.23
FIGURE 4.30 DELTA DEMODULATION COMPONENTS

4.24
 DELTA PCM (DPCM)
 Instead of using one bit to indicate positive and negative
differences, we can use more bits -> quantization of the
difference.
 Each bit code is used to represent the value of the difference.
The more bits the more levels -> the higher the accuracy.


4.25
THANK YOU
Data communications and Computer Networks
UNIT-II -Transmission modes
 4-3 TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in
either parallel or serial mode. In parallel mode, multiple bits are sent with
each clock tick. In serial mode, 1 bit is sent with each clock tick. While
there is only one way to send parallel data, there are three subclasses of
serial transmission: asynchronous, synchronous, and isochronous.

Topics discussed in this section:

Parallel Transmission
Serial Transmission

4.28
FIGURE 4.31 DATA TRANSMISSION AND MODES

4.29
PARALLEL TRANSMISSION
Binary data, consisting of Is and Os, may be organized into groups of n
bits each. Computers produce and consume data in groups of bits much
as we conceive of and use spoken language in the form of words rather
than letters. By grouping, we can send data n bits at a time instead of 1.
This is called parallel transmission.
FIGURE 4.32 PARALLEL TRANSMISSION

4.31
FIGURE 4.33 SERIAL TRANSMISSION

4.32
ASYNCHRONOUS TRANSMISSION
 Asynchronous transmission is so named because the timing of a signal is
unimportant.
 Instead, information is received and translated by agreed upon patterns. As
long as those patterns are followed, the receiving device can retrieve the
information without regard to the rhythm in which it is sent.
 Patterns are based on grouping the bit stream into bytes. Each group, usually
8 bits, is sent along the link as a unit. The sending system handles each
group independently, relaying it to the link whenever ready, without regard to
a timer.
 The start and stop bits
 NOTE
In asynchronous transmission, we send 1 start bit (0) at
the beginning and 1 or more stop bits (1s) at the end of
each byte. There may be a gap between
each byte.

4.34
 NOTE
Asynchronous here means
“asynchronous at the byte level,”
but the bits are still synchronized;
their durations are the same.

4.35
FIGURE 4.34 ASYNCHRONOUS
TRANSMISSION

4.36
 NOTE

In synchronous transmission, we send bits one

after another without start or stop bits or gaps. It
is the responsibility of the receiver to group the
bits. The bits are usually sent as bytes and many
bytes are grouped in a frame. A frame is
identified with a start and an end byte.

4.37
FIGURE 4.35 SYNCHRONOUS TRANSMISSION

4.38
 ISOCHRONOUS
 In isochronous transmission we cannot have uneven gaps
between frames.
 Transmission of bits is fixed with equal gaps.

4.39
THANK YOU!!!
DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING
DATA COMMUNICATIONS AND COMPUTER NETWORKS

TRANSMISSION MEDIA

PRESENTATION BY
Mrs. C. Kalpana, AP/CSE
Mrs. P. Bhavani, AP/CSE 1
Dr. A. Ramachandiran, Assoc.Prof./CSE
What is Transmission media?
• Transmission media is a communication channel that carries the information
from the sender to the receiver. Data is transmitted through the
electromagnetic signals.
• The main functionality of the transmission media is to carry the information in
the form of bits through LAN(Local Area Network).
• It is a physical path between transmitter and receiver in data communication.
• In a copper-based network, the bits in the form of electrical signals.
• In OSI(Open System Interconnection) phase, transmission media supports
the Layer 1. Therefore, it is considered to be as a Layer 1 component.
• Different transmission media have different properties such as bandwidth,
delay, cost and ease of installation and maintenance.
• The transmission media is available in the lowest layer of the OSI reference
model, i.e., Physical layer.
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Transmission Media

3
 Guided media

Twisted-pair cable

Coaxial cable

Fiber-optic cable
4
 Twisted Pairs
Twisted pair
• Two insulated copper wires, 1mm thick
• low cost
Application
• Can transmit either analog or digital information
• Several km without amplification
Bandwidth
• Thickness of the wire, and distance
• Typically, several Mbps for a few km 5
 Twisted Pairs

Category 5 UTP cable with four twisted pairs

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7
 Unshielded Twisted Pair:
An unshielded twisted pair is widely used in telecommunication.

