Introduction to networks — exam-style exercises with a quiz that marks itself, the switch trap and a latency worked out by hand (and why you should never tick “just in case”)
First IC2 quiz. A multiple-choice question on the characteristics of a WAN: three options I was sure about and a fourth, “belongs to the owner of the devices”, that sounded… familiar. I ticked it just in case. Result: zero on that question. The three right ones added up, but the wrong one subtracted as much as a right one, and the question’s score sank. That sentence belonged to the LAN, not the WAN.
That day I learned two things: IC2 multiple-choice questions subtract for wrong ticks, and Topic 1 is full of pairs that get mixed up (LAN/WAN, switch/router, PoP/NAP, latency/bit/s). This article is designed so you learn them by playing: an interactive quiz with every exam format that marks itself, two open questions with a model answer and the topic’s cheat sheet for a five-minute review.
If you have not read the theory yet, start with the introduction to networks theory: here I assume you know the communications model, the devices, switching and the dataset’s journey from the cloud to your laptop.
Table of Contents
How to use this article
- Do the quiz without looking at the theory. Answer each question and press “Check”: you will see whether you got it right and the explanation (why the right answer is right and why the others fail).
- In multiple-choice questions, tick only what you are sure of. They are scored like the exam: (right − wrong) / correct, with a minimum of 0. If a question has 4 correct options and you tick 3 right and 1 wrong, you get (3 − 1) / 4 = 0.5 instead of 0.75.
- Write down the ones you miss and go back to the matching section of the theory (the cheat sheet tells you which). At the end you will see your mark out of 10 and you can try again: the options are shuffled.
- Write the open answers down. Draft your answer and compare it with the model answer and the “What the marker looks for” list.
The questions are original. They cover the same concepts and formats as the exam, but with different wording and different distractors: what you learn here are the ideas, not the order of the options.
Exam-style quiz
Thirty-one questions in total: twenty-nine here, in every format (single choice, multiple choice that subtracts for wrong ticks, true or false, gaps with filler words, matching, ordering, short answer and a latency calculation), and two open questions in the next section. They follow the order of the theory: communications model, media, latency, devices, switching, types of networks, the Internet and delivery.
You download a CSV from a cloud server to your laptop. Match each thing that happens with the element of the communications model that does it:
| The server holds the CSV and decides to send it | |
| The server’s network card turns the bits into a signal | |
| Fibre, copper and Wi-Fi carry the signal from one place to another | |
| The laptop’s Wi-Fi card turns the waves back into bits | |
| Your pandas script opens the CSV and uses it |
Definitions:
- Source
- Receiver
- Transmitter
- Destination
- Router
- Transmission system
Solution
- The server holds the CSV and decides to send it → Source
- The server’s network card turns the bits into a signal → Transmitter
- Fibre, copper and Wi-Fi carry the signal from one place to another → Transmission system
- The laptop’s Wi-Fi card turns the waves back into bits → Receiver
- Your pandas script opens the CSV and uses it → Destination
The source generates the data, the transmitter turns it into signals that can be transmitted, the transmission system carries them, the receiver turns the received signal into information and the destination takes the data from the receiver.
The router is left over: it is a network device, not one of the five elements of the model.
Complete the sentence about the two ends of the model:
The source system is made up of the ____ and the ____; the destination system, of the ____ and the ____.
Words:
- destination
- transmission system
- source
- transmitter
- receiver
- switch
Solution
The source system is made up of the source and the transmitter; the destination system, of the receiver and the destination.
Each end has two pieces: the one that produces or uses the data (source, destination) and the one that converts between data and signal (transmitter, receiver). The transmission system is in the middle and belongs to neither end.
Which of these are tasks of a communication system? (each wrong tick subtracts)
- Synchronisation
- Error detection and correction
- Addressing and routing
- Network management
- CPU process scheduling
- Compiling the program that sends the data
Solution
Answer: Synchronisation · Error detection and correction · Addressing and routing · Network management
The course notes list 11 tasks: use of the transmission system, interface implementation, signal generation, synchronisation, exchange management, error detection and correction, addressing and routing, recovery, message formatting, security and network management.
Tip: if an option has nothing to do with moving data from one place to another, it does not belong.
True or false about the communications model:
- Security is one of the tasks of a communication system.
- The transmission system is what generates the data being sent.
