In Table II, at least four 9s (i.e. 99.99%) of availabilities
over the backbone links are achieved (i.e. the link failure
duration is no more than 52.56 minutes in one year) except the
link of site B7 to site B8 where they only have three 9s (i.e.
the failure duration is 8.76 hours in one year). This link
preserves the largest distance than the other sites. The longer
link distance may be more sensitive to the change of link
conditions such as the weather and attains lower availability