How Information Travels

When you watch a video on an online platform there are two actors: your cell 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.

When visiting a website, the message "404 error: page not found" appears. Who sends that message, your browser or the server? Explain what has happened during the journey of your request for that response to arrive.

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 Main Street, Apartment 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 cell 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 cell 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 cell 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 ten-word message and a $3\ \text{MB}$ photo travel over the same network to the same cell phone. Explain why the photo takes longer even though it follows "the same path", and calculate how many $1\ \text{MB}$ packets would be needed to send it, comparing it with the text message, which fits into just one.

In the Technology classroom, you are streaming a live match via video and, at the same time, downloading a 1.5 GB software update. The school network experiences momentary interference and 2 % of the transmitted data packets are lost.

1. a) Predict what will happen in each case if the application decides to continue without requesting the lost packets: will it affect the video the same as the installation file? Justify the effect on each.
2. b) Justify why the video service prefers to lose packets permanently rather than requesting their retransmission, whereas in the file download that would be unacceptable.

Two messages sent one second apart may arrive at your friend's cell phone in a different order from how you wrote them. Analyze why this can happen (each packet finds its own path through the network) and explain what the messaging application does to display them in the correct order.

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.

When thirty students download the same file at the same time in the classroom, everything slows down. Explain what resource all these computers share (the school's internet connection bandwidth) and suggest two solutions the teacher could use so that the download does not overload the class network.

During a video call with a school in Japan, your connection's download speed drops sharply from 100 Mbps to 0.5 Mbps. At that moment you are using both the text chat and the live camera simultaneously.

1. a) Predict the behavior of both services: which of the two will continue to work normally and which will fail? Justify your prediction by comparing the volume of data required by each.
2. b) To prevent the call from dropping, the application automatically lowers the video resolution to 240p. Justify how this measure allows communication to continue smoothly despite the slowness of the network.

You are going to carry out an experiment in class to empirically verify how Wi-Fi network saturation affects data download time.

1. a) Design the experiment by defining: the variable you will deliberately modify (independent variable), the variable you will measure (dependent variable), and two conditions that you must keep strictly constant in all tests so that the data is reliable.
2. b) If with 1 computer connected a 250 MB file takes 5 seconds to download, predict with justification what will happen to the download time if all 25 students in the class download that same file at the exact same time. Justify your answer by explaining how the access point distributes its capacity.

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.

You send a message from your cell phone to a friend's cell phone who lives in another city. Write down the complete chain of elements through which the message passes (your cell phone, your router, the internet, the application server, your friend's router or cell tower, their cell phone) and indicate at which point the message changes network.

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.

A science museum organizes a worldwide astronomical broadcast with its main server in Madrid. At the time of the event, 300,000 users from the Americas, Asia, and Europe connect at the same time and the website stops responding due to overload.

1. a) Propose a technical solution based on distributing website replicas (mirror servers) across different continents and justify why a user in Tokyo will see the page much faster with this system.
2. b) Predict what will happen to French users if the mirror server in Paris suffers a total power outage. Will they lose access to the website? Justify how the network should react to that failure.

An online store announces discounted tickets starting at 10:00. At 10:01, the server receives 95 000 requests per second; the page stops loading and shows a connection error to most users.

1. a) Explain what has happened to the server and what it means that a "bottleneck" has occurred in this case.
2. b) Design two complementary technical measures that the store should implement for the next sale (one aimed at reinforcing its server capacity and the other at organizing buyers' access without crashing the system).