Introduction to Computer Networks

 unit-III 

Introduction to Computer Networks

Introduction:

A computer network is a set of nodes like computers and networking devices that are connected through communication for the purpose of communication and sharing resources(hardware/software) among the users.

Networks are used to: (Benefits of computer network) 

• Facilitate communication through email / video conferencing / instant messaging or any other mode. 
• Share hardware devices like a printer or scanner 
• Enable file sharing 
• Share software or operating programs 
• Share information 

Disadvantages of computer network 

• Lack of robustness, 
• security issue, 
• cost of network

Need of Computer Network

• Resource Sharing :- Resource Sharing means to make all programs, data and peripherals available to anyone on the network irrespective of the physical location of the resources and the user. 
• Reliability :- Reliability means to keep the copy of a file on two or more different machines, so if one of them is unavailable (due to some hardware crash or any other) them its other copy can be used. 
• Cost Factor :- Cost factor means it greatly reduces the cost since the resources can be shared.

Evolution of networking

ARPANET (Advanced Research Projects Agency NETwork): In 1969, The US govt. formed an agency named ARPANET to connect computers at various universities and defense agencies. The main objective of ARPANET was to develop a network that could continue to function efficiently even in the event of a nuclear attack.

Internet (INTERconnection NETwork): The Internet is a worldwide network of computer networks. It is not owned by anybody. The internet has evolved from ARPANET. The internet is a globally connected network system that utilizes TCP/IP to transmit information. following services are instantly available through internet : Email, Web-enabled audio/video conferencing services, Online movies and gaming , Data transfer/file-sharing, Instant messaging , Internet forums , Social networking , Online shopping ,Financial services.

Interspace: is a client/server software program that allows multiple users to communicate online with real –time audio, video and text chat in dynamic 3D environments. 

Elementary Terminology of Networks :-

1. Nodes (Workstations):- The term nodes refer to the computers that are attached to a network and are seeking to share the resources.

2. Server:- A computer that facilitates the sharing of data, software and hardware resources on the network.

Computer Network: Types

1. Personal Area Network (PAN)

2. Local Area Network (LAN) 

3. Metropolitan Area Network (MAN)  

4. Wide Area Network (WAN) 

1. Personal Area Network(PAN) – Spread in the proximity of an individual. Cover an area of a few meters radius. 

• Set up using guided media(USB cable) or unguided media (Bluetooth, Infrared). Owned, controlled, and managed by a single person. 

Examples: 

• A network of devices such as computer, Phone, MP3/MP4 Player, Camera etc. 
• Transferring songs from one cell phone to another is a PAN of two phones. Transferring files from a PC to an MP3 playeris a PAN between the two.

2. Local Area Network (LAN) – LANs are the most frequently used/discussed networks.It is one of the most common one of the simplest types of network.It is designed for small physical areas such as an office, group of buildings.Any of different types of topologies can be used to design LAN like Star, Ring, Bus, Tree etc. 

Characteristics of LAN 

• private networks means no need of regulatory control. 
• Operate at relatively high speed. 
• Ethernet, Token ring etc type media access controls are used 
• Connects computers in a single building, block or campus.

Advantages of LAN 

• Resource Sharing 
• Software Applications Sharing
• Easy and Cheap Communication 
• Centralized Data 
• Data Security 
• Internet Sharing

 Disadvantages of LAN 

• High Setup Cost 
• Privacy Violations 
• Data Security Threat 
• LAN Maintenance Job 
• Covers Limited Area 

Examples: 

A networked office building, school or home. Sometimes one building can contain a few small LANs (Like some schools have independent LANs in each computer lab.).

3. Metropolitan Area Network(MAN):– 

Spread within a city . Cover an area of a few kilometres to a few hundred kilometres radius. Set up using all types of all guided and unguided media. Owned and operated by a government body or a large corporation. 

Examples: 

A network of schools, or banks, or Government offices etc. within a city. 

A MAN is usually formed by interconnecting a number of LANs and individual computers.

