Chapter 1: Introduction
Introduction
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A computer network allows computers/ devices to
exchange data.
Networked devices pass data to each other along data
connections (network links).
The information that needs to sent is converted into digital
form.
Data bits are transferred in the form of blocks/ packets.
The best-known computer network is the Internet.
Landline phone network system is an example of voice
communication network (Telephone Network)
Mobile / Cellular Phone Network
Computer Network
• A computer network consists of a set of
nodes, a set of communication links, and a
set of communication protocols needed to
facilitate the communication between the
nodes.
• Nodes: Computers; Phones; Smart Phones;
Printer; Any device capable of being
networked
• Links: Cables; Optical Fibers; Wireless
Example Network: Graphical View
Circles: Nodes
Lines : Links (or connectivity information)
Networks
• Nodes:
– Stationary (not moving)
– Mobile
• Links: aka Channels
– Wired
– Wireless (without wires)
• Air
• Water
Five components of data communication
1.6
Data Comm System Components
• Message: -> information (data) to be communicated.
Consists of text, numbers, sound, or video – or any
combination of these.
• Sender: -> device that sends the data message. Can be a
computer, telephone handset, video camera, etc.
• Receiver: -> device that receives the message. Can be
computer, telephone handset, TV, etc.
• Transmission Medium: -> physical path by which a
message travels from sender to receiver. Can be coaxial
cable, optical fiber, wireless, etc.
• Protocol: -> a set of rules that govern data
communications. Represents an agreement between
communicating devices. Without a protocol, two
devices may be connected but not communicating
(intelligibly).
Data Representation
• Data can be represented in different forms such as:
– Text
– Numbers
– Images
– Audio
– Video
Data needs to be converted into binary form in
order to transport it over a computer network.
Example: Text Data Representation
• In data communication, text is represented as a bit
pattern, a sequence of bits (0s or 1s).
• Binary representations for A-Z, a-z, 0-9,
!,?$%..etc.
• Different sets of bit patterns have been designed to
represent text symbols. Each set is called a code.
Some text codes are
– ASCII (8-bit code, with MSB (Most Significant Bit)
always set at 0)
– Extended ASCII (8-bit code)
– Unicode (8-, 16-, and 32-bit codes (UTF-8, UTF-16,
and UTF-32, respectively))
–
Example
• How many different text symbols can be
represented by
(a) Extended ASCII code (b) UTF-32 ?
• Solution:
(a) 28 = 256
(b) 232 = 4 G = 4.29 x 109
Direction of Data Flow- Simplex
•In simplex mode, the communication is
unidirectional (one way). Only one of the
two stations on a link can transmit; the
other can only receive.
–e.g. keyboards (can only introduce input),
monitors (can accept output only), Traditional
TV
Direction of Data Flow- Half-Duplex
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Each station can both transmit and receive, but
not at the same time. When one device is sending,
the other can only receive, and vice versa.
In a half-duplex transmission, the entire capacity
of channel is used by a device
Example: Walkie-talkie, CB (Citizen Band) radio
Direction of Data Flow- Full-duplex
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In full-duplex mode, both stations can transmit and
receive simultaneously
It is like a 2-way street with traffic flowing in both
directions at the same time
Signals going in either direction share the capacity of the
link. This sharing can occur in two ways: either the link
must contain two physically separate transmission paths,
one for sending and other for receiving, or the capacity of
the channel is divided between signals traveling in
opposite directions
Example: Telephone network (Two persons can talk and
listen at the same time)
Network
• A network is a set of devices (also called
nodes) connected by media links.
• A node can be computer, printer, or any
other device capable of sending and/or
receiving data generated by other nodes on
the network.
• The links connecting the devices are called
“communication channels”.
Physical Structure
A station can be connected
to a communication link in
two ways:
1.Point-to-Point
2.Multipoint (Multidrop)
Types of Connections
• A point-to-point connection provides a dedicated
link between two devices. The entire capacity of
the channel is reserved for transmission between
those two devices.
• Mutipoint (or multidrop) connection
– More than two devices share a single link.
– The capacity of the channel is shared, either spatially or
temporally.
– If several devices can use the link simultaneously, it is a
“spatially shared” connection.
– If users must take turn, it is time-shared (or temporal)
connection.
Types of connections: point-to-point and multipoint
1.17
Physical Topology
• Topology defines the physical arrangement
of links in a network.
• It refers to the way a network is laid out
physically.
• It is the geometric representation of the
relationship of all the links and the linking
devices (also called nodes) to each other.
• Two or more devices connect to a link; two
or more links form a topology.
Categories of topology
1.19
Mesh Topology
• Every device has a dedicated point-to-point
link to every other device.
• A fully connected mesh network has
n*(n-1)/2 physical channels (links) to link
‘n’ devices, assuming each physical link
allows communication in both directions
(full-duplex mode)
• Every device on the network must have
(n-1) input/output (I/O) ports.
