How information travels

When you watch a video on an online platform there are two actors: your mobile phone (the client) and the platform's computer (the server). Explain what each one does at that moment, who requests and who responds.

Indicate who acts as the client and who acts as the server in each situation: (a) you request the weather page from your browser; (b) you upload a photo to your cloud storage; (c) you watch a video on a platform. Explain in each case who requests and who responds.

Explain in your own words what a server is and reason why servers for major platforms are kept running twenty-four hours a day in air-conditioned rooms with backup power.

What is an IP address? Explain it in your own words using the analogy of a house's postal address: what role each plays and why they are needed for data to reach its destination.

The IP address 192.168.1.35 consists of four numbers separated by dots. Compare it to a postal address ("12 High Street, Flat 3A") and explain broadly what role each part plays so that data finds the correct device.

At home you have a console with private IP 192.168.1.15 and a computer with IP 192.168.1.20, both connected to the router. Your friend tries to invite you to a direct match by entering the IP "192.168.1.15" from his home, but the game gives him a connection not found error.

1. a) Explain to your friend why that address will never allow him to reach your console from outside your home.
2. b) If you access the website "cualesmiip.com" simultaneously from the computer and from the console, will you see the same address or different addresses? Justify your answer by explaining the intermediary role played by the router.

The figure shows the journey of a message from your mobile phone to a server, with an intermediate cloud covered by a question mark, as shown in the diagram.

What does the cloud marked with "?" represent? Order and describe the stages of the message's journey from the mobile phone to the server, and explain why the message travels split into several packets instead of in one piece.

A photo is sent split into numbered packets that may arrive at the destination out of order. Explain how it is possible for the recipient to reconstruct the photo perfectly and what you think happens if one of the packets gets lost along the way.

The figure shows three packets leaving a mobile phone in order and arriving at the server out of order, which reorders them, as shown in the diagram.

Explain how the server manages to reconstruct the message in the correct order by looking at the packet numbers, and reason what you think happens if packet 1 takes much longer than the others to arrive.

A video file takes up $700\ \text{MB}$ and your connection downloads at $50\ \text{MB}$ per second. Calculate the theoretical download time using the division $\frac{700\ \text{MB}}{50\ \text{MB/s}}$, and then explain two reasons why in practice it usually takes longer than the calculated time.

A film takes up $1\,400\ \text{MB}$ and your connection downloads at $40\ \text{MB}$ per second. Calculate the theoretical download time using the division $\frac{1\,400\ \text{MB}}{40\ \text{MB/s}}$. Then reason whether with a connection half as fast you could stream the film smoothly without buffering, knowing that online video consumes about $20\ \text{MB}$ per second.

The "ping" measures the time taken for a message to go and return: a nearby server records $0,02\ \text{s}$ and one on another continent, $0,25\ \text{s}$. Explain what these figures mean for an online game, where every millisecond counts, and why choosing the right server improves the game.

Put the steps of a website visit in order and explain each one: (a) the server sends the page; (b) you type the address into the browser; (c) the browser displays the page on the screen; (d) your request travels over the internet to the server.

Observe the following diagram of connections between internet routers to send a file from your school (A) to a server (F):

A ── B ── D ── F

│ │ │ │

└─── C ── E ───┘

1. a) During the transfer, a breakdown cuts the line between B and D. Justify whether your transmission will be completely cancelled or if it can be completed, indicating the alternative route the packets will follow.
2. b) A classmate claims: "It would be faster and safer to run a single direct cable from A to F without going through other intermediate nodes". Refute their claim by providing two technical drawbacks of designing the network in that way.