Exam practice

1. Sharing: many people’s packets can take turns on the same cable, so no one person’s big file blocks everyone else.
2. Mistakes cost little: if something goes wrong on the way, only the lost packet has to be sent again, not the whole message.
3. Different ways: each packet can go by whichever way is working or least busy; if a link breaks, the next packets go round it.

A packet has a header, with the address it is going to, the address it came from, and its number (for example “2 of 4”), and the data, one piece of the message (as in Figure 6.1).
The receiver reads each packet’s number and puts the pieces in order: 1, 2, 3, 4. It doesn’t matter that they arrived as 3, 1, 4, 2. If one number never arrives, it asks for that packet to be sent again.

• i.A router joins networks and passes each packet one step nearer the address in its header.
• ii.A server is a computer that keeps websites, files or messages and sends them to anyone who asks.
• iii.A fiber box (ONT) turns a home’s or office’s electrical signals into flashes of light in an optical fiber, and light back into signals.
• iv.A mobile tower carries data by radio between phones within a few kilometers and the phone company’s network.

The home has one address on the internet, the router’s. Inside, the router gives each device its own home address (such as 192.168.1.5). When a device asks for a page, the router sends the request out with the home’s address and remembers which device asked; when the answer comes back, it passes it to that device.

time = distance ÷ speed.

• i.There and back is 60 km. 60 ÷ 200,000 = 0.0003 s, about a third of a thousandth of a second.
• ii.9,000 ÷ 200,000 = 0.045 s, about one twentieth of a second.

From the server, packets travel through routers and fiber cables in that country, then through cables under the sea, to India’s networks (or through China’s). Fiber cables cross the border into Nepal, at places such as Biratnagar, Birgunj or Bhairahawa (or Rasuwagadhi from China), and run along the main roads to the internet company’s network and to Pokhara. From there the last part is either a fiber cable to a home’s fiber box and Wi-Fi router, then Wi-Fi radio to the phone, or a fiber or radio link to a mobile tower, then mobile data radio to the phone.

No single computer or building runs the internet or carries all its messages. Routers are joined in a mesh, each to several others, so there are many ways between any two places. When a link or a router fails, routers learn this from each other and send packets round it, and any packets lost are sent again. So a failure in one place doesn’t stop the rest.
The weak points are at the edges, where there is only one way: the one cable to a home, the one tower a phone reaches, a town or a country with only one link. A break there cuts off everyone behind it.

3 GB = 3 × 1,024 = 3,072 MB. Per day: 3,072 ÷ 30 ≈ 102 MB.
Video call: 15 × 6 = 90 MB a day. Left: 102 − 90 = about 12 MB a day.
Medium quality: 12 ÷ 12 = about 1 minute a day. High quality: 12 ÷ 50 = about a quarter of a minute, 15 seconds.
The calls almost fill the pack. To watch videos as well, she needs Wi-Fi for them, a bigger pack, or shorter calls (or voice calls on some days, at about 0.75 MB a minute).

Any five:

1. Set app updates and photo backups to Wi-Fi only: they are big and happen without you noticing.
2. Lower the video quality: higher quality needs several times the data for the same minutes.
3. Turn off autoplay: videos that play by themselves use data you didn’t mean to spend.
4. Turn on data saver (low data mode): it stops apps using data in the background.
5. Download videos and maps on Wi-Fi to use later: then they need no mobile data at all.
6. Use a voice call instead of a video call when the picture isn’t needed: sound alone needs about an eighth of the data (0.75 MB a minute against 6).
7. Check which apps use the most data each month, and limit them.

Turn on the hotspot in the phone’s settings, with a strong password, and give the password only to the three classmates; their laptops join it like any Wi-Fi.
Watch out for: the data, since every laptop uses her pack, and laptops use a lot (updates, backups); the battery, which a hotspot drains fast, so keep the phone charging; strangers, which the password keeps out; and switching it off when they have finished.

With an adult: find the router’s label (settings address and first admin password); open the settings page in a browser; change the admin password to a long one; choose a Wi-Fi name that doesn’t give away the family’s name or house; set a strong Wi-Fi password (random words, 12 characters or more) with WPA2 or WPA3 security; check the list of connected devices; let it install updates and restart it now and then.
The admin password must be changed because the one it came with is simple, often printed on the label and the same on thousands of routers. Anyone who joins the Wi-Fi, a guest or someone who learned the Wi-Fi password, could use it to open the settings, change everything, and lock the family out.

• i.Doesn’t work. The Wi-Fi still shows full bars, because the router is fine, but the router’s only link to the internet is cut.
• ii.Works, if a tower is in reach: mobile data goes by radio to the tower, a different way that doesn’t use the cut cable.
• iii.Works. Each home has its own cable, and the internet company’s network has other ways; only Bimal’s home’s edge link is cut.
• iv.Works. It goes through other cables and routers, nowhere near Bimal’s home. The mesh in the middle is untouched.