UNIT –I

Network A network is a set of devices (often referred to as nodes) connected by

communication links. A node can be a computer, printer, or any other device capable of
sending and/or receiving data generated by other nodes on the network.
USES OF COMPUTER NETWORKS

1.Business Applications

a)Resource Sharing:The goal is to make all programs, equipment, and especially data available to
anyone on the network without regard to the physical location of the resource or the user.

Probably even more important than sharing physical resources such as printers, and tape backup systems, is
sharing information. Companies small and large are vitally dependent on computerized information and large
are vitally dependent on computerized information.

Networks called VPNs (Virtual Private Networks) may be used to join the individual networks at different
sites into one extended network.

b)Providing Communication Medium:Every company that has two or more computers now has
email (electronic mail), which employees generally use for a great deal of daily communication.

Telephone calls between employees may be carried by the computer network instead of by the
phone company. This technology is called IP telephony or Voice over IP (VoIP) when Internet
technology is used.

Desktop sharing lets remote workers see and interact with a graphical computer screen. This
makes it easy for two or more people who work far apart to read and write a shared blackboardor
write a report together.

c)Doing Business Electronically: Airlines, bookstores, and other retailers have discovered that many
customers like the convenience of shopping from home.
2.Home Network Applications

a)Access to remote information: Internet access provides home users with connectivity to remote
computers.As with companies, home users can access information, communicate with other
people, and buy products and services with e-commerce

b)Person to person Communication: Peer-to-peer communication is often used to share

music and videos.
one of the most popular Internet applications of all, email, is inherently peer-to-peer. This form of
communication is likely to grow considerably in the future.

Any teenager worth his or her salt is addicted to instant messaging. This facility, derived from the
UNIX talk program in use since around 1970, allows two people to type messages at each other in
real time. There are multi-person messaging services too, such as the Twitter service that lets
people send short text messages called ‘‘tweets’’ to their circle of friends or other willing audiences.

person-to-person communications and accessing information are social network applications like
Facebook

c)Interactive Entertainment

d)Electronic Commerce e-commerce is widely used is access to financial institutions.

Many people already pay their bills, manage their bank accounts, and handle their investments
electronically.

3.Mobile Users: Mobile computers, such as laptop and handheld computers, are one of the

fastest-growing segments of the computer industry.

4.Social Issues

Social networks, message boards, content sharing sites, and a host of other applications allow people to
share their views with like-minded individuals.

The trouble comes with topics that people actually care about, like politics, religion, or sex. Views that are
publicly posted may be deeply offensive to somepeople.

Worse yet, they may not be politically correct. Furthermore, opinions need not be limited to text; high-
resolution color photographs and video clips are easily shared over computer networks. Some people take a
live-and-let-live view,but others feel that posting certain material (e.g., verbal attacks on particular

countries or religions, pornography, etc.) is simply unacceptable and that such content must be censored.
Different countries have different and conflicting laws in this area.

Network Hardware
The computer networks can be discussed in two dimensions
i) Transmission technology ii) Scale
There are two types of transmission technology that are in widespread use:
Broadcast links
Point-to-point links.
Point-to-point links connect individual pairs of machines. To go from the source to the destination
on a network made up of point-to-point links, short messages, called packets in certain contexts,
may have to first visit one or more intermediate machines. Often multiple routes, of different
lengths, are possible, so finding good ones is important in point-to-point networks. Point-to-point
transmission with exactly one sender and exactly one receiver is sometimes called unicasting.
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. A wireless network is a common example
of a broadcast link, with communication shared over a coverage region that depends on the wireless
channel and the transmitting machine.
When a packet with this code is transmitted, it is received and processed by every machine on the
network. This mode of operation is called broadcasting. Some broadcast systems also support
transmission to a subset of the machines, which known as multicasting.
Scale
An alternative criterion for classifying networks is by scale. Distance is important as a
classification metric because different technologies are used at different scales.
Personal Area Network
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. some companies have designed a short-range wireless
network called Bluetooth to connect these components without wires. The idea is that if our devices
have Bluetooth, then we need no cables. You just put them down, turn them on, and they work
together.
In the simplest form, Bluetooth networks use the master-slave paradigm. The system unit (the PC)
is normally the master, talking to the mouse, keyboard, etc., as slaves. The master tells the slaves
what addresses to use, when they can broadcast, how long they can transmit, what frequencies they
can use, and so on.

