AP CSP
AP Computer Science Principles / All lessons / Big Idea 4: Computer Systems and Networks
4Big Idea 4: Computer Systems and Networks
AP CSP topic 4.2

Fault Tolerance

The internet was designed to keep working even when parts of it fail. Because there are many paths between devices, packets can be routed around a broken connection. This redundancy costs extra equipment, but it makes the system fault tolerant. The more independent paths there are between two devices, the more failures the connection can survive.

Fault Tolerance vocabulary

  • Fault tolerance: The ability of a system to keep working when some parts fail.
  • Redundancy: Extra parts or paths that can take over when others fail.
  • Route: The path packets take from sender to receiver.
  • Router: A device that forwards packets toward their destination.

AP-style practice questions

Try each one, then open the answer. To save your progress, take this quiz in the interactive course.

Skill: Computing Innovations 
Network diagram:
A connects to B and C
B connects to D
C connects to D

1. If the connection between B and D fails, can A still send data to D?

  1. (A) Yes, through C
  2. (B) No, because B is the only path
  3. (C) Only if A is restarted
  4. (D) No, because A and D are not directly connected
Show answer

(A) Yes, through C There is a second path, A to C to D, so the network is fault tolerant.

Skill: Computing Innovations

2. Which of the following best describes redundancy in a network?

  1. (A) Removing extra connections to save money
  2. (B) Including extra devices or paths so the network keeps working when one fails
  3. (C) Sending each packet twice
  4. (D) Using only one router
Show answer

(B) Including extra devices or paths so the network keeps working when one fails Redundant paths let traffic route around failures.

Skill: Computing Innovations

3. Which of the following is a cost of making a network more fault tolerant?

  1. (A) It needs extra equipment and connections
  2. (B) It makes packets impossible to route
  3. (C) It removes all security
  4. (D) It stops the network from working
Show answer

(A) It needs extra equipment and connections Redundancy costs money for extra hardware and links.

Skill: Computing Innovations 
Network diagram:
P connects to Q
Q connects to R
R connects to S

4. Which single connection failure would stop P from reaching S?

  1. (A) Only P–Q
  2. (B) Only R–S
  3. (C) Any one of the three connections
  4. (D) None, the network is fault tolerant
Show answer

(C) Any one of the three connections There is only one path, so breaking any link disconnects P from S.

Open 4.2 in the interactive course