Computer Networks

1. Network
A group of devices that can communicate with each other over links. Each device is called a host. Each host has a
unique address

Transmission Modes: Transmission mode means transferring of data between two devices. It is also known as
communication mode. There are three types of transmission mode(i) Simplex Mode Half (ii)Duplex Mode Full
(iii)Duplex Mode

(i) Simplex mode, the communication is unidirectional, as on a one-way . Only one is sender or transmitter and
the other can only receive. Ex: Radio, Television

(ii) Half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is
sending, the other can only receive, and vice versa. Ex: Walky-Talky

(iii) Full-duplex mode, both stations can transmit and receive simultaneously. In full_duplex mode, signals going
in one direction share the capacity of the link with signals going in other direction.Ex. Telephone
2. Types of area networks – LAN, MAN and WAN
The Network allows computers to connect and communicate with different computers via any medium. LAN, MAN
and WAN are the three major types of the network designed to operate over the area they cover. There are some
similarities and dissimilarities between them. One of the major differences is the geographical area they cover,
i.e. LAN covers the smallest area; MAN covers an area larger than LAN and WAN comprises the largest of all.
There are other types of Computer Networks also, like :
 PAN (Personal Area Network)
 SAN (Storage Area Network)
 EPN (Enterprise Private Network)
 VPN (Virtual Private Network)

3. Data Communication Terminology

Physical medium like cables over which information is exchanged is called channel. Transmission channel may
be analog or digital. As the name suggests, analog channels transmit data using analog signals while digital channels
transmit data using digital signals.

In popular network terminology, path over which data is sent or received is called data channel. This data channel
may be a tangible medium like copper wire cables or broadcast medium like radio waves.

Data Transfer Rate

The speed of data transferred or received over transmission channel, measured per unit time, is called data transfer
rate. The smallest unit of measurement is bits per second (bps). 1 bps means 1 bit (0 or 1) of data is transferred in 1
second.

Here are some commonly used data transfer rates −

1 Bps = 1 Byte per second = 8 bits per second

1 kbps = 1 kilobit per second = 1024 bits per second

1 Mbps = 1 Megabit per second = 1024 Kbps

1 Gbps = 1 Gigabit per second = 1024 Mbps

Bandwidth

Data transfer rates that can be supported by a network is called its bandwidth. It is measured in bits per second (bps).
Modern day networks provide bandwidth in Kbps, Mbps and Gbps. Some of the factors affecting a network’s
bandwidth include −

(i) Network devices used (ii) Protocols used (iii) Number of users connected (iv)Network overheads like collision,
errors, etc.
4. Transmission Media
Classification of Transmission Media:
Guided Media

Coaxial cable
Coaxial cable is very commonly used transmission media, for example, TV wire is usually a coaxial cable

Coaxial cable is of two types:

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.

Fibre Optic

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.
 Twisted pair:

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 range for twisted pair cable is from 0 to 3.5KHz.

An unshielded twisted pair is widely used in telecommunication and it is used for telephone lines that have low-speed
data.
A shielded twisted pair is a cable that contains the mesh surrounding the wire that allows the higher transmission
rate.

Unguided Media

It is also called wireless communication or unbounded transmission, they transmit electromagnetic waves without
using a physical conductor. In this medium signals are radiated through the air (or, in a few cases, water) and
therefore, are reaching to anyone with a device capable of accepting them.

We can categorize wireless transmission into the following groups,

a. Radio waves
b. Micro waves
c. Infrared waves

Radio Waves

Radio waves are electromagnetic waves. When an antenna transports radio waves they are propagated in all
directions in free space which means the sending and receiving antennas do not have to be aligned that is any
receiving antenna can receive that transmitted wave.

The frequency of radio waves about 30 hertz (Hz) to 300 gigahertz (GHz) and like all other electromagnetic waves
radio waves travel at the speed of light in vacuum.

