Security Monitoring

Appendix C. Calculating Availability

Note

Much of the information that follows is based on the concepts presented in the book High Availability Network Fundamentals, by Chris Oggerino (Cisco Press). At the time of this writing, the book is unfortunately out of print. If you can get your hands on a copy, it is worth your while.

This appendix provides richer detail to help you evaluate the components of system availability, as an extension of what was presented in Chapter 6. You can calculate the availability of a single component with the following equation:

So, the availability of a component whose Mean Time Between Failures (MTBF) is 175,000 hours and Mean Time To Repair (MTTR) is 30 minutes would be:

In other words, according to the manufacturer’s testing results, the component is expected to have only 1.51 minutes of downtime per year.

Most systems are composed of more than one component, of course. Multicomponent systems are arranged in a serial or a parallel fashion. For a serial component-based system, each component is a single point of failure, and so each component depends on the other for system availability. In contrast, a parallel component system has redundant components built such that the failure of a single component will not cause the entire system to fail. You can calculate ...