A sensor network node is (usually) defined as a small piece of hardware including a radio, a battery, and a few simple sensors and actuators. The most important aspect to optimize against is battery life; in many applications nodes will just sleep most of the time. Academically, a sensor network comprises a distributed loosely connected network of uniform nodes. Non-academically, admittedly, sensor networks will often be small (less than a hundred nodes), consist of a sink, usually tethered to a network and a power supply, and anchor nodes may be developed in a network, for instance, for positioning/triangualization reasons. Often, just the addition of a few specialized nodes greatly simplifies design and tree-spanning protocols with a root node just work well.
However, for robust development of protocols for self-organizing networks, often the academic view is taken. So, a small, exercise:
Assume a thousand nodes, make them all blink at 1Hz at roughly the same time while preserving battery life.
