The UNET-2 modem – An extensible tool forunderwater networking research

Mandar Chitre, Iulian Topor and Teong-Beng KoayAcoustic Research Laboratory, Tropical Marine Science Institute,

National University of Singapore, Singapore 119227.{mandar, iulian, koay}@arl.nus.edu.sg

Abstract—Several decades of research in underwater commu-nication and networking has resulted in novel and innovativesolutions to combat challenges such as long delay spread,rapid channel variation, significant Doppler, high levels of non-Gaussian noise, limited bandwidth and long propagation delays.Many of the physical layer solutions can be tested by transmittingcarefully designed signals, recording them after passing throughthe underwater channel, and then processing them offline usingappropriate algorithms. However some solutions requiring onlinefeedback to the transmitter cannot be tested without real-timeprocessing capability in the field. Protocols and algorithms forunderwater networking also require real-time communicationcapability for experimental testing. Although many modems arecommercially available, they provide limited flexibility in physicallayer signaling and sensing. They also provide limited controlover the exact timing of transmission and reception, whichcan be critical for efficient implementation of some networkingprotocols with strict time constraints. To aid in our physicaland higher layer research, we developed the UNET-2 software-defined modem with flexibility and extensibility as primary designobjectives. We present the hardware and software architectureof the modem, focusing on the flexibility and adaptability that itprovides researchers with. We describe the network stack that themodem uses, and show how it can also be used as a powerful toolfor underwater network simulation. We illustrate the flexibilityprovided by the modem through a number of practical examplesand experiments.

I. INTRODUCTION

With an increasing interest in marine monitoring usingrobotic and autonomous sensor systems, the ability to com-municate effectively underwater is rapidly becoming veryimportant. Although underwater acoustic communications hasbeen explored for several decades, the achievable throughputin an underwater network today is very modest as comparedto terrestrial wireless networks. This is largely attributed tochallenges such as long delay spread, rapid channel variation,significant Doppler, high levels of non-Gaussian noise, limitedbandwidth and long propagation delays. In the past years,researchers have developed novel and innovative solutions tocombat many of these challenges [1]. In fact, some of thechallenges such as propagation delay offer new opportuni-ties that may be exploited in scheduling transmissions [2].Researchers’ ability to test some of these new ideas andinnovations is often limited by the modems available to themfor field testing. Although several off-the-shelf modems arecommercially available, they provide limited flexibility in thephysical layer signaling and sensing. Most of the modemsalso provide very limited control over the exact timing of

transmission and reception, which can be critical for effi-cient implementation of networking protocols with strict timeconstraints [2], [3], [4]. To aid in physical and higher layerresearch at the Acoustic Research Laboratory (ARL), wedeveloped the UNET-2 software-defined modem (shown inFigs. 1 and 2) with flexibility and extensibility as primarydesign considerations. This modem is now well-tested andextensively being used for communication between sensornodes, autonomous underwater vehicles (AUVs) and surfacestations in several research projects.

In this paper, we present the hardware and software architec-ture of the modem, focusing on the flexibility and adaptabilitythat it provides researchers with. We describe the networkstack that the modem uses, and show how it can be used asa powerful tool for underwater network simulation. We alsopresent some examples and experiments to demonstrate howthe modem’s flexibility can be applied in different channelconditions and application scenarios.

II. MODEM FEATURES

Designed primarily as a research tool, the UNET-2 mo-dem provides a flexible platform for a variety of underwaternetworks. With substantial computing power packed into asmall package, researchers are able to implement and deploycomplex algorithms in the modem. The modem providesoptions for customization and extension at many levels (moreon this in section III-D), allowing network protocols as well asphysical layer algorithms to be implemented and tested easily.

Fig. 1. The UNET-2 modem “dry-end” digital electronics (10×9×5 12

cm).

1

Fig. 2. The UNET-2 modem packaged as a stand-alone surface unit.

The firmware and software in the modem starts up with a con-figuration that provides robust communication performance,but can be reconfigured on the fly to meet application orprotocol demands.

