Computer Networks

A computer network is a collection of two or more computers and other devices — printers, scanners, servers, mobile phones — joined together by a communication channel so that they can exchange data and share resources. Every device attached to a network is called a node or a host. The moment two machines can pass data to each other you already have a network; scale only changes the name, from two laptops sharing one file up to the Internet itself.

Five components must be present before any communication can take place, and a question on the components of data communication expects all five. There must be a sender (the device that transmits), a receiver (the device that accepts), a message (the data being carried — text, numbers, images, audio or video), a transmission medium (the physical path, wired or wireless, along which the message travels) and a protocol. A protocol is a set of agreed rules that both machines follow; without it the receiver has no way of interpreting the bits arriving at its port. HTTP (HyperText Transfer Protocol) is the protocol a browser uses to fetch a page such as your school website, and TCP/IP (Transmission Control Protocol / Internet Protocol) is the pair of protocols on which the entire Internet is built.

Now the part that earns marks — why networks exist. Write the advantage and then a one-line reason for it:

• Resource sharing. One printer or one scanner in a school computer lab can serve forty machines instead of buying forty printers.
• File and data sharing. A student's project or a teacher's question bank can be copied from one machine to another in seconds without a pen drive.
• Communication. E-mail, chat, video conferencing and online classes all depend on a network.
• Centralised data and backup. Data kept on one server can be secured and backed up in one place instead of being scattered across every desk.
• Cost saving. Sharing hardware, software licences and one Internet connection is far cheaper than equipping each computer separately.
• Reliability. If one printer or one copy of a file becomes unavailable, another node on the network can supply it.
• Remote access. A teacher can reach school files from home, and services such as IRCTC railway booking or a UPI payment work from anywhere in the country.

A complete answer never stops at advantages. State the disadvantages too, because a longer question usually asks for both. Setting up a network costs money for cables, switches and skilled installation. It needs a network administrator to manage users, permissions and faults. Security risk rises sharply: malware that reaches one machine can spread to the rest, and shared folders can expose private data. There is dependence — if the server or the main cable fails, every user stops working at once. And a shared line can become congested, so speed falls when many users are active together.

Two more words are worth defining now because later sections reuse them. Data transfer rate is the amount of data actually moved per second, measured in bits per second. Bandwidth is the maximum data-carrying capacity of the channel — the ceiling that the transfer rate can never exceed. The final section treats both in detail.

Finally, keep the vocabulary straight. The Internet is the worldwide public network of networks. An intranet is a private network inside one organisation that uses the same technology but is closed to outsiders — a school's internal portal, for example. The World Wide Web is not the Internet; it is one service running on the Internet, the collection of linked pages that browsers display.

Networks are classified mainly by the geographical area they cover. Learn the four names in increasing order of size — PAN, LAN, MAN, WAN — and attach to each one a distance, an owner, a speed and one Indian example. That structure is exactly what a comparison question rewards.

PAN — Personal Area Network. The smallest type, organised around one person and the devices immediately around them, typically within a few metres. A mobile phone paired with wireless earphones, a laptop connected to a wireless mouse, a smart watch talking to a phone, or a phone sharing its Internet connection as a hotspot with a nearby laptop — all of these are PANs. They are usually wireless, built on Bluetooth or infrared, need no administrator, and carry small amounts of data. A PAN built without wires is sometimes called a WPAN (Wireless Personal Area Network).

LAN — Local Area Network. A network confined to one building, one campus or one small area — a school computer lab, an office floor, a cyber cafe. The defining features to write down are: it is privately owned by a single organisation, it uses the organisation's own cables and devices, it offers the highest data transfer speed of all four types, it has the lowest error rate because the cable runs are short and under one owner's control, and it is the cheapest to set up. Common media are twisted pair cable with RJ-45 connectors, or WiFi for a wireless LAN, and the usual central device is a switch.

MAN — Metropolitan Area Network. A network spread across a city or a large town, generally formed by connecting several LANs. It covers a larger area than a LAN and a smaller one than a WAN. Standard examples: the cable television network of a city, a bank's branches spread across one city linked into a single network, a university connecting its faculties located in different parts of a city, or a broadband provider serving one municipal area. A MAN is usually owned either by one large organisation or by a service provider, and its speed and error rate fall between those of a LAN and a WAN.