- Signal generation is a task of the communication system.
- The destination is what turns the received signal into information.
Solution
- T — Security is one of the tasks of a communication system.
- F — The transmission system is what generates the data being sent.
- T — Signal generation is a task of the communication system.
- F — The destination is what turns the received signal into information.
The data is generated by the source; the transmission system only carries it. And the one that turns the signal into information is the receiver: the destination gets the data already prepared.
From your laptop you send a request to the server, and 9 ms pass until the signal triggers the action on the other side. What are you measuring?
- Latency
- The speed in bit/s
- The number of nodes in the network
- The packet size
Solution
Answer: Latency
Latency: the amount of time that passes between sending a signal and the corresponding action happening. It adds up three pieces: propagation, transmission and time in the switches.
According to the course notes, which times are added up to get the latency? (each wrong tick subtracts)
- Propagation time
- Transmission time
- Time in the switches
- Program compilation time
- Operating system boot time
Solution
Answer: Propagation time · Transmission time · Time in the switches
Latency = propagation time + transmission time + time in the switches. In the theory simulator, propagation (9.045 ms) was almost 98 % of the total.
Which unit measures the number of elementary pulses (ones and zeros) sent every second?
Solution
Answer: bit/s
Bit/s: the number of elementary pulses (1 or 0) transmitted every second. Careful: bit with a lowercase b; bytes are something else (1 byte = 8 bits).
Order these transmission media from slowest to fastest, using the typical maximum speed from the course notes:
- Coaxial
- Wi-Fi
- Bluetooth
- Optical fibre
- Twisted pair
Solution
Correct order:
- Coaxial
- Bluetooth
- Wi-Fi
- Twisted pair
- Optical fibre
Coaxial: 10-20 Mbit/s · Bluetooth: 24 Mbit/s · Wi-Fi: 300 Mbit/s · twisted pair: up to 1000 Mbit/s · optical fibre: 100 Gbit/s.
The trap is coaxial: even though it is a cable, its maximum (20 Mbit/s) is below Bluetooth (24 Mbit/s).
Which of these are wireless links? (each wrong tick subtracts)
- Infrared
- Satellite microwaves
- Bluetooth
- Optical fibre
- Coaxial cable
Solution
Answer: Infrared · Satellite microwaves · Bluetooth
Wireless: infrared, radio (Wi-Fi, Bluetooth) and microwaves (terrestrial and satellite). Twisted pair, coaxial and fibre are cables.
True or false about the limits of technology:
- The speed of light sets a limit no technology can get around.
- Many users sharing the same connections does not affect performance.
- Different technologies, protocols and operating systems can live together in the same LAN.
- With optical fibre, latency becomes zero.
Solution
- T — The speed of light sets a limit no technology can get around.
- F — Many users sharing the same connections does not affect performance.
- T — Different technologies, protocols and operating systems can live together in the same LAN.
- F — With optical fibre, latency becomes zero.
The three limits in the course notes: the speed of light, many users sharing the connections and heterogeneity (different technologies, protocols and OSs in the same LAN). Fibre transmits very fast, but the signal still needs time to cover the distance: propagation does not disappear.
According to the definition in the course notes, what is needed to talk about a network?
- Devices connected to a common transmission medium through network adapters, each with a unique identifier
- All the devices being in the same building
- Access to the Internet
- Two devices joined by a single direct cable
Solution
Answer: Devices connected to a common transmission medium through network adapters, each with a unique identifier
Network: a set of devices connected to a common transmission medium through network adapters; each adapter has a unique identifier (the physical or MAC address).
Your laptop, the cloud server and the lab printer are connected to a network. What are the devices connected to a network called, in general?
Solution
Answer: Network nodes
Network nodes are the computers or devices connected to the network.
In the computer lab, which device takes care of each need?
| Phones and laptops joining the lab network without a cable | |
| Joining the 20 lab PCs to each other with cables | |
| Taking the lab network to the Internet and handing out addresses with DHCP | |
| Dealing with the signal that comes from the carrier |
Definitions:
- Network adapter
- Modem
- Access point
- Router
- Switch
Solution
- Phones and laptops joining the lab network without a cable → Access point
- Joining the 20 lab PCs to each other with cables → Switch
- Taking the lab network to the Internet and handing out addresses with DHCP → Router
- Dealing with the signal that comes from the carrier → Modem
AP: plugs into the switch and gives Wi-Fi to the LAN. Switch: connects the devices of the LAN. Router: connects the local network to the Internet; it does routing, NAT and DHCP. Modem: connects to the carrier and translates its network’s signal.
