Recall warm-up
- Before any networking: name two ways you have moved a file from one device to another.
- When you send a photo on your phone, does the photo itself fly through the air?
- Why does a whole office share one printer instead of giving every desk its own?
The Hook
You are at your office desk and want to open a report that lives on a computer in another city. Without any network, someone would have to copy it onto a pen drive and put it in a courier envelope. It would take a day, and you would need a new envelope for every change.
A computer network replaces the courier. This whole course follows one message on its trip from your laptop to a server and back. Today we answer the most basic question: what is actually travelling, and along what?
Predict First
A friend sends you a 3-second voice note. Between their phone and yours, what physically travels? Pick one: (a) sound, (b) a long pattern of two kinds of symbol, (c) a copy of the phone's memory chip.
,(b). The voice note is turned into a long string of bits — each bit is a 0 or a 1 — and only that string travels, carried by electrical, light or radio signals. Sound is not sent, and no memory chip moves. Your phone rebuilds the sound from the bits.
Core Concept
What a network is
Picture a courier company: houses where parcels start and end, and roads between them. A computer network is the same idea for data.
Everything else in this course is detail about how the exchange works: how data is wrapped, addressed, checked, passed along and confirmed.
Hosts: where messages start and end
The devices you use — laptop, phone, smart TV — and the big computers that answer them — a mail server, a video server — are called hosts (also end systems). A host runs the programs that create and read messages.
A server is still a host. "Server" only describes its job: it waits for requests and answers them.
Links: the roads
A link is the physical path between two neighbouring devices. Links come in three families:
| Link family | What carries the bits | Where you meet it |
|---|---|---|
| Copper cable | changes in voltage | office desk cable, old phone lines |
| Optical fibre | pulses of light | between cities, under oceans |
| Wireless | radio waves | Wi-Fi, mobile data |
Sharing resources
Why build networks at all? Three reasons, in the order they appeared historically:
- Share expensive things: one printer, one big disk, one powerful computer used by many people.
- Communicate: e-mail, chat, video calls.
- Share the links themselves: laying a separate cable between every pair of people is impossible, so many conversations must share a few links. Ten people would already need 45 private cables (we count this in Example 2).
That last point drives the whole subject. Because links are shared, someone has to decide who sends when and which way data goes next. Those two questions return in Phases 2 and 3.
What "sending data" really means
Text, photos, voice and video all become the same thing before they travel: a string of bits. A bit is one of two values, written 0 and 1.
- The sending host encodes the content as bits. Each letter has an agreed 8-bit pattern — for the letter H the pattern is
01001000. - The link carries each bit as a signal: high or low voltage, light on or off, one radio wave shape or another.
- The receiving host reads the signals back into bits and decodes the bits into the original content.
The link has no idea what the bits mean. Meaning exists only because sender and receiver agreed in advance how to encode and decode. Agreement on rules is the seed of the whole subject — in Phase 1 it gets a name.
The supplied notes include an interactive visual for this concept; this reading view preserves the surrounding explanation and sequence.
Putting it together
- A network is hosts joined by links; a host is where a message starts or ends.
- Everything travels as bits, carried by signals; meaning comes from an agreed encoding.
- Private links for every pair grow like n²/2, so real networks share links — which creates the turn-taking and path-choosing problems of later phases.
Visual
Worked Examples
In a small office there are 4 laptops, 1 file server and 1 printer, all connected. Which of them are hosts?
Show the full solution
Apply the test: does the device run programs where messages start or end?
- 4 laptops — yes (people run programs on them).
- File server — yes (it answers requests).
- Printer — yes: it receives documents (messages end there) and reports its status.
All 6 devices are hosts. The cables between them are links, not hosts.
Ten colleagues each want a private cable to every other colleague. How many cables are needed?
Show the full solution
Count pairs. Colleague 1 needs cables to 9 others, colleague 2 to 8 new ones (the cable to 1 is already counted), … colleague 9 to 1 new one, colleague 10 to none new.
Total = 9 + 8 + … + 1 = 45. In general n people need n(n − 1)/2 cables: 10 × 9 / 2 = 45.
A campus has 50 hosts. Plan A: a private cable for every pair of hosts. Plan B: every host has one cable to a shared central box that passes data along. How many cables does each plan need, and how many times more cables does Plan A use?
