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UNIT 2 ▪ CHAPTER 1
09
Computer Networks
Evolution · Data Communication · Media · Devices · Topologies · Internet · Protocols
A computer network is a group of two or more computers (and other devices) connected together so they can share data, files, programs and hardware resources. The largest computer network in the world is the Internet — it connects billions of devices across every country.
Real-life analogy. A computer network is like a postal system. Every house (device) has an address (IP), messages (data packets) are sent in envelopes (packets), the post office follows rules (protocols), and a chain of postmen (routers) carries each envelope from sender to receiver.
9.1 Evolution of Networking
| Year | Milestone | What it brought |
|---|---|---|
| 1969 | ARPANET — Advanced Research Projects Agency Network | The world’s first packet-switched network; connected four U.S. universities |
| 1980s | NSFNET — National Science Foundation Network | Replaced ARPANET as the research backbone; bigger and faster |
| 1989 | World Wide Web (Tim Berners-Lee, CERN) | HTML, HTTP, URL, the first web browser |
| 1990s | Commercial Internet | ISPs open the net to the public; e-mail and the Web explode |
| 1995 | Internet in India (VSNL, 15 Aug 1995) | Public access available through dial-up |
| 2000s+ | Broadband, Wi-Fi, 3G / 4G / 5G, fiber optics | Always-on, mobile, gigabit-speed networks |
9.1.1 Benefits of Networking
- Resource sharing — printers, scanners, Internet connection.
- File & data sharing — centralised servers.
- Communication — e-mail, chat, video calling.
- Reliability — backup across multiple machines.
- Cost saving — one printer for the entire office.
- Remote access — work from anywhere.
9.2 Data Communication — The Basics
9.2.1 The Five Components of any Communication
| Component | Role | Example |
|---|---|---|
| Sender | Creates the message | Your WhatsApp app |
| Receiver | Gets the message | Your friend’s phone |
| Message | The data itself | Text, image, video |
| Medium / channel | The path between sender & receiver | Wi-Fi, cable, radio |
| Protocol | The shared rules of communication | TCP/IP, HTTPS |
9.2.2 Measuring Capacity of a Communication Channel
Bandwidth is the maximum amount of data a channel can carry per second. Data transfer rate is the actual amount of data flowing through the channel per second. Both are measured in bits per second (bps).
| Unit | Full form | Value |
|---|---|---|
| bps | bits per second | 1 bit / second |
| Kbps | Kilobits per second | 1 024 bps |
| Mbps | Megabits per second | 1 024 Kbps |
| Gbps | Gigabits per second | 1 024 Mbps |
| Tbps | Terabits per second | 1 024 Gbps |
Your home Wi-Fi plan shows “100 Mbps” — that is the bandwidth. The real download speed while streaming YouTube (say 30 Mbps) is the data transfer rate.
9.3 IP Address
An IP address (Internet Protocol address) is a unique numerical label given to every device on a network, so that data packets know where to go. Just like your home has a postal address, every networked device has an IP address.
9.3.1 IPv4 vs IPv6
| IPv4 | IPv6 | |
|---|---|---|
| Address size | 32 bits | 128 bits |
| Written as | Four decimal numbers separated by dots | Eight hexadecimal groups separated by colons |
| Range per octet/group | 0 – 255 | 0000 – FFFF |
| Example | 192.168.1.5 | 2001:0db8:85a3:0000:0000:8a2e:0370:7334 |
| Total addresses | ~4.3 billion (almost exhausted) | ~3.4 × 1038 |
9.4 Switching Techniques
Before a message reaches its destination it may pass through several intermediate nodes. A switching technique decides how the route is chosen and used.
9.4.1 Circuit Switching
A dedicated physical path is reserved between sender and receiver for the entire duration of the communication — just like an old telephone call. The path is released only when the call ends.
9.4.2 Packet Switching
The message is chopped into small packets. Each packet carries the destination address and travels independently — possibly via different routes — and the packets are re-assembled at the receiver. This is how the Internet works.
9.4.3 Side-by-side comparison
| Circuit Switching | Packet Switching | |
|---|---|---|
| Path reserved? | Yes, for the whole call | No — decided per packet |
| Uses bandwidth? | Even when silent | Only when data is flowing |
| Reliability on link failure | Call drops | Re-route automatically |
| Example | Traditional telephone | Internet, mobile data |
9.5 Transmission Media
The transmission medium is the physical (or invisible) channel that carries the signal from sender to receiver. It is broadly divided into wired (guided) and wireless (unguided) media.
