Unit 1

Introduction to Computer
Networks

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 Introduction
• Each of the past three centuries was dominated by a single new technology.
• The 18th century was the era of the great mechanical systems accompanying the
Industrial Revolution.
• The 19th century was the age of the steam engine.
• During the 20th century, the key technology was information gathering,
processing, and distribution.
• Among other developments, we saw the installation of worldwide telephone
networks, the invention of radio and television, the birth and unprecedented
growth of the computer industry, the launching of communication satellites, and,
of course, the Internet.
• As a result of rapid technological progress, these areas are rapidly converging in
the 21st century and the differences between collecting, transporting, storing,
and processing information are quickly disappearing. MKK
• The old model of a single computer serving all of the organization’s
computational needs has been replaced by one in which a large number of
separate but interconnected computers do the job. These systems are
called computer networks.
• Computer network means a collection of autonomous computers
interconnected by a single technology.
• Two computers are said to be interconnected if they are able to exchange
information.
• The computers on a network may be linked through cables, telephone
links, radio waves, satellites, or infrared light beams.
• Devices in a Network are connected using wired or wireless transmission
media such as cable or air.
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Basic Set of Rules for Computer Network
• Information must be delivered reliably
• Information must be delivered consistently [unchanging]
• Multiple computers must be able to identity each other
Benefits of computer network:
• Network can increase efficiency
• It can help Standardize Policies, Procedures and Processes
• It Ensures Information consistency and Integrity
• It Ensures information security MKK
Uses of network
• Simultaneous access to data
Ex: data files, Software, hardware resources
• File Server contain documents which are used by other computers
• Personal communications
Ex: email, conferencing, voice over IP
• Easy data backup

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Applications of Networks
• Resource Sharing
 Hardware (computing resources, disks, printers)
 Software (application software)
• Information Sharing
 Easy accessibility from anywhere (files, databases)
 Search Capability (WWW)
• Communication
 Email
 Message broadcast
• Remote computing
• Distributed processing (GRID Computing) MKK
Network Topologies
• Logical layout of wires and equipment is called topology.
Topology: Shape of the network

• The way in which devices are interconnected to form a network is

called network topology.
• Some of the factors that affect choice of topology for a network are
 Cost  Reliability

 Scalability  Ease of installation

 Flexibility  Ease of maintenance MKK

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Bus Topology
• It is also called linear bus
• One wire connects all nodes
Advantages:
• Easy to setup
• Require less hardware
• Low cost
Disadvantages:
• Slow
• Easy to crash
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• If one system effected remaining systems cannot communicate with each other
Star Topology
• A star topology is a topology for a
Local Area Network (LAN) in which server
is connected to each node individually.
• Server is also called the central node.
• Any exchange of data between two nodes
must take place through the server.
Advantages
• Failure of one node does not affect the network
• Troubleshooting is easy as faulty node can be detected from central node
immediately
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Ring Topology
• In ring topology each terminal is connected to
exactly two nodes, giving the network a
circular shape.
• Data travels in only one pre-determined direction.
Advantages:
• Very high transmission speeds possible
• Small cable segments are needed to connect two
nodes
Disadvantages :
• Failure of single node brings down the whole network
• Troubleshooting is difficult as many nodes may have to be inspected before faulty one is
identified
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• Difficult to remove one or more nodes while keeping the rest of the network intact
Tree Topology
• Tree topology has a group of star networks
connected to a linear bus backbone cable.
• It incorporates features of both star and bus
topologies.
• Tree topology is also called hierarchical topology.
Advantages
• Existing network can be easily expanded
• Easier installation and maintenance
• Well suited for temporary networks
Disadvantages
• Failure of backbone cable brings down entire network
• Insecure network
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• Maintenance difficult for large networks
Mesh Topology
• In this type of topology, a host is connected to one or multiple hosts.
• This topology has hosts in point-to-point connection with every other host
• It provides the most reliable network structure among all network topologies.

