mechanic wrote:
On Fri, 18 Sep 2020 08:35:15 -0700, Ken Blake wrote:

On 9/18/2020 3:15 AM, mechanic wrote:
On Thu, 17 Sep 2020 09:51:02 -0500, Rene Lamontagne wrote:

Check, power failures do not affect pots service, they run on their own
independent power systems.

One of their ways to ensure 99.99% availability. Even after the
zombies attack you can still phone mom.

There's no way to ensure 100% availability, and not even 99.99%. It's
always possible for the phone line to be down.

As a matter of fact, in my experience, my cell phone has more
availability than my old POTS line ever had

In the UK the old Public Switched Telephone Network had service
agreements with that kind of availability baked in. That means when
you pick up the phone you get dial tone 99.99% of the time. Similar
requirements carry over to SLAs in the modern digital era. These
requirement weren't just to enable chat with your mom, but to allow
utilities to carry realtime info and instructions over the PSTN, now
over the Internet.

Ken probably objects to your use of "ensure", which implies a
guarantee. However, as Ken said, no-one can *ensure* availability, just
*strive* for that availability. And - as you imply - SLAs describe the
target for availability and what happens - often a penalty of some sort
- when that target is not achieved.

FYI, I was involved in 'Five Nines' - i.e. 99.999% [1] - 'HA' (High
Availability) computer systems.

[1] About 5 *minutes* unscheduled downtime per *year*.