9.5.1 Wired (Guided) Media
| Medium | What it is | Speed | Used for |
|---|---|---|---|
| Twisted-pair | Pairs of copper wires twisted to cancel interference (e.g. Cat-5, Cat-6) | 100 Mbps – 10 Gbps | LAN cabling, telephone lines |
| Co-axial | A central copper conductor wrapped in insulator, braided shield, outer cover | 10 – 100 Mbps | Cable TV, older LANs |
| Optical Fibre | Thin glass / plastic fibre carrying light signals | 10 – 100 Gbps | Backbone Internet, FTTH broadband |
9.5.2 Wireless (Unguided) Media
| Medium | Frequency | Needs line-of-sight? | Used for |
|---|---|---|---|
| Radio waves | 3 KHz – 1 GHz | No | AM/FM radio, Wi-Fi, Bluetooth |
| Microwaves | 1 GHz – 300 GHz | Yes | Mobile towers, satellite links |
| Infrared | 300 GHz – 400 THz | Yes, short range | TV remotes, some sensors |
9.5.3 Wired vs Wireless — Pros & Cons
Wired
- Higher, more stable speed
- Harder to intercept
- Less affected by weather
Wireless
- Mobility — no cables
- Easier installation
- Cheaper for large areas
- ❌ More interference / security risk
9.6 Network Devices
| Device | What it does |
|---|---|
| Modem | MOdulator + DEModulator — converts digital signals to analog (and back) so they can travel over telephone/cable lines |
| Hub | Dumb multi-port repeater — broadcasts data to every connected device (obsolete, used for small LANs) |
| Switch | Smart hub — forwards data only to the specific device using MAC addresses; standard in modern LANs |
| Repeater | Boosts / regenerates a weakening signal over long distances |
| Router | Connects different networks, finds the best path for a packet, uses IP addresses (your Wi-Fi router) |
| Gateway | Connects two networks that use different protocols; a protocol translator |
| Bridge | Connects two LANs (or segments) operating with the same protocol |
| Wi-Fi card / NIC | Network Interface Card — hardware inside every computer / phone that actually transmits and receives data |
Hub vs Switch vs Router — hub shouts to everyone, switch whispers only to the right person, router directs traffic between different networks.
9.7 Network Topologies
A topology is the physical or logical arrangement of devices in a network — the shape made by the cables and nodes.
9.7.1 Bus Topology
Every device connects to a single backbone cable. Simple and cheap, but the whole network fails if the main cable breaks.
9.7.2 Star Topology
Every device connects to a central hub or switch. Most common in schools/offices today. One broken cable affects only one device.
9.7.3 Tree Topology
A hierarchy of star networks — hubs of hubs. Used in large campus networks.
9.7.4 Mesh Topology
Every device connects directly to every other device. Extremely reliable, expensive. Used on the Internet backbone.
9.7.5 Summary table
| Topology | Cost | Reliability | Typical use |
|---|---|---|---|
| Bus | Low | Low (single cable) | Small, obsolete LANs |
| Star | Medium | Medium (hub is critical) | Schools, offices |
| Tree | Medium-High | Medium | Campus networks |
| Mesh | Very high | Very high | Internet backbone, military |
| Ring | Medium | Low (if node fails) | Some older LANs (Token Ring) |
9.8 Types of Networks by Size
| Type | Full form | Covers | Example |
|---|---|---|---|
| PAN | Personal Area Network | ~10 m | Bluetooth headphones + phone |
| LAN | Local Area Network | One building | Home Wi-Fi, school lab |
| MAN | Metropolitan Area Network | One city | Cable TV network, city-wide ISP |
| WAN | Wide Area Network | Country / world | The Internet itself |
9.9 Introduction to the Internet
The Internet is the global network of interconnected networks. It uses the TCP/IP protocol suite and enables services such as the Web, e-mail, file transfer, instant messaging, online games, video streaming and IoT.
9.9.1 URL — Uniform Resource Locator
A URL is the full address of any resource on the Web. Break down this example:
https://www.cbse.gov.in/curriculum/comp-science.pdf
└──┬──┘ └─────┬─────┘ └──────┬──────┘
protocol domain name path / file
| Part | Meaning |
|---|---|
https:// | Protocol — how the browser should fetch the resource |
www.cbse.gov.in | Domain name — human-friendly label that maps to an IP |
/curriculum/comp-science.pdf | Path + file on the web server |
9.9.2 WWW — World Wide Web
The WWW (1989, Tim Berners-Lee) is a collection of inter-linked web pages accessed over the Internet using HTTP. The Web is one service running on the Internet — not the Internet itself.