WAN — Wide Area Network. A network spanning a country, a continent or the whole world, formed by joining many LANs and MANs. It relies on leased telephone lines, optical fibre backbones, microwave links and communication satellites, so it is not owned by any single organisation. Consequently the WAN has the lowest transfer speed, the highest error rate and the highest setup and running cost of the four. The Internet is the largest example. Indian examples that work well in an answer are the railway reservation system, which lets a booking made in one state update a seat held in the database centrally, and the ATM network of a nationalised bank, which lets a card issued in one state be used at a machine in another.

Examiners test the boundaries, so fix the ordering in your memory as: PAN < LAN < MAN < WAN for area and cost, and exactly the reverse — LAN > MAN > WAN — for speed and reliability. If a question describes two computers in the same room, answer LAN, not WAN. If it describes several offices of one company inside one city, answer MAN. If it crosses cities or countries, answer WAN. Some textbooks add a CAN (Campus Area Network) between LAN and MAN for a college campus; mention it only if your textbook does, and never in place of the four required names.

Topology means the arrangement or layout in which the nodes of a network are connected to one another. Physical topology is how the cables actually run; logical topology is the path the data follows, which need not be the same shape. In an exam, describe the arrangement in one sentence, then give at least two advantages and two disadvantages.

Bus topology. Draw one long horizontal line across the page — this is the single main cable, called the backbone or trunk. From it, short vertical lines called drop lines run up and down to each computer, and a small box called a terminator sits at each end of the backbone to absorb the signal and stop it reflecting back. Every node receives every signal but keeps only the one addressed to it. Advantages: uses the least cable, so it is the cheapest and simplest to install; adding one more node to a small network is easy. Disadvantages: a single break anywhere in the backbone brings the entire network down; fault detection is difficult because the fault could be anywhere along the cable; performance falls sharply as nodes are added, because all of them share one cable and their signals collide; it suits only small networks.

Star topology. Draw a central box — a hub or, in modern networks, a switch — and straight lines radiating outward from it to each computer arranged in a circle around it. There is no direct cable between two computers; everything passes through the centre. Advantages: a fault in one cable affects only that one node, so the rest keep working; adding or removing a node is easy and does not disturb others; fault detection is simple because each node has its own link; with a switch at the centre, performance is high. Disadvantages: if the central device fails, the whole network stops; it needs more cable than a bus; the cost of the central device is extra. This is the topology used in nearly all school and office LANs today.

Ring topology. Draw the computers arranged in a closed loop, each connected to exactly two neighbours, with no free end and no terminator. Data travels around the ring in one direction, passing through each node in turn until it reaches the addressed node; a small control message called a token decides whose turn it is to send. Advantages: every node regenerates the signal as it passes, so the signal stays strong over longer distances; the token prevents collisions, so it performs predictably even under heavy traffic. Disadvantages: a break in the loop or the failure of one node can stop the whole network unless a dual ring is used; adding or removing a node interrupts the network; data passes through other machines, which is a privacy concern; fault diagnosis is difficult.

Mesh topology. Draw every computer joined by a separate dedicated cable to every other computer, so the diagram fills with crossing lines. For n nodes a full mesh needs n(n - 1) / 2 cables and n - 1 ports on every node. Advantages: the most reliable arrangement, since one failed link simply leaves alternative paths; dedicated links mean no traffic congestion; data does not pass through other nodes, so it is private and secure; faults are easy to locate. Disadvantages: extremely costly in cable and ports; installation and maintenance are difficult; impractical for large networks, which is why it is used mainly for critical backbone links.

Tree topology. Draw one device at the top — the root, usually a switch — connected downward to several second-level switches, each of which has its own group of computers hanging beneath it, giving the shape of an inverted tree. It is a hierarchy of star networks joined to a common backbone. Advantages: easy to expand by adding a new branch; suits large networks split by department or by floor; a fault can be isolated to one branch. Disadvantages: the whole network depends on the root and the backbone, and failure there disables everything below it; it needs a great deal of cable; configuration is complex.

A transmission medium is the physical path along which data travels from sender to receiver. Media are divided into two families. Guided (wired) media carry the signal along a solid path — a cable. Unguided (wireless) media send the signal through air or space with no physical conductor. Begin every answer with that division; it frames everything that follows.

