The transfer of data from one machine to another machine such that the sender and
receiver both interpret the data correctly is called data communication. Data communication
is concerned with the transmission of digital data. This transfer of data is in the form of bits 0
and 1. Some form of transmission medium is necessary for this exchange of data.
Components of Data Communication System:
A data communication system is made up of five components. These are:
1. Message: The message is in the information or data to be communicated. A message
may consist of text, picture, audio, video or a combination of these.
2. Sender: The device that sends the message is called the sender. This may be a
computer, a video camera, a Web camera, or a telephone instrument.
3. Medium: The physical path through which a message travels from the sender to the
receiver is called the medium. E.g., coaxial cable wire, twisted pair wire, fiber-optic
cable, radio waves, etc. The word “channel” is also used to refer to the medium
between the sender and the receiver.
4. Receiver: The device that receives the message is called the receiver. This can be a
computer, a television or radio set, or a mobile (telephone) instrument.
5. Protocol: A protocol is a set of rules that control the process of data communication.
When two or more devices exchange data, there must be agreements on how this
exchange will take place. E.g., at what speed will the sender transmit data, in which
language, what if errors take place during the process o data transmission, and how
these errors take place during the process of data transmission, and how these errors
will be corrected, how to know that all the data has been transmitted. All these issues
are handled by a set of rules called the protocol.
Distributed processing means that processing will
occur on more than one processor. These multiple
computers communicate with each other through a
computer network. In distributed processing, a task is
divided among many computers. Thus, a single
computer is not responsible for all the processing, but
each separate computer handles a part of processing
Applications of Distributed Processing:
1. Internet, World Wide Web
2. Scientific computing, including grid computing
3. Real-time process control, e.g., nuclear power plant control system, aircraft control
4. Banking system
5. Airline and railway ticket reservation system
6. Telecommunications, cellular networks
Advantages of Distributed Processing:
1. Distributed database: One computer system cannot have the capacity to store all
the data of an organization. In such a case, the database is distributed among multiple
2. Security: The system can be designed so that a user can perform a limited set of
operations and on a small set of the database. E.g., a bank may allow access to its
database through an automated teller machine (ATM) but these transactions are
limited only to the user‟s account, and not on the entire database.
3. Faster problem solving: When many computers are working at the same time on
different parts of a problem, the problem can be solved faster than by a single
machine working alone. Typical applications are breaking encryption codes,
complicated scientific calculations, weather forecasting, etc.
4. Collaborative processing: Multiple computers and multiple users may interact
on a single task.
5. Security through redundancy: Multiple computers running the same program
at the same time provide security through redundancy. If one computer fails, the other
can carry on the job.
Definition: A computer network is an interconnection of two or more computers such that
they can share resources and information. These computers can be linked together using a
wide variety of cables, telephone lines, or through satellites. The computers in a network
communicate with each other using a set of rules known as PROTOCOL.
Types of computer networks:
All computer networks can be classified into three categories based on the following factors:
a) Size (number of computers) of the network,
b) Geographical range of the network i.e., the distance it covers,
c) Ownership i.e., owned by a single organization, a public body like the municipal
corporation, a national organization like the Indian Railway or airlines, or a global
The 3 types of computer networks are: Local Area Network (LAN), Metropolitan Area
Network (MAN) and Wide Area Network (WAN).
Local Area Network (LAN):
A local area network is a network of computers in a limited geographical region, e.g., within
a room or a building. It is an interconnection of many workstations and a file server through a
central connecting device called a hub.
Metropolitan Area Network (MAN):
A metropolitan area network (MAN) is a network that
interconnects users with computer resources in a
geographic area or region larger than that covered by
even a large local area network (LAN) but smaller than
the area covered by a wide area network (WAN). The
term is applied to the interconnection of networks in a
city into a single larger network (which may then also
offer efficient connection to a wide area network). It is
also used to mean the interconnection of several local
area networks by bridging them with backbone lines.
The latter usage is also sometimes referred to as a
Wide Area Network (WAN):
A WAN covers large geographic area with
various communication facilities such as
long distance telephone services, satellite
transmission, and under-sea cables. The
WAN typically involves best computers,
network and much different type of
communication hardware and software.