 These waves are omnidirectional so they are useful for multicasting in which one sender but many receivers.
 Examples of radio waves are television, AM and FM radio, cordless phones, and paging.
Micro Waves
Micro Waves includes a line of sight transmission that is the sending and receiving antennas that need to be properly
aligned with each other. The distance is directly proportional to the height of the antenna which is covered by the
signal. In mobile phone communication and television distribution, these are majorly used. Due to the unidirectional
properties of Micro Waves, they are very useful when unicast (one-to-one) communication is needed between the
sender and the receiver. Cellular phones, satellite networks, and wireless LANs are using Micro Waves.

Infrared Waves

The frequency of Infrared waves is about 300 GHz to 430 THz, which can be used for short-range communication.
Infrared waves of high frequencies cannot penetrate walls. This characteristic of Infrared waves prevents interference
between one system and another. This means a short-range communication system in a room cannot be affected by
another system in the adjacent room. The Infrared Data Association (IrDA) has established standards for using these
signals for communication between devices such as keyboards, mouse, PCs, and printers and it is also responsible for
sponsoring the use of infrared waves.

Satellite communication, in telecommunications, the use of artificial satellites to provide communication links
between various points on Earth. Satellite communications play a vital role in the global telecommunications system.
Approximately 2,000 artificial satellites orbiting Earth relay analog and digital signals carrying voice, video, and data
to and from one or many locations worldwide.
5. Network Devices

(i) Modem: Short for modulator-demodulator. A modem is a device that enables a computer to transmit data
over, for example, telephone or cable lines. Computer information is stored digitally, whereas information
transmitted over telephone lines is transmitted in the form of analog waves.

(ii) Hub: A hub is the most basic networking device that connects multiple computers or other network devices
together. Unlike a network switch or router, a network hub has no routing tables or intelligence on where to
send information and broadcasts all network data across each connection.

(iii) Switch: Switches facilitate the sharing of resources by connecting together all the devices, including
computers, printers, and servers, in a small business network. Thanks to the switch, these connected devices
can share information and talk to each other, regardless of where they are in a building or on a campus.
Building a small business network is not possible without switches to tie devices together.

(iv) Router: A router works as a dispatcher, directing traffic and choosing the most efficient route for information,
in the form of data packets, to travel across a network. A router connects your business to the world,
protects information from security threats, and even decides which devices have priority over others.

(v) Repeater: A repeater operates at the physical layer. Its job is to regenerate the signal over the same network
before the signal becomes too weak or corrupted so as to extend the length to which the signal can be
transmitted over the same network

(vi) Bridge: It is used for interconnecting two LANs working on the same protocol. It has a single input and single
output port, thus making it a 2 port device.
(vii) Gateway : A gateway, as the name suggests, is a passage to connect two networks together that may work
upon different networking models. They basically work as the messenger agents that take data from one
system, interpret it, and transfer it to another system. Gateways are also called protocol converters and can
operate at any network layer. Gateways are generally more complex than switch or router.

(viii) RJ45 Connection: RJ45 is a type of connector commonly used for Ethernet networking. It looks similar to a
telephone jack, but is slightly wider. Since Ethernet cables have an RJ45 connector on each end, Ethernet
cables are sometimes also called RJ45 cables.

(ix) Ethernet Card: An Ethernet card is the communications hub for your computer; it connects to a network
using a network cable. Ethernet cards can also communicate one-on-one with another Ethernet card,
allowing for peer-to-peer network connections these are useful for direct file sharing.

(x) WiFi Card: Wireless Internet cards, also known as Local Area Network, or LAN, cards, are one of the many
types of adapter cards that add capabilities to your computer.
6. Network Topologies
The arrangement of a network which comprises of nodes and connecting lines via sender and receiver is referred as
network topology. The various network topologies are :

BUS Topology :Bus topology is a network type in which every computer and network device is connected to single
cable. When it has exactly two endpoints, then it is called Linear Bus topology.

Features of Bus Topology 1.It transmits data only in one direction. 2.Every device is connected to a single cable
Advantages of Bus Topology 1. It is cost effective.2. It is easy to understand.3.Easy to expand joining two cables
together.
Disadvantages of Bus Topology1.Cables fails then whole network fails.2.Cable has a limited length.