Local Area Networks

A LAN is a privately owned network that operates within and nearby a single building like a home,
office or factory. LANs are widely used to connect personal computers and consumer electronics
to let them share resources (e.g., printers) and exchange information. When LANs are used by
companies, they are called enterprise networks. Wireless LANs are also popular where each
computer communicates with the device called an AP (Access Point), wireless router, or base
station which 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 and it runs at speed from 11 to hundreds of Mbps. Wired LANs
use a range of different transmission technologies. Most of them use copper
wires, but some use optical fiber.
Metropolitan Area Network
A MAN (Metropolitan Area Network) covers a city. The best-known examples
of MANs are the cable television networks available in many cities. Recent
developments in highspeed wireless Internet access have resulted in another
MAN, which has been standardized as IEEE 802.16 and is popularly known as
WiMAX.
Wide Area Network
A WAN (Wide Area Network) spans a large geographical area, often a country
or continent. In most WANs, the subnet consists of two distinct components:
transmission lines and switching elements. Transmission lines move bits
between machines. They can be made of copper wire, optical fiber, or even
radio links. Most companies do not have transmission lines lying about, so
instead they lease the lines from a telecommunications company. Switching
elements, or just switches, are specialized computers that connect two or more
transmission lines.
Internetworks
A collection of interconnected networks is called an internetwork or internet. A
network is formedby the combination of a subnet and its hosts. The machine
that makes a connection between two or more networks and provides the
necessary translation, both in terms of hardware and software, is a gateway.

TOPOLOGY

Physical Topology
The term physical topology refers to the way in which a network is laid out physically. One or more
devices connect to a link; two or more links form a topology. The topology of a network is the
geometric representation of the relationship of all the links and linking devices (usually called
nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring
Mesh: In a mesh topology, every device has a dedicated point-to-point link to every other device.
The term dedicated means that the link carries traffic only between the two devices it connects. To
find the number of physical links in a fully connected mesh network with n nodes, we first consider
that each node must be connected to every other node. Node 1 must be connected to n - I nodes,
node 2 must be connected to n – 1 nodes, and finally node n must be connected to n - 1 nodes. We
need n(n - 1) physical links. However, if each physical link allows communication in both directions
(duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh
topology, we need n(n -1) /2 duplex-mode links.
To accommodate that many links, every device on the network must have n – 1 input/output
(VO) ports to be connected to the other n - 1 stations.
Advantages:
1. The use of dedicated links guarantees that each connection can carry its own data load, thus
eliminating the traffic problems that can occur when links must be shared by multiple
devices.
2. A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire
system. Third, there is the advantage of privacy or security. When every message travels
along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other
users from gaining access to messages. Finally, point-to-point links make fault
identification and fault isolation easy. Traffic can be routed to avoid links with suspected
problems. This facility enables the network manager to discover the precise location of the
fault and aids in finding its cause and solution.
Disadvantages:
1. Disadvantage of a mesh are related to the amount of cabling because every device must be
connected to every other device, installation and reconnection are difficult.
2. Second, the sheer bulk of the wiring can be greater than the available space (in walls,
ceilings, or floors) can accommodate. Finally, the hardware required to connect each link
(I/O ports and cable) can be prohibitively expensive.
For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a
backbone connecting the main computers of a hybrid network that can include several other
topologies.

Star Topology:
In a star topology, each device has a dedicated point-to-point link only to a central
controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh
topology, a star topology 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 .
A star topology is less expensive than a mesh topology. In a star, each device needs only one link
and one I/O port to connect it to any number of others. This factor also makes it easy to install and
reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only
one connection: between that device and the hub.
Other advantages include robustness. If one link fails, only that link is affected. All other links
remain active. This factor also lends itself to easy fault identification and fault isolation. As long as
the hub is working, it can be used to monitor link problems and bypass defective links.

One big disadvantage of a star topology is the dependency of the whole topology on one single
point, the hub. If the hub goes down, the whole system is dead. Although a star requires far less
cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is
required in a star than in some other topologies (such as ring or bus).

Bus Topology:
The preceding examples all describe point-to-point connections. A bus topology, on the
other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network

Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection
running between the device and the main cable. A tap is a connector that either splices into the main
cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal
travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes
weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of
taps a bus can support and on the distance between those taps.
Advantages of a bus topology include ease of installation. Backbone cable can be laid along the
most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus
uses less cabling than mesh or star topologies. In a star, for example, four network devices in the
same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy
is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to
reach only as far as the nearest point on the backbone.
Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be
optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection
at the taps can cause degradation in quality. This degradation can be controlled by limiting the
number and spacing of devices connected to a given length of cable. Adding new devices may
therefore require modification or replacement of the backbone.
In addition, a fault or break in the bus cable stops all transmission, even between devices on the
same side of the problem. The damaged area reflects signals back in the direction of origin, creating
noise in both directions.
Bus topology was the one of the first topologies used in the design of early local area networks.
Ethernet LANs can use a bus topology, but they are less popular.
Ring Topology In a ring topology, each device has a dedicated point-to-point connection with
only the two devices on either side of it. A signal is passed along the ring in one direction, from
device to device, until it reaches its destination. Each device in the ring incorporates a repeater.
When a device receives a signal intended for another device, its repeater regenerates the bits and
passes them along
A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either
physically or logically). To add or delete a device requires changing only two connections. The only constraints are
media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is
simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a
specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location.
However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled
station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing
off the break. Ring topology was prevalent when IBM introduced its local-area network Token Ring. Today, the need
for higher-speed LANs has made this topology less popular. Hybrid Topology A network can be hybrid. For example,
we can have a main star topology with each branch connecting several stations in a bus topology as shown in Figure
Network Software
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 number of layers, the name of each layer, the contents of each
layer, and the function of each layer differ from network to network. The purpose of each layer is
to offer certain services to the higher layers while shielding those layers from the details of how
the offered services are actually implemented. In a sense, each layer is a kind of virtual machine,
-offering certain services to the layer above it. 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. 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.
A set of layers and protocols is called a network architecture. A list of the protocols used by a
certain system, one protocol per layer, is called a protocol stack.

 In networking, a protocol defines the rules that both the sender and receiver and
all intermediate devices need to follow to be able to communicate effectively.
 A protocol provides a communication service that the process use to exchange
messages.
 When communication is simple, we may need only one simple protocol.
 When the communication is complex, we may need to divide the task between
different layers, in which case we need a protocol at each layer, or protocol
layering.
  Protocol layering is that it allows us to separate the services from the
implementation.
 A layer needs to be able to receive a set of services from the lower layer and to
give the services to the upper layer.
 Any modification in one layer will not affect the other layers.

Basic Elements of Layered Architecture

 Service: It is a set of actions that a layer provides to the higher layer.
 Protocol: It defines a set of rules that a layer uses to exchange the information
with peer entity. These rules mainly concern about both the contents and order of
the messages used.
 Interface: It is a way through which the message is transferred from one layer to
another layer.
Features of Protocol Layering
1. It decomposes the problem of building a network into more
manageable components.
2. It provides a more modular design.

Principles of Protocol Layering

1. The first principle dictates that if we want bidirectional communication,
we needto make each layer so that it is able to perform two opposite
tasks, one in each direction.
The second principle that we need to follow in protocol layering is that the two objects
under each layer at both sites should be identical
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.
Mechanism for finding errors in received information uses codes for error detection.
Information that is incorrectly received can then be retransmitted until it is received
correctly. More powerful codes allow for error correction, where the correct message is
recovered from the possibly incorrect bits that were originally received.
Another issue is finding a working path through a network. Often there are multiple paths
between a source and destination, and in a large network, there may be some links or
routers that are broken. The network should automatically make this decision. This topic
is called routing.
Since there are many computers on the network, every layer needs a mechanism for
identifying the senders and receivers that are involved in a particular message. This
mechanism is called addressing or naming, in the low and high layers.
The network should continue to work well even when the network gets large. It is said
to be scalable. Another design issue is resource allocation. Networks provide a service
to hosts from their underlying resources, such as the capacity of transmission lines. To
do this well, they need mechanisms that divide their resources so that one host does not
interfere with another too much. 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 actiontaken by a peer entity.
Connection Oriented and Connectionless
Both Connection-oriented service and Connection-less service are used for the connection
establishment between two or more two devices. These types of services are offered by the network
layer.
Connection-oriented service is related to the telephone system. It includes connection establishment
and connection termination. In a connection-oriented service, the Handshake method is used to
establish the connection between sender and receiver.

Connection-less service is related to the postal system. It does not include any connection
establishment and connection termination. Connection-less Service does not give a
guarantee of reliability. In this, Packets do not follow the same path to reach their
destination.
Service Primitives
A service is formally specified by a set of primitives (operations) available touser processes to access the
service. These primitives tell the service to perform some action or report on an action taken by a peer entity.
If the protocol stack is located in the operating system, as it often is, the primitives are normally system
calls. These calls cause a trap to kernel mode, which then turns control of the machine
over to the operating system to send the necessary packets

OSI Reference Model

o OSI stands for Open System Interconnection.

o It is a reference model that describes how information from a software application
in one computer moves through a physical medium to the software application in
another computer.
o OSI consists of seven layers, and each layer performs a particular network
function.
o OSI model was developed by the International Organization for Standardization
(ISO) in 1984, and it is now considered as an architectural model for the inter-
 computer communications.
o OSI model divides the whole task into seven smaller and manageable tasks. Each
layer is assigned a particular task.
o Each layer is self-contained, so that task assigned to each layer can be performed
independently.
ORGANIZATION OF THE OSI LAYERS
FUNCTIONS OF THE OSI LAYERS
1. PHYSICAL LAYER