E.g. Interstate banking networks and
Airline reservation systems. WAN typically
operate at lower link speeds (about 1
The way in which individual computers (called nodes) on a network interact with each other
computers depends on how they are organized. The two basic types of network structures (or
architectures) are: server-based (client-server model), and peer-to-peer network.
1. Client-Server model (Server-based): In this type of network, certain
computers called servers are dedicated to provide various services to the other
computers, called clients. The client and server are connected through a central hub.
2. Peer-to-Peer Network:
In a peer-to-peer network (also called P2P
network), there are no dedicated servers.
No single computer is assigned the role of
providing services to all other computers.
Each machine works as both a server as
well as a client. The word “peer” means
“equal”, and therefore in this type of
network, all computers are equal
The geometrical arrangement of computer resources, remote devices, and communication
facilities is known as network topology. A computer network is comprised of nodes and
links. A node is the end point of any branch in the computer, a terminal device; workstation
etc. a link is a communication path between two nodes.
There are 4 main topologies which are discussed below:
1. Star Topology: This is the most common arrangement of computer systems and
links between them. All devices are connected to central hub. The function of the hub
is to deliver the data to and from the computers. The hub may be a server computer
2. Bus Topology: This topology is very popular with local networks. A single cable
runs in the building and all nodes are linked along with this communication line. This
cable is called the bus or backbone. The two ends of the cable are terminated with
terminators. We can imagine a bus picking up various people from one bus stop and
dropping off people as it travels, and then picking up a few more.
3. Ring Topology: Ring topology is also used for local area networks. In this
arrangement, a network cable passes from one node to another until all odes are
connected in the form of a ring or loop. There is a direct point-to-point link between
4. Mesh Topology: In this, there is network random connection of nodes using
communication links. This network lines are planned carefully after serious thoughts
to minimize cost.
Network media refers to the medium used in computer networks for transmission of data. It is
the actual path over which an electrical signal travels as it moves from one computer/device
to another. This data transmission can take place either through wired media or wireless.
There are 2 basic categories of Transmission Media:
1. Guided and
A. Guided : Guided Transmission Media uses a "cabling" system that guides the data signals
along a specific path. The data signals are bound by the "cabling" system. Guided Media is
also known as Bound Media. Cabling is meant in a generic sense in the previous sentences
and is not meant to be interpreted as copper wire cabling only.
B. Unguided: Unguided Transmission Media consists of a means for the data signals to
travel but nothing to guide them along a specific path. The data signals are not bound to a
cabling media and as such are often called Unbound Media.
Wireless Media: Wireless communication uses radio frequencies (RF) or infrared (IR)
waves to transmit data between devices on a Local Area Network. In a wireless LAN, a major
component is the wireless hub, or access point, used for signal distribution.
Wireless Internet transfers digital data using radio frequency signals.
Wireless connectivity is useful for business travelers who need access to Internet in
cafes & airport.
Millions of computers of different types are connected on the Internet. Some of these
computers may be running UNIX operating systems while others may be running under
Windows operating systems. For all these computers to communicate with each other, they
must speak a common language and follow some common rules.
The set of rules, which enables different types of computers to communicate with each other,
is called protocol. The protocol used by the Internet for transferring messages from one
computer to another is called the Transmission Control Protocol/Internet Protocol (TCP/IP).
This term actually refers to two parts:
Transmission Control Protocol (TCP): The job of the TCP is to break down every
piece of data into small parts called packets. Each such packet also contains the
address of the sender and the receiver.
Internet Protocol: The job of the Internet Protocol (IP) is to find out how the data
will go from point A to point B passing through a series of routers. These packets of
data are sent over different paths until they reach their final destination. This is
somewhat similar to the way regular mail passes through several post offices before it
reaches its final destination. As the packets arrive at the destination, TCP takes over
again, identifying each packet and checking to see if it is intact. Once it has received
all the packets, TCP reassembles them into the original.
TCP/IP (TRANSMISSION CONTROL PROTOCOL/INTERNET PROTOCOL):
The protocol used on the internet is called TCP/IP. The TCP/IP has four layers:
1. The Application layer
2. The Transport layer
3. The Internet layer
4. The Network Interface layer
OSI Model (OPEN SYSTEM INTERCONNECTION MODEL):
This has been outline by international organization of standardization (ISO) to facilitate
communication of software platforms each other. OSI MODEL is an abstract description for
layered communication and computer network protocol design. The seven types of layers are
1. Application layer.
2. Presentation layer.
3. Session layer.
4. Transport layer.
5. Network layer.
6. Data Link layer.
7. Physical layer.
Definition: The Internet is a worldwide collection of computer networks, co-operating with
each other to exchange data using a common software standard. It can also be defined as “the
global communications systems of diverse, INTER connected computer NET works for the
exchange of information”. The Internet is also called as the Information Superhighway.
