CompTIA Network+ (N10-009)Network OperationsHard
A vendor datasheet for a switch lists a Mean Time Between Failures (MTBF) of 100,000 hours and a Mean Time To Repair (MTTR) of 4 hours. Using the standard availability formula, what is the calculated availability of this device?
- A96.000%
- B25.000%
- C99.600%
- D99.996%
Show answer & explanationAnswer & explanation
Correct answer: D. 99.996%
Availability = MTBF / (MTBF + MTTR) = 100,000 / (100,000 + 4) = 100,000 / 100,004 ≈ 0.99996, or 99.996%. This shows the device is expected to be operational nearly all the time relative to its repair downtime.
Why the other options are wrong
- A. 96% would require a much larger MTTR relative to MTBF, not matching the given numbers.
- B. 25% would result if MTTR and MTBF were treated as a simple ratio without proper formula application.
- C. 99.6% miscalculates by using the wrong ratio scale (off by a factor of 10).
Availability Formula (MTBF/MTTR)
Availability measures the proportion of time a system is operational, calculated as MTBF divided by the sum of MTBF and MTTR.
- Formula: Availability = MTBF / (MTBF + MTTR)
- MTBF = average time between failures; MTTR = average repair time
- Higher MTBF and lower MTTR both increase availability
Memory trick: Availability = Uptime over Uptime-plus-fix-time.