The physical layer coordinates the functions required to transmit a bit stream over a
physical medium.
The physical layer is concerned with the following functions:
 Physical characteristics of interfaces and media - The physical layer defines
the characteristics of the interface between the devices and the transmission
medium.
 Representation of bits - To transmit the stream of bits, it must be encoded to
signals. The physical layer defines the type of encoding.
 Signals: It determines the type of the signal used for transmitting the information.
 Data Rate or Transmission rate - The number of bits sent each second –is also
defined by the physical layer.
 Synchronization of bits - The sender and receiver must be synchronized at the
bit level. Their clocks must be synchronized
  Line Configuration - In a point-to-point configuration, two devices are
connected together through a dedicated link. In a multipoint configuration, a link
is shared between several devices.
 Physical Topology - The physical topology defines how devices are connected to
make a network. Devices can be connected using a mesh, bus, star or ring
topology.
 Transmission Mode - The physical layer also defines the direction of
transmission between two devices: simplex, half-duplex or full-duplex.

2. DATA LINK LAYER

It is responsible for transmitting frames from one node to the next node.
The other responsibilities of this layer are
 Framing - Divides the stream of bits received into data units called frames.
 Physical addressing – If frames are to be distributed to different systems on the
network , data link layer adds a header to the frame to define the sender and
receiver.
 Flow control- If the rate at which the data are absorbed by the receiver is less
than the rate produced in the sender, t he Data link layer imposes a flow
ctrl mechanism.
 Error control- Used for detecting and retransmitting damaged or lost frames and
to prevent duplication of frames. This is achieved through a trailer added at the
end of the frame.
 Medium Access control -Used to determine which device has control over the
link at any given time.

3. NETWORK LAYER
This layer is responsible for the delivery of packets from source to destination.
It determines the best path to move data from source to the destination based on the
network conditions, the priority of service, and other factors.
The other responsibilities of this layer are
 Logical addressing - If a packet passes the network boundary, we need another
addressing system for source and destination called logical address. This
addressing is used to identify the device on the internet.
 Routing – Routing is the major component of the network layer, and it determines
the best optimal path out of the multiple paths from source to the destination.

4. TRANSPORT LAYER

It is responsible for Process to Process delivery. That is responsible for source-to-

destination (end-to-end) delivery of the entire message, It also ensures whether the
message arrives in order or not.
The other responsibilities of this layer are
 Port addressing / Service Point addressing - The header includes an address
called port address / service point address. This layer gets the entire message to
the correct process on that computer.
 Segmentation and reassembly - The message is divided into segments and each
segment is assigned a sequence number. These numbers are arranged correctly on
the arrival side by this layer.

 Connection control - This can either be connectionless or connection oriented.

 The connectionless treats each segment as an individual packet

and delivers to the destination.
 The connection-oriented makes connection on the destination side
before the delivery. After the delivery the termination will be terminated.
 Flow control - The transport layer also responsible for flow control but it
 is performed end-to-end rather than across a single link.
 Error Control - Error control is performed end-to-end rather than across the
single link..

5. SESSION LAYER

This layer establishes, manages and terminates connections between applications.

The other responsibilities of this layer are
 Dialog control - Session layer acts as a dialog controller that creates a dialog
between two processes or we can say that it allows the communication between
two processes which can be either half-duplex or full-duplex.
 Synchronization- Session layer adds some checkpoints when transmitting the
data in a sequence. If some error occurs in the middle of the transmission of data,
then the transmission will take place again from the checkpoint. This process is
known as Synchronization and recovery.

6. PRESENTATION LAYER

It is concerned with the syntax and semantics of information exchanged between two
systems.
The other responsibilities of this layer are
 Translation – Different computers use different encoding system, this layer is
responsible for interoperability between these different encoding methods. It will
change the message into some common format.
 Encryption and decryption-It means that sender transforms the original
information to another form and sends the resulting message over the n/w. and
vice versa.
 Compression and expansion-Compression reduces the number of bits contained
in the information particularly in text, audio and video.
 7. APPLICATION LAYER
This layer enables the user to access the network. It handles issues such as network
transparency, resource allocation, etc. This allows the user to log on to remote user.
The other responsibilities of this layer are
 FTAM (File Transfer, Access, Management) - Allows user to access files in
a remote host.
 Mail services - Provides email forwarding and storage.
 Directory services - Provides database sources to access information about
various sources and objects.

