How does Node Down Work?

I'm not certian I understand how NPM determines if a node is down.

Node is pinged in a regular ping cycle(120 seconds). If the node does not respond, NPM sets the node status to warning and starts a fast ping cycle for that node. NPM fast pings the device for the time set in the Setting interface as Node Warning Level. This is 120 seconds by default.

That means that the node would be down for 240 seconds before we get a node down alert.Am I correct? How many ping packets are sent during each ping sweep?

Any clarification would be appreciated.

-John