CBSE Class 12 Computer Science Computer Network Notes Set 01

1. Computer Network

A computer network is an interconnected collection of autonomous computers and other devices that can share data and resources. Autonomous means that no computer on the network can start, stop or control another.

Need for networking:
1. To share files, data and peripherals such as printers and scanners.
2. To communicate quickly (email, chat, voice and video calls).
3. To reduce the cost of sharing and transferring information.
4. Reliability: a file can be kept on two or more machines, so if one is unavailable another copy can be used.

2. Evolution of Networking

1. ARPANET (Advanced Research Projects Agency Network): started in 1969 by ARPA of the US Department of Defense. It connected a few computers of universities and defence offices and is regarded as the first network of its kind.
2. NSFNET (National Science Foundation Network): set up in the mid-1980s by the US National Science Foundation to connect universities and research centres. It was faster than ARPANET and became the backbone of the early Internet.
3. Internet: a worldwide network of networks. ARPANET, NSFNET and many other networks joined together using common protocols (TCP/IP) to form the Internet.

3. Data Communication Terminology

Components of data communication:
1. Sender: the device that sends the message.
2. Receiver: the device that receives the message.
3. Message: the data or information being communicated (text, numbers, images, audio, video).
4. Communication media: the path through which the message travels (cable or wireless).
5. Protocol: the set of rules that sender and receiver follow so that they understand each other.

Measuring capacity of a channel:
1. Bandwidth: the range of frequencies a channel can carry, i.e. the difference between its highest and lowest frequencies. It is measured in Hertz (Hz, kHz, MHz, GHz). A higher bandwidth means more data can be carried.
2. Data transfer rate: the amount of data transferred per second over a channel. It is measured in bits per second: bps, Kbps, Mbps, Gbps and Tbps.

IP address: a unique numeric address given to every device connected to a network or the Internet. An IPv4 address is 32 bits long and is written as four numbers (0 to 255) separated by dots, for example 192.168.1.2. (A newer version, IPv6, uses 128-bit addresses.)

Switching techniques are used to send data from a source to a destination through intermediate nodes.
1. Circuit switching: a complete physical path between sender and receiver is set up first, and then the whole data is sent along it. The path stays reserved for the entire session. Example: a traditional telephone call.
2. Packet switching: the message is broken into small packets. Each packet travels independently, possibly by different routes, and the packets are reassembled in order at the destination. No path is reserved in advance, so the lines are shared efficiently. The Internet uses packet switching.

4. Transmission Media

The medium through which data travels from one place to another is called the transmission (communication) medium. It is of two types: wired (guided) and wireless (unguided).

Wired (guided) media

1. Twisted pair cable: two insulated copper wires twisted around each other in a double helix; the twisting reduces interference. Ethernet (LAN) cables such as Cat-5 and Cat-6 are twisted pair cables with RJ45 connectors.
Merits: simple, cheap, light, flexible, and easy to install and maintain.
Demerits: low bandwidth, picks up noise easily, and cannot carry a signal beyond about 100 m without a repeater.

2. Co-axial cable: a central copper conductor surrounded by insulation, a braided metal shield and an outer cover. It is used in cable TV networks.
Merits: higher bandwidth and better noise protection than twisted pair; carries signals over longer distances.
Demerits: more expensive and less flexible than twisted pair.

3. Optical fibre cable: thin strands of glass or plastic that carry data as pulses of light.
Merits: very high bandwidth and speed, immune to electrical interference and noise, very secure, and suitable for long distances and harsh environments.
Demerits: most expensive, difficult to install, and joining two fibres is difficult.

Wireless (unguided) media

1. Radio waves (about 3 kHz to 1 GHz): omnidirectional, so the sending and receiving antennas need not be aligned; can travel long distances and pass through walls. Used in AM/FM radio, cordless phones and Wi-Fi.
Merits: offer mobility, are cheaper than laying cables over long distances, and penetrate buildings.
Demerits: insecure (anyone can tune in) and affected by weather such as rain and storms.

2. Microwaves (about 1 GHz to 300 GHz): unidirectional, line-of-sight transmission, so the antennas must face each other, usually on tall towers. Used for long-distance telephone links, television and satellite communication.
Merits: carry large amounts of data, and no cables are needed over hills, rivers or oceans.
Demerits: cannot pass through walls or solid obstacles, insecure, costly towers and antennas, and affected by weather.

3. Infrared waves (about 300 GHz to 400 THz): used for very short-range communication such as TV remote controls. They cannot pass through walls, so communication stays within a room, which makes it fairly secure.

