A communication device (300) operates in a training mode to develop parameters based on estimates related to at least two different instances of channel conditions, such as indicated by bit error rates for channel transmissions (610, 620, 710, 720). The parameters are used to define a function that characterizes an optimized throughput or energy consumption per bit through varying channel conditions. The function is preferably of the form a0+a1*Log(1+L.sub.p), where a0 and a1 are fitted parameters, and L.sub.p the frame size. The device responds to changes in channel conditions by transmitting messages on the communication channel using a frame size adapted for channel conditions based on the particular function and the developed parameters (640 680, 740 780).