I've been away from the NOC for a couple of years now, but one way that
made sense for reporting a general availability number is to consider the
backbone as one component and each leaf node as an identical independent
component (leased line, router and CSU/DSU).
Then you can define the end to end availability as the concatentation of
two independent leaf nodes and the backbone.
Highly redundant backbones always have a high availability (>99%). Leased
lines are abominable (low 90s or less -- don't ask me about circuits to
Italy) and two routers and two CSU/DSUs lead to an overall end to end of
95% or less.
In my experience with leased lines, they always dominated this equation and
overall availability is strictly limited by actual leased line
So keep that CPE in the equation and figure out how to get highly available
bit pipes. I think actual networks like frame relay have a chance of being
much more available since the network actually understands frames. Leased
lines are simply electrical signalling conduits to the phone company -- not
I'm skeptical of any end-to-end availability figures over 97%. I don't
think they reflect the reality of leased line circuits today, or else they
don't include the leaf node circuits and only report backbone availability.
For a highly redundant backbone, almost any definition of availability
should result in a number like 99.mumble%. Remember 99.9% availability
means less than 9 hours outage per year. Routing hiccups take that much.
One or two leased lines outages is all you get for 9 hours. The real world
is a lot less available than that.
But since half the web servers I try to talk to refuse me half the time,
I'm not sure that network availability per se (HWB's complaints duly
acknowledged) is the tallest pole in the tent.