Twisted pair cable. Pairs of insulated copper wires twisted around each other and bundled inside an outer sheath. The twisting is not decoration — it cancels much of the electrical interference and the crosstalk that neighbouring wires would otherwise induce in each other, and that is the standard one-line answer to why the wires are twisted. It comes as UTP (Unshielded Twisted Pair), the cable used with RJ-45 connectors in most school and office LANs, and STP (Shielded Twisted Pair), which adds a metal shield for better noise protection at higher cost. Merits: cheapest cable, thin and flexible, easy to install and repair. Limits: lowest bandwidth of the three cables, high attenuation (weakening of the signal with distance) so repeaters are needed on long runs, and it is the most affected by noise.

Coaxial cable. A single central copper conductor, surrounded by an insulating layer, then a braided metal mesh acting as a shield, then an outer plastic jacket. The mesh gives it much better noise immunity than twisted pair. Merits: higher bandwidth than twisted pair, less affected by interference, can carry signals over longer distances. Limits: costlier, thicker and less flexible, so it is harder to install and route. It is the cable familiar from cable television connections and was used in older bus-topology networks.

Fibre optic cable. Very thin strands of glass or plastic — a core wrapped in a cladding — that carry data as pulses of light, which stay inside the core by total internal reflection. Merits: by far the highest bandwidth; completely immune to electromagnetic interference because it carries light, not electricity; very low attenuation, so it runs over long distances without repeaters; hard to tap, so it is the most secure cable. Limits: the most expensive; fragile and can break if bent sharply; joining two fibres requires special equipment and trained technicians. It is the medium of choice for undersea and long-distance backbone links and for broadband to the home.

Unguided media are classified by frequency. Radio waves are omnidirectional — they spread in every direction and pass through walls — so the transmitter and receiver need not be aligned. They cover long distances and are used for FM radio, mobile telephony and wireless networking; the drawbacks are interference from other transmitters, poor security since anyone in range can receive, and the need for a government licence in many bands. Microwaves travel in a narrow beam and need a clear line of sight, so tall towers are built with dish antennas aimed at each other; they carry high bandwidth over long distances but are blocked by hills and buildings and disturbed by heavy rain. Beaming through a satellite extends the same idea across continents. Infrared waves work only over short distances and cannot pass through walls, which is both the limitation and the security benefit — a signal in one room stays in that room. A television remote control is the standard example.

Two named short-range technologies must be defined separately. Bluetooth is a short-range wireless standard, usually effective within about ten metres, used to build PANs between devices that have been paired — earphones, speakers, a wireless mouse, file transfer between two phones. It uses little power but offers modest speed. WiFi (a brand name of the Wi-Fi Alliance, usually expanded in textbooks as Wireless Fidelity) uses radio waves to build a wireless LAN over a room, a floor or a building through a device called an access point or a hotspot. It is much faster and covers a wider area than Bluetooth, but consumes more power, and its range and speed fall when walls, distance or crowding come in the way. Ranges quoted for either vary with the hardware and the version, so state them as approximate.

Every device below exists to solve one specific problem. Learn the problem and the definition follows.

NIC — Network Interface Card. The hardware, fitted inside or built into a computer, that physically connects it to the network; also called a network adapter or LAN card. Every NIC carries a unique hardware address burnt in at manufacture, called the MAC address (Media Access Control address), which identifies that device on the local network. Without a NIC a machine cannot join a network at all.

Modem — MOdulator-DEModulator. A modem converts the digital signal produced by a computer into an analog signal that an ordinary telephone or cable line can carry (modulation), and converts the incoming analog signal back into digital form for the computer (demodulation). That two-way conversion is the definition; write both halves. Modems may be internal (a card inside the cabinet) or external (a separate box), and today the home broadband box usually combines a modem, a router and a WiFi access point in one unit.

Repeater. A signal weakens as it travels — that is attenuation. A repeater receives the weak signal, regenerates and amplifies it, and sends it on, so the network can extend beyond the maximum length of a single cable. It does not read addresses and does not filter anything; it simply strengthens whatever arrives.

Hub. A device with several ports used as the centre of a star network. It is essentially a multiport repeater: data arriving on one port is broadcast to every other port, and the machines themselves discard what is not addressed to them. A hub has no memory of who is connected where.

Switch. A device that looks like a hub but is intelligent. It maintains a MAC address table of which device is on which port, so a frame arriving for one machine is forwarded only to that machine's port. Nothing else on the network sees it.

