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Practical VoIP Using VOCAL by Luan Dang, Cullen Jennings, David Kelly

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Setting Up a Redundant System

At its peak, our old office had about 60 people using the internal trial as their phone system. In terms of capacity, we could have handled all the office phone traffic through a single host. There were only five or six people who might have been considered power phone users. The rest could have been considered light users. Therefore, if we had been running a single host, capacity would not have been an issue.

The issue would have been availability. Although Linux is a stable operating system, to ensure system availability, we needed to build redundancy into the system.

What Is Redundancy?

A truly redundant network has no single point of failure within its system. For phone systems, the goal is to achieve what is known as carrier class reliability , in which the system is up and running 99.999% of the time. Another term used to describe this is five nines reliability. The difference between a system being 99.999% or 99% reliable translates into a potential downtime of just over 5 minutes compared to 3.5 days over an average year.

Availability is measured by:

(Total User Time - Total User Outage Time) x 100 / Total User Time

In this equation, Total User Time is equal to the total number of end users multiplied by the time that transpires during the reporting period.

Some organizations prefer to measure availability in Defects Per Million (DPM), which is measured by:

Total User Outage Time x 1,000,000 / Total User Time

In this equation, Total ...

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