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Hybrid Topology
It is the combination of two or more
different topologies.
Advantages:
• It is extremely flexible.
• It is very reliable.
• Error detecting and troubleshooting
are easy.
• Handles a large volume of traffic.
• It is used to create large networks.
Disadvantages:
• It is a type of network expensive.
• The design of a hybrid network is very complex. MKK
• There is a change in the hardware to connect one topology with another topology.
 Network Hardware
Network Hardware can classify either by transmission technology
or by scale
• There are two types of transmission technology that are in widespread use:
broadcast links and point-to-point links.
• Point-to-point transmission with exactly one sender and exactly one receiver is
sometimes called unicasting.
• In contrast, on a broadcast network, the communication channel is shared by all the machines on the network; packets sent
by any machine are received by all the others. An address field within each packet specifies the intended recipient. Upon
receiving a packet, a machine checks the address field. If the packet is intended for the receiving machine, that machine
processes the packet; if the packet is intended for some other machine, it is just ignored.
• Broadcast systems usually allow the possibility of addressing a packet to all
destinations by using a special code in the address field. This mode of operation
is called broadcasting.
• Some broadcast systems also support transmission to a subset of the machines,
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which known as multicasting.
• An alternative criterion for classifying networks is by scale.
• Distance is important as a classification metric because different technologies are used
at different scales.

Classification of interconnected processors by scale.

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Personal Area Networks
• PANs (Personal Area Networks) let devices communicate over the range of a person. A
common example is a wireless network that connects a computer with its peripherals.
• Almost every computer has an attached monitor, keyboard, mouse, and printer. Without
using wireless, this connection must be done with cables.
• Also short-range wireless network called Bluetooth
to connect these components without wires.
– Bluetooth networks use the master-slave paradigm

Bluetooth PAN configuration

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Local Area Networks
• A LAN is a privately owned network that operates within and nearby a single building
like a home, office or factory.
• When LANs are used by companies, they are called enterprise networks.
• Wireless LANs are very popular these days where it is too much trouble to install cables.
• In these systems, every computer has a radio modem and an antenna that it uses to
communicate with other computers.
• This device, called an AP (Access Point), wireless router, or base station, relays packets
between the wireless computers and also between them and the Internet.
• There is a standard for wireless LANs called IEEE 802.11, popularly known as WiFi.
• It runs at speeds anywhere from 11 to hundreds of Mbps.
• Wired LANs use a range of different transmission technologies like copper wires, optical
fiber etc., and LANs are restricted in size
• The topology of many wired LANs is built from point-to-point links. IEEE 802.3, popularly
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called Ethernet
Features
• Local Area Network is a group of computers connected to each other in a small area
such as building, office.
• LAN is used for connecting two or more personal computers through a communication
medium such as twisted pair, coaxial cable, etc.
• It is less costly as it is built with inexpensive hardware such as hubs, network adapters,
and ethernet cables.
• The data is transferred at an extremely faster rate in Local Area Network.
• Local Area Network provides higher security.

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Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet.

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Metropolitan Area Networks
A MAN (Metropolitan Area Network) covers a city.
• Design to extend over a large area.
• Connecting number of LAN's to form larger network, so that resources can be shared.
• Networks can be up to 5 to 50 km.
• Owned by organization or individual.
• Data transfer rate is low compare to LAN.
Example: Organization with different branches
located in the city.

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Uses Of Metropolitan Area Network:
• MAN is used in communication between the banks in a city.
• It can be used in an Airline Reservation.
• It can be used in a college within a city.
• It can also be used for communication in the military.

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Wide Area Networks
• A WAN (Wide Area Network) spans a large geographical area, often a country or
continent
• The network that connects these hosts is then called the communication subnet, or just
subnet for short.
• The job of the subnet is to carry messages from host to host
• In most WANs, the subnet consists of two distinct components: transmission lines and
switching elements. Transmission lines move bits between machines.
• Switching elements, or just switches, are specialized computers that connect two or
more transmission lines.
– When data arrive on an incoming line, the switching element must choose an outgoing line on which
to forward them.
– These switching computers have been called by the name router (Unfortunately, some people
pronounce it ‘‘rooter’’)
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Originally subnet is the collection of routers and communication lines that moves packets from the source host to the
destination host.
• In a WAN, the hosts and subnet are owned and operated by different people.
• The WAN in Fig. is a network that connects offices in Perth, Melbourne, and Brisbane.
Each of these offices contains computers intended for running user (i.e., application)
programs.

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WAN that connects three branch offices in Australia.
Features of WAN
• A Wide Area Network is a network that extends over a large geographical area such as
states or countries.
• A Wide Area Network is quite bigger network than the LAN.
• A Wide Area Network is not limited to a single location, but it spans over a large
geographical area through a telephone line, fibre optic cable or satellite links.
• The internet is one of the biggest WAN in the world.
• A Wide Area Network is widely used in the field of Business, government, and
education.