True or false about the devices:
- The switch is what gives Wi-Fi to the LAN.
- The access point plugs into the switch.
- The router does NAT.
- The access point routes packets towards the Internet.
- The modem translates the signal of the provider’s network.
Solution
- F — The switch is what gives Wi-Fi to the LAN.
- T — The access point plugs into the switch.
- T — The router does NAT.
- F — The access point routes packets towards the Internet.
- T — The modem translates the signal of the provider’s network.
Look at what each one does not do: the switch does not give Wi-Fi or connect to the Internet; the AP does not route or reach the Internet. Routing is the router’s job.
You are on the lab Wi-Fi: you can see the printer and your classmates’ PCs, but no Internet page loads. Which device connects the LAN to the Internet?
- The router
- The access point
- The switch
- Your Wi-Fi adapter
Solution
Answer: The router
If everything works inside the LAN and nothing outside, the problem is with whoever leaves the LAN: the router (or, beyond it, the modem and the carrier).
Why don’t we join every device to every other one with point-to-point links? (each wrong tick subtracts)
- Because the devices may be very far apart
- Because with many devices we would need a huge number of connections
- Because a point-to-point cable cannot transmit bits
- Because a point-to-point link can only be wireless
Solution
Answer: Because the devices may be very far apart · Because with many devices we would need a huge number of connections
With 20 PCs, joining each one to all the others would take 20 × 19 / 2 = 190 cables. The solution is a communication network: each device connects once and the network does the rest.
In the traditional telephone network, a call reserves an end-to-end path that stays busy even when nobody speaks. What kind of switching is it?
- Circuit switching
- Packet switching
- Broadcast
- Point-to-point link
Solution
Answer: Circuit switching
Circuit switching: a dedicated, continuous communication route (traditional telephone network). Packet switching: the data is split into packets and there is no dedicated route (Internet, IP).
True or false about switching:
- In packet switching there is no dedicated route.
- The Internet is based on circuit switching.
- Two packets of the same CSV can arrive along different paths.
- In circuit switching the route is released after each packet.
Solution
- T — In packet switching there is no dedicated route.
- F — The Internet is based on circuit switching.
- T — Two packets of the same CSV can arrive along different paths.
- F — In circuit switching the route is released after each packet.
The Internet (IP) uses packet switching: each packet carries its header and the network devices decide where it goes. In circuit switching the route is reserved for the whole communication.
Match each transfer mode with its example:
| Synchronous transfer mode | |
| Asynchronous transfer mode |
Definitions:
- A point-to-point Bluetooth link
- IP (and ATM as a historical example)
- TDM: time is split into fixed frames
Solution
- Synchronous transfer mode → TDM: time is split into fixed frames
- Asynchronous transfer mode → IP (and ATM as a historical example)
Synchronous mode splits the medium into fixed time slots (TDM, time-division multiplexing). Asynchronous mode sends whenever there is data, with no fixed slots: it is IP’s mode, and ATM was a historical example.
What type of network is each one?
| Your home network, with the laptop, the phone and the TV | |
| A carrier’s network joining the Canary Islands with mainland Spain | |
| The network of networks joining every carrier in the world | |
| Several laptops passing notes to each other, with no central server | |
| A hundred laptops asking a single server for the same dataset |
Definitions:
- LAN
- Point-to-point link
- Client-server
- WAN
- Internet
- P2P
Solution
- Your home network, with the laptop, the phone and the TV → LAN
- A carrier’s network joining the Canary Islands with mainland Spain → WAN
- The network of networks joining every carrier in the world → Internet
- Several laptops passing notes to each other, with no central server → P2P
- A hundred laptops asking a single server for the same dataset → Client-server
LAN: small, in a limited area. WAN: large geographical distances. Internet: global network of networks. P2P: everyone shares resources directly. Client-server: central servers with many clients.