TCP / IP Protocol
 The TCP/IP architecture is also called as Internet architecture.
 It is developed by the US Defense Advanced Research Project Agency (DARPA)
for its packet switched network (ARPANET).
 TCP/IP is a protocol suite used in the Internet today. 
 It is a 4-layer model. The layers of TCP/IP are
1. Application layer
2. Transport Layer (TCP/UDP)
3. Internet Layer
4. The Host - to - Network Layer
 APPLICATION LAYER
 An application layer incorporates the function of top three OSI layers. An
application layer is the topmost layer in the TCP/IP model.
 It is responsible for handling high-level protocols, issues of representation.
 This layer allows the user to interact with the application.
 When one application layer protocol wants to communicate with another
application layer, it forwards its data to the transport layer. 
 Protocols such as FTP, HTTP, SMTP, POP3, etc running in the application layer
provides service to other program running on top of application layer
TRANSPORT LAYER
 The transport layer is responsible for the reliability, flow control, and correction
of data which is being sent over the network.
 The two protocols used in the transport layer are User Datagram protocol and
Transmission control protocol.
o UDP – UDP provides connectionless service and end-to-end delivery of
transmission. It is an unreliable protocol as it discovers the errors but
not specify the error.
o TCP – TCP provides a full transport layer services to applications. TCP
is a reliable protocol as it detects the error and retransmits the
 damaged frames.

INTERNET LAYER
 The internet layer is the second layer of the TCP/IP model.
 An internet layer is also known as the network layer.
 The main responsibility of the internet layer is to send the packets from any
network, and they arrive at the destination irrespective of the route they take.
 Internet layer handle the transfer of information across multiple networks through
router and gateway .
 IP protocol is used in this layer, and it is the most significant part of the entire
TCP/IP suite.

HOST - TO - NETWORK LAYER

 The network interface layer is the lowest layer of the TCP/IP model. 
 This layer is the combination of the Physical layer and Data Link layer defined
in the OSI reference model.
 It defines how the data should be sent physically through the network.
 This layer is mainly responsible for the transmission of the data between two
devices on the same network.
 The functions carried out by this layer are encapsulating the IP datagram into
frames transmitted by the network and mapping of IP addresses into physical
addresses.
 The protocols used by this layer are Ethernet, token ring, FDDI, X.25, frame
relay. 
Following are the differences between OSI and TCP/IP Reference Model −

OSI TCP/IP

OSI represents Open System TCP/IP model represents the Transmission

Interconnection. Control Protocol / Internet Protocol.

OSI is a generic, protocol independent TCP/IP model depends on standard protocols

standard. It is acting as an interaction gateway about which the computer network has created.
between the network and the final-user. It is a connection protocol that assigns the
network of hosts over the internet.

The OSI model was developed first, and then The protocols were created first and then built
protocols were created to fit the network the TCP/IP model.
architecture’s needs.

It provides quality services. It does not provide quality services.

The OSI model represents defines It does not mention the services, interfaces,
administration, interfaces and conventions. It and protocols.
describes clearly which layer provides
services.

The protocols of the OSI model are better The TCP/IP model protocols are not hidden,
unseen and can be returned with another and we cannot fit a new protocol stack in it.
appropriate protocol quickly.

It is difficult as distinguished to TCP/IP. It is simpler than OSI.

It provides both connection and connectionless It provides connectionless transmission in the

 OSI TCP/IP

oriented transmission in the network layer; network layer and supports connecting and
however, only connection-oriented connectionless-oriented transmission in the
transmission in the transport layer. transport layer.

It uses a horizontal approach. It uses a vertical approach.

The smallest size of the OSI header is 5 bytes. The smallest size of the TCP/IP header is 20
bytes.