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 NETWORK SOFTWARE
• The first computer networks were designed with the hardware as the main concern and
the software as an afterthought.
• This strategy no longer works. Network software is now highly structured.
Functions of Network Software
• Helps to set up and install computer networks
• Enables users to have access to network resources in a seamless manner
• Allows administrations to add or remove users from the network
• Helps to define locations of data storage and allows users to access that data
• Helps administrators and security system to protect the network from data breaches,
unauthorized access and attacks on a network
• Enables network virtualizations
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Protocol Hierarchies
• To reduce their design complexity, most networks are organized as a stack of layers or
levels, each one built upon the one below it.
• The purpose of each layer is to offer certain services to the higher layers
• When layer n on one machine carries on a conversation with layer n on another
machine, the rules and conventions used in this conversation are collectively known as
the layer n protocol.
• Basically, a protocol is an agreement between the communicating parties on how
communication is to proceed.
• A five-layer network is illustrated in the following Fig. The entities comprising the
corresponding layers on different machines are called peers.

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 Layers, protocols, and interfaces.

• In reality, no data are directly transferred from layer n on one machine to layer n on another machine.
• Below layer 1 is the physical medium through which actual communication occurs.
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Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services
the lower layer makes available to the upper one.
Design Issues for the Layers
• Reliability: is the design issue of making a network that operates correctly even though
it is made up of a collection of components that are themselves unreliable.
• Scalability: Networks are continuously evolving. The sizes are continually increasing
leading to congestion. Hence, the design should be done so that the networks are
scalable and can accommodate such additions and alterations.
• Addressing: At a particular time, innumerable messages are being transferred between
large numbers of computers. So, a naming or addressing system should exist so that
each layer can identify the sender and receivers of each message.
• Error Control: Unreliable channels introduce a number of errors in the data streams
that are communicated. So, the layers need to agree upon common error detection and
error correction methods
• Flow Control: If the rate at which data is produced by the sender is higher than the rate
at which data is received by the receiver, a proper flow control mechanism needs to be
implemented. MKK
• Routing: There may be multiple paths from the source to the destination. Routing
involves choosing an optimal path among all possible paths
• Congestion: overloading of the network because too many computers want to send too
much traffic
• Quality of service: provide service to applications that want high throughput
• Security: Major factor of data communication is to defend it against threats like
eavesdropping and surreptitious alteration of messages by confidentiality and
authentication

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Connection-Oriented Versus Connectionless Service
Connection-Oriented Service
A connection-oriented service is a network service that was designed and
developed after the telephone system. A connection-oriented service is used to create an
end to end connection between the sender and the receiver before transmitting the data
over the same or different networks. In connection-oriented service, packets are
transmitted to the receiver in the same order the sender has sent them. It uses a
handshake method that creates a connection between the user and sender for
transmitting the data over the network. Hence it is also known as a reliable network
service.
Connectionless Service
A connection is similar to a postal system, in which each letter takes along different
route paths from the source to the destination address. Connectionless service is used in
the network system to transfer data from one end to another end without creating any
connection. So it does not require establishing a connection before sending the data from
the sender to the receiver. It is not a reliable network service because it does not
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guarantee the transfer of data packets to the receiver, and data packets can be received in
any order to the receiver.
Six different types of service.

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Service Primitives
• A service is formally specified by a set of primitives (operations) available to user
processes to access the service.
• These primitives tell the service to perform some action or report on an action taken by
a peer entity.
• The set of primitives available depends on the nature of the service being provided.
• The primitives for connection-oriented service are different from those of
cconnectionless service.

Six service primitives that provide a simple connection-oriented service.

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The Relationship of Services to Protocols
Services and protocols are distinct concepts.
• A service is a set of primitives (operations) that a layer provides to the layer above it.
– A service relates to an interface between two layers, with the lower layer being the service provider
and the upper layer being the service user.
• A protocol, in contrast, is a set of rules governing the format and meaning of the
packets, or messages that are exchanged by the peer entities within a layer.