Following are the categories of the unshielded twisted pair cable:

• Category 1: Category 1 is used for telephone lines that have
low-speed data.
• Category 2: It can support upto 4Mbps.
• Category 3: It can support upto 16Mbps.
• Category 4: It can support upto 20Mbps. Therefore, it can be
used for long-distance communication.
• Category 5: It can support upto 200Mbps.
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Advantages Of Unshielded Twisted Pair:
• It is cheap.
• Installation of the unshielded twisted pair is easy.
• It can be used for high-speed LAN.
Disadvantage:
• This cable can only be used for shorter distances
because of attenuation.

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 Shielded Twisted Pair
A shielded twisted pair is a cable that contains the mesh surrounding the
wire that allows the higher transmission rate.
Characteristics Of Shielded Twisted Pair:
• The cost of the shielded twisted pair cable is not very high and not very
low.
• An installation of STP is easy.
• It has higher capacity as compared to unshielded twisted pair cable.
• It has a higher attenuation.
• It is shielded that provides the higher data transmission rate.
Disadvantages
• It is more expensive as compared to UTP and coaxial cable.
• It has a higher attenuation rate. 10
 COAXIAL CABLE
• Coaxial cable is very commonly used transmission media, for example,
TV wire is usually a coaxial cable.
• The name of the cable is coaxial as it contains two conductors parallel to
each other.
• It has a higher frequency as compared to Twisted pair cable.
• The inner conductor of the coaxial cable is made up of copper, and the
outer conductor is made up of copper mesh. The middle core is made up
of non-conductive cover that separates the inner conductor from the
outer conductor.
• The middle core is responsible for the data transferring whereas the
copper mesh prevents from the EMI(Electromagnetic interference).
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COAXIAL CABLE

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 Coaxial Cable
• It has better shielding and
greater bandwidth than
unshielded twisted pairs

• It can span longer distances

at higher speeds.

• Modern cables have a

bandwidth of up to a few
GHz. 13
 Coaxial Cable
Coaxial cable is of two types:
• Baseband transmission: It is defined as the process of
transmitting a single signal at high speed.
• Broadband transmission: It is defined as the process of
transmitting multiple signals simultaneously.

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 Two kind of Coaxial Cable
• 50 – ohm (Ω) (Used for digital transmission)
• 75 – ohm (Ω) (used for analog transmission and
cable television)

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Coaxial Cable

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 Coaxial Cable
Advantages Of Coaxial cable:
• The data can be transmitted at high speed.
• It has better shielding as compared to twisted pair cable.
• It provides higher bandwidth.
Disadvantages Of Coaxial cable:
• It is more expensive as compared to twisted pair cable.
• If any fault occurs in the cable causes the failure in the
entire network.
17
 Fiber Optics
• Fibre optic cable is a cable that uses electrical signals for
communication.
• Fibre optic is a cable that holds the optical fibres coated in plastic that
are used to send the data by pulses of light.
• The plastic coating protects the optical fibres from heat, cold,
electromagnetic interference from other types of wiring.
• Fibre optics provide faster data transmission than copper wires.

18
 Basic elements of Fibre optic cable:
• Core: The optical fibre consists of a narrow strand of glass or plastic
known as a core. A core is a light transmission area of the fibre. The
more the area of the core, the more light will be transmitted into the
fibre.
• Cladding: The concentric layer of glass is known as cladding. The
main functionality of the cladding is to provide the lower refractive
index at the core interface as to cause the reflection within the core
so that the light waves are transmitted through the fibre.
• Jacket: The protective coating consisting of plastic is known as a
jacket. The main purpose of a jacket is to preserve the fibre strength,
absorb shock and extra fibre protection.

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Fiber Optics

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21
 Fiber Optics
• Information passed in the form of Light source
• Transmission medium: ultra-thin fiber of glass
• Achievable bandwidth with fiber technology is in
excess of 50,000 Gbps (50 Tbps)
• Light can propagate only in a straight line, without
bouncing, yielding a single-mode fiber.
• many different light rays are bounced at different angles is
called multimode fiber.
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Fiber Cables

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• Light propagates through Glass core
• Core is typically 50 microns in diameter

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 Wireless Transmission media
People who need to be online all the time.

