Communicating over a network of communication stations includes monitoring for reception of an electromagnetic wave that has a power above a threshold and a frequency in a predetermined frequency interval, and selecting carrier frequencies for modulating information onto signals transmitted over the network. The carrier frequencies are selected based at least in part on whether an electromagnetic wave having a power above the threshold and a frequency in the frequency interval has been received in a predetermined time interval, and based at least in part on a demand by one or more stations in the network for capacity for communicating over the network.