Availability
Link availability is typically expressed as "n-nines" where n is "five" for 99.999% availability (< 5.3 minutes of outage per year)(< 53 minutes of outage per year) and so on. Note that an "outage" may not actually mean that data is no longer delivered by the link, but rather indicates that the data bit error rate (BER) is elevated above a specified level which varies by manufacturer.