Pipeline Publishing, Volume 3, Issue 11
This Month's Issue:
The Long Arm of Telecommunications Law
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Carrier Grade: The Myth and the Reality of Five Nines
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Standard 1037C and MIL-ST-188 define telecommunications availability as a ratio of the time a module can be used (if a use request existed) over a period of time. It is a ratio of uptime to total time. Expressed otherwise, it is the proportion of time a system is functioning. But what is overlooked, and actually somewhat funny, is that five-nines is actually referring to a logarithmic unit derived from the number of nines following the decimal point.

Since 100% availability is very difficult to achieve, SLAs become a kind of amortization game.


However you define it, five-nines of availability is a pretty tough target to hit. This corresponds to 5.26 minutes of unplanned downtime in a year. It is rather unlikely that if the NOC engineers must be involved in a restoration it will take less than 6 minutes to get notification of the outage, auto process the alarm, display the alert information, look


Further, is availability an average or a probability? Generally, if you are building a network element, you would define availability as a “probability of failure”. But if you are an engineer in a NOC running a report, you would define availability as an average. Whereas, according to Cisco, the number of defects in a million is used to calculate software availability and time does not enter directly into the calculation.

While NEBS is mostly concerned with safety, the NEBS standards - Telcordia (GR-63 CORE and GR-1089 CORE NEBS Level 3) also provide goals for central office reliability of network hardware.

Setting Availability Requirements

Cisco says: “Increased availability translates into higher productivity, and perhaps higher revenues and cost savings. Reliability implies that the system performs its specified task correctly; availability, on the other hand, means that the system is ready for immediate use. Today's networks need to be available 24 hours a day, 365 days a year. To meet that objective, 99.999 or 99.9999 percent availability is expected.”

You see this figure everywhere. “Carrier grade means extremely high availability. In fact, the requirement for availability in commercial telecom networks is 99.999%... and is known as fine nines reliability.” [Carrier Grade Voice Over IP, a book by Daniel Collins]


up and display any related information, have a NOC engineer acknowledge the alert, open a trouble ticket, diagnose the root cause, determine the likely fix, launch the device interaction tools, change the necessary items on the device, and restart the service. Hence the overwhelming need to automate the entire process, relying heavily on redundancy to provide seamless transfer from the failing element to a healthy element. But of course, one expects that any specific element module will actually go more than a year without fault, so the availability target can be averaged out to meet the NOC goal of five-nines.

This averaging and predicting of element reliability becomes very clear with another precisely defined measure from MIL-HDBK-217 and Bellcore/Telecordia TR332:

FITS - the total number of failures of the module in a billion hours (i.e. 1000,000,000 hours).

Our civilization likely will not last a billion hours which is 117,041 years, so of course this measure must be estimated, computed from component parts, or derived from very large sample sets (like number of chips produced in a year). Bellcore (Telcordia) standards TR-332 Issue 6 and SR-332 Issue 1 provide guideline models for predicting failure.


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