RING Topology It is called ring topology because it forms a ring as each computer is connected to another computer,
with the last one connected to the first. Exactly two neighbours for each device.

Features of Ring Topology 1. The transmission is unidirectional, but it can be made bidirectional by having 2
connections between each Network Node, 2.Data is transferred in a sequential manner that is bit by bit. Data
transmitted, has to pass through each node of the network, till the destination node.
Advantages of Ring Topology 1.Transmitting network is not affected by high traffic or by adding more nodes, as only
the nodes having tokens can transmit data.2.Cheap to install and expand
Disadvantages of Ring Topology 1.Troubleshooting is difficult in ring topology.2.Failure of one computer disturbs the
whole network.

STAR Topology: In this type of topology all the computers are connected to a single hub through a cable. This hub is
the central node and all others nodes are connected to the central node.
 Features of Star Topology:1.Every node has its own dedicated connection to the hub.2.Hub acts as a repeater for data
flow.
Advantages of Star Topology:1.Fast performance with few nodes and low network traffic.2.Easy to
troubleshoot.3.Easy to setup and modify.4.Only that node is affected which has failed, rest of the nodes can work
smoothly.
Disadvantages of Star Topology:1.Cost of installation is high.2.If the hub fails then the whole network is stopped
because all the nodes depend on the hub.

TREE Topology It has a root node and all other nodes are connected to it forming a hierarchy. It is also called
hierarchical topology. It should at least have three levels to the hierarchy.

Features of Tree Topology:1. Ideal if workstations are located in groups.2.Used in Wide Area Network.
Advantages of Tree Topology:1.Expansion of nodes is possible and easy.2.Error detection is easily done.
Disadvantages of Tree Topology:1.Heavily cabled.2.Costly.3.Central hub fails, network fails.

MESH Topology It is a point-to-point connection to other nodes or devices. All the network nodes are connected to
each other. Mesh has n(n-1)/2 physical channels to link n devices.

Features of Mesh Topology: 1.Fully connected.2.Robust.3.Not flexible.

Advantages of Mesh Topology: 1.Each connection can carry its own data load.2.Provides security and privacy.
Disadvantages of Mesh Topology: 1.Installation and configuration is difficult.2.Cabling cost is more.3.Bulk wiring is
required.

7. Sending data across the network (Switching Technique)

 Circuit Switched Networks − Circuit switched networks are connection-oriented networks. Here, a dedicated route is
established between the source and the destination and the entire message is transferred through it.
Packet Switched Networks − Packet switched networks are connectionless networks. Here, the message is divided
and grouped into a number of units called packets that are individually routed from the source to the destination.

8. Internet
Introduction to Internet: The Internet is a global network of billions of computers and other electronic devices. With
the Internet, it's possible to access almost any information, communicate with anyone else in the world, and do much
more.You can do all of this by connecting a computer to the Internet, which is also called going online. When
someone says a computer is online, it's just another way of saying it's connected to the Internet.

The World Wide Web: WWW usually called the Web for short is a collection of different websites you can access
through the Internet. A website is made up of related text, images, and other resources. Websites can resemble
other forms of media like newspaper articles or television programs or they can be interactive in a way that's unique
to computers.

ARPANET: Advanced Research Projects Agency Network, experimental computer network that was the forerunner
of the Internet. The Advanced Research Projects Agency (ARPA), an arm of the U.S. Defence Department, funded the
development of the Advanced Research Projects Agency Network (ARPANET) in the late 1960s. Its initial purpose was
to link computers at Pentagon-funded research institutions over telephone lines.

URL: A Uniform Resource Locator (URL), otherwise known as a Universal Resource Locator, is the address of a
resource on the Internet and the protocol used to access it. It indicates the location of a web resource like a street
address indicates where a person lives physically because of this, an URL is often referred to as: “web address”.

