The Holy Grail of Five Nines
What the popular "5 nines"
really means and how reliability and availability are measured.
By: John Shepler
Five nines reliability. It's the standard
often quoted for traditional telephone service, a goal for new
competitors, and a bragging point for equipment vendors. But
what does it really mean? Just how close to perfect is five nines
The spoken term five-nines refers to the
number 99.999%. Count the nines. This number is generally referred
to as a reliability figure. Actually, what people call reliability
is more correctly called "availability." It's not just
how often a piece of equipment has a software crash or a power
supply that bursts into flames that is really important. It's
how much of the time you actually get to use it. In other words,
how much of the time is this particular device available. Availability
includes how often it breaks and then how fast it gets put back
into service. You also have to include how often it is out of
service or unavailable due to routine maintenance.
Here's an example. Say your softswitch
has a software glitch that only shows up under obscure combinations
of events. When the glitch occurs once every six months, the
software crashes and automatically reboots. That takes a minute.
Does this switch have five nines uptime? Yes. Now if a power
supply smokes once a year and it takes 20 minutes to fix it,
that's not good enough for five nines even though the power supply
failure occurs less frequently.
Here are some handy numbers to give you
perspective on the whole nines issue:
Five nines or 99.999% availability means
5 minutes, 15 seconds or less of downtime in a year.
Or, if you are really ambitious, shoot
for six nines or 99.9999% availability, which allows 32 seconds
or less downtime per year.
Otherwise, four nines or 99.99% availability
allows 52 minutes, 36 seconds downtime per year.
Three nines or 99.9% availability allows
8 hours, 46 minutes downtime per year.
Two nines or 99% availability allows 3
days, 15 hours and 40 minutes downtime per year.
One nine or 9% availability allows over
332 days of downtime per year. That's right. You're only up and
running about a month out of the year on average. Good grief.
Zero nines is totally, absolutely useless.
It's 100% downtime per year. Perhaps you can get a little something
for it from the recycler.
How do you get more nines? Buy the best
equipment that's the easiest to repair. Then add redundancy.
Highly reliable systems often include multiple power supplies
and processors, battery backup, diesel or natural gas generators
for longer power outages than batteries can handle, multiple
diverse communication lines and extras of whatever else is likely
to fail. Buggy software that crashes all the time is going to
hurt your reliability. If it goes down a lot and takes a long
time to get back online your availability will be hurt also.
One thing to be aware of is the five nines
criteria tends to apply to whatever the person quoting it says
it applies to. PBX systems that meet five nines availability
may only do so for the core system and might not include individual
line cards and certainly not the phones themselves and their
wiring. If it's REALLY important that you minimize downtime,
you need to consider EVERYTHING that can fail and make sure it
is backed up and/or very easy to fix.
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