Our user experience of applications, devices and networks is far from the “five nines” standards (99.999 percent availability) telcos used to tout.
As a practical matter, today’s heterogenous, edge-powered, internet transport fabric, IP-based application environment absolutely means user experience cannot approach 99.999-percent availability for any applications.
That might not apply to core systems in banking, financial trading or some security-critical use cases, but only to the core systems, not the end user access of those systems.
The problem is that no matter what any single participant in the value chain might claim for its own availability, and even if that availability is between 99 percent and 99.99 percent, the entire end-to-end value chain depends on the sum total of availability across the whole value chain, and that math is challenging.
Consider an example where contributor availabilities are:
Device: 99%
Home broadband access: 99.5%
Internet backbone: 99.99%
App server: 99.9%
Local power: 99.5%
The end-to-end availability requires multiplying all those discrete availabilities. So the formula is
0.99 × 0.995 × 0.9999 × 0.999 × 0.995 ≈ 97.4 percent. That means 229 hours of downtime per year, not the 5.26 minutes per year allowed by "five nines” standard.
The only reason end users seem unaware of the change is that much of the downtime happens when they are not actively using their connections (devices not present; devices in “do not disturb” mode; user is sleeping; apps not in immediate use).