Cambridge 9618 · International A Level Computer Science · ~16 min read
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Circuit Switching
In circuit switching, a dedicated physical path is established between sender and receiver before any data is transmitted. That path is reserved for the entire duration of the communication and released only when the connection ends. This is how traditional telephone networks (PSTN — Public Switched Telephone Network) work.
✓ Guaranteed, constant bandwidth for the call ✗ Path is occupied even during silence/pauses — wasteful
How circuit switching works
Connection setup: A dedicated path is established through the network. All intermediate switches reserve resources along the route.
Data transfer: All data travels along the same fixed path. Bandwidth is dedicated — no other traffic can use this portion of the link.
Connection teardown: When the call ends, the dedicated path is released and resources become available to others.
Packet Switching
In packet switching, data is divided into small units called packets. Each packet is sent independently through the network and may travel via a different route to reach the destination. Packets are reassembled into the original data at the destination.
This is how the internet works. No dedicated path is reserved — routers dynamically decide the best route for each packet based on current network conditions.
Packet structure
Structure of a packet
HEADERSource IP · Dest IP · Seq No · Protocol
+
PAYLOADActual data (fragment of the message)
+
TRAILERError check (checksum/CRC)
Header contains: source IP address, destination IP address, packet sequence number, total packet count, protocol type. Trailer contains a checksum for error detection.
How packet switching works
Segmentation: The original message is divided into fixed-size packets. Each gets a header containing destination address and sequence number.
Independent routing: Each packet is sent separately. Routers examine the destination IP and choose the best available route — different packets may take different paths.
Reassembly: The destination device receives all packets (possibly out of order), uses sequence numbers to reorder them, and reassembles the original message.
Different routes — same destination
Packets may travel different routes
Packet 1
A
→
B
→
D
→
Dest
Packet 2
A
→
C
→
D
→
Dest
Packet 3
A
→
B
→
C
→
D
→
Dest
Packets 1 and 2 may arrive before Packet 3, which took a longer route. Sequence numbers allow the destination to reorder them correctly.
Comparison
📞 Circuit Switching
Dedicated path reserved for entire session
Guaranteed, constant bandwidth
No delay variation (no jitter)
Path is wasted during idle periods (silence)
Setup time required before data can flow
If a link on the path fails, entire call drops
Used by: traditional telephone networks (PSTN)
Billing: by time connected (not data volume)
🌐 Packet Switching
No dedicated path — bandwidth shared dynamically
More efficient use of network resources
Variable delay (packets may arrive out of order)
No wasted bandwidth during idle periods
No setup time — packets sent immediately
If a router fails, packets automatically rerouted
Used by: the internet (TCP/IP)
Billing: by data volume transferred
Full Comparison Table
Feature
Circuit Switching
Packet Switching
Path
Dedicated, fixed for whole session
Dynamic — each packet routed independently
Setup
Required before data flows
Not required — immediate
Bandwidth
Reserved, constant, guaranteed
Shared, variable
Efficiency
Low — bandwidth wasted when idle
High — bandwidth used by all when available
Reliability
Single path — whole session fails if broken
Multiple routes — packets rerouted around failures
Ordering
Data always arrives in order
Packets may arrive out of order — resequenced
Delay
Consistent (no jitter)
Variable (congestion, different routes)
Best for
Real-time voice calls (traditional phone)
Internet data, email, web, file transfer
Why the Internet Uses Packet Switching
The internet was designed to be fault-tolerant — it should continue working even if parts of the network are destroyed or fail. Packet switching achieves this because:
If one router fails, packets automatically find alternative routes
No single point of failure for any connection
Bandwidth is shared efficiently among many users simultaneously
Scales to millions of simultaneous connections without reserving paths for each
Cambridge 9618 exam tip: Know that circuit switching uses a dedicated path (PSTN/traditional phone) while packet switching divides data into packets that take independent routes (internet). Key advantages of packet switching: efficient use of bandwidth, fault-tolerant (rerouting if a node fails), no setup time. Key advantage of circuit switching: guaranteed constant bandwidth, no jitter. Know packet structure: header (source/dest IP, sequence number), payload (data), trailer (checksum). Be able to state what sequence numbers are used for (reordering packets at destination).
⚠️ Common Mistakes
Saying circuit switching is "faster" — it has consistent delay, but wastes bandwidth and needs setup time; packet switching often has lower total latency for internet data
Forgetting that packets can take different routes — the internet's packet switching allows this; circuit switching uses a single fixed path
Confusing sequence numbers with IP addresses — sequence numbers are used to reorder packets at the destination; IP addresses are for routing
Saying the internet uses circuit switching — the internet uses packet switching with the TCP/IP protocol suite
Forgetting that packet switching can have variable delay (jitter) — this is a disadvantage for real-time voice calls, which is why VoIP sometimes sounds choppy
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Worksheet — 3.2.2 Circuit & Packet Switching
8 questions · Cambridge 9618 standard
Q1State two characteristics of circuit switching.[2]
✅ Mark scheme
Any two from: a dedicated path is established before data is sent [1]; the path is reserved for the entire duration of the session [1]; bandwidth is guaranteed and constant [1]; the same route is used for all data [1]; setup time is required before communication begins [1].