In the next few sections, we outline the flexibility providedby the modem in various aspects of communication andnetworking:

A. Carrier frequency, bandwidth & source level

The transmit and receive carrier frequencies of the modemcan be changed dynamically, up to a maximum carrier fre-quency of 78 kHz. By independently and dynamically control-ling these, we can compensate for mean Doppler at the analogfront-end if desired. The digital filters and baseband samplingrate are dynamically adjusted to provide a bandwidth of upto 2f/3 where f is the carrier frequency. The source level ofeach physical layer packet transmission can be dynamicallycontrolled in steps of 1 dB.

B. Packet detection & logical channels

Each physical layer packet transmitted by the modem isprefixed by a packet detection preamble and an optional guardtime. Up to 3 user-defined preambles may be configured inthe modem. When reception is enabled, the modem usesa bank of sign-correlators [5] to monitor the water for allconfigured preambles and triggers a packet reception whenany of the preambles is detected. The preamble can be usedto dynamically determine the signaling and forward errorcorrection (FEC) schemes to be used for the packet thatfollows. This allows multiple logical channels to be created.For example, we can configure a control channel to use anincoherent signaling scheme with a low-rate FEC for robustcommunication, and a data channel to use coherent modula-tion with a high-rate FEC for high-speed communication. Linktuning algorithms may dynamically adapt the parameters ofthe data channel to provide good performance in time-varyingchannels [6].

C. Collision detection

The preamble detection operates independently of thepacket acquisition and processing. This enables the modemto notify the network stack of a collision if a preamble isdetected while a packet is being acquired. Medium access

control (MAC) protocols which require collision detectioncan therefore be supported on the modem. To avoid falsecollision alarms due to a multiple arrivals arising from thedelay spread of the channel, the preamble detector has a user-settable blanking time immediately after a detection.

D. Modulation & FEC schemes

The modem is preconfigured with two modulation schemes.The incoherent OFDM modulation scheme uses multicarrierdifferential energy detection signaling to provide Doppler-resilient robust performance in doubly-spread channels. Thecoherent OFDM modulation scheme employs multicarrierphase-coherent differential signaling with a cyclic prefix toprovide high data rate communication [7]. Modulation schemeparameters such as number of carriers, prefix length, suffixlength, null carriers, peak-to-average power ratio (PAPR) re-duction, signal constellation size (binary or quadrature phase-shift-keying, i.e., BPSK or QPSK), differential mode (timeor frequency), packet length, etc are dynamically controllableby the user. The modulation schemes may be used with noFEC, a 1/3-rate convolution code, a 1/2-rate Golay code, ora 1/6-rate concatenated code. All codes are implemented withbit-interleaving to ensure uncorrelated errors. The convolutioncode is decoded using a 1-norm Viterbi decoder for robust per-formance in non-Gaussian noise [8]. The user may customizethe modem to add new modulation and/or FEC schemes.

E. Ranging, synchronization & timestamping

The modem maintains a clock with a resolution of 1 µs.If the modem is equipped with an optional oven-controlledcrystal oscillator (OCXO), this clock provides a low-drifttiming reference for one-way travel time (OWTT) ranging andfor network protocols that need accurate time synchronization.

The time of transmission or detection of each packet isnoted and made available to the network stack. Physical layerpackets may be queued for transmission at a predeterminedtime. This is useful for slotted protocols and necessary forprotocols which require the transmission time of a packet to beencoded into the packet. The modem generally automaticallycontrols the transmit/receive mode of the analog front-endfor half-duplex communications. The switching between themodes can take several tens of milliseconds for some front-end designs. Timing sensitive protocols wishing to controlthe transmit/receive mode directly are empowered to do so.This allows extraneous timing information about the protocolor application behavior to be used to improve the protocolperformance.