9.9.3 Website vs Web Server vs Web Browser
| Term | What it is |
|---|---|
| Website | Collection of related web pages hosted under one domain |
| Web Server | A computer running software (Apache, Nginx, IIS) that serves web pages on request |
| Web Browser | Program on the user’s device (Chrome, Firefox, Edge, Safari) that requests and displays web pages |
| Web Hosting | Service of storing your website files on a server that is always online |
9.9.4 Web Scripting
| Client-side | Server-side | |
|---|---|---|
| Runs where | In the browser | On the web server |
| Purpose | Interactive pages, validation | Database access, business logic |
| Examples | JavaScript, VBScript | PHP, Python, Node.js, JSP |
9.9.5 Web 2.0
Web 2.0 is the second generation of the Web, where ordinary users also create content — not just read it. Hallmarks: social networking (Facebook, Twitter), blogs, wikis (Wikipedia), video-sharing (YouTube), collaborative editing (Google Docs) and user-generated reviews.
9.10 Network Protocols
A protocol is the set of rules that computers follow when they communicate — what format to use, how to acknowledge messages, how to handle errors. Without a shared protocol, computers cannot talk, just as two people with no common language cannot hold a conversation.
| Protocol | Full form | Default port | Purpose |
|---|---|---|---|
| TCP/IP | Transmission Control / Internet Protocol | — | The core protocol suite of the Internet; breaks data into packets, addresses them, re-assembles them reliably |
| HTTP | Hyper-Text Transfer Protocol | 80 | Transfers web pages between browser and server |
| HTTPS | HTTP Secure | 443 | HTTP over an encrypted SSL/TLS channel — modern standard for websites |
| FTP | File Transfer Protocol | 21 | Upload / download files to and from a server |
| SMTP | Simple Mail Transfer Protocol | 25 | Sending e-mail from client to mail server / between servers |
| POP3 | Post Office Protocol v3 | 110 | Downloading e-mail from server to client; usually deletes server copy |
| IMAP | Internet Message Access Protocol | 143 | Reading e-mail while keeping it on the server — used by Gmail / Outlook |
| PPP | Point-to-Point Protocol | — | Carries TCP/IP over serial / dial-up / DSL links |
| Telnet | Tele-networking | 23 | Remote login to another computer in plain text (insecure) |
| SSH | Secure Shell | 22 | Secure, encrypted replacement for Telnet |
Quick way to remember SMTP vs POP3: SMTP = Sending, POP = Picking Out Post.
9.10.1 The TCP/IP four-layer model (brief)
| Layer | Job | Examples |
|---|---|---|
| Application | End-user services | HTTP, FTP, SMTP, DNS |
| Transport | Reliable delivery between hosts | TCP, UDP |
| Internet | Routing & addressing | IP, ICMP |
| Network Access | Physical link | Ethernet, Wi-Fi, PPP |
9.11 Common Mistakes & Confusions
| # | Mistake / Confusion | Correction |
|---|---|---|
| 1 | “Internet” and “WWW” mean the same | Internet is the network; Web is just one service on it |
| 2 | Confusing hub, switch and router | Hub → broadcasts · Switch → targets a device · Router → connects networks |
| 3 | Confusing bandwidth and data rate | Bandwidth = maximum; data rate = actual |
| 4 | Writing an IP octet as 256 | Each octet is 0–255 only |
| 5 | Mixing SMTP & POP3 roles | SMTP sends · POP3/IMAP receives |
| 6 | Calling Wi-Fi “a protocol” | Wi-Fi is a technology; TCP/IP is the protocol suite running over it |
| 7 | Saying fibre optic carries electricity | It carries light signals through glass/plastic |
Quick-revision summary
- Networks evolved: ARPANET (1969) → NSFNET → WWW (1989) → Commercial Internet → 5G / fibre today.
- Data communication = sender · receiver · message · medium · protocol.
- Bandwidth (max capacity) is measured in bps, Kbps, Mbps, Gbps, Tbps.
- IPv4 uses 32 bits (four 0–255 numbers); IPv6 uses 128 bits (eight hex groups).
- Switching: circuit (dedicated path, phone) vs packet (independent packets, Internet).
- Wired media: twisted-pair, co-axial, fibre optic. Wireless media: radio, microwave, infrared.
- Devices: modem · hub · switch · repeater · router · gateway · bridge · NIC / Wi-Fi card.
- Topologies: bus · star · tree · mesh · (ring). Network types by size: PAN · LAN · MAN · WAN.
- A URL has three parts — protocol, domain name, path.
- Web scripting — client-side (JavaScript) runs in the browser; server-side (PHP, Python) runs on the web server.
- Web 2.0 = user-generated content (social media, blogs, wikis).
- Key protocols: TCP/IP · HTTP/HTTPS · FTP · SMTP (send mail) · POP3/IMAP (read mail) · PPP · Telnet · SSH.
Unit 3
Database
Management
Management
Relational Model · SQL (DDL + DML) · Aggregates · Joins · MySQL + Python
Chapter 10 • Database Concepts & SQL Basics
Chapter 11 • Advanced SQL
Chapter 12 • Python ⇄ MySQL Connectivity
Chapter 11 • Advanced SQL
Chapter 12 • Python ⇄ MySQL Connectivity