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For any Internet of things(IOT) system, internet has to be present. The sensors should send their information to the internet. This data will be processed in real time and a command may be isssued via Internet to the sensor based IoT system to perform a task. For example, an image processing sensor whenever sees a child at your door on thanksgiving, transmits the data to your phone and you may push a button to dispense a chocolate. If treats are over you may push a button that makes a speaker give a message. All this happens on Internet. WSNs on the contrary may or may not require internet to achieve their tasks.
A WSN is basically a sensor interfaced with a microcontroller based platform mainly for data collection. The on site memory on microcontroller board can be programmed to collect data in afternoon 1 - 2pm. They may route that data to a central node. For Example : In a fireforest prediction system, several sensor based platforms are laid throughout jungles in Sweden which collect data and route it to a central Node.