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Ref. Page Chapter 17: Data Communications and Computer Networks Slide 1/57

Computer Fundamentals: Pradeep K. Sinha & Priti Sinha
In this chapter you will learn about:
 Basic elements of a communication system
 Techniques, channels, and devices used to transmit
data between distant locations
 Types of computer networks
 Communication protocols and their use in computer
networks
 Internetworking tools and their use in building large
computer networks
 Characteristics and advantages of distributed data
processing
Ref. Page 320 Chapter 17: Data Communications and Computer Networks Slide 2/57
Learning Objectives

Sender
(source)
Receiver
(sink)
Medium
Carries the message
Creates and sends
a message
Receives the
message
Basic Elements of a Communication
System

Data Transmission Modes
Sender Receiver
(a) Simplex
Sender
(or Receiver)
Receiver
(or Sender)
OR
(b) Half-duplex
Sender
(and Receiver)
Receiver
(and Sender)
AND
(c) Full-duplex

 Bandwidth: Range of frequencies available for data
transmission. It refers to data transmission rate. Higher
the bandwidth, the more data it can transmit
 Baud: Unit of measurement of data transfer rate.
Measured in bits per second (bps)
Data Transmission Speed

 Narrowband: Sub-voice grade channels in range from
45 to 300 baud. Mainly used for telegraph lines and
low-speed terminals
 Voiceband: Voice grade channels with speed up to
9600 baud. Mainly used for ordinary telephone voice
communication and slow I/O devices
 Broadband: High speed channels with speed up to 1
million baud or more. Mainly used for high-speed
computer-to-computer communication or for
simultaneous transmission of data
Data Transmission Speed Category

The most commonly used ones are:
 Twisted-pair wire (UTP cable)
 Coaxial cable
 Microwave system
 Communications satellite
 Optical fibers
Data Transmission Media

Unshielded Twisted-Pair (UTP)
Cable

Coaxial Cable
Central copper wire
PVC insulation
Copper mesh
Outer PVC shield

Microwave Communication System
Transmitting
station
Receiving
station
In between
repeaters
Transmitting antennas Receiving
antennas
Line of sight Line of sight Line of sight

Satellite Communication System
Satellite in space
Transmitting
station on earth
6 GHz 4 GHz
Uplink Downlink
Receiving
station on earth

Optical Fiber Communication System
Electrical
signal
Optical fiber
Amplifier
Electrical
signal
Light to
electrical wave
converter
Electrical to
light wave
converter
Light waves
Sender
Receiver

Digital and Analog Data Transmission
 Analog signal: Transmitted power varies over a
continuous range. Example: sound, light, and radio
waves
 Digital signal: Sequence of voltage pulses represented
in binary form
 Computer generated data signal is digital, whereas
telephone lines carry analog signals
(Continued on next slide)

(Continued from previous slide)
 When digital data is to be sent over an analog facility,
digital signals must be converted to analog form
 Conversion of digital signal to analog form is known as
modulation
 Conversion of analog signal to digital form is known as
demodulation
 Digital transmission of data is preferred over analog
transmission of data due to lower cost, higher
transmission speeds, and lower error rate
Digital and Analog Data Transmission

Analog and Digital Signals
v
t
0
1 1 1 1
0 0 0
(b) Digital signal
-v
0 1/f 2/f
(a) Analog signal
Voltage
+v
t

Modulation Techniques
 Amplitude Modulation (AM): Two binary values (0 and
1) of digital data are represented by two different
amplitudes of the carrier signal, keeping frequency and
phase constant
 Frequency Modulation (FM): Two binary values of
digital data are represented by two different frequencies,
while amplitude and phase are kept constant
 Phase Modulation (PM): Two binary values of digital
data are represented by shift in phase of carrier signal

Modems
 Modem is short for MOdulator/DEModulator
 Special device used for conversion of digital data to
analog form (modulation) and vice-versa (demodulation)
 Essential piece of hardware where two digital devices
(say two computers) want to communicate over an
analog transmission channel (say a telephone line)