Q2Describe what happens to data in packet switching, from the sender to the receiver. Include what information is contained in a packet header.[5]
✅ Mark scheme
Data is divided into fixed-size packets [1]; each packet is given a header containing the source IP address, destination IP address, and a sequence number [1]; packets are sent independently through the network — each may take a different route [1]; routers examine the destination address and forward each packet along the best available path [1]; at the destination, packets are reordered using their sequence numbers and reassembled into the original message [1].
Q3Give two reasons why packet switching makes more efficient use of network bandwidth than circuit switching.[2]
✅ Mark scheme
In circuit switching, bandwidth is reserved even when no data is being sent (e.g. during silence in a phone call) — packet switching only uses bandwidth when data is actually being transmitted [1]; packet switching allows multiple users to share the same network links simultaneously — bandwidth is not reserved for one connection, so spare capacity is always available to other packets [1].
Q4A traditional telephone call uses circuit switching. A VoIP (Voice over Internet Protocol) call uses packet switching. Give one advantage and one disadvantage of VoIP packet switching compared to a traditional circuit-switched call.[2]
✅ Mark scheme
Advantage: more efficient use of bandwidth — the call uses network resources only when speaking; cheaper to route over the internet than traditional phone lines [1]; Disadvantage: variable delay (jitter) because packets may take different routes and arrive out of order — can cause distorted or choppy audio; no guaranteed quality of service like a circuit-switched call [1].
Q5Why is packet switching more fault-tolerant than circuit switching? Give a specific example.[3]
✅ Mark scheme
In packet switching, each packet is routed independently — if a router or link fails, subsequent packets are automatically routed via a different path [1]; example: if Router B in the path A→B→D fails, later packets are automatically sent via A→C→D instead — the communication continues [1]; in circuit switching, if any link in the dedicated path fails, the entire call is immediately dropped and must be re-established [1].
Q6Explain the purpose of the sequence number in a packet header.[2]
✅ Mark scheme
Packets may arrive at the destination out of order because they can take different routes [1]; the sequence number identifies the position of each packet within the original message so the destination device can reorder them and correctly reassemble the original data [1].
Q7A message is divided into 4 packets labelled P1, P2, P3, P4. They are sent across the internet and arrive at the destination in the order P3, P1, P4, P2. Describe how packet switching enables this out-of-order arrival and how the receiving device reconstructs the original message.[4]
✅ Mark scheme
Each packet is routed independently — different packets may take different routes through the network depending on congestion, so they can arrive in any order [1]; each packet contains a sequence number in its header that indicates its position in the original message [1]; the destination device buffers incoming packets and uses the sequence numbers to reorder them: P1, P2, P3, P4 [1]; once all packets have arrived (or any missing ones have been retransmitted), the payloads are reassembled in sequence to reconstruct the original message [1].
Q8Compare packet switching with circuit switching. State two advantages of packet switching over circuit switching for internet data transmission, and identify one scenario where circuit switching is preferable, justifying your choice.[5]
✅ Mark scheme
Circuit switching: a dedicated physical path is established for the entire duration of the communication; bandwidth is reserved even when no data is being sent [1]; Advantage 1 of packet switching: no dedicated path required — multiple communications share the same links, making more efficient use of bandwidth [1]; Advantage 2: resilience — if one node or link fails, packets are rerouted automatically; in circuit switching, a failed node breaks the entire connection [1]; Scenario where circuit switching is preferable: real-time voice calls (PSTN/traditional telephony) [1]; justification: circuit switching guarantees consistent, predictable latency and no packet reordering delays — critical for real-time audio quality [1].
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Mini Test — 3.2.2 Circuit & Packet Switching
10 questions · 10 marks · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1Which switching method is used by the internet?
Q2In packet switching, what is the purpose of the sequence number in the packet header?
Q3What is a disadvantage of circuit switching compared to packet switching?
Q4In a packet, which section contains error-checking information?
Q5Traditional telephone networks (PSTN) use:
Section B — Short Answer [5 marks]
Q6Describe the three stages of circuit switching.
Mark schemeConnection setup — a dedicated path is established through the network and resources are reserved [1]; data transfer — all data travels along the fixed path with guaranteed bandwidth [1]; connection teardown — when communication ends, the path is released and resources become available to others [1]. One mark per stage.
Q7State two items of information stored in a packet header.
Mark schemeAny two from: source IP address; destination IP address; sequence number; packet number / total number of packets; protocol type [1 each, max 2].
Q8Explain why packet switching is described as more fault-tolerant than circuit switching.
Mark schemeIn packet switching, packets are routed independently — if a node/router on one route fails, subsequent packets are automatically sent via a different route [1]; circuit switching uses a single dedicated path — if any link fails, the entire connection is lost and must be re-established [1].
Q9What is "jitter" in the context of packet switching and why does it occur?
Mark schemeJitter is variation in packet arrival delay/timing [1]; it occurs because different packets may take different routes through the network (some longer than others) and because network congestion varies — so some packets are delayed more than others [1].
Q10Give one reason why circuit switching is still preferred for traditional voice telephone calls over long distances.
Mark schemeCircuit switching provides a guaranteed, constant-bandwidth connection with no jitter — the call quality is consistent and there is no variable delay; for real-time voice communication, consistent low latency is essential [1]. Accept: no risk of packets arriving out of order causing audio distortion.