S4 ICT | Cloud Computing and Virtualisation: Tutor Prep Guide (45-minute flow, script, common errors)

2026-11-15

Tutor lesson-prep guides · Secondary ICT

Every student uses the cloud, yet few can say which layer they are responsible for. This lesson uses one responsibility line and two calculations to turn a slogan into an answerable question.

Objectives — By the end of the lesson students can: (1) state who manages the operating system, the runtime and the application across IaaS, PaaS and SaaS, with one example each; (2) explain that a hypervisor divides one physical machine CPU, memory and storage among several virtual machines; (3) compute a saving of 800 W or 40 per cent when eight 250 W servers become two 600 W servers, then 8,409.6 dollars of annual energy cost; (4) compute 8.76 hours of downtime a year from 99.9 per cent availability.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Percentages	Eight servers draw 2,000 W and save 800 W, so what share is saved	40 per cent
Unit conversion	800 W for 24 hours is how many kWh	19.2 kWh
Multiplication	19.2 kWh a day over 365 days	7,008 kWh
Decimals	What is 99.9 per cent as a decimal	0.999, leaving 0.001 as downtime

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Entry check	Ask 40 per cent, 19.2 kWh, the annual figure and 0.001; keep answers on the board	Spoken answers, working on paper
6-16 min	Act one: three service layers, one responsibility line	IaaS runs your own OS, PaaS runs only your code, SaaS you only use; draw the line bottom to top	Students give a SaaS example they use
16-26 min	Act two: virtualisation is slicing a cake	A hypervisor slices one 16-core 64 GB machine into four virtual machines, each with its own OS	Students contrast a VM with a physical machine
26-36 min	Act three: consolidation power and availability	Eight 250 W become two 600 W, saving 800 W; 19.2 kWh a day and 8,409.6 a year; 99.9 per cent gives 8.76 hours	Students compute the saving and downtime
36-42 min	Question types: layers, slicing, power, availability	The same numbers asked four ways	Students do one and write the unit
42-45 min	Exit check	Three spoken questions; note which act each error comes from	All three correct

03 Teaching script (read it out)

Scene 1: Act one: three layers, differing in how far your responsibility reaches

The cloud is not one cloud but a responsibility line. IaaS rents machines: the provider supplies CPU, memory, storage and network, and you run everything above the operating system, such as renting a virtual server for your own website. PaaS rents a runtime: the provider also runs the OS and the database, so you upload code, as with an application platform. SaaS rents software: you only sign in and use it, as with webmail or online meetings. So when a question asks whether the user installs updates, ask which layer first: IaaS users update the OS, PaaS users do not, and SaaS users never touch it.

A company rents a virtual server and installs its own database: is that IaaS or PaaS, and why?

Scene 2: Act two: virtualisation slices one machine for several users

A physical server has 16 cores and 64 GB of memory, yet one website uses only two cores and 8 GB, so the rest is wasted. Virtualisation inserts a layer called the hypervisor, which divides the physical resources to create four virtual machines, each with pretend CPU, memory and disk and its own operating system. Inside, the system believes it owns a whole machine. So remember one sentence: the hypervisor slices, and a virtual machine is a slice. The gain is consolidation, four workloads on one physical box, buying fewer machines and burning less power. The cost is a single point of failure: if that box dies, all four stop, so production spreads across physical hosts.

What is the biggest risk of four virtual machines on one physical host, and how do you fix it?

Scene 3: Act three: consolidation is subtraction, availability is multiplication

Eight old servers at 250 W draw 2,000 W; two at 600 W draw 1,200 W, so 800 W is saved, which is 800 over 2,000, or 40 per cent. That 0.8 kW runs 24 hours for 19.2 kWh; at 1.2 dollars a unit that is 23.04 a day and 8,409.6 a year over 365 days. Availability works the other way: 99.9 per cent means that of 8,760 hours a year, 0.1 per cent is down, and 8,760 times 0.001 is 8.76 hours, so almost nine hours of downtime. Ask for 99.99 per cent and downtime falls to 0.876 hours at several times the price, which is why choosing a service level trades money for downtime.

How many hours of downtime separate 99.9 from 99.99 per cent, and why is the price so different?

04 Question types and worked steps

Type	Example	Steps	Answer
Classify the layer	Renting a virtual server and installing the OS and database yourself	Ask how far the user responsibility reaches	IaaS, everything above the OS is yours
Virtualisation concept	One 16-core 64 GB machine split into four virtual machines	The hypervisor slices, each VM has its own OS	Four VMs share one physical host
Consolidation saving	Eight 250 W servers become two at 600 W	Subtract the totals, divide by the original total	800 W saved, that is 40 per cent
Availability	Downtime in a year at 99.9 per cent	8,760 hours times 0.001	8.76 hours

Sanity check (teach this): Four checks: (1) layer questions are settled by asking who updates the operating system; (2) consolidated power must be lower than the original, and a larger figure means a multiplication slip; (3) kWh is power times hours, so watts divide by 1,000 first; (4) higher availability means less downtime, so 99.99 per cent must come out below 99.9 per cent.

05 Independent practice (3 questions)

Question	Answer	What to watch
Eight servers draw 2,000 W and consolidation gives 1,200 W: what share is saved	40 per cent, that is 800 W	Check the denominator is the original 2,000 W
0.8 kW saved runs for a day: how many kWh	19.2 kWh	0.8 times 24, after converting watts to kilowatts
99.9 per cent availability: downtime a year	8.76 hours	8,760 times 0.001

06 Common mistakes and what to say

Mistake	Student writes	Say this
Confusing IaaS with PaaS	Calling a rented virtual server with a self-installed database PaaS	One line settles it: do you install and update the operating system? If yes, it is IaaS.
Treating a VM as a physical machine	Saying one host can hold only one virtual machine	Virtualisation exists to put several VMs on one host, otherwise there is no consolidation at all.
Slipping on kWh	Multiplying 800 by 24 to get 19,200 kWh	800 is watts, so divide by 1,000 first to get 0.8 kW, then multiply by 24 for 19.2 kWh, a thousandfold difference.
Reading higher availability as more downtime	Answering 87.6 hours for 99.99 per cent	The higher the percentage, the smaller the leftover share, so 99.99 per cent leaves 0.876 hours. Count the decimal places.

07 Homework (paper, section and question numbers)

S4 ICT Practice Paper 1, cloud computing and virtualisation section — Section A, questions 1 to 4

08 Exit check (last 5 minutes)

Question	Answer	Decision
A rented virtual server with your own OS: which service layer	IaaS	Wrong: reteach act one, who runs the operating system
Eight 250 W servers become two at 600 W: the saving	800 W, that is 40 per cent	Wrong: reteach act three, divide by the original total
Downtime a year at 99.9 per cent availability	8.76 hours	Wrong: reteach act three, 8,760 times 0.001

09 Tutor FAQ

Which year level is this for?

Hong Kong S4 ICT uses cloud computing and virtualisation to test service layer judgement and resource arithmetic. The S3 work on percentages and unit conversion is the prerequisite, so revise the difference between kW and kWh first.

Is 45 minutes enough?

Yes, provided act one gets ten minutes. The layers are settled by one responsibility line, so never read the acronyms out one by one or students only memorise letters.

Are these a real company figures?

No. 250 W, 600 W and 1.2 dollars a unit are teaching numbers. Real data centres differ widely in power density and tariffs, and the method and direction of the judgement are the point.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.