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IP Telephony Solutions And Network Availability
By: Mohamed Shaker

One aspect of voice communications that requires serious consideration during the design stage is a high service availability expectation on the part of telephony users. You need to obtain answers to the following questions:

• How high an availability expectation is reasonable?
• What price does an SMB need to pay to push the availability as close to the 100 percent mark as possible?

Most telephone companies (telcos) pride themselves on offering five 9s (99.999 percent) or six 9s (99.9999 percent) reliability for their central office (CO) switches or PBX products. That level of equipment reliability translates into approximately 5.25 minutes of downtime per year for the five 9s and 31 seconds of downtime for the six 9s reliability levels. These are statistical numbers, however. Despite their value, statistics can be deceiving.

From the end-user perspective, what is the meaning of this level of reliability for a singular piece of equipment in the context of an entire communications solution? Multiple components and pieces of equipment are involved in any telephony solution. The factors in total network availability calculations must include the following:

• Mean time between failures (MTBF) for all of the relevant equipment and/or communications links that make up the collective infrastructure to support the act of making a phone call

• Mean time to repair (MTTR) for all of the relevant equipment and/or communications links that compose the collective infrastructure to support the act of making a phone call

• Software reliability.

• Required time for scheduled maintenance and upgrades.

• Power availability.

Assume, for example, that the probability of being functional through the course of a year has been calculated for every piece of hardware and software required for making phone calls. Equipment and software upgrades have been reduced to a component of the entire solution and assigned a level of probability as well.

The product resulting from the multiplication of all of the individual probabilities translated into time would give the end users a perspective of the level of service availability that they could expect.

Assume that the product of all of the individual probabilities is 0.995, or a 99.5 percent probability of the IP Telephony network being available for use. Conversely, this translates into 0.005 or 0.5 percent probability of the network not being available. Translating 0.5 percent unavailability into time over the course of the year (365.25 days x 24 hours/day x 0.005) results in approximately 44 hours (43.83 hours, to be exact) of downtime per year, or less than 2 days.

Two days per year of service unavailability in no way implies that any single piece of equipment would have this kind of downtime, because the individual components have not even been specifically identified in this calculation. The number serves only as an example of what an end user might expect. And when you put it in the context of a typical work schedule, it's not that drastic. Most SMB employees take vacations that are longer than 2 days per year.

When approaching an IP Telephony design, the level of availability expectation on the part of the customer should be ascertained beforehand in the context of the existing and proposed solutions. This means that you need to identify solution components, assign to each a level of probability of being functional throughout the year, and translate the final product into time. It's not a bulletproof calculation, but it's a mechanism that can give a realistic perspective to the customer and avoid bad feelings following deployment.

Reference:
Cisco Network Design Solutions for Small-Medium Businesses



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Posted by ROOT Technologies


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