Use of Modems in Data Communications
Sender
Computer
Receiver
0 1 1 0
0 0
Computer
1 0 0
Demodulator 0 1 1 0
Digital signals
Digital signals
Analog signals on
telephone line
Modulator
Modulator
1 0 0 Demodulator
modem at
sender
computer end
modem at
receiver
computer end

Factors for Modem Selection
 Transmission speed
 Internal versus external
 Facsimile facility

Data Transmission Services
 Data transmission service providers are popularly
known as common carriers
 Various types of services offered by common carriers
are:
 Dial-up line: Operates in a manner similar to a
telephone line
 Leased line: Special conditioned telephone line
that directly and permanently connects two
computers
 Integrated Services Digital Network (ISDN):
Telephone system that provides digital (not analog)
telephone and data services

 Value Added Network (VAN): Provides value-added
data transmission service. Value added over and
above the standard services of common carriers may
include e-mail, data encryption/decryption, access to
commercial databases, and code conversion for
communication between computers
Data Transmission Services

Multiplexing
 Method of dividing physical channel into many logical
channels so that a number of independent signals may
be simultaneously transmitted
 Electronic device that performs multiplexing is known
as a multiplexer
 Multiplexing enables a single transmission medium to
concurrently transmit data between several
transmitters and receivers

Two Basic Methods of Multiplexing
 Frequency-Division Multiplexing (FDM): Available
bandwidth of a physical medium is divided into several
smaller, disjoint logical bandwidths. Each component
bandwidth is used as a separate communication line
 Time-Division Multiplexing (TDM): Total time
available in a channel is divided among several users,
and each user of the channel is allotted a time slice
during which he/she may transmit a message

40 KHz
50 KHz
60 KHz
70 KHz
80 KHz
Signal-1
Signal-2
Signal-3
Signal-4
Signal-5
Channel
40 KHz
50 KHz
60 KHz
70 KHz
80 KHz
Signal-1
Signal-2
Signal-3
Signal-4
Signal-5
Sending end Receiving end
Modulator Demodulator
Frequency-Division Multiplexing
Frequency-Division Multiplexing

A3 A2 A1
B3 B2 B1
C3 C2 C1
Signal
A
Signal
B
Signal
C
C3 C2 C1
B3 B2 B1
A3 A2 A1
Channel
… C2 B2 A2 C1 B1 A1
Time sliced
signals
Reassembled
signals
Sending
end
Receiving
end
Demulti-
plexer
Time-Division Multiplexing

Asynchronous and Synchronous
Transmission
 Two modes of data transmission on a communication
line are asynchronous and synchronous
 Asynchronous transmission
 Sender can send data at any convenient time and
the receiver will accept it
 Data is transmitted character by character at
irregular intervals
 Well suited to many keyboard type terminals

 Synchronous transmission
 Sender and receiver must synchronize with each
other to get ready for data transmission before it
takes place
 Entire blocks of characters are framed and
transmitted together
 Well suited to remote communication between a
computer and such devices as buffered terminals
and printers
Asynchronous and Synchronous
Transmission

Irregular time intervals
between two characters
Each character framed by
start and stop bits
Character Character Character
(a) Asynchronous transmission
Char Char Char Char Char Char
Indefinite time interval
between two blocks of data
A block of characters may
consist of hundreds of
characters
Trailer containing end of block
indication
Header containing synchronizing
and other information
(b) Synchronous transmission
Data Transmission

Switching Techniques
 Data is often transmitted from source to destination
through a network of intermediate nodes
 Switching techniques deal with the methods of
establishing communication links between the sender
and receiver in a communication network
 Three commonly used switching techniques are:
 Circuit switching: Dedicated physical path is
established between sending and receiving stations
through nodes of the network for the duration of
communication

 Message switching: Sender appends receiver’s
destination address to the message and it is
transmitted from source to destination either by
store-and-forward method or broadcast method
 Packet switching: Message is split up into fixed size
packets and each packet is transmitted independently
from source to destination node. Either store-and-
forward or broadcast method is used for transmitting
the packets. All the packets of a message are re-
assembled into original message at the destination
node
Switching Techniques

Source
node
Destination
node
Dotted line
indicates
establishment of
physical path
Switching
nodes
Circuit Switching Method

Store-and-Forward Method of
Message Switching
C
D
A
B
C
1
3
2
4
5
Either path 1-2-3-4 or 1-5-4 may be used to
transmit a message from A to B.