Application Web:
E-mail is Electronic Mail. E-mail works on Simple Network Management Protocol (SNMP). Using e-mail we can send
text, audio and video files to one person or a group of persons. Free E-mail can be created in Google, Yahoo, Hotmail,
Netscape, Lycos, etc., the following shows sequence of steps to create email account in Gmail which is a part of
Google.com. To create email account in yahoo site, hotmail site or any other site same sequence of steps can be
followed

INTERNET CHAT SESSION Chat is meant for live discussion using text and multimedia features. On the left bottom of
the window, click on chat option; add user email identity with whom we want to chat. Confirmation comes from the
recipient and the chat becomes a two way process. Internet chat provides a facility to have live chat between many
persons or group of persons. It is a multi way chat technique. Internet Relay Chat (IRC) provides the facility of internet
chat. Each chat group has as separate channel maintained my IRC architecture. It is normally not meant for one-to-
one chat.

TELNET (TErminal NETwork) is a protocol that enables the user to log on a different system and use the resources.
The different system (remote) can be located in same building, city, and country or geographically located in another
place of the world. The system where the user works physically (local system) is called client and the remote system
is called host. Therefore, telnet follows client/server model. To access the host system, user should know the IP
address of that system. If user has login account and password on remote system, then it is called full-privilege telnet
session, otherwise user can login as guest and share resources (files, memory and CPU) Restricted telnet connection
is called anonymous/guest telnet session

web page: A document which can be displayed in a web browser such as Firefox, Google Chrome, Opera, Microsoft
Internet Explorer or Edge, or Apple's Safari. These are also often called just "pages."

Website: A collection of web pages which are grouped together and usually connected together in various ways.
Often called a "web site" or simply a "site."

web server :A computer that hosts a website on the Internet.

Web hosting : web hosting is an online service that enables you to publish your website or web application on the
internet. When you sign up for a hosting service, you basically rent some space on a server on which you can store all
the files and data necessary for your website to work properly.

Difference between Static and Dynamic Web Pages

Static Web pages:Static Web pages are very simple. It is written in languages such as HTML, JavaScript, CSS, etc. For
static web pages when a server receives a request for a web page, then the server sends the response to the client
without doing any additional process. And these web pages are seen through a web browser. In static web pages,
Pages will remain the same until someone changes it manually.

Dynamic Web Pages:Dynamic Web Pages are written in languages such as PHP,JSP, ASP, ASP.NET, etc. In dynamic
web pages, the Content of pages is different for different visitors. It takes more time to load than the static web page.
Dynamic web pages are used where the information is changed frequently, for example, stock prices, weather
information, etc.

Difference Between Server-side Scripting and Client-side Scripting: A script is generally a series of program or
instruction, which has to be executed on other program or application. As we know that the web works in a client-
server environment. The client-side script executes the code to the client side which is visible to the users while a
server-side script is executed in the server end which users cannot see. . The main difference between server-side
scripting and client-side scripting is that the server side scripting involves server for its processing. On the other hand,
 client-side scripting requires browsers to run the scripts on the client machine but does not interact with the server
while processing the client-side scripts.

A web browser, or simply "browser," is an application used to access and view websites. Common web browsers
include Microsoft Internet Explorer, Google Chrome, Mozilla Firefox, and Apple Safari.The primary function of a web
browser is to render HTML, the code used to design or "mark up" webpages. Each time a browser loads a web page,
it processes the HTML, which may include text, links, and references to images and other items, such as cascading
style sheets and JavaScript functions. The browser processes these items, then renders them in the browser window.

9. Network Protocol
A protocol is a set of rules. A network protocol is a set of rules followed by the network. Network protocols are
formal standards and policies made up of rules, procedures and formats that defines communication between two or
more devices over a network.

Transmission Control Protocol (TCP):The Transmission Control Protocol is the core protocol of the internet protocol
suite. It originated in the network implementation in which it complemented the Internet Protocol. Therefore the
entire suite is commonly referred to as TCP/IP. TCP provides reliable delivery of a stream of octets over an IP
network.

Internet Protocol(IP):The Internet Protocol is the principal protocol in the Internet protocol suite for relaying data
across networks. Its routing function essentially establishes the internet. Historically it was the connectionless
datagram service in the original Transmission Control Program; the other being the connection oriented
protocol(TCP). Therefore, the Internet protocol suite is referred as TCP/IP.

