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By Wedge
Greene and
Barbara
Lancaster, LTC International
Carrier grade power
What is the
hard fast requirement for Carrier-Grade?
Is “five-nines” or 99.999%
up or 0.99999 available a
hard, fast requirement of telecommunications
or is it the telecommunications equivalent
of an Urban Myth? From
a common sense perspective, the meaning
of availability is clear,
and given the essential nature of
telecommunications, the necessity
of five-nines is easily understood.
But when you want to measure it,
and hold someone accountable for
delivering that availability, you
must establish an operational
definition for it. We asked
a cross section of telecom and OSS
experts if they knew the origin of
five-nines and surprisingly some
answered that in reality there was ‘no
such thing.’ (We suspect that
these folks skipped their Statistical
Analysis classes, or perhaps meant
to say that measuring five-nines
is not sufficient to achieve Carrier
Grade performance…) Wikipedia
has a useful entry for “The
myth of the nines” which supports
the latter:
So we set out to track down just
what carrier-grade is and
where it comes from.
For the thirty years we have been
in this industry, five-nines was
the number one rule-of-thumb: the
most basic underlying tenant of hardware
construction, network design, and
the expressed goal of every network
system. It is the very heart of the
operational war stories on which
we were raised. For example, there
is the case of telecom power requirements – the
very foundation of the ‘set
apart’ and ‘we are different’ basis
of Point of Presence (POP – the
local building where telecom switching
equipment was placed) construction.
Power loss was once identified as
the single greatest threat to telecom
network reliability as alternating
circuit equipment was prone to costly
service variations (telecom could
not rely on cycle timing from a power
utility) and unacceptable equipment
failure rates. So DC rail-based power
was mandated for telecom POP construction
and every carrier-grade network element
had to run off that DC power. For
certain, it made power failovers
remarkably straightforward with all
those batteries. Even computing servers
(where they were lightly scattered
across voice POPs) had to have DC
powered versions. Never mind that
high voltage DC rail power can fry
your personnel six ways from Sunday,
once all POPs were powered by DC,
the domestic
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Is “five-nines” or 99.999%
up or 0.99999 available a
hard, fast requirement of telecommunications
or is it the telecommunications equivalent
of an Urban Myth?
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network “failures attributable
to power outages over the last n years
(N being variable in the stories)
could be counted on one hand.” And
this is true, DC power in POPs is remarkably
reliable, probably six-nines reliable
considered, in full, as a redundant
system. But the story of DC power and
five-nines availability is an oral
tradition handed down from those who
taught us and to those we taught.
NOC Targets
Also it is very real that the yardstick
standard for availability of networks
to be delivered by the Network Operations
Center was 99.999%. This was not a
soft target. Performance reviews of
NOC management were frequently based
on hitting that target. Availability
was the number one KPI for networks
and NOCs were tasked with restoring
any outages so that network availability
would be maintained. But early on in
the then-named Network Management Forum
(now the TeleManagement Forum), participants
of the newly formed SLA group (though
it was not called SLA back in 1994)
realized that each of them had different
ways of computing that figure for availability.
It took years to hammer this out to
an agreement on what to count and how
to calculate it. (For more details,
see the current SLA Handbook Solution
Suite Version 2.0) But in reality,
every standards group, and even every
industry, seems to have a different
way of defining availability. What-to-count
as an outage becomes “key”.
Wedge Greene has a personal experience
with the traps lying in wait for those
trying to measure and report on availability.
Back in the early nineties when he
was a signaling and routing specialist,
but because he had software and systems
experience, he was yanked out of his
comfortable theoretical work and tasked
with developing a system to
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