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How 5 Nines Means High Availability
Five Nines ( 5 nines) is an industry benchmark long used to describe highly reliable telecommunications systems and network equipment. Here's how it's calculated and what it means to your service uptime.

By: John Shepler

Five nines reliability. It's the standard often quoted for traditional telephone service and top network equipment vendors. But what does it really mean? Just how close to perfect is five nines anyway? Let’s take a look.

Reliability or Availability
The spoken term five-nines refers to the number 99.999%. 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 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.

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.

By the Numbers
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. This is often acceptable for many non-critical applications.

Two nines or 99% availability allows 3 days, 15 hours and 40 minutes downtime per year. That’s somewhat marginal for business.

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.

Zero nines, of course, is totally useless. It's 100% downtime per year.

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 & processors, plus 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 also be hurt.

Include Everything
One thing to be aware of is that 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.

On Cloud 9’s
Another option to consider seriously is to move all those hardware availability issues to the cloud. Cloud services aren’t perfect, either, but they often can provide higher service availability than you can afford locally. That’s because the cloud architecture is designed for considerable redundancy and fast failover when problems occur. If you do opt for the cloud, though, be sure you have highly reliable bandwidth connections to your provider. A 5 nines hosted PBX system isn’t all that available if you have a flakey internet connection from your facility to the cloud. A dedicated SIP Trunk or a set of redundant trunks is a much better solution.

Nines for Your Needs
How many nines do you need and what’s the cost for WAN or cloud availability? Get the best price vs performance options for your particular requirements now.

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