Which are characteristics of a WAN? (each wrong tick subtracts)
- It covers a wide geographical area
- It needs to cross public access routes
- It partly uses circuits from telecom providers
- It usually belongs to the same organisation as the connected devices
- It covers a building or a group of nearby buildings
Solution
Answer: It covers a wide geographical area · It needs to cross public access routes · It partly uses circuits from telecom providers
WAN: wide geographical area, public access routes, providers’ circuits and technologies such as circuit switching, packet switching, TDM and ATM. The distractors are characteristics of a LAN: if you learn them in pairs, they will not catch you out.
True or false about LANs:
- A LAN usually uses broadcast systems.
- A LAN usually belongs to a telecom carrier.
- A LAN transmits a larger amount of data than a WAN.
- A LAN needs to cross public access routes.
Solution
- T — A LAN usually uses broadcast systems.
- F — A LAN usually belongs to a telecom carrier.
- T — A LAN transmits a larger amount of data than a WAN.
- F — A LAN needs to cross public access routes.
LAN: small coverage, belongs to the owner of the devices, larger amount of data transmitted, broadcast systems and, more recently, switching and ATM. Crossing public routes is a WAN thing.
Complete the sentence about the origin of the Internet:
The Internet grew out of ____ to let very different ____-switching networks talk to each other, and it relies on the ____ protocols.
Words:
- packet
- TCP/IP
- ARPANET
- Ethernet
- circuit
- NAP
Solution
The Internet grew out of ARPANET to let very different packet-switching networks talk to each other, and it relies on the TCP/IP protocols.
The Internet is the evolution of ARPANET, designed to connect many arbitrary packet-switching networks, and it is based on TCP/IP (Transport Control Protocol / Internet Protocol).
Match each description with its acronym:
| The company that gives Internet access to your home or university | |
| The physical place where your network enters that company’s network | |
| The point where two different carriers exchange traffic |
Definitions:
- ISP
- NAP
- PoP
- LAN
Solution
- The company that gives Internet access to your home or university → ISP
- The physical place where your network enters that company’s network → PoP
- The point where two different carriers exchange traffic → NAP
ISP (Internet Service Provider): connects users’ and companies’ LANs to the Internet. PoP (Point of Presence): physical point where the ISP has its network and connects to other networks. NAP (Network Access Point): point where different ISPs interconnect and share traffic.
The dataset’s server uses a different carrier from yours. What is the point where the two carriers pass traffic to each other called (acronym)?
Solution
Answer: NAP
At a NAP different ISPs interconnect and share traffic. Do not confuse it with the PoP, which is where one ISP has its network and connects its customers.
Order the devices the download goes through, from the cloud server to your laptop:
- Server
- Your carrier’s PoP
- Modem
- Switch
- NAP
- Router
- Laptop
- Access point
Solution
Correct order:
- Server
- NAP
- Your carrier’s PoP
- Modem
- Router
- Switch
- Access point
- Laptop
The server goes out through its carrier, which meets yours at the NAP; your carrier brings it to its PoP; from there it reaches your modem, the router puts it into the LAN, the switch takes it to the access point and the AP hands it to you over Wi-Fi.
True or false about addresses and delivery:
- The header carries, among other fields, the address of whoever sends the data.
- Every adapter in the LAN keeps the frame, whoever it is for.
- If the destination station is on another network, the message is handed to a router.
- A network adapter’s physical address is its IP address.
Solution
- T — The header carries, among other fields, the address of whoever sends the data.
- F — Every adapter in the LAN keeps the frame, whoever it is for.
- T — If the destination station is on another network, the message is handed to a router.
- F — A network adapter’s physical address is its IP address.
It is received by the device whose physical (MAC) address matches the destination address. If the destination is on my network, I deliver it directly; if not, I hand it to a router, which carries it to the destination network.
Your laptop (MAC 08002B00EE0B) wants to send something to a station that is not on its network. Who does it hand the message to?
- A router
- The switch, so it takes it out of the network
- Directly to the destination station
- The access point
Solution
Answer: A router
The rule in the course notes: if the destination is on my network, I deliver it; if not, I hand it to a router. The classic exam trap is swapping “router” for “switch”.