4. Wide Area Network (WAN) –Slightly more complex than a LAN, a WAN connects computers across longer physical distances. The Internet is the most basic example of a WAN, connecting all computers together around the world. Because of a WAN’s vast reach, it is typically owned and maintained by any single person or owner. 

Characteristics of WAN 

• Covers large distances(states, countries, continents). 

• Communication medium like satellite, public telephone networks etc and routers are used establish connection. 

Examples: 

A network of ATMs, BANKs, National Government Offices, International Organizations' Offices etc., spread over a country, continent, or covering many continents.

Advantages of WAN 

• Long distance business can connect on the one network. 
• Shares software and resources 
• Messages can be sent very quickly to wide range of nodes 
• Hardware devices can be shared. 

Disadvantages of WAN 

• Need a good firewall to restrict unauthorized access 
• Setting up a network can be an expensive, slow and complicated. 
• Maintaining a network is a full-time job 
• Security is a major issue when many different people have the ability to use information

Network Devices

1. Modem:-A modem (modulator/demodulator) is a hardware device that allows a computer to send and receive information over telephone lines. When sending a signal, the modem converts (modulates) digital data to an analog signal and transmit it over a telephone line. Similarly when an analog signal is received, the modem converts it back (demodulates) to a digital signal.

2. Hub :- A Hub is an electronic device that connects several nodes to form a network and redirects the received information to all the connected nodes in broadcast mode. The computer(s) for which the information is intended receive(s) this information and accept(s) it. Other computers on the network simply reject this information. 

Types of Hub :

• Passive Hub: This type of does not amplify or boost the signal. It does not manipulate or view the traffic that crosses it. 

• Active Hub: It amplifies the incoming signal before passing it to the other ports

3. Switch :- A switch is a network device which is used to interconnect computers or devices on a network. It filters and forwards data packets across a network. The main difference between hub and switch is that hub replicates what it receives on one port onto all the other ports while switch keeps a record of the MAC addresses of the devices attached to it and forwards data packets onto the ports for which it is addressed across a network, that’s why switch is intelligent Hub. 

4. Repeater :- A Repeater is a device that is used to amplify and regenerate a signal which is on its way through a communication channel. A repeater regenerates the received signal and re-transmits it to its destination.

5. Router :- A device that forwards data packets from one network to another by finding the shortest route, based on an internal routing table and the address of the destination network in the incoming packet, the router determines whether to send the packet out or keep it within the network. 

6. Gateway :- A gateway is a device that connects dissimilar networks. 

Network Topologies

The geometrical arrangement of computer resources, network devices along with communication channel is known as Network structure or Network topology.

Topology can be physical or logical.

• Physical Topology-physical layout of nodes and cables in the network. 
• Logical topology - the way information flows between different components.

Types of Physical Network Topologies 

• Bus Topology 
• Star Topology 
• Tree Topology
• Mesh Topology 

Bus Topology :-Bus topology is a topology for a Local Area Network (LAN) in which all the nodes are connected to a single cable. The starting and ending point of cable is called terminator. 

ADVANTAGES 

• It is easy to install. 
• It requires less cable length and hence it is cost effective. 
• Failure of a node does not affect the network.

DISADVANTAGES 

• In case of cable or terminator fault, the entire network breaks down. 
• Not suitable for large number of computers. 
• At a time only one node can transmit data.

STAR Topology: -A STAR topology is based on a central node which acts as a hub. A STAR topology is common in homes networks where all the computers connect to the single central computer using it as a hub. 

Advantages: 

• Easy to troubleshoot 
• A single node failure does not affect the entire network. 
• Fault detection and removal of faulty parts is easier. 
• In case a workstation fails, the network is not affected. 

Disadvantages:- 

• Difficult to expand. 
• Longer cable is required.
• The cost of the hub and the longer cables makes it expensive over others.
• In case hub fails, the entire network 

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 

• Ideal if workstations are located in groups. 

• Used in Wide Area Network. 

Advantages  

• Extension of bus and star topologies. 
• Expansion of nodes is possible and easy. 
• Easily managed and maintained. 
• Error detection is easily done

Disadvantages 

• Heavily cabled. 
• Costly. 
• If more nodes are added maintenance is difficult. 
• Central hub fails, network fails.

