At present, the realities of acoustic communications and undersea networks differ substantially from the expectations arising from academic theory and investigation. The issue is not that the fundamentals of communications theory do not apply, but rather the fact that the difficulties of the acoustic channel do not necessarily fit the assumptions underlying conventional applications ofthat theory. In this paper we describe many of the practical problems that have been addressed over the past thirteen years of modem development at Teledyne Benthos and its predecessor companies. 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 impact on physically small, battery powered, DSP-based, omni-directional modems. We describe a statistics-based model of the channel impulse response which reveals many of impediments to effective communications in the underwater environment. 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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