Link availability is the percentage of time a link is useable when considering real-world causes for outage, such as weather conditions in in local climates, outages due to equipment failure and other system problems. Availability is typically quoted in nines. For example, 99.9%, or three-nines (3-9’s) availability, means, on average, the link is expected to be not available 0.1% of the time, or an average of 8.76 hours per year. Four-nines (4-9’s) availability translates into only 52.6 minutes per year of down-time and five-nines averages just 5.26 minutes of downtime per year.