At present, the realities of acoustic communications and undersea netwotks differ substantially from the espectations arising from academic theory and inverstigation. The issue is not that the fundamentals of communications theory do not apply.but rather the fact that the difficulties of teh acoustic channel do nor necessarily fit the assumptions underlying conventional applications of that theory.In this paper we describe many of the practical problems that have been addressed over the past ten years of modem development.Some of the issues addressed have an RF analog.but the severity of the channel and the combination of channel constraints and modem-platform operations makes acoustic communications a very different problem.In particular,we address these issues via their impatce on physically small,battery powered,DSP-based,ommidirectional modems.Next we decribe progress toward using commercial acoustic modems as the basis for underwater networks.We discuss design choices at the physical,link,and network layers that are consistent with the compound constraints of the transmission channel and modem.
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