Abstract: Data is exchanged by periodically broadcasting at predetermined first short intervals from each of a plurality of data sources on a radio channel a communication-initiating radio signal indicating that the respective source has data ready for transfer. The radio channel is monitored by that can transmit to one of the sources a signal triggering data exchange, whereby subsequently the respective data is transmitted from the one source to the receiver. Thereafter periodic broadcast of the communication-initiating radio signal from the one source is suspended for a predetermined long interval substantially greater than the short interval.