AP CSP Topic 4.2 Guided Notes - Fault Tolerance
Big Idea 4: Computer Systems and Networks · Topic 4.2 · Guided Notes (Student)
Fault Tolerance — Guided Notes
Fill these in during class or catch up here if you were absent. Print this page or work on paper — then check yourself with the CFUs on the Topic 4.2 page.
Print these notesTopic 4.2 lesson pageAll CSP topics
Today’s objectives
- Describe the benefits of fault tolerance — why a network built to survive failure keeps users connected even when components fail unexpectedly and in groups (LO CSN-1.E)
- Explain how a given system is fault-tolerant, pointing to its redundancy: the extra components and alternate paths that let data take a different route when something fails (LO CSN-1.E)
- Identify a system's vulnerabilities to failure — the single point of failure every path depends on — and decide where added redundancy would remove them (LO CSN-1.E)
Bell ringer
The town of Cedar Bend has 5 sites on its fiber network, joined by 5 lines: School–Library, Library–Town Hall, Town Hall–Clinic, School–Town Hall, and Clinic–Fire Station. A backhoe on Main Street is about to cut exactly ONE line.
Sketch the map. Which single cut does the most damage, and why that one? Then: the town can afford ONE new line — where do you add it to best protect Cedar Bend?
01. Redundancy: Extra Paths on Purpose
Key Vocabulary (LO CSN-1.E)
| Term | Definition (write it) |
|---|---|
| Redundancy | |
| Fault-tolerant |
Extra Components, Extra Paths
- Redundancy protects a system by building in .
- On a network map, redundancy looks like .
- When a connection dies mid-transfer, the data .
Stop and think
- In your bell-ringer map, explain why cutting School–Library barely mattered while cutting Town Hall–Clinic was a disaster. Use the phrase 'different route' in your answer.
- Mid-download, a router along your path to a game server dies completely — yet the download finishes. Explain what happened to the rest of your data, and name the property of the Internet that made it possible.
- The Internet's designers assumed its components WOULD fail and planned around it, instead of trying to build parts that never fail. Why is that the smarter engineering bet for a complex system?
Answer in complete sentences. Then check yourself with the matching CFUs on the Topic 4.2 page.
02. The Benefit, the Bill, and the Weak Points
What Redundancy Buys vs. What It Costs
The benefit: fault tolerance
The bill: additional resources
- A system earns the name fault-tolerant when it .
- The price of redundancy is .
- Redundant routing helps the Internet grow because .
AP TIP: is a two-sided sentence — redundancy often requires additional resources BUT can provide the benefit of fault tolerance. Full-credit answers name both halves.
Network Triage: The Three-Question Audit
For each network, run the audit the exam expects: Is it fault-tolerant? Where is the vulnerability — the single point of failure? Where would ONE addition create redundancy?
Complete the empty cells.
| Network | Fault-tolerant? | Vulnerability (single point of failure) | Where to add redundancy |
|---|---|---|---|
| A chain: five routers connected in a line, A–B–C–D–E | Every middle link and device; cutting B–C strands D and E | Link E back to A, closing the chain into a loop | |
| A ring: six routers in a closed loop, each linked to two neighbors | Yes, for any single failure — traffic reroutes the long way around | A cross-link through the middle of the ring guards against double failures | |
| A star: every device in a school connects to one central switch | No — one device failure can take down every connection at once | ||
| A mesh of six offices, each linked to three others — all reaching the Internet through ONE ISP line | Inside the mesh, yes; as a whole system, no | A second ISP connection from a different provider on a different physical path |
Watch out — “Redundancy Is Wasteful Duplication”
Myth: A second line that sits unused all year is money down the drain. In plain English 'redundant' literally means unnecessary — so cutting the duplicate is just good budgeting.
Explain why this is wrong:
Deep Dive · Beyond the AP Exam
Deep Dive: The Backhoe Test, Formalized
Bridges: links you cannot lose
Independent paths: counting your spares
- A link is a bridge if removing it .
- Two paths between the same endpoints are independent only if they share .
- With k fully independent paths, the network survives .
Deep dive only — the AP exam asks you to spot the single point of failure in a described system, never to prove graph theorems.
Stop and think
- A gaming café connects 30 PCs through one router, which reaches the Internet through one cable modem. State whether the system is fault-tolerant, identify its vulnerability to failure, and prescribe ONE addition — with the resource cost the owner must accept.
- A friend says Cedar Bend should cancel its second fiber line since 'it did nothing all year.' Respond in two sentences using the exact trade described in.
- Name the TWO distinct benefits that redundant routing between two points provides to the Internet, and explain in one sentence why the second one is about growth, not failure.
Answer in complete sentences. Then check yourself with the matching CFUs on the Topic 4.2 page.
Common AP Traps
Three ways Topic 4.2 loses points on the exam — one minute now, real points in May.
Vulnerability = the SPOF — in your own words:
Redundancy has a price — in your own words:
Tolerating ≠ preventing — in your own words:
Fault Tolerance, in One Slide
- The Internet has been engineered to be fault-tolerant, with abstractions for routing and transmitting data.
- Redundancy — extra components, including more than one path between any two devices — is how that tolerance is built.
- When a device or connection fails, subsequent data is sent via a different route, if possible.
- Fault-tolerant = supports failures and continues to function — vital because complex systems fail unexpectedly, often in groups.
- Redundancy costs additional resources but buys fault tolerance — and redundant routing increases reliability AND helps the Internet scale.
Exit check — I can…
- ☐ describe the benefits of fault tolerance for users and for scaling (LO CSN-1.E a)
- ☐ explain how a system is fault-tolerant by pointing to its redundancy (LO CSN-1.E b)
- ☐ identify a system's single point of failure and where to add redundancy (LO CSN-1.E c)
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