Techniques involving anonymous content promotion are described. The child node includes: a signal transmission unit transmitting a parent node search signal; a signal reception unit receiving a response signal; and a parent node selection unit that selects a parent node for routine communication on the basis of a wait time from the transmission of a parent node search signal to the reception of a response signal. A handpiece winding disposed along the bore interacts with the instrument transmission line winding to form an inductive coupling with the instrument transmission line winding.