The relationship between a service and a protocol

service defines operations that can be performed on an object but does not specify how
these operations are implemented. In contrast, a protocol relates to the implementation
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of
the service
 REFERENCE MODELS
• The two important network architectures:
The OSI reference model and The TCP/IP reference model.
The OSI Reference Model
• This model is based on a proposal developed by the International Standards
Organization (ISO) as a first step toward international standardization of the protocols
used in the various layers
• The model is called the ISO OSI (Open Systems Interconnection) Reference Model
because it deals with connecting open systems—that is, systems that are open for
communication with other systems. We will just call it the OSI model for short.
• The OSI model has seven layers.

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OSI Reference Model principles:
• 1. A layer should be created where a different abstraction is needed.
• 2. Each layer should perform a well-defined function.
• 3. The function of each layer should be chosen with an eye toward defining
internationally standardized protocols.
• 4. The layer boundaries should be chosen to minimize the information flow across the
interfaces.
• 5. The number of layers should be large enough that distinct functions need not be
thrown together in the same layer out of necessity and small enough that the
architecture does not become unwieldy.

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The Physical Layer
• The physical layer is concerned with transmitting raw bits over a communication
channel.
The Data Link Layer
• The main task of the data link layer is to transform a raw transmission facility into a line
that appears free of undetected transmission errors.
• It accomplishes this task by having the sender break up the input data into data frames
(typically a few hundred or a few thousand bytes) and transmit the frames sequentially.
• If the service is reliable, the receiver confirms correct receipt of each frame by sending
back an acknowledgement frame.
• Another issue is how to keep a fast transmitter from drowning a slow receiver in data.
Some traffic regulation mechanism like Flow control is needed
• Broadcast networks have an additional issue to control access to the shared channel. A
special sublayer of the data link layer, the medium access control sublayer, deals with
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this problem.
The Network Layer
• The network layer controls the operation of the subnet.
• A key design issue is determining how packets are routed from source to destination.
• Handling congestion is also a responsibility of the network layer, in conjunction with
higher layers that adapt the load
• More generally, the quality of service provided (delay, transit time, jitter, etc.) is also a
network layer issue.
The Transport Layer
• The basic function of the transport layer is to accept data from above it, split it up into
smaller units call segments
• The transport layer also determines what type of service to provide to the session layer
• This layer can be termed as an end-to-end layer as it provides a point-to-point
connection between source and destination to deliver the data reliably.
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The Session Layer
• The session layer allows users on different machines to establish sessions between them.
• Sessions offer various services, including
• dialog control: keeping track of whose turn it is to transmit
• token management: preventing two parties from attempting the same critical operation
simultaneously
• Synchronization: checkpointing long transmissions to allow them to pick up from where they
left off in the event of a crash and subsequent recovery
The Presentation Layer
• The presentation layer is concerned with the syntax and semantics of the information
transmitted.
The Application Layer
• The application layer contains a variety of protocols that are commonly needed by users. One
widely used application protocol is HTTP (HyperTextTransfer Protocol), which is the basis for
the World Wide Web. MKK
• Other application protocols are used for file transfer (FTP), electronic mail(SMTP), and so on.
The TCP/IP Reference Model:
• The ARPANET was a research network sponsored by the DoD (U.S. Department of
Defense).
• It eventually connected hundreds of universities and government installations, using
leased telephone lines.
• When satellite and radio networks were added later, the existing protocols had trouble
interworking with them, so a new reference architecture was needed.
• Thus, from nearly the beginning, the ability to connect multiple networks in a seamless
way was one of the major design goals.
• This architecture later became known as the TCP/IP Reference Model.
• The DoD wanted connections to remain intact as long as the source and destination
machines were functioning, even if some of the machines or transmission lines in
between were suddenly put out of operation.
• Ranging from transferring files to real-time speech transmission, a flexible architecture
was needed.
• All these requirements led to the choice of a packet-switching network based on aMKK
connectionless layer that runs across different networks.
The TCP/IP Reference Model:
• TCP/IP Reference Model is a four-layered suite of communication protocols. It was
developed by the DoD (Department of Defence) in the 1960s
• TCP stands for Transmission Control Protocol and IP stands for Internet Protocol.
• TCP/IP Protocol suite have 4 layers
1. The Link Layer
2. The Internet Layer
3. The Transport Layer
4. The Application Layer

The TCP/IP reference model. MKK

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The Link Layer
• It is also called a network interface layer or link layer. It can be considered as the combination of
physical layer and data link layer of the OSI model.
• It defines how bits are to be encoded into optical or electrical pulses.
• It accepts IP packets from the network layer and encapsulates them into frames.
• It is not really a layer at all, in the normal sense of the term, but rather an interface between hosts and
transmission links.
Protocols used in this layer are:
ARPANET:-
• Advanced Research Project Agency Software
• It is world first packet switching Software & core Software came to compose the global Internet.
SATNET:-

• SATNET, also known as satellite network that formed as an initial segment of the Internet.

