Before we get into this, we need to talk about what the network as a sensor was before it was new. Conceptually, instead of having to install a bunch of sensors to generate telemetry, the network itself (routers, switches, wireless devices, etc.) would deliver the necessary and sufficient telemetry to describe the changes occurring on the network to a collector and then Stealthwatch would make sense of it.
The nice thing about the network as a sensor is that the network itself is the most pervasive.