1 2 3
Message
Broadcast Channel
Nodes n
Broadcast Method of Message
Switching

Routing Techniques
 In a WAN, when multiple paths exist between the source
and destination nodes of a packet, any one of the paths
may be used to transfer the packet
 Selection of path to be used for transmitting a packet is
determined by the routing technique used
 Two popularly used routing algorithms are:
 Source routing: Source node selects the entire path
before sending the packet
 Hop-by-hop routing: Each node along the path
decides only the next node for the path

Network Topologies
 Term network topology refers to the way in which
the nodes of a network are linked together
 Although number network topologies are possible,
four major ones are:
 Star network
 Ring network
 Completely connected network
 Multi-access bus network

Host
Node
Star Network

Ring Network

Completely Connected Network

Computers (nodes)
Single communication line shared by all nodes
Multi-Access Bus Network

Ring
Star
Completely
connected
Hybrid Network

Network Types
 Networks are broadly classified into two types: Local
Area Network (LAN) and Wide Area Network (WAN)
 Local Area Network (LAN) as compared to WAN:
 Limited to a small geographic coverage
 Has much higher data transmission rate
 Experiences fewer data transmission errors
 Has lower data communication cost
 Typically owned by a single organization
 Networks that share some of the characteristics of both
LANs and WANs are referred to as Metropolitan Area
Network (MAN)

Communication Protocols
 Protocol is a set of formal operating rules, procedures,
or conventions that govern a given process
 Communication protocol describes rules that govern
transmission of data over communication networks
 Roles of communication protocol:
 Data sequencing
 Data routing
 Data formatting
 Flow control
 Error control

Communication Protocols
 Precedence and order of transmission
 Connection establishment and termination
 Data security
 Log information.
 Communication protocols are normally split up into a
series of modules logically composed of a succession of
layers.
 Terms protocol suite, protocol family, or protocol stack
are used to refer to the collection of protocols (of all
layers) of a network system

Network Interface Card (NIC)
 Hardware device that allows a computer to be
connected to a network, both functionally and
physically
 Printed circuit board installed on to one of the
expansion slots of computer
 Provides a port on the back to which network cable is
attached

The OSI Model
 The Open System Interconnection (OSI) model is
framework for defining standards for linking
heterogeneous computers in a packet switched
network
 Standardized OSI protocol makes it possible for any
two heterogeneous computer systems, located
anywhere in the world, to easily communicate with
each other
 Separate set of protocols is defined for each layer in its
seven-layer architecture. Each layer has an
independent function

Network
Layer 7
(application)
Layer 6
(presentation)
Layer 5
(session)
Layer 4
(transport)
Layer 3
(network)
Layer 2
(data link)
Layer 1
(physical)
Application protocol
Presentation protocol
Session protocol
Transport protocol
Network protocol
Data-link protocol
Physical protocol
Layer 7
(application)
Layer 6
(presentation)
Layer 5
(session)
Layer 4
(transport)
Layer 3
(network)
Layer 2
(data link)
Layer 1
(physical)
Process A Process B
Node 1 Node 2
Interface
Interface
Interface
Interface
Interface
Interface Interface
Interface
Interface
Interface
Interface
Interface
Layers, Interfaces, and Protocols
in the OSI Model

An example illustrating transfer of message M from sending node to the
receiving node in the OSI model: Hn, header added by layer n:Tn, trailer
added by layer n.
7
H7 M
H6 H M
H5 H6 H
7
M
Sending node
H4 H5 H6 H7 M1 5 7
H4 H H6 H M2
H2 H3 H4 H5 H6 H7 M
1
T2 5 2
H2 H3 H4 H H6 H7 M T2
H
3
H4 H5 H6 H
7
M2
H3 H4 H5 H6 H7 M1
Process A
H7 M
H6 H7 M
H5 H6 H7 M
Receiving node
H4 H5 H6 H7 M1 H4 H5 H6 H7 M2
H2 H3 H4 H5 H6 H7 M1 T2 2
H2 H3 H4 H5 H6 H7 M T2
5
H3 H4 H H6 H7 M2
6
H3 H4 H5 H H7 M1
Process B

