"Why am I still down? I have an SLA! I'm losing so much money!"
When building a network, you always want to be sure that it is available as much as possible to the users and systems on it. When you buy a commercial network service, internet access or private WAN, it is typically backed up by a Service Level Agreement (SLA), which specifies the "uptime" of the service. A service with a 99.9% uptime guarantee means that it can be down up to 0.1% of the time without penalty to the provider. But that doesn't mean the service will actually be available 99.9% of the time. It just means the provider has to pay you some measly credits for any extra downtime beyond their Mean Time To Repair. An hour of extra downtime turns into a credit of 1/720th of your monthly bill. It's insulting. Plus, your connection was down during that time, and you surely lost more money than the credit will compensate.
"How can I prevent this?"
One important way to mitigate this issue is by adding additional connections and configuring them for automatic failover. This way, at least one of the connections should be operational at nearly all times. Generally, the odds of two different connections going down at the same time are pretty unlikely, particularly when they are from different providers, using different paths on the street, different mediums (fibre/DSL/DOCSIS/LTE), and even different entrances to the building. The more diverse the connections are, the less likely it is that something will happen to them simultaneously.
"How good can I make it?"
I wrote this little tool to calculate what the uptime would be when you add additional connections. Adding more connections and/or a higher uptime percentage will result in the decreased likelihood of all the connections being down simultaneously.
Enter the uptime percentages of your individual connections and click Submit.