Skip to main content
CSP LabBy Educator Today
AP CSP course

C56 · Topic 4.2 · 40 minutes · CSN-1.E

Testing network resilience

By the end you can:

  • Evaluate each single-link failure.
  • Analyse combined failures.
  • Assess an improvement against its actual new graph.

Before this lesson: topics 4.1.

Redundancy
Additional components or paths that can preserve a function when some fail.
Fault tolerance
A system’s ability to continue specified operation despite some failures.

Understand

Test a resilience claim systematically. A network that survives every single-link failure can still fail when two links break together. Count which links reach an endpoint and examine whether remaining routes can use them, rather than assuming a dense-looking picture must stay connected.

An improvement changes the graph and therefore the relevant failure sets. If a new link is added to the source, a statement that the old two source links isolate it may become false. Explain a specific protected failure and a specific remaining limitation for the actual revised graph.

Worked example

Remove C-T and D-T, then add B-T.

  1. Before the addition, both destination links are removed and no path reaches T.
  2. B-T creates a new reachable destination link and S-B-T works.
  3. If S-A and S-B instead fail, B-T does not help; adding a third source link would change that source-isolation analysis.

Explore each concept

Choose a concept, change its inputs and follow the explanation. Each example models the stated rule or scenario; completing it is practice rather than evidence of mastery.

  • Redundancy and resource cost
  • Single-link versus multiple failures
  • Endpoint isolation
Open this example on its own

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to explore: Test every single failure and combined failures
  1. Failure testeach-single-linkWaiting
  2. Add direct S-T linknoWaiting

Ready to explore: Test every single failure and combined failures

Why: Change an input, predict the first effect, then advance one step.

Try this: Change one input or choose another listed case. Predict the effect, reset and compare the states and governing rule.

Step through and experiment

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to experiment
  1. Input 1C-TWaiting
  2. Input 2(empty)Waiting

Ready to experiment

Why: Predict the first change, then advance one step.

Try this: Fail C-T, then also D-T. Add a link and predict whether it reconnects S to T.

Check a misconception

Claim: Surviving each single failure implies surviving every pair of failures.

Combined failures can remove all remaining paths even when each individual failure was tolerated.

Guided practice

Explain why all seven single-link failures leave some S-to-T path in the original graph.

Work with a partner. Make a prediction, compare your reasoning, then identify the rule that supports your answer.

Independent transfer

Propose one added link that protects against losing S-A and S-B, then give a remaining failure.

Exit check

Why are route existence and actual network availability not identical numerical predictions?

Write your reasoning before comparing with a classmate. If your teacher assigned this task, submit it through My classes to save your work and receive feedback.

Check yourself

  1. Question 1 of 2C-T and D-T both fail with no added link. What follows?

    Select one answer.

  2. Question 2 of 2After adding S-D, which failure certainly isolates S?

    Select one answer.

Your practice so far

Check an answer to see its rationale.

Teacher guidance and scoring