Mesh topology is a group of nodes which are all connected to each other and many types of connections are possible in a mesh topology.

Advantage 

• The arrangement of the network nodes is such that it is possible to transmit data from one node to many other nodes at the same time. 
• Fault is diagnosed easily

Disadvantage 

•  Installation and configuration is difficult.
•  Cabling cost is more. 
•  Bulk wiring is required. 

Introduction to Internet

The Internet is a massive network of networks, a networking infrastructure. It connects million of computers together globally, forming a network in which any computer can communicate with another computer as long as they are both connected to the Internet. The World Wide Web, or simply Web is a way or medium of accessing information over the Internet. It is an information sharing model that is built on top of the Internet.

” The Internet emerged in the United States in the 1970s but did not become visible to the general public until the early 1990s. By 2020, approximately 4.5 billion people, or more than half of the world’s population, were estimated to have access to the Internet. 

The Internet provides a capability so powerful and general that it can be used for almost any purpose that depends on information, and it is accessible by every individual who connects to one of its associated networks.

URL :- URL stands for Uniform Resource Locator. A URL is nothing more than the address of a given unique resource on the Web or address of a website. The URL is an address that matches users to a specific resource online, such as webpage. 

Example- https://www.bsfdabla.kvs.ac.in

WWW : The World Wide WEB (WWW), commonly known as the ‘Web’. It is an information system all the web resources are identified by Uniform Resource Locator (URL). Tim Berners-Lee invented the WWW in 1989. He wrote the first web browser in 1990. 

Application of WWW : 

• Publishing, Marketing and advertising 
• Research and Development  
• Communication 
• Collaboration 
• Industrial classification of the sample companies etc. 
• Gaming 

e-mail : email (or e-mail) is defined as the transmission of messages over communications networks. Typically the messages are notes entered from keyboard and sent over internet using computer or mobile.

Chat : Chat may refer to any kind of communication over the Internet that offers a real-time transmission of text messages from sender to receiver. Chat messages are generally short in order to enable other participants to respond quickly. 

Voip :- Voice over Internet Protocol (VoIP), is a technology that allows you to make voice calls using a broadband Internet connection instead of a regular (or analog) phone line. 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. VoIP can allow you to make a call directly from a computer.

Website :- a website is a group of web pages, containing text, images and all types of multi-media files. 

Difference between Website and Webpage 

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."

Webpage:  A document which can be displayed in a web browser such as Firefox, Google Chrome, Opera, Microsoft Internet Explorer etc.

 Static webpage  v/s  dynamic webpage

Static webpage

• The static web pages display the same content each time when someone visits it.
• It takes less time to load over internet.
• No Database used. 

dynamic webpage

• Dynamic web pages the page content changes according to the user.
• Dynamic web pages take more time while loading.
• A database is used in at the server end in a dynamic web page. 

Web Server :- a web server is a computer that stores web server software and a website's component files (e.g. HTML documents, images, CSS style sheets, and JavaScript files). 

When client sends request for a web page, the web server search for the requested page if requested page is found then it will send it to client with an HTTP response. 

If the requested web page is not found, web server will the send an HTTP response :Error 404 Not found.

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. 

A server is a physical computer that runs without any interruption so that your website is available all the time for anyone who wants to see it.

Web Browser :- 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.

Plug-ins:- a plug-in (or plugin, add-in, add-on) is a software component that adds a specific feature to an existing computer program. When a program supports plug-ins, it enables customization. Plug-ins are commonly used in Internet browsers but also can be utilized in numerous other types of applications. 

Add-ons : An Add-on is either a hardware unit that can be added to a computer to increase the capabilities or a program unit that enhances primary program. Some manufacturers and software developers use the term add-on. Examples of add-ons for a computer include card for sound, graphic acceleration, modem capability and memory. Software add-ons are common for games, word-processing and accounting programs.

Cookies :- cookies are small files which are stored on a user’s computer and contains information like which Web Pages visited in the past, logging details Password etc. They are designed to hold a modest amount of data specific to a particular client and website and can be accessed by the web server or the client computer.

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