• It was implemented by BBN(Body-to-Body Network) Technologies under the direction of the Advanced
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Research Projects Agency.
Packet radio

• Packet radio is a digital radio communications mode used to send packets of data.

• Packet radio uses packet switching to transmit datagrams.

• Packet radio can be used to transmit data long distances.

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The Internet Layer
The Internet layer is responsible for logical transmission of data packets over the internet.
Main functions of the internet layer are −
• It transmits data packets to the link layer.
• It routes each of the data packets independently from the source to the destination, using the optimal
route.
• It reassembles the out-of-order packets when they reach the destination.
• It handles the error in transmission of data packets and fragmentation of data packets.

The protocols used in this layer are −

• Internet Protocol, IP − It is a connectionless and unreliable protocol that provides a best effort delivery
service. It transports data packets called datagrams that travel over different routes across multiple
nodes.
• Address Resolution Protocol, ARP −This protocol maps the logical address or the Internet address of a
host to its physical address, as printed in the network interface card.
• Reverse Address Resolution Protocol, RARP − This is to find the Internet address of a host when its
physical address is known.
• Internet Control Message Protocol, ICMP − It monitors sending the queries as well as the error
messages.
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• Internet Group Message Protocol, IGMP −It allows the transmission of a message to a group of recipients
simultaneously.
The Transport Layer
• The transport layer is responsible for error-free, end-to-end delivery of data from the source
host to the destination host.

In Transport layer there are two protocols namely, TCP and UDP.

• Transmission Control Protocol, TCP − It is a reliable connection-oriented protocol that transmits

data from the source to the destination machine without any error. A connectio
• User Datagram Protocol, UDP − It is a message-oriented protocol that provides a simple
unreliable, connectionless, unacknowledged service. It is suitable for applications that do not
require TCP’s sequencing, error control or flow control. It is used for transmitting a small
amount of data where the speed of delivery is more important than the accuracy of delivery.
• Stream Control Transmission Protocol, SCTP − It combines the features of both TCP and UDP. It is
message oriented like the UDP, which providing the reliable, connection-oriented service like
TCP. It is used for telephony over the Internet.
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The Application Layer
• The application layer is the highest abstraction layer of the TCP/IP model that provides the
interfaces and protocols needed by the users. It combines the functionalities of the session
layer, the presentation layer and the application layer of the OSI model.

This layer uses a number of protocols, the main among which are as follows −
• Hyper Text Transfer Protocol, HTTP − It is the underlying protocol for world wide web. It defines
how hypermedia messages are formatted and transmitted.
• File Transfer Protocol, FTP − It is a client-server based protocol for transfer of files between
client and server over the network.
• Simple Mail Transfer Protocol, SMTP − It lays down the rules and semantics for sending and
receiving electronic mails (e-mails).
• Domain Name System, DNS − It is a naming system for devices in networks. It provides services
for translating domain names to IP addresses.
• TELNET − It provides bi-directional text-oriented services for remote login to the hosts over the
network.
• Simple Network Management Protocol, SNMP − It is for managing, monitoring the network MKK
and for organizing information about the networked devices.
 EXAMPLE NETWORKS
• The subject of computer networking covers many different kinds of networks, large and
small, well known and less well known. They have different goals, scales, and
technologies. They are:
• Internet
• Third-Generation Mobile Phone Networks
• Wireless LANs (IEEE 802.11)
• RFID and Sensor Networks

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The Internet
• In simplest terms, the Internet is a global network comprised of smaller networks that
are interconnected using standardized communication protocols. The Internet
standards describe a framework known as the Internet protocol suite. This model
divides methods into a layered system of protocols.
• The Internet provides a variety of information and communication facilities; contains
forums, databases, email, hypertext, etc. It consists of private, public, academic,
business, and government networks of local to global scope, linked by a broad array of
electronic, wireless, and optical networking technologies.

