“Carrier-grade is a term for public
network telecommunications products that require up to 5 or 6 nines (or 99.999 to
99.9999 percent) reliability…. The term "5 nines" is usually associated with carrierclass
servers, while "6 nines" is usually associated with carrier-class switches.“
[Wikipedia] As software and servers now press for five-nines, network elements are
expected to support six-nines.