• The Electromagnetic Spectrum

• Radio Transmission
• Microwave Transmission
• Infrared Transmission
• Light Transmission
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 Radio Transmission
• Easy to generate
• Travel long distances
• Can penetrate buildings easily
• Omnidirectional
• Travel in all directions from the source
• Work well from 100 to 500 km Height from earth

26
• The properties of radio waves are frequency dependent.

• Path loss: At low frequencies, power falls off sharply with

distance from the source—at least as fast as 1/r 2 in air—as
the signal energy is spread more thinly over a larger
surface.

• Problem: Low bandwidth

• Application Area: radio and television broadcasting, cell

phones
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29
THANK YOU!!!!

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 Microwave Transmission
• Above 100 MHZ, the waves travel in nearly straight lines
• Transmitting and receiving antennas must be accurately
aligned with each other.
• Repeaters are needed periodically
• Microwaves do not pass through buildings well.

• Application Area: satellite communications, radar signal,

phones, and navigation applications
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32
 Infrared Transmission
• Infrared rays are directional, cheap, and easy to
build
• Drawback: they do not pass through solid objects
• Used for short-range communication.
• Application area: the remote controls used for
televisions

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34
 Light Transmission
• Unidirectional
• Offers very high bandwidth at very low cost
• Laser beam 1 mm wide can travel 500 meters
• Application Area: Laser Treatment, Laser Printing

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36
 Communication Satellites
• A communication satellite can be thought of as a big
microwave repeater in the sky.

• It contains several transponders, each of which listens to

some portion of the spectrum

• It amplifies the incoming signal, and then rebroadcasts it at

another frequency to avoid interference with the incoming
signal.
37
 Communication Satellites Types
• Geostationary Satellites
• Medium-Earth Orbit Satellites
• Low-Earth Orbit Satellites

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39
 Geostationary Satellites
• A geostationary satellite is an earth-orbiting satellite, placed
at an altitude of approximately 35,800 kilometers over the
equator

• Three such satellites, each separated by 120 degrees of

longitude, can provide coverage of the entire planet

• Application area are television broadcasting and weather

forecasting etc.,
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Geostationary Satellites

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 Medium-Earth Orbit Satellites
• Between the two Van Allen belts, we find the MEO
(Medium-Earth Orbit) satellites (5,000 km to 15,000
km over the earth).

• 10 MEO satellites are needed to cover the entire earth.

• Application area is navigation systems. Eg: GPS (Global

Positioning System) satellites
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Medium-Earth Orbit Satellites

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 Low-Earth Orbit Satellites
• LEO satellites are placed between 2000 km to 5000 km
above the earth.

• 50 LEO satellites are needed to cover the entire earth.

• Used for communications, military reconnaissance,

spying and other imaging applications
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Low-Earth Orbit Satellites

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Thank You !!!

46
Data communications and Computer Networks
Topic: SWITCHING
PRESENTATION BY
Mrs. C. Kalpana, AP/CSE
Mrs. P. Bhavani, AP/CSE
Dr. A. Ramachandiran, Assoc.Prof./CSE
SWITCHING
•In large networks, there can be multiple paths
from sender to receiver. The switching
technique will decide the best route for data
transmission.
• A switched network consists of a series of
interlinked nodes, called switches.
•Switching technique is used to connect the
systems for making one-to-one communication.
 Switched network
Network: Series of interlinked nodes, called switched network

• Switches: Devices capable of creating temporary connections between

two or more devices linked to the switches . Some of these switches are
connected to the end systems ( computers or telephones) . Others used
only for routing
Switched networks
 1. CIRCUIT-SWITCHED NETWORKS
• A circuit switched network consists of a set of
switches connected by physical links.
• A connection between 2 stations is a dedicated path made of one or
more links.
• However each connection uses only one dedicated
channel on each link.
• Each link is divided into n channels using FDM or TDM.
• FDM - Frequency Division Multiplexing
• TDM – Time Division Multiplexing
Three phases are need to communicate two
parties or multiple parties in a conference call):
• Connection setup
• data transfer
• Connection teardown
• The setup phase: means creating dedicated channels
between the switches.

• Data Transfer Phase: After the establishment of the

dedicated circuit (channels), the two parties can
transfer data.