E-mail is the short form of electronic mail. It can be defined as the exchange of messages
and computer files between computers over a computer network. It is the method through
which messages can be sent to other Internet users. It is the most frequently used component
of the Internet. It has also been termed as the “killer app” of the Internet.
The word „blog‟ is a short form of the term weblog. It is a website, usually maintained by an
individual. The various features of a blog can be summarized as follows:
A blog is a personal diary, but put up on the Web so as to share information
A weblog is a collection of text, images, and media files, arranged in reverse
The activity of updating a blog is called “blogging”
A person who keeps a blog is called a “blogger”
Blogs are often written on a particular topic – e.g., photography, politics,
entertainment, sports, etc
Blogs are updated very frequently – often everyday
As new entries are put up on a blog, the older entries are archived. These old entries
are not available on the mail blog website but can be seen in the Archives section of
Comments: This is a special feature of a blog. Visitors to a blog can leave behind their
comments on a form
The term IP stands for Internet Protocol. Each device connected to the Internet must be
uniquely identified. This is similar to the telephone system where each subscriber has a
unique telephone number. An IP address is a unique number assigned to every device that is
on the Internet. A typical address may be look like this: 184.108.40.206. This is called as
DOTTED QUAD NOTATION. There are 4 parts (hence the word quad) of the address,
separated by the dot (.) symbol. In each part, the number can vary from 0 to 255. There are no
leading zeros in an IP address. Thus, the following is an invalid address: 123.45.067.89. A
mixture of decimal and binary notation is not allowed. So the following address in invalid:
11011000.27.61.137. IP address is unique. Each address defines one & only one connection
on the internet. Two devices on the internet can never have the same address. But if a device
has two connections to the internet, via two networks, it has two IP address.
Every computer on the Internet has a unique IP address. But this numeric IP address is
difficult to remember. Just imagine if you have to call all your friends by their mobile
numbers instead of their names! It would be impossible. The domain name is the computer
name that can be used instead of an IP address. Domain names are easier to remember for
human beings; e.g., it is easier to remember the address of Microsoft website as
www.microsoft.com rather than IP address 220.127.116.11. The Domain Name System (DNS
was developed in 1984 by Dr. Paul Mockapetris)
Domain Name System: DNS is the short form of Domain Name System. It is an
Internet services that translates domain names into IP addresses. Because domain
names are alphabetic, they‟re easy to remember. The Internet is however based on IP
Uniform Resource Locator (URL):
Each web page has a unique address called a Uniform Resource Locator (URL). Web address
looks like this:
Each URL consists of 3 parts:
i. A PROTOCOL part
ii. A HOST part
iii. A DOCUMENT part
http:// tells the Web browser that it is making a World Wide Web connection. http is
the abbreviation of Hyper Text Transfer Protocol
www.economictimes.com is the location of the web server, that is, the computer on
which the web pages are stored
Index.html is the filename of the web page that you are trying to access. Usually, this
is the default page loaded by the browser when you enter a website. This is also called
the home page.
Search engines allow users to find specific documents through keyword searches. If you have
to search for, say information on monuments then just typing monuments will result in
thousands of sites being listed that contain information about monuments. However, if you
need information only about monuments in India, in South India, then entering the key words
monuments, South India will result in more focused search. A search engine is a tool that
searches for websites based on words that we give as search terms. Search engines look
through their own databases of information in order to find what it is that you are looking for.
Meta Search Engines:
A Meta search engine is a search tool that doesn‟t create its own database of information, but
instead searches those of other engines. “Metacrawler” and Dogpile, for instance search the
databases of each of the other search engines such as Lycos, WebCrawler, Excite, AltaVista
and Yahoo. Using multiple databases will mean that the search results are more
comprehensive, but slower to obtain. The advantage of using a meta search engine is that the
user does not have to type the same query separately in different search engines.