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Reliability is the most critical aspect of a business phone system: You pick up the phone and you get a dial tone, period. ShoreTel delivers IP telephony systems with unmatched reliability, using an approach that is fundamentally different from that of any other IP PBX supplier in the world.
ShoreTel's architecture and design not only deliver high reliability, but do so in a very simple and cost-effective manner. This paper describes ShoreTel's unique approach to providing extremely reliable VoIP.
We start by defining reliability and availability, and then compare the approaches ShoreTel and other IP telephony vendors take to ensure high availability of their IP PBX hardware. You will see how the underlying system design and architecture dictate the type of redundancy that can be deployed to increase reliability, and why ShoreTel's n+1 redundancy is so much simpler and more cost-effective than the 1:1 redundancy used by other systems. The paper also addresses the reliability of the underlying data network, and the challenges of implementing a virtually always available voice system on an infrastructure that has a much lower availability rating.
No enterprise voice system today is without such applications as auto attendant and voicemail, so we examine application reliability and the need to hold applications to the same reliability and availability standards as the voice system hardware. And we finish by looking at soft reliability issues?th e impact that software problems, administrative and maintenance activities, and network quality can have on voice system availability.
2.0 Five-Nines Availability
When voice system reliability is discussed, people are typically talking about the reliability of the hardware. Without reliable hardware, you don't have a reliable system. We begin our reliability story by defining hardware reliability and how it is achieved.
Classically, reliability is measured by determining how often the hardware in a system fails, and then computing the percentage of time the system is available. In telephony, the accepted benchmark is "five-nines" reliability, or a system that is available at least 99.999% of the time. We should note here that, while availability is what is actually computed, it is often mistakenly referred to as reliability, and spoken of as "five-nines of reliability." Availability can be predicted based on the probability of hardware component failure, as detailed below.
ShoreTel ShoreGear Family
Availability is predicted by taking into account the type and number of hardware components in a system and calculating the mean time between failure (MTBF). Currently shipping ShoreGear IP PBX units have a predicted MTBF of approximately 135,600 hours, and each failure requires 1 hour of mean time to repair (MTTR). Using these variables, we can now do a simple computation to estimate the availability of ShoreTel hardware:
Predicted Availability of a Typical ShoreGear Unit
Availability = ____MTBF____ = 135,600 = 99.9993% MTBF + MTTR 135,600 + 1
This availability equation represents the standard definition of "reliability," and shows that a typical ShoreGear unit will achieve five-nines of availability. Stated another way, a ShoreGear switch on average is unavailable for 1 hour every 10 years.
2.1 ShoreTel vs. Other IP PBXs
Other IP PBXs can achieve five-nines of availability, but they do so by using redundancy and thus adding to the cost and complexity of the solution. The base units of competing IP telephony systems offer significantly lower availability than ShoreGear units, because they lack the unique architecture and design of the ShoreTel system.
ShoreTel Call Control Architecture
All of the major IP PBX vendors use a call control server to set up phone calls and provide telephony features. The most common method is to use a centralized call control server that provides dial tone for all phones and trunks, as shown in the figure below:
In distinct contrast, ShoreTel uses a distributed call control model. A portion of the end points are handled by each call control server as illustrated in the Distributed Call Control figure below:
Call control is provided by each ShoreGear switch comprising a ShoreTel system. In competing IP telephony systems, vendors implement call control on a separate computer system. This computer system is typically embedded in the IP PBX chassis, but some vendors put it in a separate box.
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