The shape of the lesson
By the end of the lesson, learners can
- 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.
- Packet switching and the router: In this lab, send a message and watch it split into packets.
- Which one when?: Exam questions give a scenario and ask you to justify one method.
- 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.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One path or many?. A traditional phone call books a whole path through the network for as long as you talk, even while you are silent. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: 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. Project tab 7 of the lab (Circuit or packet switching) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Packet switching and the router | In this lab, send a message and watch it split into packets. Project tab 1 of the lab (Packets taking different routes) and let a student drive it while the class predicts. Model the worked example Put the packets back together on the board, one line at a time. Your turn: Why do two packets from the same message sometimes take different routes across the internet? | Pairs |
| 21 to 30 min | Teach and check: Which one when? | Exam questions give a scenario and ask you to justify one method. 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. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Which one when?, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| "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. |
| 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. |
| 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. |
Circuit switching and packet switching: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how data is transmitted using circuit switching.
Explain how packet switching is used to send a message across the internet.
Describe the function of a router in packet switching.
Give two benefits and two drawbacks of packet switching compared with circuit switching.
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.
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.
Answer key and marking notes
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What does a router do with each packet it receives in a packet-switched network?
- AReads the destination IP address and forwards the packet to the next hop
- BReads the sequence number and holds the packet until the earlier packets have arrived
- CReserves a dedicated path to the destination before passing the packet along it
- DJoins the packet to the other packets from the same message and forwards the whole message
A message is split into packets numbered 1 to 6. The receiving computer gets them in the order 3, 1, 6, 2, 5. What does it do?
- ABuffers the packets, requests packet 4 again, then reassembles them in order 1 to 6
- BRebuilds the message in the order the packets arrived, then requests packet 4 again
- CDiscards every packet and asks the sender to send the whole message again on one route
- DAsks the last router to put the packets in order and supply the missing packet 4
A packet leaves the sender with a hop limit of 64. Each router reduces the hop limit by 1. The packet passes through 9 routers and reaches its destination. What is its hop limit when it arrives?
- A55
- B56
- C64
- D54
Which is a benefit of circuit switching compared with packet switching?
- AData arrives in order and at a constant rate
- BBandwidth is not wasted when the users have no data to send
- CData is automatically re-routed if one link in the path fails
- DNo time is needed to set up a connection before sending data
A circuit-switched telephone call reserves 64 kbit/s (64 000 bits per second) for 5 minutes. The two callers are both silent for 60% of the call. How many bits of reserved capacity are wasted?
- A11 520 000
- B7 680 000
- C19 200 000
- D192 000
An internet (VoIP) phone call works well at night but breaks up and lags in the evening when many people are online. Which explanation is correct?
- APacket-switched links are shared, so busy links delay packets unevenly or lose them
- BThe call uses circuit switching, and the circuit is released whenever too many users join the network
- CRouters reassemble the voice packets in the wrong order when they have too many packets to sort
- DThe callers' IP addresses change during busy periods, so the packets are sent to the wrong computer
Quiz key
A router reads the destination IP address, looks up the best next hop in its routing table and forwards the packet. Putting packets in order by sequence number happens at the destination computer, not at routers. Reserving a path is circuit switching, and routers never reassemble whole messages.
Packets may take different routes and arrive out of order. The receiver holds them in a buffer, uses the sequence numbers to see that packet 4 is missing, requests it again and then reassembles 1 to 6. Using arrival order scrambles the message; there is no need to discard packets that arrived correctly; and routers neither reorder packets nor keep copies of them to resend.
Each of the 9 routers subtracts 1, so the hop limit arrives as 64 - 9 = 55. 56 counts only 8 routers; 64 assumes the value never changes; 54 counts the destination computer as a hop as well, but only routers reduce it.
A dedicated circuit gives reserved bandwidth, so data arrives in order with a constant, low delay. The other three are benefits of packet switching: circuit switching wastes the reserved bandwidth when idle, loses the connection if a link fails, and needs time to set up the circuit first.
5 minutes is 300 s, so the circuit reserves 64 000 x 300 = 19 200 000 bits, and 60% of that, 11 520 000 bits, is wasted during the silence. 7 680 000 is the 40% that is used; 19 200 000 is the whole reservation; 192 000 comes from using 5 instead of 300 seconds (60% of 64 000 x 5). With packet switching, the idle link time could carry other users' packets.
VoIP is carried by packet switching, so links are shared. When they are busy, packets queue at routers, giving varying delay, and some are dropped, so the audio breaks up. A circuit-switched call would have reserved bandwidth and would not be affected by other users; routers do not reassemble packets; and IP addresses do not change because the network is busy.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.