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Unit 1 was one machine talking to itself. Unit 2 is two machines, a thousand kilometres apart, agreeing on how to talk.
Data in memory, then data on a disk. Every byte stayed inside one computer.
Chapter 4 wrote marks.csv. How does that file get from the school
office to the board's server 800 km away — reliably, in seconds, over cables
and air owned by dozens of different companies?
Chapter 6 — the hardware: what is connected, how it is wired, and
which box sits where.
Chapter 7 — the rules: the protocols the two ends agree on.
Computer network — a collection of two or more independent computing devices connected through a transmission medium, so that they can exchange data and share resources.
Almost entirely by drawing and naming. "Draw the star topology." "Name the device." "Suggest a suitable topology and justify." A word list scores nothing; a labelled diagram scores full marks. Every diagram in this deck is one you should be able to reproduce.
| Component | In this chapter |
|---|---|
| Nodes — the devices | §2, §3 |
| Medium — what carries the signal | §6 |
| Layout — how they are joined | §5 |
| Protocol — the agreed rules | Chapter 7 |
Resource sharing (one printer, many users) · File sharing · Communication (email, chat, VoIP) · Centralised data and backup · Cost saving · Reliability — a copy exists elsewhere.
The Internet is the global network of networks — the wires, routers and addresses. The World Wide Web is one service that runs on it, made of linked pages served over HTTP.
Email, FTP and VoIP also run on the Internet and are not part of the Web. The Internet is the road; the Web is one kind of traffic on it.
ARPANET — Advanced Research Projects Agency Network, 1969. The first packet-switched network, funded by the US Department of Defense, connecting four university computers. The ancestor of the Internet.
NSFNET — National Science Foundation Network, 1986. A high-speed backbone that linked regional networks together and opened the network beyond defence research.
Internet — the worldwide interconnection of networks using the TCP/IP protocol suite.
World Wide Web (WWW) — a collection of interlinked hypertext documents accessed over the Internet using HTTP, proposed by Tim Berners-Lee in 1989–90 at CERN.
Interspace — a client/server software environment that allows users to communicate using multiple media — text, video, audio — in a dynamic three-dimensional environment, in real time.
It appears in the syllabus and almost nowhere else in life. Learn the wording above; you will not need to reason about it.
| Type | Full form | Spread | Owned by | Example |
|---|---|---|---|---|
| PAN | Personal Area Network | up to ~10 m | one person | phone + earbuds over Bluetooth |
| LAN | Local Area Network | up to ~1 km | one organisation | a school computer lab |
| MAN | Metropolitan Area Network | up to ~50 km | shared / a provider | all branches of a bank in a city; cable TV |
| WAN | Wide Area Network | unlimited | many organisations | the Internet |
| LAN | MAN | WAN | |
|---|---|---|---|
| Data transfer speed | Very high | Moderate | Lower |
| Error rate | Lowest | Moderate | Highest |
| Cost to set up | Low | High | Very high |
| Ownership | private | private or public | usually public |
| Typical medium | twisted pair, Wi-Fi | optical fibre | fibre, satellite, leased lines |
| Maintained by | the owner | a provider | many providers |
Signals weaken and take time over distance. A LAN's cables are short, so the signal is strong, fast and rarely corrupted. A WAN's path crosses continents, so it is slower, more error-prone, and needs equipment at every hop — which is exactly why it costs more.
Classify each: (a) two laptops sharing files over Bluetooth in one room · (b) the computers of one college campus with three buildings · (c) the branches of a bank across Delhi · (d) a company with offices in Mumbai, London and Tokyo.
(a) PAN · (b) LAN — a campus network is still a LAN · (c) MAN — one city · (d) WAN.
(b) is the one people get wrong. Several buildings on one campus, owned by one organisation, is a LAN — sometimes called a CAN, Campus Area Network. It becomes a MAN only when it spreads across a city on someone else's infrastructure.
NIC — lets a computer join a network at all. Repeater — makes a weak signal strong again. Hub — joins devices, shouts to everyone. Switch — joins devices, speaks to one. Router — joins networks and chooses the route. Gateway — joins networks that speak different protocols. Modem — converts digital to analog and back.
Network Interface Card — the hardware that physically connects a computer to a network. Every NIC carries a permanent, globally unique MAC address burnt into it by the manufacturer.
48 bits, written as six hex pairs —
00:1B:44:11:3A:B7. It identifies the device; an IP address
identifies its position on the network. Also called the physical or
Ethernet address.
Repeater — a device that receives a weakened (attenuated) signal, regenerates it to its original strength, and retransmits it, so the network can span a longer distance.
Modem — Modulator–Demodulator. It converts the computer's digital signal into an analog signal for transmission over a telephone or cable line, and converts incoming analog signals back to digital.
Telephone lines were built to carry the analog waveform of a human voice. A computer produces digital square pulses, which such a line cannot carry faithfully. The modem translates in both directions — which is why there is one at each end.
A hub has no idea who is who. It copies whatever arrives to every other port. C and D must inspect the data and discard it.
