A2 · Practice questions · Circuit switching and packet switching

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Six original Cambridge-style questions on circuit switching, packet switching and routers, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Path first, or packets first.

Circuit: set up a dedicated path, send in order at a constant rate, release; bandwidth wasted when idle.
Packet: split, header (IPs, sequence number, hop limit), route independently, reassemble.
Router: read destination IP, look up routing table, forward to next hop.
Justify: real-time and constant rate favours circuit; shared, bursty, resilient favours packet.

Explain

[3 marks]

Describe how data is transmitted using circuit switching.

A dedicated circuit/path is established between the sender and the receiver before transmission. ✓
All the data follows the same path, in order, using the reserved bandwidth. ✓
The circuit is released when the communication ends. ✓

Explain

[5 marks]

Explain how packet switching is used to send a message across the internet.

The message is split into packets. ✓
Each packet is given a header including source and destination IP addresses and a sequence number. ✓
Each packet is sent independently and may take a different route. ✓
Routers read the destination IP address and forward each packet to the next hop using their routing tables. ✓
At the destination the packets are reordered using the sequence numbers and reassembled; missing packets are requested again. ✓

Recall

[3 marks]

Describe the function of a router in packet switching.

Receives a packet and reads the destination IP address in its header. ✓
Uses its routing table to find the best next hop / route. ✓
Forwards the packet to the next router (or the destination network), avoiding congested or failed links. ✓

Analysis

[4 marks]

Give two benefits and two drawbacks of packet switching compared with circuit switching.

Benefit: links are shared by many users, so bandwidth is used more efficiently. ✓
Benefit: packets can be re-routed round failed or congested links, so the network is more resilient / no set-up time. ✓
Drawback: packets may arrive out of order and must be reassembled, and some may be lost and need resending. ✓
Drawback: delay varies with congestion, which can affect real-time data such as live video or voice. ✓

Calculate

[3 marks]

A circuit-switched telephone call reserves 64 kbit/s (64 000 bits per second) for 3 minutes. The caller is silent for 50% of the call. Calculate the total capacity reserved in bits, and the capacity wasted. Explain why this waste would not happen with packet switching.

Reserved: 64 000 x 180 = 11 520 000 bits. ✓
Wasted: 50% of 11 520 000 = 5 760 000 bits. ✓
With packet switching no capacity is reserved; packets are only sent when there is data, so idle link time can carry other users' packets. ✓

Apply

[4 marks]

A television company needs a live link from a sports stadium to its studio for a 2-hour match. The video must arrive at a constant rate with no gaps. Recommend circuit switching or packet switching and justify your answer.

Circuit switching. ✓
A dedicated path with reserved bandwidth gives a constant data rate for the whole broadcast. ✓
Data arrives in order with a low, constant delay, so there is no need to buffer or reassemble packets. ✓
Performance is not affected by other users' traffic; the higher cost is justified for a one-off, high-value live event. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.