Recall warm-up
- What is a host, in one line?
Stuck? Peek at 0.1.1 Why networks exist: hosts, links and sending data
- Network = devices + links that exchange data.
- Host = where a message starts or ends (laptop, phone, server).
- Link = physical path: copper (voltage), fibre (light), wireless (radio).
- All content → bits → signals → bits → content; meaning comes from an agreed encoding.
- One link per pair of n hosts: n(n − 1)/2 links — too many, so links are shared.
- How many private cables do 7 hosts need if every pair gets its own?
Stuck? Peek at 0.1.1 Why networks exist: hosts, links and sending data
- Network = devices + links that exchange data.
- Host = where a message starts or ends (laptop, phone, server).
- Link = physical path: copper (voltage), fibre (light), wireless (radio).
- All content → bits → signals → bits → content; meaning comes from an agreed encoding.
- One link per pair of n hosts: n(n − 1)/2 links — too many, so links are shared.
- Name the three families of link and what each uses to carry bits.
Stuck? Peek at 0.1.1 Why networks exist: hosts, links and sending data
- Network = devices + links that exchange data.
- Host = where a message starts or ends (laptop, phone, server).
- Link = physical path: copper (voltage), fibre (light), wireless (radio).
- All content → bits → signals → bits → content; meaning comes from an agreed encoding.
- One link per pair of n hosts: n(n − 1)/2 links — too many, so links are shared.
The Hook
Think of a city's roads. Some roads join exactly two houses (a private driveway). Others are a busy street shared by every house along it. Junctions in the middle decide where traffic goes. A network has the same three ingredients: private links, shared links, and boxes in the middle that pass data along. Today we name them and learn the standard shapes.
Predict First
Twenty people sit in one classroom and anyone may speak to the whole room. Fifty such classrooms in different cities are joined together. Guess: is "the classroom" and "the set of classrooms" the same kind of network, or two different kinds?
,Two different kinds. One classroom is a small network in one building — a LAN. The joined classrooms spread over cities form a WAN, and networks of networks joined world-wide form the Internet. The classroom where everyone hears everyone is a broadcast link.
Core Concept
Nodes and links
Any device attached to the network is a node. Hosts are nodes; so are the boxes in the middle that pass data along. A link joins nodes.
Point-to-point vs broadcast links
- Point-to-point link: exactly two nodes, one at each end. What one sends, only the other receives. (A private driveway.)
- Broadcast (shared) link: many nodes on the same link. What one sends, every node on it receives. (A classroom where everyone hears the speaker.)
On a broadcast link, if two nodes talk at once, their signals mix and both are spoiled. Who may speak when on a shared link is a whole chapter of Phase 2.
Direction: simplex, half-duplex, full-duplex
- Simplex: bits flow one way only (a TV broadcast tower to your TV).
- Half-duplex: both directions, but only one at a time (a walkie-talkie: "over").
- Full-duplex: both directions at the same time (a phone call).
| Mode | Directions | Both at once? | Everyday example |
|---|---|---|---|
| Simplex | one | — | TV tower → TV |
| Half-duplex | two | no, take turns | walkie-talkie |
| Full-duplex | two | yes | phone call |
Topologies: the shapes
The topology is the layout of nodes and links. For n nodes:
| Topology | Shape | Links needed | If one link breaks… |
|---|---|---|---|
| Mesh (full) | every pair directly linked | n(n − 1)/2 | only that pair is affected; reroute through others |
| Star | every node linked to one central box | n | only that node is cut off; if the centre fails, everyone is |
| Bus | all nodes tap one shared backbone cable | 1 backbone (+ n short taps) | a backbone break splits the network |
| Ring | each node linked to its two neighbours in a circle | n | a single break can stop a one-way ring |
| Tree | stars joined in levels | n − 1 between n nodes | a branch is cut off below the break |
LAN, WAN, the Internet
- LAN (Local Area Network): nodes in one room, floor or building, owned by one organisation. Short links, high speed.
- WAN (Wide Area Network): links that span cities or countries, often rented from a telecom company.
- internet (small i): any network of networks. The Internet (capital I): the single world-wide network of networks we all use.
Hosts vs hubs vs switches vs routers (plain words)
The boxes in the middle, in plain words for now (each gets its precise layer in Lesson 1.1.3):
- Hub: repeats every signal it receives out of all its other cables. Dumb and noisy — it turns a star of cables into one big broadcast link.
- Switch: joins hosts inside one LAN and sends data only out of the cable that leads to the right host.
- Router: joins different networks and chooses which way data should go next on its trip across networks.
A laptop is a host, not a box in the middle: it starts and ends messages. Your home "Wi-Fi router" is really several boxes in one case (a router, a switch and a wireless access point).
Putting it together
- Nodes are all devices; hosts are the ends. Links are point-to-point (two nodes) or broadcast (everyone hears).
- Topologies trade cost against reliability: mesh n(n − 1)/2 links, star/ring n, tree n − 1.
- Switches deliver inside a LAN; routers choose paths between networks; LANs joined by WANs make the Internet.
Visual
Worked Examples
8 hosts are connected in a full mesh. How many links are there, and how many link connections does each host need?
Show the full solution
Links = 8 × 7 / 2 = 28. Each host links to every other host: 7 connections per host.