A switch keeps a table of which MAC address is on which port, so it forwards to one port only.
| Hub | Switch | |
|---|---|---|
| Sends data to | every port | only the destination port |
| Uses addresses? | No | Yes — MAC addresses |
| Bandwidth | shared by all | full, per port |
| Collisions | frequent | avoided |
| Security | poor — all can see the data | better |
| Cost | cheaper | more expensive |
Router — a device that connects two or more networks and determines the best path along which to forward a data packet towards its destination, using the IP address in the packet.
Gateway — a device that connects two networks that use different protocols or architectures, translating between them so that they can communicate. It is the entry and exit point of a network.
A router joins networks that already speak the same language and just need directions. A gateway joins networks that speak different languages and need a translator. Every gateway does routing; not every router is a gateway.
What people call "the router" at home is usually a modem, router, switch and wireless access point in one case. In the exam, name the function being asked about, not the box.
| Device | Job | Works using | Connects |
|---|---|---|---|
| NIC | attaches a computer to a network | MAC address | one computer → the medium |
| Repeater | regenerates a weakened signal | nothing — pure signal | two lengths of cable |
| Hub | joins devices, broadcasts | nothing — no addresses | devices in one LAN |
| Switch | joins devices, forwards precisely | MAC address | devices in one LAN |
| Router | chooses the path between networks | IP address | network ↔ network |
| Gateway | translates between protocols | protocol conversion | dissimilar networks |
| Modem | digital ⇄ analog conversion | modulation | computer ↔ phone/cable line |
Name the device for each: (a) the lab's 30 computers must be connected so that data reaches only the intended machine · (b) a cable run of 150 m has a signal too weak at the far end · (c) the school LAN must be connected to the Internet · (d) a computer has no way to connect to a network at all.
(a) Switch — "only the intended machine" rules out a hub.
(b) Repeater — the problem is signal strength over distance, nothing else.
(c) Router — it joins two different networks and picks the path. (A modem is
also needed if the link is over a telephone or cable line.)
(d) NIC — without it the machine cannot join any network.
Technique: the question always contains the keyword. "only the intended" → switch · "weak / long distance" → repeater · "two networks" → router · "different protocols" → gateway.
Network topology — the arrangement or layout in which the nodes of a network are physically or logically connected to one another.
Every advantage and disadvantage in this section is one of these four. Learn the four questions and you can derive the answers.
Three ways, and you need all three:
① Draw it — a labelled diagram, four to six nodes.
② Two advantages and two disadvantages.
③ Choose one for a described situation and justify the
choice.
Always label the nodes and the central device. An unlabelled sketch of circles and lines is worth nothing, and the labels take four seconds.
Bus topology — all nodes are connected to a single common backbone cable through drop lines. A signal placed on the bus travels to every node; only the addressed node accepts it.
① Uses the least cable of any topology, so it is the cheapest.
② Easy to install for a small network laid out in a line.
③ Failure of one node does not affect the others.
① If the backbone breaks, the whole network fails — a single
point of failure.
② A fault is hard to locate — you must test the whole cable.
③ All nodes share the bandwidth, and collisions rise sharply as
nodes are added.
Star topology — every node is connected by a separate point-to-point link to a central device (a hub or switch). All communication passes through that centre.
① Failure of one node or one cable affects only that node.
② Faults are easy to locate — the problem is on one link.
③ Adding or removing a node does not disturb the rest.
④ With a switch, no collisions and full bandwidth per node.
① If the central device fails, the whole network fails.
② Needs more cable than a bus — one run per node.
③ Cost of the central hub or switch.
If a question asks which topology to use inside a building or lab and gives no special constraint, star is the expected answer — and "easy fault isolation, one node's failure does not affect others" is the justification.
Tree topology — a hierarchical arrangement combining the features of bus and star: groups of star-configured nodes are connected to a linear backbone, forming levels like the branches of a tree.
① Scales well — new branches are added without redesigning the
network.
② A fault in one branch does not affect the others.
③ Natural fit where the organisation itself is hierarchical.
① If the root or the backbone fails, the whole network fails.
② Uses a lot of cable and is harder to configure.
③ More devices needed, so higher cost.
A campus or a multi-storey building: one switch per floor or per block, all joined back to a central one. That is exactly the shape of the case-study question in §7.5.
Mesh topology — every node has a dedicated point-to-point link to every other node. For n nodes the number of links is n(n − 1) / 2, and each node needs n − 1 ports.
① Most reliable — if one link fails, data takes another route.
② No traffic congestion; each link carries only its own pair's data.
③ Best security and privacy — data does not pass through other nodes.
① Enormous cabling — the count grows roughly as n².
② Highest cost of all topologies.
③ Very difficult to install and reconfigure.
How many cables does a full mesh of 8 nodes need? And how many ports on each node?
