Resiliency and Redundancy - NOT the same


But when was the last time you thought about resiliency and redundancy in your dial tone? I would venture to say that if you've done it at all, it was for Y2K, and it probably hasn't been looked at since.

The reason that I bring this up is that, although the incidents are limited, severe outages are when we need connectivity the most. If you are planning to keep your voice environment running reliably during a disaster, in addition to power, consider the resiliency of your trunking, and connectivity to 911 at the same time.

Resiliency vs. Redundancy

Dictionary.Com defines resiliency as the ability to recover readily from illness, depression, adversity, or the like;

Dictionary.Com defines redundancy as the provisioning of additional or duplicate systems, equipment, etc., that function in case an operating part or system fails.

You achieve resiliency by employing redundant systems, but resilient systems, do not necessarily provide redundancy. Let me give you an example:

You decide to get 2 T1's for your PBX, and you order them from different carriers for redundancy, That gives you resiliency too, right? If they are running into your building through a single conduit, or a single cable delivered by your LEC, then a single back hoe, can dig up both lines, leaving your fiber connectivity in a huge mess of spaghetti at the bottom of a hole. So you have some level of redundancy, but the resiliency is questionable as there is a single point of failure because of the single entrance. Putting those circuits in separate conduits, on separate paths, gives you the resiliency piece of the equation.

For many years, landscapers have been putting in sprinkler lines with a machine they call a "Ditch Witch". The telecom industry calls these "Fiber Finders" since these machines seem to have an uncanny knack for finding fiber optic cable buried in the ground, and tear it to shreds. In many cases, people learn the meaning, and difference, in resiliency and redundancy.

I also get a lot of customers asking about answering their own E911 calls. In addition to the obvious liability this brings about, I ask them what kind of resiliency and redundancy they plan to have in their system. Most of the time, what I get back is the same look your dog gives you when you squeeze his squeaky toy, and a "Huh? What do you mean?" What I mean is what level of redundancy have you built into your power and trunking to be sure that you have a path out of the system to 911 that is also resilient? After all, the very reason you are calling 911 may be the exact same reason your trunks are down.

Don't just think about redundant paths out of your building. Understand the route your circuits take on their way to the central office, and make sure your network is resilient as well. I remember back in the late 90's, while preparing for Y2K, a large financial institution in NYC found that although they had multiple entrances from their 4 buildings, and the circuits went to 3 separate central offices, there was a single manhole in Manhattan that about 80% of their circuits all passed through! One errant move by a backhoe could take out literally thousands of their lines.

Fortunately, with a little planning they achieved a network design, from the PBX to the CO that minimized the single points of failure. The simple rules that were followed were as follows:

  • Diverse carriers
  • Diverse building entrances
  • Diverse conduits and paths
  • Diverse CO Buildings
  • Diverse CO switches on separate floors (Yes many carriers have more than 1 switch)
  • Diverse CO switches
  • Diverse CO Switch Cabinets
Much of the work to achieve diversity is the analysis of the existing infrastructure, and understanding where specific circuits were routed. Back before 9/11, it was easier to get street level fiber maps from the carriers and see the actual routing of your circuits, but even today, you should be able to request diversity from the carrier, and have it written into your contract. The sad part is that you'll just have to take their word they are actually giving it to you.

Once you achieve the level of diversity, you need to ensure that it stays that way. Bandwidth is not unlimited, and carriers need to manage it. As new customers come on the network, and others leave, they regularly re-evaluate the network load, and "groom" the circuits to even out distribution. The point I am making here is that contract negotiations need to include very specific clauses related to grooming. What you don't want to have happen, is to have your circuits, once you have established the diversity you want, moved; putting you right back into a situation you just engineered around.

I sincerely hope all of my readers and listeners on the east coast made it through Irene, safe and only inconvenienced. Events of the past week get us thinking about resiliency, redundancy, and access to communications, and I hope I've provided some additional food for thought about your network.


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Until next week. . . dial carefully.