This is the comparison to have ready, so memorise the five contrasts. Forwarding: a hub broadcasts to all ports, a switch sends only to the intended port. Intelligence: a hub keeps no address table, a switch maintains one. Bandwidth: all devices on a hub share the available bandwidth, whereas each port of a switch gets its own. Collisions and speed: a hub causes many collisions and works in half duplex, while a switch largely eliminates collisions and supports full duplex, so the network is faster. Security and cost: a hub is less secure because every machine receives every frame, but is cheaper; a switch is more secure and costlier. In practice hubs have been almost entirely replaced by switches.

Bridge. A device that connects two segments of the same network and filters traffic between them using MAC addresses. If a frame is meant for a machine on the same side, the bridge does not let it cross; only frames meant for the other side pass. That filtering reduces unnecessary traffic and improves performance.

Router. A device that connects two or more different networks and forwards data between them using IP addresses (Internet Protocol addresses). A router examines the destination address of each packet and chooses the best available path towards it — that path selection, called routing, is what separates a router from a switch. The box that connects your home or school LAN to the Internet provider is a router.

Gateway. A device — sometimes a computer running special software — that connects two networks working on different protocols or different architectures, translating between them so they can communicate. Because it converts, a gateway is often called a protocol converter, and it acts as the entry and exit point of a network. Keep the distinction sharp: a router joins networks that already speak the same protocol; a gateway joins networks that do not.

Beyond cables and boxes, a network must be organised. There are two models, and questions ask you to compare them.

In a client-server network one powerful computer, the server, is dedicated to providing services — storing files, running a database, hosting a website, managing printing. The other computers, the clients, request those services and display the results. When you open the IRCTC website, your browser is the client and the machine that stores the trains and seats is the server. Advantages: data is centralised, so backup, updating and security are handled in one place; access rights can be controlled user by user; the network is easy to manage as it grows; a powerful server gives better performance for shared work. Disadvantages: a dedicated server and its software are expensive; a trained administrator is required; if the server fails, every client stops working; heavy traffic can congest the server.

In a peer-to-peer (P2P) network all computers are equal — there is no dedicated server, and every machine can act as both client and server, offering its own files or printer to the others. Two students sharing a folder between laptops over the same WiFi, or sending a file phone-to-phone over Bluetooth, are working peer-to-peer. Advantages: cheap, since no server is bought; simple to set up; no administrator needed; the failure of one machine does not stop the others. Disadvantages: no central backup, so data is scattered; security is weak because each user controls their own sharing; performance drops as machines serve each other while also being used; it becomes unmanageable beyond a small number of computers. The rule of thumb: peer-to-peer suits a home or a small office; client-server suits a school, a bank or any organisation that must protect its data.

Bandwidth is the maximum amount of data that a communication channel can carry in a given time — its capacity. In digital data communication it is measured in bits per second (bps). Be careful with a common confusion: in analog communication bandwidth means the range of frequencies a channel can carry and is measured in hertz (Hz), and some textbooks introduce both meanings. Higher bandwidth does not mean the signal travels faster; it means more data can be pushed through per second, exactly as a wider road carries more vehicles at the same speed. The data actually delivered, which is always lower than the bandwidth because of noise, sharing and overheads, is called throughput.

The units multiply by one thousand at each step. 1 Kbps (kilobit per second) = 1,000 bps. 1 Mbps (megabit per second) = 1,000 Kbps = 10⁶ bps. 1 Gbps (gigabit per second) = 1,000 Mbps = 10⁹ bps, and 1 Tbps (terabit per second) = 1,000 Gbps. Note that data-transfer units are counted in powers of ten, while memory sizes are traditionally counted in powers of two, where 1 KB = 1,024 bytes. A few school textbooks take 1 Kbps as 1,024 bps as well; follow whichever your textbook uses and state the value you have assumed in the answer.

The trap examiners set is the difference between a bit and a byte. A lowercase b means bit, an uppercase B means byte, and 1 byte = 8 bits. Network speeds are quoted in bits per second, while file sizes are given in bytes, so a connection sold as 8 Mbps delivers at best about 1 MB (megabyte) of file per second. Work such sums in three steps: convert the file size to bits, convert the speed to bits per second, then divide. For a 40 MB file on an 8 Mbps line — 40 MB = 40 × 8 = 320 megabits; 320 ÷ 8 = 40 seconds. Write the conversion line; it is where the marks are.