A 1500-byte packet crosses a 100 Mbit/s, 200 m twisted-pair cable (the signal travels at 2·10⁸ m/s) and goes through a switch that takes 0.05 ms. Calculate each piece of the latency in milliseconds:
| Piece | How it is calculated | ms |
|---|---|---|
| Transmission | bits / (bit/s) | |
| Propagation | metres / (m/s) | |
| Switch | given | 0.05 |
| Total latency | sum |
Solution
| Piece | How it is calculated | ms |
|---|---|---|
| Transmission | bits / (bit/s) | 0.12 |
| Propagation | metres / (m/s) | 0.001 |
| Switch | given | 0.05 |
| Total latency | sum | 0.171 |
Transmission: 1500 × 8 = 12,000 bits; 12,000 / 100,000,000 = 0.00012 s = 0.12 ms.
Propagation: 200 / 200,000,000 = 0.000001 s = 0.001 ms.
Total: 0.12 + 0.001 + 0.05 = 0.171 ms. On a short cable propagation hardly counts; over the simulator’s 1,800 km, it was almost everything.
If you got question 29 (the calculation) wrong, review the order: first turn bytes into bits (× 8), then divide by the bit/s for transmission and by the m/s for propagation, and turn seconds into milliseconds (× 1000) before adding the switch time, which is already in ms.
Open questions
The IC2 written exam includes questions where you have to write. From Topic 1, the ones that make most sense are the comparison between circuits and packets and the communications model explained with an example. Write your answer before opening the solution:
Explain the difference between circuit switching and packet switching. Give an example of each and say which one the Internet uses.
Solution
Model answer:
In circuit switching a dedicated, continuous communication route is set up between the two ends before starting, and that route stays reserved for the whole communication, even when nothing is being sent at some point. The typical example is the traditional telephone network.
In packet switching the data is split into packets that carry their own header (with the destination address) and there is no dedicated route: the network devices decide where each packet goes, so two packets of the same message can take different paths and the network is shared by many users.
The Internet uses packet switching (IP). Related to this, synchronous transfer mode (TDM) splits the medium into fixed slots, while asynchronous mode (IP; ATM as a historical example) sends whenever there is data.
What the marker looks for:
- Dedicated, continuous route in circuits, reserved for the whole communication
- Data split into packets with no dedicated route
- An example of each (telephone network / Internet)
- Saying that the Internet uses packet switching (IP)
- Extra: linking it to the synchronous (TDM) and asynchronous modes
If you only have time for one sentence per concept: circuits = reserved path (telephone); packets = each packet on its own (Internet).
Describe the simplified communications model: name its elements, say what each one does and apply it to a real example.
Solution
Model answer:
The model has five elements. The source generates the data to be transmitted. The transmitter turns that information into signals that can be transmitted. The transmission system carries the signal (cables or wireless links). The receiver turns the received signal back into information. The destination takes the data from the receiver.
Source and transmitter form the source system; receiver and destination, the destination system.
Example: when you download a dataset, the cloud server is the source; its network card, the transmitter; fibre, copper and Wi-Fi form the transmission system; the laptop’s Wi-Fi card is the receiver, and the program that opens the CSV is the destination. Along the way, the communication system has to handle tasks such as synchronisation, error detection and correction, or addressing and routing.
What the marker looks for:
- The five elements with their function
- Source system = source + transmitter; destination system = receiver + destination
- An example where every element is identified
- Extra: mentioning some tasks of the communication system
Compare your answer with the model answer point by point using “What the marker looks for”. If one is missing, review the first diagram in the theory.
In an open answer, a well-chosen example is worth as much as the definition. If you use the same example from start to finish (the dataset travelling from the cloud), your answer reads in order and the examiner sees that you understand it, not that you are reciting it.
The whole topic in one drawing
Before the cheat sheet, a review in a single picture. It is the download’s journey again, but this time with all the topic’s vocabulary on top:
Read it in three passes. At the top, which part is WAN and Internet (everything outside the faculty, with packet switching) and which part is the LAN. In the middle, the medium of each link: fibre up to the modem, twisted pair inside the faculty and Wi-Fi on the last hop, which is the bottleneck. At the bottom, the role of each piece in the model: the server is source and transmitter, the laptop is receiver and destination, and everything in between is the transmission system.
Cheat sheet: introduction to networks
The essentials in 30 seconds
- Network: devices connected to a common medium through adapters with a unique identifier (MAC). The devices are network nodes.
- Model: source → transmitter → transmission system → receiver → destination. Source system = source + transmitter; destination system = receiver + destination.