• Teardown Phase: When one of the parties needs to

disconnect, a signal is sent to each switch to release
the resources.
A trivial circuit-switched network
• Example : when system A needs to connect to system M:
• A sends a setup request that includes the address of system M, to
switch I. Switch I finds a channel between itself and switch IV.
Switch I then sends the request to switch IV, which finds a
dedicated channel between itself and switch III. Switch III informs
system M of system A's intention at this time.

• An acknowledgment from system M needs to be sent in the

opposite direction to system A.

• After system A receives this acknowledgment the connection

established.
 Circuit-switched network
• Circuit switching takes place at the physical layer

• Data transferred between the two stations are not

packetized. The data are a continuous flow sent by the source
station and received by the destination station

• There is no addressing involved during data transfer. Of

course, there is end-to-end addressing used during the setup
phase.
 Efficiency
• circuit-switched networks are not so efficient as
the resources are allocated during the entire
duration of the connection.

• These resources are unavailable to other

connections.
Delay
2. PACKET SWITCHED NETWORK
• Messages need to be divided into packets.
• Size of the packet is determined by the network and the governing
protocol.
• no resource reservation, but allocated on demand.

• The allocation is done first come, first served based

• When a switch receives a packets , the packet must wait if there are
other packets being processed, this lack of reservation may create
delay
 DATAGRAM NETWORKS
• Each packet (called as datagrams in this approach) is treated
independently of all others
• All packets (or datagrams) belong to the same message may
travel different paths to reach their destination .
• Datagram Switching is done at the network layer
• This approach can cause the datagrams of a transmission to arrive
at their destination out of order with different delays between the
packets.
• Packets may also be lost or dropped because of a lack of resources.
• In most protocols, it is the responsibility of an upper- layer
protocol to reorder the datagrams or ask for lost datagrams before
passing them on to the application.
• The datagram networks are referred to connectionless networks.
There are no setup or teardown phases.
• How are the packets routed to their destination??
Routing table in a datagram network
• Each packet switch has a routing table
which is based on the destination address.
• The routing tables are dynamic and are updated
periodically.
• The destination addresses and the corresponding
forwarding output ports are recorded in the tables.
• The destination address in the header of a packet
in a datagram network remains the same during
the entire journey of the packet.
• When the switch receives the packet, this
destination address is examined; the routing table is
consulted to find the corresponding port through
which the packet should be forwarded.
 Efficiency
• Better than that of a circuit-switched network.
• Resources are allocated only when there are packets to be
transferred. If a source sends a packet and there is a delay of a few
minutes before another packet can be sent, the resources can be
reallocated during these minutes for other packets from other
sources.
• Switching in the Internet is done by using the datagram approach to
packet switching at the network layer
 Delay in a datagram network

• Total delay =3T + 3t + WI + W2

T: transmission times t: propagation delays
(WI + W2) : waiting times
2.2 VIRTUAL-CIRCUIT NETWORKS
• It’s a cross between circuit
switched network and
datagram network, and has some
characteristics of both.
 Characteristics:
 Packets from a single message travel along the same path.
 Three phases to transfer data (set up, data transfer and tear
down)
 Resources can be allocated during setup phase
 Data are packetized and each packet carries an address in the
header
 Implemented in data link layer
Virtual-circuit network Addressing
 Global addressing:
- Source and destination needs unique addresses (used by the switches
only to create a virtual-circuit identifier ) during the set up phase
• Local addressing (virtual-circuit identifier –VCI):
- Actually used for data transfer
- A small address used by a
frame between two switches.
 Setup request

• The switch, in the setup phase acts as a packet switch ; it has

a routing table used to know the output port number
Setup acknowledgement
• When Destination B receives the up frame , and it is ready to
receive frames from A, it assign a VCI (in this case :77). This VCI lets
the destination know that the frames comes from A not other
sources.
Data Transfer and Tear down Phase

• After sending all frames, a special frame is send to end the connection
• Destination B responds with a teardown confirmation frame
Delay in Virtual Circuit switching
• In datagram network, each
packet may experience a wait at
a switch before it is forwarded.
In addition, the delay is not
uniform for the packets of a
message.
• In a virtual-circuit network,
there is a one- time delay for
setup and a one-time delay for
teardown. If resources are
allocated during the setup
Total delay = 3T+ 3t +setup delay + teardown delay phase, there is no wait time for
individual packets.
Thank You!!!!