On Tuesday, September 2, 2003, at 11:10 AM, Harald Tveit Alvestrand
wrote:
[note - the 2 sets of expectations for VoIP rollouts is a very true
obsevation. however....]
paradox? what paradox?
the fact that VoIP has two largely orthogonal qualities at the same
time is not a cause for calling it paradoxical; only if they have to
have two incompatible values of the same metric should the "P" word be
applicable.
the metric is availability.
--On 31. august 2003 02:04 -0400 S Woodside <sbwoodside(_at_)yahoo(_dot_)com>
wrote:
internet
The public internet. Availability on the public internet is /best
effort/ which means that other factors (such as price and
bandwidth)
can be more important than availability.
"best effort" means that you get no better service than what you pay
for.
Why shouldn't that achieve five nines?
Because it's local-network reliability only. The remote party and
everyone in between would also need to pay for the higher availability
as well to ensure edge-to-edge availability at five nines. My
assumption is that is a non-goal of the "best effort" network.
telephony
The public and private branch exchanges of the telephone network.
Availability in telephony is counted as /five nines/. So,
99.999% of
the time when you pick up the phone you get a dialtone. That's
all
but five minutes per year.
and one has to carefully tune the measures of "availability" to
achieve that figure.
for instance (personal experience from Norway, mostly):
- pabx availability is dependent on the janitor not tripping over the
power cord, and not measured
- a cell phone that is out of range is not counted (of course!)
- a subscriber line that is down is not counted until the time the
subscriber reports the error.
- "acts of God" like fires in telephone exchanges (and even some
"backhoe fade" cable faults) are not counted.
- if you try to call Liberia, all bets are off.
in practice, five nines is a shared illusion between the operator and
the regulator.
Perhaps, perhaps not. I live in Ontario Canada and in the recent
blackout, my phone kept working. It was as one might say an "object
lesson" in the difference between best-effort and five nines.
simon
Conclusion:
statistics always lie.
what exists, exists.
paradox is in the mind of the beholder.
--
simonwoodside.com -- openict.net -- 99% Devil, 1% Angel