An office of 12 hosts uses a star around one switch. How many links? What happens if the switch fails? What if one host's cable fails?
Show the full solution
Star needs one link per host: 12 links. If the switch fails, every host loses the network (single point of failure). If one cable fails, only that one host is cut off.
A company has a core of 5 routers connected as a full mesh. Three more hosts are each connected by one link to one router (a star hanging off the core). Count the links. Which single failure disconnects the most hosts?
Show the full solution
Mesh part: 5 × 4 / 2 = 10 links. Star part: 3 links. Total 13.
No single link failure in the mesh disconnects anyone (traffic goes round). The worst single failure is a router that has hosts hanging off it: all hosts attached to it are cut off. A host's own link failing cuts off only that host.
GATE Traps
- "Wi-Fi router" at home is a router + switch + wireless access point in one case. In questions, judge a box by its job, not its marketing name.
- Star: if the centre fails, everyone is disconnected — the centre is a single point of failure.
- A hub does not make a star private: a hub repeats to everyone, so the star behaves like one shared broadcast link.
- Mesh link count is n(n − 1)/2, not n(n − 1) — each link serves two nodes.
Memory Hook
Hub = loudspeaker (everyone hears). Switch = receptionist (walks the message to the right desk in the building). Router = travel agent (picks the next city).
Connect the Dots
From 0.1.1 we knew what travels (bits) and between whom (hosts). Now we know along what shapes and through which boxes. Next (0.2.1): bits are counted and sent at some speed — how do we measure "how many bits" and "how many bits per second" without falling into the most common unit trap in GATE?
Self-Test
- NAT
How many links does a full mesh of 9 nodes need?
Write your answer before opening the key.
- MCQ
Which box joins different networks and chooses where data goes next?
- Hub
- Switch
- Router
- Host
- MSQ
Which statements are true?
- On a broadcast link every attached node receives what one node sends.
- A point-to-point link joins exactly two nodes.
- Half-duplex allows both directions at the same time.
- In a star, failure of the centre disconnects all nodes.
- MCQ
A ring of 10 nodes and a star of 10 outer nodes need how many links respectively?
- 10 and 10
- 10 and 9
- 45 and 10
- 9 and 10
- MCQ
Five hosts are connected to a hub. Which description is correct?
- Each host has a private point-to-point path to every other host.
- The hub sends each signal only to the intended host.
- Physically a star, but it behaves like one shared broadcast link.
- It is a WAN.
Rule Card
- Node = any device; host = node where messages start/end.
- Point-to-point (2 nodes) vs broadcast (all hear all).
- Simplex (one way) · half-duplex (two ways, one at a time) · full-duplex (both at once).
- Links: mesh n(n−1)/2, star n, ring n, tree n−1, bus 1 backbone.
- LAN (building) ⊂ WAN (cities) ⊂ the Internet (network of networks).
- Hub repeats to all · switch delivers within a LAN · router picks the next network.
Concept Ledger
- node
- point-to-point link
- broadcast (shared) link
- simplex / half-duplex / full-duplex
- topology (mesh, star, bus, ring, tree)
- LAN
- WAN
- internet / the Internet
- hub
- switch
- router
Answer Vault
Try the questions first. Open when you are ready to check.
Warm-up answers
- A device at the edge of the network where a message starts or ends (laptop, phone, server).
- 7 × 6 / 2 = 21.
- Copper (voltage), optical fibre (light), wireless (radio waves).
Self-test answers
- 36 — 9 × 8 / 2 = 36.
- C — A router connects networks and picks the next way to send data.
- A: A hub just repeats signals to everyone.
- B: A switch delivers inside one LAN.
- D: A host is where messages start or end, not a box in the middle.
- ABD — Broadcast = everyone hears; point-to-point = two ends; star centre is a single point of failure.
- C: Half-duplex allows both directions but only one at a time; both at once is full-duplex.
- A — A ring of n nodes has n links; a star with n outer nodes has n links to the centre.
- B: A star with 10 outer nodes needs 10 links, one per node.
- C: 45 is a full mesh of 10.
- D: n − 1 links is a tree / chain, not a ring (a ring closes the circle).
- C — Cables form a star, but a hub repeats everything to everyone, so all hosts share one broadcast link.
- A: Nothing is private through a hub.
- B: That is what a switch does, not a hub.
- D: Five hosts on a hub in one place form a LAN.
Flashcards
- Links in a full mesh of n nodes?
Reveal answer
n(n − 1)/2. - Hub vs switch vs router?
Reveal answer
Hub repeats to everyone; switch delivers to the right host inside a LAN; router chooses the next network. - Half-duplex vs full-duplex?
Reveal answer
Half: both directions, one at a time. Full: both directions simultaneously. - Single point of failure of a star?
Reveal answer
The central box. - LAN vs WAN?
Reveal answer
LAN: one building, one owner. WAN: spans cities or countries.
Exam shortcuts
- Links by topology: mesh n(n − 1)/2 · star n · ring n · tree n − 1 · bus 1 backbone. — n nodes, simple (one link per connected pair) layouts. For a star, n counts the outer nodes, not the centre.
- Who decides inside vs between networks: switch inside one LAN, router between networks. — classifying the plain-word role of a box; the precise layer view comes in 1.1.3.