AP CSP Day 19: Fault Tolerance & Redundancy
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Fault tolerance is the ability of a system to continue operating correctly even when some components fail. Redundancy, achieved by duplicating critical connections or hardware, is the primary way networks achieve fault tolerance. AP CSP exam questions about fault tolerance often present network diagrams and ask whether all nodes remain connected after one or more connections are removed. A network is fault tolerant when there is more than one path between any two nodes, ensuring no single failure can isolate a node.
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Fault Tolerance and Network Redundancy
What Is Fault Tolerance?
A fault-tolerant system continues operating correctly even when some components fail. The internet achieves fault tolerance through redundant connections: multiple paths exist between nodes, so a single link failure does not disconnect the network.
How to Evaluate Fault Tolerance
A network is fault tolerant between two nodes if there is more than one path connecting them. If removing any single connection still leaves at least one alternative path, the network tolerates single-link failures.
Practice Question
In a network, four devices (A, B, C, D) are connected: A to B, A to C, B to D, and C to D. If the connection between A and B fails, can device A still communicate with device D?
The network has redundant paths. Although the A-B connection fails, A can still reach D through the alternative path A → C → D. This is fault tolerance through redundancy — multiple paths ensure communication continues despite individual failures.
A) The existence of an alternative path (A-C-B) means communication is still possible. B is correct. C) No repair is needed because the alternate path through C works immediately. B) Device D remains connected to C and B; it is not isolated.
Students only consider the most direct path and forget to check for alternative routes. In fault tolerance questions, always trace ALL possible paths.
For network fault tolerance questions, draw the diagram and systematically trace every possible path between the two endpoints. If at least one path exists, communication is possible.
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