Protocols are unknown in the OSI model and In TCP/IP, returning protocol is not difficult.
are returned while the technology modifies.
 THE INTERNET
The Internet has revolutionized many aspects of our daily lives. It has affected the way we do
business as well as the way we spend our leisure time. Count the ways you've used the Internet
recently. Perhaps you've sent electronic mail (e-mail) to a business associate, paid a utility bill, read
a newspaper from a distant city, or looked up a local movie schedule-all by using the Internet. Or
maybe you researched a medical topic, booked a hotel reservation, chatted with a fellow Trekkie,
or comparison-shopped for a car. The Internet is a communication system that has brought a wealth
of information to our fingertips and organized it for our use.
 A Brief History
A network is a group of connected communicating devices such as computers and printers. An
internet (note the lowercase letter i) is two or more networks that can communicate with each other.
The most notable internet is called the Internet (uppercase letter I), a collaboration of more than
hundreds of thousands of interconnected networks. Private individuals as well as various
organizations such as government agencies, schools, research facilities, corporations, and libraries
in more than 100 countries use the Internet. Millions of people are users. Yet this extraordinary
communication system only came into being in 1969.
In the mid-1960s, mainframe computers in research organizations were standalone devices.
Computers from different manufacturers were unable to communicate with one another. The
Advanced Research Projects Agency (ARPA) in the Department of Defense (DoD) was interested
in finding a way to connect computers so that the researchers they funded could share their findings,
thereby reducing costs and eliminating duplication of effort.
In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented its ideas
for ARPANET, a small network of connected computers. The idea was that each host computer
(not necessarily from the same manufacturer) would be attached to a specialized computer, called
an inteiface message processor (IMP). The IMPs, in tum, would be connected to one another. Each
IMP had to be able to communicate with other IMPs as well as with its own attached host. By 1969,
ARPANET was a reality. Four nodes, at the University of California at Los Angeles (UCLA), the
University of California at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the
University of Utah, were connected via the IMPs to form a network. Software called the Network
Control Protocol (NCP) provided communication between the hosts.
In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group,
collaborated on what they called the Internetting Projec1. Cerf and Kahn's landmark 1973 paper
outlined the protocols to achieve end-to-end delivery of packets. This paper on Transmission
Control Protocol (TCP) included concepts such as encapsulation, the datagram, and the functions
of a gateway. Shortly thereafter, authorities made a decision to split TCP into two protocols:
Transmission Control Protocol (TCP) and Internetworking Protocol (lP). IP would handle datagram
routing while TCP would be responsible for higher-level functions such as
segmentation, reassembly, and error detection. The internetworking protocol became
known as TCPIIP.

The Internet Today

The Internet has come a long way since the 1960s. The Internet today is not a simple
hierarchical structure. It is made up of many wide- and local-area networks joined by
connecting devices and switching stations. It is difficult to give an accurate
representation of the Internet because it is continually changing-new networks are being
added, existing networks are adding addresses, and networks of defunct companies are
being removed. Today most end users who want Internet connection use the services of
Internet service providers (lSPs). There are international service providers, national
service providers, regional service providers, and local service providers. The Internet
today is run by private companies, not the government. Figure 1.13 shows a conceptual
(not geographic) view of the Internet.
 International Internet Service Providers:
At the top of the hierarchy are the international service providers that connect nations together.

National Internet Service Providers:

The national Internet service providers are backbone networks created and maintained by specialized
companies. There are many national ISPs operating in North America; some of the most well known are SprintLink,
PSINet, UUNet Technology, AGIS, and internet Mel. To provide connectivity between the end users, these backbone
networks are connected by complex switching stations (normally run by a third party) called network access points
(NAPs). Some national ISP networks are also connected to one another by private switching stations called peering
points. These normally operate at a high data rate (up to 600 Mbps).
Regional Internet Service Providers:
Regional internet service providers or regional ISPs are smaller ISPs that are connected to one or more
national ISPs. They are at the third level of the hierarchy with a smaller data rate. Local Internet Service Providers:
Local Internet service providers provide direct service to the end users. The local ISPs can be connected to regional
ISPs or directly to national ISPs. Most end users are connected to the local ISPs. Note that in this sense, a local ISP can
be a company that just provides Internet services, a corporation with a network that supplies services to its own
employees, or a nonprofit organization, such as a college or a university, that runs its own network. Each of these local
ISPs can be connected to a regional or national service provider
Transmission Media

Transmission media is a communication channel that carries the information from the sender to the receiver
o Data is transmitted through the electromagnetic signals.
o The main functionality of the transmission media is to carry the information
in the form of bits (Either as Electrical signals or Light pulses).
o It is a physical path between transmitter and receiver in data communication.
o The characteristics and quality of data transmission are determined by the
characteristics of medium and signal.
o Transmission media is of two types : Guided Media (Wired) and UnGuided
Media (wireless).
o In guided (wired) media, medium characteristics are more important whereas, in

unguided (wireless) media, signal characteristics are more important.

o Different transmission media have different properties such as bandwidth, delay,
cost and ease of installation and maintenance.
o The transmission media is available in the lowest layer of the OSI reference
model, i.e., Physical layer.