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Third-Generation Mobile Phone Networks
• People love to talk on the phone even more than they like to surf the Internet, and this
has made the mobile phone network the most successful network in the world.
• It has more than four billion subscribers worldwide roughly 60% of the world’s
population and more than the number of Internet hosts and fixed telephone lines
combined
• First-generation mobile phone systems transmitted voice calls as continuously varying
(analog) signals rather than sequences of (digital) bits, i.e., AMPS (Advanced Mobile
Phone System) in 1982.
• Second-generation mobile phone systems switched to transmitting voice calls in digital
form to increase capacity, improve security, and offer text messaging i.e., GSM (Global
System for Mobile communications) in 1991. [most widely used mobile phone system
in the world, is a 2G system]
• The third generation, or 3G, systems were initially deployed in 2001 and offer both
digital voice and broadband digital data services. UMTS (Universal Mobile
Telecommunications System), also called WCDMA (Wideband Code Division Multiple MKK
Access), is the main 3G system that is being rapidly deployed worldwide.
Cellular design of mobile phone networks.

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Wireless LANs (IEEE 802.11)
• Wireless LANs (WLANs) are wireless computer networks that use high-frequency radio waves
instead of cables for connecting the devices within a limited area forming LAN (Local Area
Network). Users connected by wireless LANs can move around within this limited area such as
home, school, campus, office building, railway platform, etc.
• Most WLANs are based upon the standard IEEE 802.11 standard or WiFi.
Components of WLANs:
• Wireless Access Point (WAP or AP)
• Client
Types of WLANS
Wireless network with an access point:
Mobile devices or clients connect to an access point (AP) that
in turn connects via a bridge to the LAN or Internet.
(a) Wireless network with an access point. (b) Ad hoc network.
The client transmits frames to other clients via the AP.
Ad hoc network: Clients transmit frames directly to each other
in a peer-to-peer fashion. MKK
RFID and Sensor Networks
• The networks we have studied so far are made up of computing devices that are easy to
recognize, from computers to mobile phones.
• With Radio Frequency IDentification (RFID), everyday objects can also be part of a
computer network.
• An RFID tag looks like a postage stamp-sized sticker that can be affixed to (or embedded
in) an object so that it can be tracked.
• The tag consists of a small microchip with a unique identifier and an antenna that
receives radio transmissions.
• RFID readers installed at tracking points find tags
when they come into range and interrogate them
for their information as shown in Fig.
• Applications include checking identities, managing the supply chain, timing races, and
replacing barcodes. MKK
Internet Based Applications
• Internet applications are server-based applications. Following are a few Internet
Applications −
• World Wide Web (WWW)
• Electronic mail (e-mail)
• File Transfer Protocol (FTP)
• Telnet (i.e., log-in to the computer located remotely)
• Internet Relay Chat (IRC) (Real time video chatting)

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 Network Devices
Hardware devices that are used to connect computers, printers, fax machines and
other electronic devices to a network are called network devices.
These devices transfer data in a fast, secure and correct way over same or different
networks.
Network devices may be inter-network or intra-network.

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Repeaters
• Repeaters are network devices operating at physical layer of the OSI model that amplify
or regenerate an incoming signal before retransmitting it. They are incorporated in
networks to expand its coverage area. They are also known as signal boosters.
• When an electrical signal is transmitted via a channel, it gets attenuated (loss of
transmission signal strength) depending upon the nature of the channel or the
technology. This poses a limitation upon the length of the LAN or coverage area of
cellular networks. This problem is alleviated by installing repeaters at certain intervals.

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Bridges
• A bridge is a network device that connects multiple LANs (local area networks) together
to form a larger LAN. The process of aggregating networks is called network bridging. A
bridge connects the different components so that they appear as parts of a single
network. Bridges operate at the data link layer of the OSI model and hence also referred
as Layer 2 switches.

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Routers
A router is a network layer hardware device that transmits data from one LAN to another
if both networks support the same set of protocols.
So a router is typically connected to at least two LANs
and the internet service provider (ISP).
It receives its data in the form of packets, which
are data frames with their destination address added.
Router also strengthens the signals before
transmitting them. That is why sometimes it is also called
repeater.