8 × 7 / 2 = 28 cables, and 7 ports on every node. Adding a ninth node would need 8 more cables and a new port on all eight existing nodes — which is exactly why mesh is never used for an office LAN.
| Bus | Star | Tree | Mesh | |
|---|---|---|---|---|
| Cable needed | Least | Moderate | High | Most |
| Cost | Lowest | Moderate | High | Highest |
| One node fails | others fine | others fine | others fine | others fine |
| Single point of failure | the backbone | the central device | the root | none |
| Fault detection | Difficult | Easy | Moderate | Easy |
| Adding a node | Easy | Easy, no disturbance | Easy | Very difficult |
| Reliability | Low | Moderate | Moderate | Highest |
| Best used for | small, cheap, linear | a lab or office | a campus or multi-floor site | critical backbones |
Reliability costs cable. Bus is cheapest and most fragile; mesh is most reliable and most expensive; star and tree sit in between, which is why almost every real network is one of those two.
Transmission medium — the physical path along which a
signal travels from sender to receiver.
Guided (wired) — the signal is confined within a solid
medium: twisted pair, coaxial cable, optical fibre.
Unguided (wireless) — the signal travels through air or
space: radio waves, microwaves, infrared, satellite.
Cheapest, easiest to install, used for almost every LAN (the RJ-45 Ethernet cable). Twisting cancels interference. Lowest bandwidth and shortest range of the three — about 100 m per run.
A core conductor inside a braided shield. Better bandwidth and far less interference than twisted pair; carries a signal further. Thicker, costlier, harder to install. Used for cable TV and older LANs.
Carries light, not electricity. Highest bandwidth, longest distance, immune to electrical interference, and the most secure — it cannot be tapped without detection. Most expensive; needs skilled installation.
| Medium | How it travels | Range | Used for | Limitation |
|---|---|---|---|---|
| Radio waves | omni-directional; passes through walls | long | Wi-Fi, FM radio, cordless phones | insecure — anyone in range receives it |
| Microwaves | line of sight, tower to tower | up to ~50 km per hop | mobile networks, long-distance links | needs unobstructed line of sight; rain fade |
| Infrared | very short range, line of sight | a few metres | TV remotes, some device pairing | cannot pass through walls |
| Satellite | up to a satellite and back down | global | remote areas, TV broadcast, GPS | very costly; noticeable delay |
Never write just "optical fibre". Write "optical fibre, because the distance exceeds 100 m and it offers the highest bandwidth with no electromagnetic interference". The justification is where the mark is.
A hub is a non-intelligent device: it broadcasts incoming data to every connected port, so all nodes share the bandwidth, collisions are common and any node can see all traffic.
A switch is intelligent: it maintains a table of MAC addresses and forwards data only to the destination port. This gives full bandwidth per port, avoids collisions and is more secure.
A router connects two or more networks that use the same protocol and selects the best path for a packet using its IP address.
A gateway connects two networks that use different protocols or architectures and performs the necessary protocol translation. It acts as the entry and exit point of a network.
Sunrise Public School is setting up its campus network in Pune, with four blocks and one branch office in Nagpur.
(i) Suggest the most suitable place to install the server, with
reason.
(ii) Suggest a suitable cable layout for connecting the blocks.
(iii) Where should a switch/hub be placed, and why?
(iv) Which device is needed to connect the campus to the Nagpur
office?
(v) Which medium would you use between Academic and Hostel, and why?
(i) Admin block. It has the largest number of computers (150), so placing the server there minimises total network traffic across the blocks and gives the most users the fastest access.
(ii) Star topology, with all three other blocks cabled back to the Admin block. It gives easy fault isolation — a break in one cable affects only that block — and the distances are all short enough for a single run.
(iii) A switch in every block. Each block has multiple computers that must be interconnected, and a switch forwards data only to the intended machine, avoiding collisions and wasted bandwidth.
(iv) A router, since the two sites are separate networks and the packets must be routed between them. A modem is also required at each end if the link uses a telephone or cable line.
(v) Optical fibre. The distance is 120 m, beyond the ~100 m limit of twisted pair, and fibre offers the highest bandwidth with no electromagnetic interference.
Server → the block with the most computers.
Layout → star, centred on the server block, unless told otherwise.
Switch → in every block, because each has several computers.
Remote site → router (plus a modem if over a phone/cable line).
Medium → check the distance: under 100 m twisted pair, over 100 m
optical fibre, and wireless where cabling is impossible.
Always give the reason — the reason is the mark.
Bus — one backbone cable with terminators at both ends and four nodes connected by drop lines. Advantage: it uses the least cable, so it is the cheapest to install.
Star — four nodes each connected by its own cable to a central switch. Advantage: failure of one node or cable does not affect the others, and faults are easy to locate.
Marks are for the labels: "backbone", "terminator", "switch", and the node names.
Each node connects to every other node, so each needs n − 1 = 5 ports.
You now know what is connected and how. Chapter 7 is the other half:
the rules the two ends agree on — HTTP, FTP, TCP/IP, SMTP, POP3 — and how a
name like ncert.nic.in becomes a machine you can actually reach.
① A router uses the IP address — Chapter 7 explains what one
actually looks like.
② A switch uses the MAC address; keep the two apart, because
the paper tests exactly that confusion.
③ The Internet is the network, the Web is a service on it —
and Chapter 7 is about the services.