A traditional phone call books a whole path through the network for as long as you talk, even while you are silent. The internet does the opposite: it chops data into packets and lets every packet find its own way. This lesson compares the two methods, explains what a router does with each packet, and shows where each method fits.
In circuit switching a dedicated path (circuit) between sender and receiver is set up before any data is sent; all the data follows that path, in order, and the bandwidth is reserved until the circuit is released. In packet switching the message is split into packets, each with a header holding the source and destination IP addresses and a sequence number. Each packet is routed independently: every router reads the destination address and uses its routing table to forward the packet to the next hop, so packets may take different routes and arrive out of order. The receiver reassembles them using the sequence numbers. Links are shared by everyone, so packet switching uses the network far more efficiently.
Fig. 1: circuit switching reserves one path (orange) for the whole call; in packet switching each numbered packet may take a different route
Section 01
Circuit switching.
In the lab, switch to "Circuit or packet": in the top network one route is reserved and every piece of data follows it.
Set up: before any data is sent, a dedicated circuit (a path through the switches/nodes) is established between the two devices.
Transfer: all the data travels along the same path, in order, using the bandwidth reserved for this connection only.
Release: when the communication ends, the circuit is closed and its links are freed for others.
Benefits
Drawbacks
data arrives in the order it was sent, so no reassembly is needed
time is needed to set up the circuit before any data can be sent
the whole bandwidth of the circuit is available, at a constant rate with little delay: good for real-time voice and video
bandwidth is wasted while no data is being sent (e.g. silence in a call), because nobody else can use the circuit
no other users share the circuit, so performance does not fall when the network is busy
if any link in the circuit fails, the connection is lost and must be set up again
no packet headers or reordering, so less processing at each end
a limited number of circuits: when they are all in use, new connections are refused ("line busy"); dedicated lines are expensive
Where applicable: traditional landline telephone calls (the PSTN), and dedicated leased lines where a guaranteed, constant bandwidth is needed, e.g. a live broadcast link.
Examiner trap
"Circuit switching is faster" is not enough. Say why: once set up, the bandwidth is dedicated, so the data arrives at a constant rate, in order, without queuing at routers.
Section 02
Packet switching and the router.
In this lab, send a message and watch it split into packets. Click a router to make it busy: the packets re-route around it and arrive out of order.
Split: the message is broken into packets. Each packet has a header (source IP address, destination IP address, sequence number, total number of packets, hop limit / time to live, error check) and a payload (part of the data).
Send: each packet is sent independently. No route is set up in advance.
Route: each router reads the destination IP address, checks its routing table and forwards the packet to the next router (hop) on the best available route at that moment, avoiding busy or failed links.
Hop limit: each router reduces the hop limit by 1; a packet that reaches 0 is discarded, so lost packets do not circulate for ever.
Reassemble: packets can take different routes and arrive out of order. The receiver uses the sequence numbers to put them back in order, checks them for errors and requests any missing or corrupted packets again (TCP).
Function of a router: receives a packet; reads the destination IP address in its header; looks up the next hop in its routing table; forwards the packet along the chosen link; routers also exchange information so their routing tables reflect congestion and failed links.
Worked example Put the packets back together
A message is split into packets numbered 1 to 5. They arrive in the order 2, 5, 1, 4. What does the receiving computer do?
It reads the sequence number in each packet header and holds the packets in a buffer.
The header says there are 5 packets, but packet 3 has not arrived.
After waiting, the receiver requests packet 3 to be sent again.
When packet 3 arrives, the packets are reassembled in the order 1, 2, 3, 4, 5 to rebuild the message.
Answer: Buffer and reorder the packets by sequence number, request the missing packet 3, then reassemble 1 to 5
Your turn
Why do two packets from the same message sometimes take different routes across the internet?
Each packet is routed independently. Each router chooses the best next hop at the moment the packet arrives, so if a link becomes congested or fails, a later packet is sent a different way.
Examiner trap
A router does not read the whole message, and it does not reassemble the packets. It looks only at the destination IP address of each packet; reassembly happens at the destination computer.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
What happens before any data is sent using circuit switching?
Quick check+10 XP
In the packet lab, what happens when a router on the route is made busy?
Quick check+10 XP
How does the receiver rebuild a message sent using packet switching?
Quick check+10 XP
Which is a drawback of circuit switching?
Section 03
Which one when?
Exam questions give a scenario and ask you to justify one method. Learn the comparison as pairs.
Circuit switching
Packet switching
Route
one dedicated path, set up first
no set-up; each packet routed separately
Bandwidth
reserved for one connection, wasted when idle
shared by all users, so used efficiently
Order of arrival
always in order
may arrive out of order; reassembled by sequence number
Delay
constant, low once set up
varies with congestion; packets can be delayed or lost
If a link fails
the connection is lost
packets are re-routed round the failure
Security
data follows one path, harder to intercept in many places
packets pass through many routers; encryption is needed for sensitive data
Applicable to
traditional telephone calls, dedicated leased lines
the internet: web pages, email, file transfer, streaming, VoIP calls
Your turn
A company sends large files and emails between its offices all day, but traffic is very uneven. Circuit or packet switching? Give a reason.
Packet switching: the links are shared and only used when there is data, so bursty traffic does not waste reserved bandwidth; and packets can be re-routed if a link fails. File transfer and email do not need a constant rate.
Examiner trap
Modern internet phone calls (VoIP) use packet switching, not circuit switching. They cope with variable delay by buffering, and quality drops when the network is congested.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 3 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
What is the function of a router in packet switching?
Exam style+20 XP
Why does packet switching make more efficient use of a network than circuit switching?
Exam style+20 XP
Which item is NOT normally in a packet header?
Exam style+20 XP
Why does a packet carry a hop limit?
Skill unlocked
Circuit and packet switching, mastered.
This skill is now recorded on your profile. Keep the chain going.