Show the full solution
Plan A: 50 × 49 / 2 = 1225 cables.
Plan B: one cable per host = 50 cables.
Ratio 1225 / 50 = 24.5 → Plan A uses 24.5 times as many cables. That is why real networks share links through boxes in the middle — the boxes are named in the next lesson.
GATE Traps
- A server is a host. "Server" is a job description, not a different kind of network device.
- Wireless is still a link. A link does not need a cable.
- The network carries bits, not meaning. Two hosts that do not agree on the encoding receive the bits correctly and still cannot understand them.
Memory Hook
Hosts are houses, links are roads, and the parcels are made of 0s and 1s.
Connect the Dots
This is the start of the story: a message is just bits, and it must cross links between hosts. Next question (0.1.2): once many hosts share a few links, what are the possible shapes of the network, and which boxes in the middle pass the bits along?
Self-Test
- MCQ
Which statement about what travels on a link is correct?
- The original sound or picture travels.
- A string of bits travels, carried as signals.
- Only text can travel; pictures are sent separately.
- The sending device's memory is copied byte by byte as a physical object.
- MSQ
Which of these are hosts (end systems)?
- A mail server
- A smartphone
- An optical fibre between two cities
- A smart TV streaming a film
- NAT
Six hosts each get a private cable to every other host. How many cables are needed?
Write your answer before opening the key.
- MCQ
Two hosts exchange bits correctly over a link, but the receiver shows nonsense. What is the most likely cause?
- The link was too slow.
- Sender and receiver do not use the same agreed encoding.
- The receiver is not a host.
- Wireless links cannot carry text.
- MCQ
The main reason many conversations share a few links instead of every pair having its own link is:
- Shared links are faster than private links.
- The number of private links grows roughly with the square of the number of hosts.
- Private links cannot carry bits.
- Hosts can only have one link.
Rule Card
- 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.
Concept Ledger
- network
- host / end system
- server (a host that answers)
- link
- bit (0 or 1)
- signal
- encode / decode
- sender / receiver
- sharing resources
Answer Vault
Try the questions first. Open when you are ready to check.
Warm-up answers
- For example a pen drive (carry it by hand) and sending it through a messaging app. The second one is what this course explains.
- No. Something that describes the photo travels — a long pattern that the receiving phone turns back into the same picture. This lesson says exactly what that pattern is.
- Cost: one shared printer is cheaper than fifty. Sharing an expensive resource is the first reason networks exist.
Self-test answers
- B — All content is encoded as bits; the link carries the bits as voltage, light or radio signals.
- A: Sound is converted to bits before sending; sound itself never travels on the link.
- C: Pictures, voice and video are bits just like text.
- D: Nothing physical moves except the signal.
- ABD — Hosts run the programs where messages start or end: servers, phones, TVs. The fibre is a link.
- C: A fibre is a link: it only carries signals between its two ends.
- 15 — n(n − 1)/2 = 6 × 5 / 2 = 15.
- B — Meaning comes only from the agreed encoding. Correct bits decoded with the wrong rules give nonsense.
- A: Speed changes how long it takes, not what the bits mean.
- C: It received and displayed data, so it runs programs: it is a host.
- D: Any link carries bits; text is bits.
- B — n(n − 1)/2 links grows like n²: 50 hosts would need 1225 private cables.
- A: Sharing does not make a link faster; it makes the network cheaper.
- C: Private links carry bits perfectly well — they are just too many.
- D: A host can have several links (a laptop can use both a cable and Wi-Fi).
Flashcards
- What is a host?
Reveal answer
A device at the edge that runs user programs; a message starts or ends there (laptop, phone, server). - What physically travels on a link?
Reveal answer
Bits, carried as signals (voltage, light or radio). - Private links for every pair of n hosts?
Reveal answer
n(n − 1)/2. - Where does the meaning of bits come from?
Reveal answer
From an encoding agreed in advance by sender and receiver.
Exam shortcuts
- Pairs formula: n devices, one link per pair → n(n − 1)/2 links. — every pair must have its own dedicated link (a full mesh). Not valid when devices share a link or a central box.
- Host test: "Does a message start or end here?" Yes → host. — classifying devices; a box that only passes data along is not a host.