A fully connected mesh topology (five devices)
1.21
Mesh Topology
• Major Advantages:
– Use of dedicated links guarantees that each
connection can carry its own data, thus
eliminating the traffic problems that can occur
when the links must be shared by multiple
devices.
– Mesh topology is robust. If one link becomes
unusable, the whole system does not fail
– Privacy and security (as dedicated lines used)
• Main Disadvantage: Amount of cabling (~
no. of links) and no. of I/O ports required
Star Topology
• Each device (or node or host) has a dedicated
point-to-point link only to a central controller,
which is a “hub” or a “switch”.
• It does not allow direct traffic between devices.
The controller acts as an exchange: If one device
wants to send data to another, it sends the data to
the controller, which then relays the data to the
other connected device.
• Hub sends the incoming message to all other
connected nodes; while the switch sends the
incoming message only to the intended destination
node.
Example Star Topology
Star Topology
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Advantages:
Each device needs only
one link and one I/O
port.
Easy to install and
reconfigure.
Small amount of cabling.
Robust; if one link fails,
other links remain active
Disadvantages:
• The hub (or switch)
is the bottleneck.
If hub goes down,
the whole network
is dead
• The hub (or switch)
is a single-pointof-failure
Star Topology (contd.)
• Although a star requires far less cable than a
mesh, each node must be linked to a central
hub.
• Star topology is used in local-area network
(LANs).
Example LAN with a Switch
Bus Topology
•Mutipoint topology
•All devices are attached to one long cable (backbone).
•Nodes are connected to the bus by drop lines and taps.
•Drop line: a cable running from the device to the bus.
•A tap is a connector that creates a contact with the
metallic core of the cable.
A bus topology connecting three stations
1.29
Bus Topology
• As the signal travels along the backbobne, some of its
energy is transformed into heat. Thus, it becomes weaker and
weaker as it travels farther and farther. Therefore, there is
a limit on the number of taps a bus can support and on the
distance between those taps.
Advantages:
• Ease of installation.
• Less cabling than mesh, or star.
Disadvantages:
• Difficult reconfiguration and fault isolation. Adding new
devices may require modification or replacement of the
backbone.
• Signal reflection at the taps can cause degradation.
• The bus is a single point-of-failure
Bus Topology (contd.)
• Bus topology was one of the first topologies
used in the design of early local-area
networks (LANs).
• Ethernet LANs can use a bus topology, but
they are less popular now.
Ring Topology
• Each device has a dedicated point-to-point line
configuration only with the two devices on either side
of it.
• The signal is passed along the ring in one direction,
from device to device, until it reaches the
destination (Unidirectional traffic).
• Each device incorporates a repeater. When a device
receives a signal intended for another device, its
repeater regenerates the bits and passes them
along
A ring topology connecting six stations
1.33
Ring Topology
Disadvantages:
• Unidirectional
traffic.
• The ring is a
single point-offailure (break in
the ring, disabled
station)
Advantages:
• Easy to install and
reconfigure. To
add or delete a
device requires
changing only two
connections
• Simple fault
isolation.
Categories of networks
Categories of Networks
Classification based on:
1. Size
3. Distance covered
2. Ownership
4. Physical architecture
Local Area Networks (LANs)
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Privately-owned networks within a single building or
campus of up to few kilometers in size.
• Characteristics:
1.
Restricted in size
2.
Transmission technology usually consists
of a single cable
3.
Topologies: bus, ring, and star
LANs
• LANs are designed to allow resources to be
shared between PCs or workstations. The
resources to be shared can include hardware
(e.g., a color printer), software (e.g., an
application program), or data.
• Early LANs had data rates ~ 4—16 Mbps
(Mega bits per second) range; while, today,
the speeds are normally 100 or 1000 Mbps.
• Wireless LANs are the newest trends in
LAN technology
Wide Area Network (WAN)
• Provides long-distance transmission of data, voice,
image, and video information over a large area,
often a country or continent.
• Utilizes public, leased, or private communication
devices.
• Enterprise Network: a WAN that is wholly owned
and used by a single company.
Metropolitan Area Network (MAN)
• MAN is a network with a size between a LAN and
a WAN.
• Covers larger area than LAN such as a city.
• It may be a single network or it may be used to
connect a number of LANs.
• MAN is designed for customers who need a highspeed connectivity, normally to the Internet, and
have endpoints spread over a city.
• It may be owned and operated privately, or it may
be service provided by a public company.
Example MAN
MAN
• MAN is designed for customers who need a
high-speed connectivity, normally to the
Internet, and have endpoints spread over a
city or part of city.
• Example: Cable TV network that was
originally designed for cable TV, but today
can also be used for high-speed data
connection to the Internet
Internetworks
• When two or more networks are connected,
they become an internetwork, or internet.