Using the timing services at the physical layer, a rangingprotocol is implemented in the network stack. The clocks onthe modems are usually not synchronized, although synchro-nization is possible using an external digital input. Initiallytwo-way travel time measurement is used to estimate range andclock offset between a pair of modems. The clock offset is thencommunicated to the peer modem and stored for future use.Once this is done, the modems are considered to be synchro-nized. Subsequent range measurements between synchronized

Dry-end

Wet-end

FPGA DSP SBCOCXO

PB↔BB

PacketDetectors

Time-stamping

Modulation,Signaling

FEC

JADE

UWNetwork

StackData/

CommandMessages

ADC/

DAC

Ethernet

TX/RX,Gain

AnalogSignals

Command Handler AT Commands

and Data

1

23

4

Fig. 3. The UNET-2 modem architecture. Customization/extension points are shows as numbered ovals as per section III-D.

modems can be achieved through OWTT measurements ontimestamped packets. The ranging protocol is able to providea range resolution of 0.1 m.

F. Diagnostic informationBeing a research modem, it is important to be able to

collect valuable diagnostic information on the environment andmodem performance. A logging service in the modem allowsterse or verbose logs to be collected on modem operations.These logs may be stored in volatile memory or in Flashmemory on the modem and downloaded over Ethernet. Themodem also provides online access to a plethora of diag-nostic information including noise statistics, packet detectoroutput, delay spread estimates, bit error rate (BER) estimates,baseband packet recordings and passband acoustic recordings.These information may potentially be used by the networkstack for selection and configuration of appropriate protocols.

III. ARCHITECTURAL OVERVIEW

A. Hardware architectureThe UNET-2 modem comprises a dry-end (shown in Fig. 1)

that interfaces with a wet-end (analog front-end). The dry-endcontains a Toradex Collibri PXA270 single board computer(SBC), Analog Devices TigerSHARC TS201 digital signalprocessor (DSP), Altera Stratix-III field-programmable gatearray (FPGA), 24-bit Σ-∆ analog-to-digital converter (ADC),16-bit digital-to-analog converter (DAC) and an optional10 MHz OCXO (±0.05 ppm drift). The wet-end containsthe preamplifier, power amplifier, analog matching circuitryand transducer. The dry-end can easily be interfaced withany of several off-the-shelf or custom wet-ends, depending onthe desired frequency band and transmit power. The interfaceuses differential lines for analog input/output noise resilience.More than one wet-end may be used to provide multiple-input-multiple-output (MIMO) capability (using wet-ends withthe same frequency band), or multi-band capability [9], [10](using wet-ends with different frequency bands). In this paper,our focus is on the dry-end electronics, signal processingand networking software in the modem. An off-the-shelf 18–36 kHz wet-end capable of transmitting at a maximum sourcelevel of about 185 dB RMS re 1µPa @ 1m was used in allexperiments presented in section V.

B. Functional blocks

An overview of the functional architecture of the modem isshown in Fig. 3. The FPGA is primarily responsible for packetdetection, passband-baseband conversion and time-stamping.The DSP implements the physical layer signal processing andFEC coding. All higher layer functionality is implemented onthe SBC running TinyCore Linux and the Java agent develop-ment (JADE) framework [11]. The optional OCXO providesa low-drift clock for accurate timing for OWTT ranging andnetwork protocols that require accurate time synchronization.The 24-bit ADC provides 138 dB of dynamic range makingautomatic gain control (AGC) at the wet-end unnecessary inmost circumstances. The modem can use a digital gain controlif the wet-end supports it. The 18–36 kHz wet-end provides adigital gain control with 48 dB of dynamic range, but a fixedgain has been sufficient in all our experiments.

C. Underwater network stack

A network stack based on the underwater networking ar-chitecture (UNA) [12] was used in the UNET-1 modem,a predecessor of the UNET-2 modem. Although the stackprovided a powerful platform that enabled network simulationcode to be field tested using the UNET-1 modem withoutany need for porting [13], we felt that the UNA has a fewshortcomings. Adding new protocol implementations requiredcross-compilation of the code. Due to lack of support fordynamic loading, only one implementation of a layer couldbe deployed in the modem at a time. The layered architectureput some unnecessary restrictions that made implementationof protocols requiring cross-layer information awkward. Toaddress these shortcomings, we extended the layered architec-ture in UNA into an agent based architecture for the UNET-2 modem. The agents are similar to layers in conventionalcommunication stacks, but provide richer interaction and infor-mation flow to achieve cross-layer (cross-agent) optimization.

