Erlang's 99.9999999% (nine nines) reliability

Question

Erlang was reported to have been used in production systems for over 20 years with an uptime percentage of 99.9999999%.

I did the math as the following:

Answer 1

My understanding of those statistics is that it is computed over ALL AXD301 systems in production. We can expect that when an AXD301 has a severe problem, it would be down for more than 0.631 seconds. During this pediod, other AXD301 will take over to keep the network operational.

However, when you sum the total numbers of hours of all running AXD301, make the ratio for the one failing AXD301, you find 99.999999%

That's how I understand this figure.

Hope this help.