Jamming can happen at all layers. The most basic of jamming is to drown out all other communications with high-power noise. But in the case of illegal/covert a

Jamming can happen at all layers. The most basic of jamming is to drown out all other communications with high-power noise. But in the case of illegal/covert activities this isn't ideal as your transmitter will be pretty easy to find. So then you have to pretend to look like a legitimate signal. Which is where encryption and modulation can help. But then application level encryption isn't going to help against basic jamming.