Hypertext Transfer Protocol (HTTP):The HTTP is the foundation of data communication for the World Wide Web. The
hypertext is structured text that uses hyperlinks between nodes containing texts. The HTTP is the application protocol
for distributed and collaborative hypermedia information system.

File Transfer Protocol (FTP):The FTP is the most common protocol used in the file transferring in the Internet and
within private networks.

Simple Mail Transfer Protocol (SMTP):SMTP is used for two primary functions. It is used to transfer email from source
to destination between mail servers and it is used to transfer email from end users to a mail system.

Domain Name System (DNS):Domain name system is used to convert the domain name to IP address. There are root
servers, TLDs and authoritative servers in the DNS hierarchy.

Post Office Protocol version 3 (POP 3):The Post Office Protocol version 3 is one of the two main protocols used to
retrieve mail from the internet. It is very simple as it allows the client to retrieve complete content from the server
mail box and deletes contents from the server.

The Point-to-Point Protocol (PPP): is an encapsulation protocol for transporting IP traffic across point-to-point links.
PPP is made up of three primary components
 Voice Over Internet Protocol :VoIP services convert your voice into a digital signal that travels over the Internet. If you
are calling a regular phone number, the signal is converted to a regular telephone signal before it reaches the
destination.

10. Mobile Technology:

A mobile phone is a wireless handheld device that allows users to make and receive calls and to send text messages,
among other features. The earliest generation of mobile phones could only make and receive calls. Today’s mobile
phones, however, are packed with many additional features, such as web browsers, games, cameras, video players
and even navigational systems.A mobile phone may also be known as a cellular phone or simply a cell phone.

GSM stands for Global System for Mobile Communication. GSM digital mobile telephony system which is widely used
in all over the world.GSM digitizes and compresses the data and then send it to the down channel with two other
streams of user data. GSM operates on two frequency bands i.e either 900MHZ or 1800 MHz.

GPRS stands for General Packet Radio Service. These are packet-based wireless communication service which
provides data rates ranging from 56 to 114kbps The key element of GPRS technology is that it uses packet-switched
data technique instead of circuit switching, and this technique makes much more efficient use of the available
capacity.GPRS operates on one or more frequencies out of frequency bands which supports the radio
(850,900,1800,900MHZ).

CDMA(Code Division Multiple Access) –A technology used to transmit wireless calls by assigning them codes. Many
users share the same frequency

WLL: Wireless Local Loop Initially, the telephones were connected via copper cables buried in the ground where
speech was converted into Electromagnetic Signals and sent via these cables. The other end will receive this Signals
and convert them again in the voice.

MOBILE GENERATION TECHNOLOGY

First Generation (1G):First generation mobile networks were reliant upon analog radio systems which meant that
users could only make phone calls, they couldn’t send or receive text messages. The 1G network was first introduced
in Japan in 1979.

Second Generation (2G):This mobile network ran on digital signal, which vastly improved its security but also its
capacity. On 2G, users could send SMS and MMS messages (although slowly and often without success) and when
GPRS was introduced in 1997, users could receive and send emails on the move.

Third Generation (3G): 3G was much faster and could transmit greater amounts of data. This means that users could
video call, share files, surf the internet, watch TV online and play online games on their mobiles for the first time.
 Fourth Generation (4G): It’s five times faster than the 3G network – and can in theory provide speeds of up to
100Mbps. Under 4G, users can experience better latency (less buffering), higher voice quality, easy access to instant
messaging services and social media, quality streaming and make faster downloads.

Fifth Generation (5G):The 5G network is yet to be released but is widely anticipated by the mobile industry. Many
experts claim that the network will change not just how we use our mobiles, but how we connect our devices to the
internet. The improved speed and capacity of the network will signal new IoT trends, such as connected cars, smart
cities and IoT in the home and office.

11. Network Security Concepts

Network security is the process of taking preventative measures to protect the underlying networking infrastructure
from unauthorized access, misuse, modification, destruction or improper disclosure.

Threat can be anything that can take advantage of a vulnerability to breach security and negatively alter, erase, harm
object or objects of interest.

