Availability (or Uptime) refers to the percentage of a specified period of time during which a computer system is capable of doing the things it is supposed to do. It came into use to describe the opposite of downtime, times when a system was not operational. For a networked system to be considered “available,” it has to have service, host and network working. It must have both connectivity (be physically attached) and functionality (actually working.) Typically, one reports availability as a single monthly figure. For example, 99% availability would mean that the system was not working for a little over 7 hours during the course of a month. Availability can be tested by sending a group of test packets and seeing if an answer is received.

Uptime — Time Lost in a year

• 98% – 7.3 days
• 99.0% – 3.7 days
• 99.9% – 8 hours
• 99.99% – 1 hour
• 99.999% – 5 minutes

Users of Microsoft Windows systems can type ‘systeminfo’ at the Command Prompt to display all system information, including the System Uptime. Users of UNIX and Linux systems can use the ‘uptime’ utility to get the uptime.