5. Network Devices

1. Modem (Modulator–Demodulator): converts the computer's digital signal into an analog signal for the telephone/cable line (modulation) and converts the incoming analog signal back into digital (demodulation). It connects a computer or network to the Internet.
2. Ethernet card (Network Interface Card, NIC, or LAN card): a card fitted in a computer that connects it to a wired network through an Ethernet cable. Each card has a unique physical (MAC) address.
3. RJ45 (Registered Jack-45): an 8-pin connector at the end of an Ethernet (twisted pair) cable that plugs into the Ethernet card, hub or switch.
4. Repeater: a device that regenerates (amplifies) a weak signal so that it can travel further. It is needed when a cable is too long for the signal to stay strong (about 100 m for twisted pair).
5. Hub: connects several computers in a LAN. It sends the data it receives to all the connected devices.
Active hub: amplifies (regenerates) the signal before passing it on.
Passive hub: only passes the signal on, without amplifying it.
6. Switch: also connects computers in a LAN, but it is intelligent. It sends data only to the device for which it is meant (using its MAC address), so there is less unnecessary traffic than with a hub.
7. Router: connects two or more different networks, for example a LAN to the Internet, and chooses the best path for each packet to reach its destination.
8. Gateway: connects networks that use different protocols, translating between them. It is the entry and exit point of a network.
9. Wi-Fi card: a wireless network adapter that lets a computer connect to a wireless network (Wi-Fi) without cables.

6. Types of Network

1. PAN (Personal Area Network): connects devices around one person, within about 10 metres. Example: a phone connected to a laptop, earbuds or a smartwatch using Bluetooth, or devices on a mobile hotspot.
2. LAN (Local Area Network): connects computers in a small area such as a room, building, school or campus, usually up to about 1 to 10 km. It has high speed, low error rates and is owned by one organisation.
3. MAN (Metropolitan Area Network): covers a city or town, for example the offices of one company spread across a city, or a city's cable TV network.
4. WAN (Wide Area Network): spreads across countries or continents and connects many LANs and MANs. It is usually owned by several organisations. The Internet is the largest WAN.

7. Network Topologies

The arrangement (pattern) in which the nodes of a network are connected is called its topology.

Bus topology

All the nodes are connected to a single cable called the backbone (bus), with a terminator at each end. A message put on the bus travels along it and is received by the node it is meant for.

Advantages: short cable length and low cost; easy to add a node.
Disadvantages: only one message can travel at a time; a break in the backbone stops the whole network; faults are difficult to find and isolate.

Star topology

Every node is connected by its own cable to a central device (hub or switch).

Advantages: easy to install and manage; failure of one node or its cable does not affect the others; faults are easy to find; nodes can be added easily.
Disadvantages: needs more cable than bus topology; if the central hub/switch fails, the whole network fails.

Tree topology

A combination of star and bus: several star networks are connected to a main (root) cable or hub, so the network branches out like an inverted tree.

Advantages: easy to expand; supports a hierarchical flow of data; suits large organisations with departments.
Disadvantages: long cable length and high cost; if the root or a branch hub fails, all the nodes below it are cut off.

8. Network Protocols

A protocol is a set of rules that devices follow to communicate over a network. It defines the format of the data, how it is sent and how errors are handled.

1. HTTP (HyperText Transfer Protocol): the rules for transferring web pages (hypertext, images, sound, video) between a web server and a browser on the WWW.
2. HTTPS (HTTP Secure): the secure version of HTTP. The data is encrypted, so it is used for logins, banking and online payments.
3. FTP (File Transfer Protocol): used to transfer (upload and download) files between computers over a network.
4. PPP (Point-to-Point Protocol): used to make a direct connection between two nodes, for example between a home computer and the ISP's server.
5. SMTP (Simple Mail Transfer Protocol): used to send email from an email client to a mail server and between mail servers.
6. POP3 (Post Office Protocol version 3): used by an email client to download email from the mail server to the user's device.
7. TCP/IP (Transmission Control Protocol / Internet Protocol): the basic protocols of the Internet. TCP breaks a message into packets at the sender, makes sure they all arrive, and reassembles them in order at the receiver. IP gives each packet the destination address and routes it across the networks.
8. TELNET: lets a user log in to a remote computer over a network and work on it through a text-based terminal (remote login).
9. VoIP (Voice over Internet Protocol): converts voice into data packets and sends them over the Internet, so that phone calls can be made over an IP network (for example WhatsApp or Skype calls).

9. Introduction to Web Services

1. WWW (World Wide Web): a system of interlinked web pages on the Internet, accessed through browsers using HTTP and addressed by URLs.
2. HTML (HyperText Markup Language): the language used to create web pages. It uses tags to define the structure and layout of a page.
3. XML (eXtensible Markup Language): a markup language for storing and transporting data. Users can define their own tags, and the data is both human-readable and machine-readable.

4. Domain name: the text name of a website, used instead of its numeric IP address, for example cbse.gov.in. The last part is the top-level domain, such as .com, .org, .gov, .edu, .net or a country code such as .in. The Domain Name System (DNS) converts domain names into IP addresses.
5. URL (Uniform Resource Locator): the complete address of a resource on the Internet. In https://www.cbse.gov.in/index.html, "https" is the protocol, "www.cbse.gov.in" is the domain name and "/index.html" is the path of the page.
6. Website: a collection of related web pages under one domain name. A web page is a single document on the WWW.
7. Web browser: software used to open and view web pages, for example Google Chrome, Mozilla Firefox, Microsoft Edge, Safari and Opera.
8. Web server: a computer, with server software, that stores websites and sends web pages to browsers when they request them.
9. Web hosting: a service that stores a website on a web server connected to the Internet, so that anyone can access it.

10. Important Full Forms