The shape of the lesson
By the end of the lesson, learners can
- Wired media: Compare media using the same four headings every time: bandwidth (bit rate), distance, interference and cost.
- Wireless media: Wireless media are all electromagnetic waves; they differ in frequency, range and whether they need line of sight.
- Ethernet and CSMA/CD: In the lab, choose CSMA/CD, then make two PCs send at almost the same moment.
- Compare media by: bandwidth, distance (attenuation), interference, cost.
- Wireless: mobility and easy set-up against interception, interference and lower speed.
- CSMA/CD: listen, transmit, detect collision, jam signal, random wait, retry.
- Avoidance: switches with full-duplex links stop collisions happening.
One picture every idea returns to
Copper cable (twisted pair or coaxial) carries electrical signals: cheap and easy to install, but it suffers from interference and attenuation. Fibre-optic cable carries pulses of light: very high bandwidth over long distances, immune to electrical interference, but more expensive. Wireless media (radio waves including WiFi, microwaves, satellites) give mobility without cables but are slower, less secure and more affected by interference. Ethernet is the standard protocol for wired LANs; on a shared cable it uses CSMA/CD: listen before transmitting, detect any collision, send a jam signal, then wait a random time before trying again.
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, Copper, glass and air.. Data can travel as electrical pulses in copper, as light in glass fibre, or as radio waves, microwaves and satellite signals. 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: Wired media | Compare media using the same four headings every time: bandwidth (bit rate), distance, interference and cost. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Wireless media | Wireless media are all electromagnetic waves; they differ in frequency, range and whether they need line of sight. | Pairs |
| 21 to 30 min | Teach and check: Ethernet and CSMA/CD | In the lab, choose CSMA/CD, then make two PCs send at almost the same moment. Project tab 6 of the lab (CSMA/CD on a shared cable) and let a student drive it while the class predicts. Model the worked example Why random? on the board, one line at a time. Your turn: What does CSMA/CD stand for? | Connect-Extend-Challenge |
| 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, 21 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 Ethernet and CSMA/CD, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Wireless is not insecure by nature, but its signals spread beyond the building. | The mark is for saying that signals can be intercepted, so data must be encrypted. |
| CSMA/CD does not prevent every collision: it detects them and recovers. | Make sure the answer includes the jam signal and the random wait, which are the commonly missed marks. |
Wired and wireless media, Ethernet and CSMA/CD: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe two differences between copper cable and fibre-optic cable.
Describe the characteristics of satellite communication.
Explain how CSMA/CD is used to manage the transmission of frames on a shared Ethernet cable.
A café wants to provide internet access to its customers. Discuss whether it should use a wired or a wireless network.
Explain why a device waits a random amount of time after a collision.
A rural farm 15 km from the nearest town needs an internet connection. There are no cables to the farm. Evaluate the use of a microwave link and a satellite link.
Answer key and marking notes
- Copper carries electrical signals; fibre carries pulses of light.
- Fibre has a higher bandwidth / bit rate than copper.
- Fibre suffers less attenuation, so it can carry data further without a repeater.
- Fibre is immune to electromagnetic interference / harder to tap; copper is cheaper and easier to install (any two differences, both sides).
- Microwave signals are sent up to a satellite and relayed back down to another location.
- Covers very large areas, including oceans and remote regions without cables.
- Long distance (about 36 000 km to a geostationary satellite) causes a noticeable delay / latency; signals can be affected by weather.
- A device listens to the cable to check whether it is idle before transmitting (carrier sense).
- If the cable is busy, it waits until it is idle; then it transmits.
- While transmitting, it monitors the cable to detect a collision.
- If a collision is detected, it stops transmitting and sends a jam signal to all devices.
- Each device waits a random time before trying to transmit again.
- Wireless: customers can use their own phones and laptops anywhere in the café without cables.
- Easy and cheap to install, and any number of customers can join.
- But signals can be intercepted, so the network should be encrypted / separated from the café's own systems.
- Wireless is the better choice because mobility for customers matters more than the higher speed of cables.
- If both devices waited the same time, they would transmit again at the same moment ...
- ... and collide again; a random wait makes it likely one starts first and the other senses the cable is busy.