Internetworking
 Interconnecting two or more networks to form a single
network is called internetworking, and the resulting
network is called an internetwork
 Goal of internetworking is to hide details of different
physical networks, so that resulting internetwork
functions as a single coordinated unit
 Tools such as bridges, routers, brouters, and gateways
are used for internetworking
 The Internet is the best example of an internetwork

Bridges
 Operate at bottom two layers of the OSI model
 Connect networks that use the same communication
protocols above data-link layer but may use different
protocols at physical and data-link layers

Routers
 Operates at network layer of the OSI model
 Used to interconnect those networks that use the same
high-level protocols above network layer
 Smarter than bridges as they not only copy data from
one network segment to another, but also choose the
best route for the data by using routing table

Gateways
 Operates at the top three layers of the OSI model
(session, presentation and application)
 Used for interconnecting dissimilar networks that use
different communication protocols
 Since gateways interconnect dissimilar networks,
protocol conversion is the major job performed by
them

Wireless Computing Systems
 Wireless computing system uses wireless communication
technologies for interconnecting computer systems
 Enhances functionality of computing equipment by freeing
communication from location constraints of wired
computing systems
 Wireless computing systems are of two types:
 Fixed wireless systems: Support little or no
mobility of the computing equipment associated with
the wireless network
 Mobile wireless systems: Support mobility of the
computing equipment to access resources associated
with the wireless network

 2G and 3G
 Wireless LAN
 WiMAX
 Wireless Local Loop (WLL)
 Radio-router
 Multihop Wireless Network
 Wireless Application Protocol (WAP)
Wireless Technologies

Distributed Computing Systems
 Configuration where many independent computer
systems are connected, and messages, processing task,
programs, data, and other resources are transmitted
between cooperating computer systems
 Such an arrangement enables sharing of many
hardware and software resources as well as information
among several users who may be sitting far away from
each other

Main Advantages of Distributed
Computing Systems
 Inherently distributed applications
 Information sharing among distributed users
 Resource sharing
 Shorter response times and higher throughput
 Higher reliability
 Extensibility and incremental growth
 Better flexibility in meeting users’ needs

 Amplifier
 Amplitude Modulation (AM)
 Application layer
 ARPANET
 Asynchronous transmission
 Bandwidth
 Baud
 Bridge
 Broadband
 Broadcast
 C-band transmission
 Circuit switching
 Coaxial cable
 Common Carriers
 Communication protocol
 Communications satellite
 Completely connected network
 Computer network
 Concentrators
 Data-link layer
 Demodulation
 Dial-up line
 Distributed Computing System
 Ethernet
 Fax modem
 File Transfer Protocol (FTP)
 Font-End Processors (FEP)
 Frequency Modulation (FM)
 Frequency-Division Multiplexing (FDM)
 Full duplex
 Gateway
 Half duplex
 Hop-by-hop routing
 Hybrid network
 Internet Protocol (IP)
 Internetworking
 ISDN (Integrated Services Digital Network)
 Ku-band transmission
 Leased line
 Local Area Network (LAN)
 Message switching
Keywords/Phrases

 Metropolitan Area Network (MAN)
 Microwave system
 Mobile computing
 Modem
 Modulation
 Multi-access Bus network
 Multiplexer
 Narrowband
 Network Interface Card (NIC)
 Network layer
 Network topology
 Nomadic computing
 Optical fibers
 OSI Model
 Packet switching
 Phase Modulation (PM)
 Physical layer
 POTS (Plain Old Telephone Service)
 Presentation layer
 Protocol family
 Protocol stack
 Protocol suite
 Repeater
 Ring network
 Router
 Session layer
 Simplex
 Source routing
 Star network
 Store-and-forward
 Synchronous transmission
 Time-Division Multiplexing (TDM)
 Transport Control Protocol (TCP)
 Transport layer
 Twisted-pair
 Unshielded twisted-pair (UTP)
 User Datagram Protocol (UDP)
 Value Added Network (VAN)
 Voiceband
 VSAT (Very Small Aperture Terminals)
 Wide Area Network (WAN)
 Wireless network
Keywords/Phrases

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