FACTORS FOR DESIGNING THE TRANSMISSION MEDIA

o Bandwidth: All the factors are remaining constant, the greater the bandwidth of a

medium, the higher the data transmission rate of a signal.

o Transmission impairment: When the received signal is not identical to the

transmitted one due to the transmission impairment. The quality of the signals
will get destroyed due to transmission impairment.
o Interference: An interference is defined as the process of disrupting a signal

when it travels over a communication medium on the addition of some unwanted

signal
 GUIDED MEDIA
 It is defined as the physical medium through which the signals are transmitted.
 It is also known as Bounded media.
 Types of Guided media: Twisted Pair Cable, Coaxial Cable, Fibre Optic Cable

TWISTED PAIR CABLE

 Twisted pair is a physical media made up of a pair of cables twisted with each
other.
 A twisted pair cable is cheap as compared to other transmission media.
 Installation of the twisted pair cable is easy, and it is a lightweight cable.
 The frequency ranges for twisted pair cable is from 0 to 3.5KHz.
 A twisted pair consists of two insulated copper wires arranged in a regular spiral
pattern.
Unshielded Twisted Pair
An unshielded twisted pair is widely used in telecommunication.
Following are the categories of the unshielded twisted pair cable:
o Category 1: Suports low-speed data.
o Category 2: It can support upto 4Mbps.
o Category 3: It can support upto 16Mbps.
o Category 4: It can support upto 20Mbps.
o Category 5: It can support upto 200Mbps.

Advantages :
o It is cheap.
o Installation of the unshielded twisted pair is easy.
o It can be used for high-speed LAN.

Disadvantage:
o This cable can only be used for shorter distances because of attenuation.

Shielded Twisted Pair

A shielded twisted pair is a cable that contains the mesh surrounding the wire that allows
the higher transmission rate.
Advantages :
o The cost of the shielded twisted pair cable is not very high and not very low.
o Installation of STP is easy.
o It has higher capacity as compared to unshielded twisted pair cable.
o It has a higher attenuation.
o It is shielded that provides the higher data transmission rate.

Disadvantages:
o It is more expensive as compared to UTP and coaxial cable.
o It has a higher attenuation rate.

COAXIAL CABLE

o Coaxial cable(Coax) is a very commonly used transmission media, for example,

TV wire is usually a coaxial cable.

o The name of the cable is coaxial as it contains two conductors parallel to each
other.
o It has a higher frequency as compared to Twisted pair cable.
o The inner conductor of the coaxial cable is made up of copper, and the outer
conductor is made up of copper mesh.
o The middle core is made up of non-conductive cover that separates the inner
conductor from the outer conductor.
o The middle core is responsible for the data transferring whereas the copper mesh
 prevents from the EMI(Electromagnetic interference).
o Common applications of coaxial cable are Cable TV networks and traditional
Ethernet LANs.

Coaxial Cable Standards

 Coaxial cables are categorized by their Radio Government (RG) ratings.
 Each RG number denotes a unique set of physical specifications, including the
wire gauge of the inner conductor, the thickness and type of the inner insulator,
the construction of the shield, and the size and type of the outer casing.
 Each cable defined by an RG rating is adapted for a specialized function.

Types of Coaxial cable :

1. Baseband transmission: It is defined as the process of transmitting a single
signal at high speed.
2. Broadband transmission: It is defined as the process of transmitting multiple
 signals simultaneously.
Advantages :

o The data can be transmitted at high speed.

o It has better shielding as compared to twisted pair cable.
o It provides higher bandwidth.
Disadvantages :
o It is more expensive as compared to twisted pair cable.
o If any fault occurs in the cable causes the failure in the entire network.

FIBRE OPTIC CABLE

o Fibre optic cable is a cable that uses electrical signals for communication.
o Fibre optic is a cable that holds the optical fibres coated in plastic that are used to
send the data by pulses of light.
o The plastic coating protects the optical fibres from heat, cold, electromagnetic
interference from other types of wiring.
o Fibre optics provide faster data transmission than copper wires.
Basic elements of Fibre optic cable:
o 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.
o 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.
o 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.

Advantages:
o Greater Bandwidth
o Less signal attenuation
o Immunity to electromagnetic interference
o Resistance to corrosive materials
o Light weight
o Greater immunity to tapping
 Disadvantages :
o Requires Expertise for Installation and maintenance
o Unidirectional light propagation.
o Higher Cost.

Propagation Modes of Fibre Optics

 Current technology supports two modes (multimode and single mode) for
propagating light along optical channels, each requiring fiber with different
physical characteristics.
 Multimode can be implemented in two forms: step-index or graded-index.

Multimode Propagation
 Multimode is so named because multiple beams from a light source move through
the core in different paths.
 How these beams move within the cable depends on the structure of the core.

