A computer network is a collection of two or more computers and devices interconnected by communication links (wired or wireless) that allow them to share data, resources, and services. Networks are the backbone of the internet, corporate offices, schools, and modern digital life.
Why Networks?
- Resource sharing: Printers, scanners, storage, and software can be shared across multiple users.
- Data sharing: Files and databases can be accessed from any connected device.
- Communication: Email, video conferencing, instant messaging.
- Cost reduction: One expensive printer shared by many is cheaper than individual printers.
Types of Networks by Geographical Spread
PAN (Personal Area Network): The smallest network; covers a few metres around a person. Examples: Bluetooth headphones, smartwatch connected to a phone.
LAN (Local Area Network): Covers a single building, floor, or campus. Owned and managed by a single organisation. High speed (100 Mbps to 10 Gbps). Examples: office network, school computer lab.
MAN (Metropolitan Area Network): Covers a city or large campus. Often used by cable TV providers or city-wide Wi-Fi. Speed: 10–100 Mbps.
WAN (Wide Area Network): Spans countries or continents. Uses leased telephone lines, satellites, or submarine cables. The internet is the largest WAN. Speed varies widely. Maintained by telecom operators.
Network Topologies
Topology is the arrangement of nodes and communication links in a network.
Bus Topology: All devices connect to a single shared cable (the bus). Simple and cheap; failure of the main cable breaks the entire network.
Star Topology: All devices connect to a central hub/switch. Most common in LANs. Failure of one device does not affect others; failure of the hub brings down the whole network.
Ring Topology: Each device connects to exactly two others, forming a closed loop. Data travels in one direction. Failure of one node can disrupt the network (in single ring).
Tree Topology (Hierarchical): A combination of star topologies connected in a hierarchy. Scalable; used in large organisations.
Mesh Topology: Every device connects directly to every other device. Highly reliable (multiple paths) but expensive due to the large number of cables. A full mesh of n nodes requires n(n-1)/2 connections.
Hybrid Topology: A combination of two or more basic topologies.
Network Devices
Hub: A basic device that broadcasts data to all ports. Works at the Physical layer. Old technology, now mostly replaced by switches.
Switch: A smarter device that sends data only to the intended recipient by learning MAC addresses. Works at the Data Link layer. More efficient than a hub.
Router: Connects different networks (e.g., your home LAN to the internet). Determines the best path for data using IP addresses. Works at the Network layer.
Modem (Modulator-Demodulator): Converts digital signals to analogue (modulation) for transmission over telephone lines and back to digital (demodulation) at the receiving end.
Network Interface Card (NIC): Hardware installed in a computer to connect it to a network. Every NIC has a unique MAC address (Media Access Control address).
Gateway: A node that acts as an entrance/exit between two networks using different protocols. Routers often serve as gateways.
Repeater: Amplifies and retransmits weakened signals to extend network range.
Access Point (AP): Provides wireless connectivity in a LAN (Wi-Fi).
Transmission Media
- Wired (Guided) Media:
- Twisted Pair Cable: Two insulated copper wires twisted together. Cheap and flexible. Types: UTP (Unshielded) and STP (Shielded). Bandwidth: up to 10 Gbps for Cat 6A.
- Coaxial Cable: Central copper conductor surrounded by insulating layer, metallic shield, and outer sheath. Used in cable TV. Less susceptible to interference.
- Optical Fibre: Transmits data as pulses of light through glass/plastic fibres. Extremely high bandwidth, immune to electromagnetic interference, used for long-distance high-speed backbone links.
- Wireless (Unguided) Media:
- Radio Waves: Low frequency; penetrate walls; used in Wi-Fi, Bluetooth, AM/FM radio.
- Microwaves: Line-of-sight transmission; used in mobile phones and satellite communication.
- Infrared: Very short range; used in TV remotes and older data transfer between nearby devices.
OSI Reference Model
The OSI (Open Systems Interconnection) model divides network communication into 7 layers. Each layer has specific functions and communicates only with the layers immediately above and below it.
| Layer | Name | Key Functions |
|-------|------|---------------|
| 7 | Application | User interface, HTTP, FTP, SMTP |
| 6 | Presentation | Encryption, compression, format conversion |
| 5 | Session | Establishing, managing, terminating sessions |
| 4 | Transport | End-to-end delivery, error control (TCP/UDP) |
| 3 | Network | Routing, IP addressing |
| 2 | Data Link | Framing, MAC addressing, error detection |
| 1 | Physical | Bits over cable/wireless, voltage levels |
Memory aid for layers (top to bottom): All People Seem To Need Data Processing.
TCP/IP Model
- 1.The practical model used on the internet has 4 layers:
- 2.Application (merges OSI layers 5, 6, 7)
- 3.Transport (TCP / UDP)
- 4.Internet / Network (IP)
- 5.Network Access / Link (merges OSI layers 1, 2)
IP Address: A unique numerical label assigned to each device on a network. IPv4 uses 32-bit addresses (e.g., 192.168.1.1). IPv6 uses 128-bit addresses.
TCP (Transmission Control Protocol): Connection-oriented; ensures reliable, ordered delivery. Used for web pages, email.
UDP (User Datagram Protocol): Connectionless; faster but no guarantee of delivery. Used for video streaming, online gaming, DNS.
Common mistakes
- Confusing hub (broadcasts to all) with switch (sends only to intended device).
- Confusing modem (converts signal type) with router (connects networks).
- Mixing up the OSI layers and their functions.
- Stating that optical fibre uses electrical signals — it uses light signals.
Summary
Computer networks are classified by size (PAN, LAN, MAN, WAN) and organised by topology (bus, star, ring, mesh, tree). Networking devices like switches, routers, and modems each operate at specific OSI layers. Understanding transmission media types and the OSI/TCP-IP model is essential for CBSE board exams and forms the foundation for understanding the internet.