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Practise 28 exam-style questions on Packets and packet switching for Cambridge IGCSE Computer Science (Paper 1). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
State the name of the part of a packet that comes after the payload and shows where the packet ends.
The trailer.
The order of a packet is header, payload, trailer. The trailer marks the end of the packet and usually contains an error check, such as a checksum, so the receiver can tell whether the packet has been corrupted.
Trailer
accept 'packet trailer'; do not accept 'header'
Explain why each packet that is sent across a network includes a packet number.
Packets can take different routes, so they may arrive out of order. The receiver uses the packet numbers to put the packets back into the correct order once the last one has arrived, and it can also tell from the numbers if a packet is missing.
The packet number records each packet's position in the original data. Without it the receiver could not reassemble packets that arrive out of order, and it would not know that a packet had gone missing.
Packets may arrive out of order (because they take different routes)
max 2
The receiver uses the packet numbers to reorder/reassemble the packets into the original data
accept 'to put the packets back in the right order'
The receiver can tell if a packet is missing (and request it again)
Describe the role of a router when data is sent across the internet using packet switching.
A router receives each packet and reads the destination address in its packet header. It then decides the best route for the packet at that moment, for example avoiding congested links, and forwards the packet to the next router or to the destination device.
Routers control the route each packet takes. The key idea is that each router makes a decision for each packet using the destination address, so packets from the same data can be sent different ways.
It receives packets (from a device or another router)
max 3
It reads/examines the destination address in the packet header
It decides the best/fastest/least congested route or next router for the packet
accept 'uses a routing table to decide where to send it'
It forwards/sends the packet on to the next router or to the destination device
Give the missing terms A to D in this description of packet switching.
Before it is transmitted, data is broken down into small units called A. Each one has a B that contains the destination address, the packet number and the originator's address. Each one can take a different route across the network, and the route it takes is controlled by a C. Because they can take different routes they may arrive out of order, so once the last one has arrived they are reordered using the D.
A: packets
B: packet header
C: router
D: packet numbers
This is the packet switching process in order: data is broken into packets, each packet header holds the addresses and packet number, routers control the route of each packet, and the packet numbers are used to reorder the packets once the last one has arrived.
A: packets
B: (packet) header
C: router
D: packet number(s)
accept 'sequence numbers'
Ready to try the rest? Every answer you write is marked against the mark scheme, so you can see exactly where the marks are.
Hi! Stuck? Ask me for a hint and I’ll help you work it out. Once you’ve checked your answer I can explain exactly where the marks are.