Software attacks means attack by Viruses, Worms, Trojan Horses etc. Many users believe that malware, virus,
worms, bots are all same things. But they are not same, only similarity is that they all are malicious software that
behave differently.

Malware is a combination of 2 terms- Malicious and Software. So Malware basically means malicious software that
can be an intrusive program code or a anything that is designed to perform malicious operations on system. Malware
can be divided in 2 categories: (i) Infection Methods (ii) Malware Actions

(i) Malware on the basis of Infection Method are following:

Virus – They have the ability to replicate themselves by hooking them to the program on the host computer like
songs, videos etc and then they travel all over the Internet Ex: File Virus, Boot Sector Virus etc

Worms – Worms are also self replicating in nature but they don’t hook themselves to the program on host
computer. Biggest difference between virus and worms is that worms are network aware. They can easily travel
from one computer to another if network is available and on the target machine they will not do much harm,
they will for example consume hard disk space thus slowing down the computer

Trojan – The Concept of Trojan is completely different from the viruses and worms. They often provide backdoor
gateway for malicious programs to enter your system and steal your valuable data without your knowledge and
permission. Examples include FTP Trojans, Proxy Trojans, Remote Access Trojans etc.

(ii) Malware on the basis of Actions:

Adware – Adware is not exactly malicious but they do breach privacy of the users. They display ads on
computer’s desktop or inside individual programs. They come attached with free to use software, thus main
source of revenue for such developers.

Spyware – It is a program or we can say a software that monitors your activities on computer and reveal
collected information to interested party. Ex: KEYLOGGER
 Network Protection:

Firewall:A firewall is software or firmware that prevents unauthorized access to a network. It inspects incoming
and outgoing.

HTTP VS. HTTPS – WHAT’S THE DIFFERENCE

HTTP stands for Hypertext Transfer Protocol, and is an application layer protocol. In simpler terms, it is the
protocol over which information is sent from a user’s web browser to the website they are visiting. The data that
is being communicated between the browser and the website is sent over in plain text, meaning that if someone
intercepted the connection between the two, they would easily be able to see the information you were both
viewing and sending on the website. This is especially dangerous when users are filling out sensitive information,
like a credit card number at checkout on Amazon, or entering location information on Facebook.

While HTTPS is essentially the same concept as HTTP, the “S” harbors one big difference between the two:
security. HTTPS stands for Hypertext Transfer Protocol Secure, and instead of acting as its own application layer
protocol, it uses separate protocols called SSL (Secure Sockets Layer) and TLS (Transport Layer Security).The SSL
also encrypts the information that is being sent, which means that the true meaning of the data (credit card
numbers, personal information, etc.) is very difficult to be cracked by anyone trying to see the information.
Nowadays, the majority of web browsers support HTTPS for more secure Internet browsing.

Cookie: A cookie is information stored on your computer by a website you visit. In some browsers, each cookie is
a small file. A cookie (called an Internet or Web cookie) is the term given to describe a type of message that is
given to a web browser by a web server.

Tips to use internet banking safely

1. Download mobile payment apps only from official stores such as Google Play and Apple Store

2. Carefully read the permissions that the app asks for. If you think a mobile payment app is asking for more
than what is required, do not install it. If you have any doubts regarding the permissions, just contact the
app’s manufacturer via their Twitter handle.

3. Never visit an online banking or shopping website by clicking on a link received in an email or text message
mostly these are phishing site or link

4. Always choose a strong password for accounts for net banking or online payment apps. It should be a mix of
uppercase and lowercase letters and special characters.

5. Do not use unsecured, public Wi-Fi networks for onling banking or shopping. Doing so may let an attacker
steal your information.

6. Only use established and well-known websites to for online shopping and paying for utilities.

7. Ensure your banking transactions are OTP (one time password) enabled. While paying a purchase through net
banking, debit/credit card, you will enter your login ID and password (or card details) and also an OTP (code
sent to your registered mobile number) before you can make the final payment. So, even if an attacker
manages to steal your net banking/card details, payment won’t go through without the OTP.