What Does 'five 9s' Mean? - 404 Tech Support

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You are here: Home / Articles / Talking Points / What does ‘five 9s’ mean?

My ISP has suffered two fiber cuts in the past month, taking out Internet access for all of their customers in a large region for hours at a time with an average repair time of 8 hours according to the company. While twiddling my thumbs waiting for the Internet to return, I began thinking of the five-nines commonly cited in high availability for systems. Is it time for ISPs to offer residential Internet customers a service-level agreement with credit for downtime?

The five-9s term indicates how many ‘9’s there are in the uptime calculation. For example, saying something is up (and available) 99.99% of the time would be four nines. Five nines mean something is available 99.999% of the time or it only has downtime of 5.26 minutes per year.

Availability %	Downtime per year	Downtime per 30 days	Downtime per week
90% (“one nine”)	36.5 days	72 hours	16.8 hours
95%	18.25 days	36 hours	8.4 hours
97%	10.96 days	21.6 hours	5.04 hours
98%	7.30 days	14.4 hours	3.36 hours
99% (“two nines”)	3.65 days	7.20 hours	1.68 hours
99.5%	1.83 days	3.60 hours	50.4 minutes
99.8%	17.52 hours	86.23 minutes	20.16 minutes
99.9% (“three nines”)	8.76 hours	43.2 minutes	10.1 minutes
99.95%	4.38 hours	21.56 minutes	5.04 minutes
99.99% (“four nines”)	52.56 minutes	4.32 minutes	1.01 minutes
99.999% (“five nines”)	5.26 minutes	25.9 seconds	6.05 seconds
99.9999% (“six nines”)	31.5 seconds	2.59 seconds	0.605 seconds

Similar terms discussing the same topic can include high availability, uptime, five-9s, class of 9s, carrier grade, and reliability. Traditional telephone landlines, or POTS, are often cited when discussing high reliability and 99.999% uptime may be required by state law for new technologies replacing landlines like VoIP for calling emergency services from public buildings.

High availability is commonly achieved through redundancy with backups, load balancing and failover methods, high quality components, use of best practices, and control of the system from endpoint to endpoint.

While I can’t blame an ISP for a fiber cut when some random person crashes their car into transmission box or a contractor digs without calling 811 (JULIE in Illinois) and cuts through a fiber line, is the next step in broadband advancement to improve its reliability? Should a credit be automatically offered to customers for less than 99.99% uptime per month? 99.9%? 99%?

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