- Microwave link: higher bandwidth and less delay than satellite, but needs line of sight between masts/dishes, which hills or trees may block.
- Satellite link: works anywhere with a view of the sky, but has high latency (delay) because of the distance to the satellite and can be affected by weather.
- Conclusion: microwave if a line-of-sight path to the town is available, otherwise satellite.
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 CSMA/CD stand for?
- ACarrier Sense Multiple Access with Collision Detection
- BCarrier Signal Multiple Access with Collision Deletion
- CCollision Sense Multiple Access with Carrier Detection
- DCarrier Sense Multiple Address with Collision Detection
A telecoms company is laying a new cable under the sea between two countries. Which medium should it use, and why?
- AFibre-optic cable, because it has very high bandwidth and low attenuation over long distances
- BTwisted-pair copper cable, because it is cheaper and easier to join than any other cable
- CCoaxial copper cable, because its shielding makes it immune to all interference
- DFibre-optic cable, because it is cheaper and less fragile than copper cable
Two devices on a shared Ethernet cable transmit at the same moment and their frames collide. What do they do next?
- AStop sending, transmit a jam signal, then each wait a random time before listening and trying again
- BCarry on sending their frames, and the receiver puts the two frames back in order
- CStop sending, then both wait the same fixed time before sending again
- DSend a jam signal, then retransmit straight away while the cable is still free
A signal is relayed by a geostationary satellite. It travels about 72 000 km in total, up to the satellite and back down. Radio signals travel at about 300 000 km per second. What is the delay?
- A0.24 s
- B0.12 s
- C4.2 s
- D0.024 s
Why does each device wait a random time after a collision, rather than a fixed time?
- AWith a fixed time both devices would sense the cable free together and collide again; a random wait makes it likely one starts first
- BA random wait gives the jam signal time to reach the destination device before the frame is resent
- CA random wait lets the switch decide which device has the higher priority
- DA random wait is shorter on average than a fixed one, so the network is faster
Why do WiFi networks use collision avoidance (CSMA/CA) instead of the collision detection used on a shared Ethernet cable?
- AA wireless device cannot reliably detect a collision while it is transmitting, so it tries to avoid collisions instead
- BRadio signals cannot collide, because each device uses a different frequency
- CWiFi devices are connected to their own switch port, so there is only one sender per link
- DCollision detection would let anyone in range read the data, so it would be insecure
Quiz key
CSMA/CD is Carrier Sense Multiple Access with Collision Detection: devices sense the carrier, share (multiply access) one medium and detect collisions. The second option garbles "sense" and "detection", the third swaps carrier and collision, and the fourth replaces "access" with "address".
Fibre carries light with very high bandwidth and low attenuation, so it can cover long distances with few repeaters, which is why undersea and backbone links use it. Twisted pair is cheap but attenuates quickly and has far lower bandwidth. Coaxial shielding reduces interference but does not remove it, and coax still attenuates much faster than fibre. Fibre is more expensive and more fragile than copper, so the last reason is wrong even though the medium is right.
Under CSMA/CD each device stops, sends a jam signal so every device knows there was a collision, then waits a random back-off time before sensing the cable and retrying. Collided frames are corrupted, so carrying on achieves nothing. A fixed wait would make the two devices retry together and collide again, and so would retransmitting immediately.
Time = 72 000 ÷ 300 000 = 0.24 s, the noticeable latency of a satellite link. 0.12 s uses only one leg of the journey (36 000 km); 4.2 s divides the speed by the distance; 0.024 s is a power-of-ten slip.
The two devices detected the collision together; equal waits would put them back on the cable together. Random back-off makes it likely that one device starts first, so the other senses a busy cable and waits. The jam signal is for every device and does not need a random delay. A shared bus has no switch choosing priorities. The point is avoiding a repeat collision, not a shorter average wait.
A transmitting wireless device cannot hear a weak collision over its own strong signal, so detection does not work; WiFi avoids collisions instead. Devices on one WiFi channel share the same frequencies, so collisions can happen. Separate full-duplex switch ports describe modern wired Ethernet. Collision detection has nothing to do with whether data can be intercepted.
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.