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Gateways
• Gateway is a network device used to connect two or more dissimilar networks. A
gateway usually is a computer with multiple NICs(Network Interface Card) connected to
different networks. A gateway can also be configured completely using software. As
networks connect to a different network through gateways, these gateways are usually
hosts or end points of the network.
• Gateway uses packet switching technique to transmit
data from one network to another. In this way it is similar
to a router, the only difference being router can transmit
data only over networks that use same protocols.

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Multiprotocol Routers
• Multiprotocol Routers uses Multiprotocol Label Switching (MPLS) routing technique that
augments speed and control of the network traffic by directing data from one node to
the next node based on short path labels. Instead of being routed using long network
addresses, the data packets are routed through path labels that identify virtual paths
between the nodes rather than endpoints. MPLS speeds up traffic flows by avoiding
complex lookups in the routing table at each node as in conventional routing
algorithms.

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Hubs
• Hubs are networking devices operating at a physical layer of the OSI model that are
used to connect multiple devices in a network. They are generally used to connect
computers in a LAN.
• A hub has many ports in it. A computer which intends
to be connected to the network is plugged in to one
of these ports. When a data frame arrives at a port,
it is broadcast to every other port, without considering
whether it is destined for a particular destination
device or not.

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Switches
• Switches are networking devices operating at layer 2 or a data link layer of the OSI
model. They connect devices in a network and use packet switching to send, receive or
forward data packets or data frames over the network.
• A switch has many ports, to which computers are
plugged in. When a data frame arrives at any port of a
network switch, it examines the destination address,
performs necessary checks and sends the frame to the
corresponding device(s).It supports unicast, multicast
as well as broadcast communications.

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Modems
• Modem is a device that enables a computer to send or receive data over telephone or
cable lines.
• The main function of the modem is to convert digital signal into analog and vice versa.
Modem is a combination of two devices − modulator and demodulator.
The modulator converts digital data into analog data when the data is being sent by the
computer. The demodulator converts analog data signals into digital data when it is
being received by the computer.

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CSU/DSU (Channel Service Unit/Data Service Unit)
• A CSU/DSU (Channel Service Unit/Data Service Unit) is a hardware device that converts
a digital data frame from the communications technology used on a local area network
(LAN) into a frame appropriate to a wide-area network (WAN) and vice versa.
To connect to a local area network, the DSU communicates with the Channel Service Unit.
• The data encoded in the digital circuit is converted into synchronous serial data by the
digital service unit.
• Wherever the digital communication line is utilized, the digital service unit also
electrically separates it from the networking equipment.
• The Channel Service Unit and the Digital Service Unit must be owned or manufactured
by the same company or manufacturer for the digital service unit to function correctly.

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NIC
• NIC or network interface card is a network adapter that is used to connect the computer
to the network. It is installed in the computer to establish a LAN. It has a unique id that
is written on the chip, and it has a connector to connect the cable to it. The cable acts as
an interface between the computer and the router or modem. NIC card is a layer 2
device which means that it works on both the physical and data link layers of the
network model.
Wireless Access Points
• A Wireless Access Point (WAP) is a networking device that allows connecting the devices
with the wired network. A Wireless Access Point (WAP) is used to create the WLAN
(Wireless Local Area Network)
• A wireless AP connects the wired networks to the wireless client. It eases access to the
network for mobile users which increases productivity and reduces the infrastructure
cost.
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Transceivers
• IrDA communication involves a transmitter unit for transmitting the data over IR and a receiver
for receiving the data.
• Infrared Light Emitting Diode (LED) is the IR source for transmitter and at the receiving end a
photodiode acts as the receiver.
• Both transmitter and receiver unit will be present in each device supporting IrDA
communication for bidirectional data transfer. Such IR units are known as ‘Transceiver’.
Firewalls
• A firewall can be defined as a special type of network security device or a software program that
monitors and filters incoming and outgoing network traffic based on a defined set of security
rules. It acts as a barrier between internal private networks and external sources (such as the
public Internet).
• The primary purpose of a firewall is to allow non-threatening traffic and prevent malicious or
unwanted data traffic for protecting the computer from viruses and attacks.

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Proxies/Proxy server
• The proxy server is a computer on the internet that accepts the incoming requests from
the client and forwards those requests to the destination server.
• The proxy server allows us to access any websites with a different IP address. It
separates the client system and web server from the global network.
• It plays an intermediary role between users and targeted websites or servers. It collects
and provides information related to user requests. The most important point about a
proxy server is that it does not encrypt traffic.

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