- Latency = propagation + transmission + time in the switches. Bit/s = pulses (1 or 0) per second.
- AP gives Wi-Fi · switch joins the LAN · router reaches the Internet (routing, NAT, DHCP) · modem talks to the carrier.
- Circuits: dedicated route (telephone, synchronous/TDM). Packets: no dedicated route (Internet, IP, asynchronous).
- ISP connects you · PoP where you enter its network · NAP where ISPs meet.
- Destination on my network → I deliver it; outside → to the router.
If they ask… / Answer / Typical trap
| If they ask… | Answer | Typical trap |
|---|---|---|
| What makes up the source system? | Source and transmitter | Adding the transmission system |
| Who turns the signal into information? | The receiver | Saying “the destination” |
| How many tasks does a communication system have? | 11 | Sneaking in “CPU scheduling” |
| What is latency? | Time between sending the signal and the action happening | Confusing it with bit/s |
| Which media are wireless? | Infrared, radio (Wi-Fi, Bluetooth), microwaves | Including fibre |
| Who connects the LAN to the Internet? | The router | The switch or the AP |
| What does the router do besides routing? | NAT and DHCP | Giving it the modem’s job |
| What switching does the Internet use? | Packet switching (IP) | Saying circuits |
| Where do different ISPs interconnect? | At the NAP | Saying PoP |
| What did the Internet evolve from? | ARPANET, on TCP/IP | Saying “UDP/IP” or “circuits” |
| Destination outside my network: who gets it? | The router | Saying “the switch” |
Typical speeds
| Medium | Speed | Family |
|---|---|---|
| Twisted pair | 1 – 1000 Mbit/s | Cable |
| Coaxial | 10 – 20 Mbit/s | Cable |
| Optical fibre | 100 Gbit/s | Cable |
| Wi-Fi | 300 Mbit/s | Wireless (radio) |
| Bluetooth | 24 Mbit/s | Wireless (radio) |
Template: calculating a latency
| Step | Calculation | Example (1500 bytes, 100 Mbit/s, 200 m, switch 0.05 ms) |
|---|---|---|
| 1. Packet bits | bytes × 8 | 1500 × 8 = 12,000 bits |
| 2. Transmission | bits / (bit/s) | 12,000 / 100,000,000 = 0.00012 s = 0.12 ms |
| 3. Propagation | metres / (m/s) | 200 / 200,000,000 = 0.000001 s = 0.001 ms |
| 4. Devices | sum of their times | 0.05 ms |
| 5. Latency | 2 + 3 + 4 | 0.12 + 0.001 + 0.05 = 0.171 ms |
LAN versus WAN, in pairs
| LAN | WAN |
|---|---|
| A building or nearby buildings | Wide geographical area |
| Belongs to the owner of the devices | Crosses public access routes |
| Larger amount of data transmitted | Uses circuits from telecom providers |
| Broadcast systems; recent switching and ATM | Circuit switching, packet switching, TDM and ATM |
Typical distractor / Why it is false
| Typical distractor | Why it is false |
|---|---|
| “The switch gives Wi-Fi” | That is the access point’s job |
| “The AP routes towards the Internet” | The AP neither routes nor reaches the Internet |
| “With fibre, latency is zero” | Propagation always exists: the speed of light is the limit |
| “The Internet uses circuit switching” | It uses packets (IP) |
| “The physical address is the IP” | The physical address is the adapter’s MAC |
| “Every adapter keeps the frame” | Only the one whose MAC matches the destination receives it |
| “The PoP is where ISPs meet” | That is the NAP; a PoP belongs to a single ISP |
Before you hand it in, ask yourself
- In multiple-choice questions, did I tick only what I am sure of, knowing each wrong tick subtracts?
- Did I check whether the statement is about what a device does or what it does not do?
- Did I separate the pairs: LAN/WAN, switch/router, PoP/NAP, latency/bit/s, circuits/packets?
- In calculations, did I turn bytes into bits and seconds into milliseconds before adding?
Once you have it, the next step is to organise all this into layers: in the next topic we will see the OSI and TCP/IP models, where the switch and the router find their exact place. And if anything is still unclear, go back to the introduction to networks theory.
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The hardware that holds up the operating system — theory with my IC2 assignment on the printer, an interrupt that stops everything and a cache that hits (and the infinite loop that didn’t freeze my laptop)