## 2 Modern Physical Layer
### Commercial and Research Modems  
In contrast to RF wireless sensor networks, only a few companies are engaged in developing modems for underwater sensor networks. The most reliable modems are shown in Table 1. The prices of the listed modems are above $1,000 [2], which makes the deployment of dense sensor networks similar to terrestrial networks very expensive. These modems have been included in this study, because they are the result of modern ongoing research, with several publications in relevant international conferences [34, 49, 50]. Moreover, these companies have collaborated with underwater researchers in recent marine projects, such as: European FP5, FP6 and FP7 in the case of Aquatec [51]; ONRU.S. projects in the case of WHOI [52]; and private contractors in the case of Teledyne Bentnos [7]. \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Modem**} & \multicolumn{2}{c}{**Frequency (kHz)**} & \multicolumn{2}{c}{**Bit Rate (bps)**} & \multicolumn{2}{c}{**Distance (km)**} & \multicolumn{2}{c}{**Power**} \\ \hline
**Company** & **Model** & **Initial Version** & **Center** & **Bandwidth** & **Bit Rate (bps)** & **Distance (km)** & **TX(W)** & **RX(W)** & **Sleep (mW)** & **AT-WU (mW)** \\ \hline \multirow{2}{*}{**Aquatec** [51]} & AQUAModem 500 & 1998 & 29 & 4 & 100 & - & - & - & - & - \\ \cline{2-11}  & AQUAModem 1000 & 2009 & 9.75 & 4.5 & 2,000 & 5 & 20 & 0.6 & 1 & 5 \\ \hline \multirow{2}{*}{**DSPComm**} & AquaComm Marlin & 2003 & 23 & 14 & 480 & 1 & 1.8 & 0.252 & 1.8 & 25.2 \\ \cline{2-11}  & AquaComm Mako & 2003 & 23 & 14 & 240 & 1 & 1.8 & 0.252 & 1.8 & 25.2 \\ \cline{2-11}  & AquaComm Orca & 2003 & 23 & 14 & 100 & 3 & 1.8 & 0.252 & 1.8 & 25.2 \\ \hline \multirow{2}{*}{**TriTech** [54]} & MicronModem & 2008 & 22 & 4 & 40 & 0.5 & 7.92 & 0.72 & & - \\ \cline{2-11}  & MicroModem (FSK) & 2005 & 25 & 4 & 80 & 2 & 100 & 0.23 & 0.2 & - \\ \cline{2-11}  & MicroModem (PSK) & 2005 & 25 & 5 & 5,388 & 2 & 100 & 2.23 & 0.2 & - \\ \hline \multirow{2}{*}{**WHOI** [52]} & ATM9X (PSK) & 2011 & 11.5/18.5/24.5 & 5 & 2,400 & 6 & 20 & 0.768 & 16.8 & - \\ \cline{2-11}  & ATM9X (MFSK) & 2011 & 11.5/18.5/24.5 & 5 & 15,360 & 6 & 20 & 0.768 & 16.8 & - \\ \cline{2-11}  & ATM9X (MFSK) & 2011 & 11.5/18.5/24.5 & 5 & 15,360 & 6 & 20 & 0.768 & 16.8 & - \\ \cline{2-11}  & ATM9X & 2002 & 18.5 & 5 & 15,360 & 0.7 & 84 & 0.7 & - & - \\ \hline \multirow{2}{*}{**EvoLogics** [6]} & S2 CR 48/78 & 2005 & 63 & 30 & 31,200 & 1 & 18 & 1.1 & 2.5 & 5/285 \\ \cline{2-11}  & S2 CR 40/80 & 2005 & 51 & 26 & 27,700 & 1 & 40 & 1.1 & 2.5 & 5/285 \\ \cline{2-11}  & S2 CR 18/34 & 2005 & 26 & 16 & 13,900 & 3.5 & 35 & 1.3 & 2.5 & 5/285 \\ \cline{2-11}  & S2 CR 12/24 & 2005 & 18.5 & 11 & 9,200 & 6 & 15 & 1.1 & 2.5 & 5/285 \\ \cline{2-11}  & S2 CR 7/17 & 2005 & 12 & 10 & 6,900 & 8 & 40 & 1.1 & 2.5 & 5/285 \\ \hline \multirow{2}{*}{**LinkQuest** [56]} & UWM1000 & 2000 & 35,695 & 17.85 & 17,800 & 0.35 & 1 & 0.75 & 8 & 8 \\ \cline{2-11}  & UWM2000 & 2000 & 35,695 & 17.85 & 17,800 & 1.5 & 2 & 0.8 & 8 & 8 \\ \cline{2-11}  & UWM2000H & 2000 & 35,695 & 17.85 & 17,800 & 1.5 & 2 & 0.8 & 8 & 8 \\ \cline{2-11}  & UWM20020 & 2000 & 71.4 & 35.7 & 35,700 & 1 & 6 & 1 & 12 & 8 \\ \cline{2-11}  & UWM3000 & 2000 & 10 & 5 & 5,000 & 3 & 12 & 0.8 & 8 & 8 \\ \cline{2-11}  & UWM3000H & 2000 & 10 & 5 & 5,000 & 3 & 12 & 0.8 & 8 & 8 \\ \cline{2-11}  & UWM4000 & 2000 & 17 & 8.5 & 8,500 & 4 & 7 & 0.8 & 8 & 8 \\ \cline{2-11}  & UWM10000 & 2000 & 10 & 5 & 5,000 & 10 & 40 & 0.8 & 9 & 8 \\ \hline \multirow{2}{*}{**Desert Star** **Systems** [56]} & SAM-1 & 2008 & 37.5 & 9 & 154 & 1000 & 32 & 0.168 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Analysis of commercial modems. AT-WU, acoustic-triggered wake-up; FSK, frequency-shift keying; PSK, phase-shift keying.