A method is disclosed for determining whether to transmit from a first unit to a second unit, where the transmission is one of a first type and a second type. In the method, a signal from the second unit is received at the first unit, and the strength of the signal as received at the first unit is measured. A determination is made of whether the transmission is of the first type or the second type, and a first threshold is selected if the transmission is of the first type, or else a second threshold is selected if the transmission is of the second type. The measured signal strength is then compared to the selected threshold, and the transmission proceeds only if the measured signal strength exceeds the selected threshold.