Single-Mode Propagation

 Single-mode uses step-index fiber and a highly focused source of light that limits
beams to a small range of angles, all close to the horizontal.
  The single-mode fiber itself is manufactured with a much smaller diameter than
that of multimode fiber, and with substantially lower density (index of refraction).
 The decrease in density results in a critical angle that is close enough to 90° to
make the propagation of beams almost horizontal.
 In this case, propagation of different beams is almost identical, and delays are
negligible. All the beams arrive at the destination “together” and can be
recombined with little distortion to the signal.
UNGUIDED MEDIA
o An unguided transmission transmits the electromagnetic waves without using any
physical medium. Therefore it is also known as wireless transmission.
o In unguided media, air is the media through which the electromagnetic energy
can flow easily.

Unguided transmission is broadly classified into three categories : Radio Waves, Microwaves ,
Infrared

RADIO WAVES

o Radio waves are the electromagnetic waves that are transmitted in all the
directions of free space.
o Radio waves are omnidirectional, i.e., the signals are propagated in all the
directions.
o The range in frequencies of radio waves is from 3Khz to 1Ghz.
o In the case of radio waves, the sending and receiving antenna are not aligned, i.e.,
 the wave sent by the sending antenna can be received by any receiving antenna.
o An example of the radio wave is FM radio.

Applications of Radio waves:

o A Radio wave is useful for multicasting when there is one sender and many
receivers.
o An FM radio, television, cordless phones are examples of a radio wave.

Advantages of Radio waves:

o Radio transmission is mainly used for wide area networks and mobile cellular
phones.
o Radio waves cover a large area, and they can penetrate the walls.
o Radio transmission provides a higher transmission rate.

MICROWAVES
Microwaves are of two types - Terrestrial microwave & Satellite microwave

Terrestrial Microwave
o Terrestrial Microwave transmission is a technology that transmits the focused
beam of a radio signal from one ground-based microwave transmission antenna to
another.
 o Microwaves are the electromagnetic waves having the frequency in the range
from 1GHz to 1000 GHz.
o Microwaves are unidirectional as the sending and receiving antenna is to be
aligned, i.e., the waves sent by the sending antenna are narrowly focused.
o In this case, antennas are mounted on the towers to send a beam to another
antenna which is km away.
o It works on the line of sight transmission, i.e., the antennas mounted on the
towers are at the direct sight of each other.

Characteristics of Terrestrial Microwave:

o Frequency range: The frequency range of terrestrial microwave is from 4-6 GHz
to 21-23 GHz.
o Bandwidth: It supports the bandwidth from 1 to 10 Mbps.
o Short distance: It is inexpensive for short distance.
o Long distance: It is expensive as it requires a higher tower for a longer distance.
o Attenuation: Attenuation means loss of signal. It is affected by environmental
conditions and antenna size.

Advantages of Terrestrial Microwave:

o Microwave transmission is cheaper than using cables.
o It is free from land acquisition as it does not require any land for the installation
of cables.
o Microwave transmission provides an easy communication in terrains as the
 installation of cable in terrain is quite a difficult task.
o Communication over oceans can be achieved by using microwave transmission.

Disadvantages of Terrestrial Microwave:

o Eavesdropping.
o Out of phase signal
o Susceptible to weather condition
o Bandwidth limited

Satellite Microwave
o A satellite is a physical object that revolves around the earth at a known height.
o Satellite communication is more reliable nowadays as it offers more flexibility
than cable and fibre optic systems.
o We can communicate with any point on the globe by using satellite
communication.
o The satellite accepts the signal that is transmitted from the earth station, and
it amplifies the signal. The amplified signal is retransmitted to another earth station.

Advantages of Satellite Microwave:

o The coverage area of a satellite microwave is more than the terrestrial microwave.
o The transmission cost of the satellite is independent of the distance from
 the centre of the coverage area.
o Satellite communication is used in mobile and wireless
communication applications.
o It is easy to install.
o It is used in a wide variety of applications such as weather forecasting, radio/TV
signal broadcasting, mobile communication, etc.

Disadvantages of Satellite Microwave:

o Satellite designing and development requires more time and higher cost.
o The Satellite needs to be monitored and controlled on regular periods so that
it remains in orbit.
o The life of the satellite is about 12-15 years. Due to this reason, another launch of
the satellite has to be planned before it becomes non-functional.

INFRARED WAVES
o An infrared transmission is a wireless technology used for communication over
short ranges.
o The frequency of the infrared in the range from 300 GHz to 400 THz.
o It is used for short-range communication such as data transfer between two cell
phones, TV remote operation, data transfer between a computer and cell phone
and devices that resides in the same closed area.

Characteristics of Infrared:
o It supports high bandwidth, and hence the data rate will be very high.
o Infrared waves cannot penetrate the walls. Therefore, the infrared communication
in one room cannot be interrupted by the nearby rooms.
o An infrared communication provides better security with minimum interference.
o Infrared communication is unreliable outside the building because the sun rays
will interfere with the infrared waves.