We adopted a subset of the foundation for intelligent phys-ical agents (FIPA) specifications [14] as a framework for theUNET-2 modem network stack, since it provides the necessaryinfrastructure and is well supported by a large community.JADE [11] is an implementation of the FIPA specifications

UW Network Stack

Physical

Link

Ranging MAC

DSP / Simulator

Repeater

Fig. 4. Underwater network stack in the UNET-2 modem. The Physical agentis a proxy for the physical layer provided by the modem or simulator. TheLink agent provides addressing and error checking. The MAC agent providesmedium access control. The Ranging agent provides time synchronization andranging capability. The Repeater agent can be enabled to configure a modemas a repeater node to extend communication range.

that runs on our SBC and manages the all the agents in thenetwork stack. Using this framework, we now have the abilityto dynamically load agents into the modem and interact withthem in various ways outlined in section III-D. In Fig. 4, wesummarize the default agents in the UNET-2 modem networkstack. As the network stack matures, we expect the agents tobecome more sophisticated and the number of agents to grow.

Although the network stack was developed in conjunctionwith the UNET-2 modem, it is quite independent of the mo-dem. The Physical agent in the modem provides an interface tothe DSP and FPGA which implement the physical layer in theUNET-2 modem. To operate with a new modem, a one-timeeffort is required to develop the Physical agent for that modem.All other agents in the network stack are able to operate withany modem with minimal or no porting effort. Thus we havedeveloped a general underwater network stack that may beused with any modem.

D. Customization and extensionFlexibility and extensibility was one of the primary objec-

tives in developing the UNET-2 modem. Access to the modemis provided at four levels (depicted graphically in Fig. 3)depending on the required degree of flexibility:

1) For most applications, it is sufficient to access themodem over a TCP/IP socket interface using text com-mands loosely based on the Hayes AT command struc-ture [15]. The command set has been extended to pro-vide access to sophisticated functionality specific to theUNET-2 modem. The main advantage of this interfaceis that users and applications can directly interact withthe modem in a human-readable form using a simpletelnet connection. To customize the modem, a start upscript with AT commands can be stored on the SBC.

2) A message-based application programming interface(API) is provided for applications that require more so-phisticated control over the behavior of the modem. Themessages comply to the agent communication language(ACL) defined in the FIPA specifications [14]. We havedeveloped a simple Java API that allows Java applica-tions to communicate with the modem using ACL. Thirdparty ACL/FIPA implementations are available for otherprogramming languages.

3) New behaviors (such as networking protocols) can beadded to the modem as FIPA-compliant agents. Theeasiest way to implement an agent is to develop in Javausing the JADE framework. The resulting compiled jarfile is simply copied into the SBC over Ethernet. Asimple Java extension API is also available to extend theAT command set to support the new behaviors added tothe modem.

4) New modulation/signaling schemes and FEC schemescan be implemented in C on the DSP. A plug-in API isavailable to add such schemes to the modem. The imple-mentation of such schemes requires cross-compilation ofthe C code for the TS201 DSP. The resulting binary issimply copied to the SBC over Ethernet.

IV. REAL-TIME SIMULATION

It is often useful to simulate network protocols beforetesting them in the field. A unified simulation frameworkcan be of great value in this effort [13], especially if itsimulates the behavior of the modem accurately. Since onlythe Physical agent in the underwater network stack describedin section III-C depends on the UNET-2 modem, we havedeveloped a simulation version of the Physical agent thatemulates the behavior of the UNET-2 physical layer (DSP,FPGA and wet-end) and the underwater channel. With theJADE framework that runs on any Java-enabled computingplatform and our simulated Physical agent, we can deploymany virtual UNET-2 modems in a simulated environment ona single computer (or on different computers in a network).The virtual modems communicate with each other usingTCP/IP multicast packets but provide appropriate propagation

Fig. 5. STARFISH AUV with the UNET-2 modem.

UW Network Stack

Physical

Link

Ranging MAC

EComms

STARFISH Positioning

System

STARFISH Command &

Control System

Range

Commands& Status

Fig. 6. STARFISH EComms agent deployed in the UNET-2 modemcoordinates acoustic communication and ranging activities of the command& control system and positioning system.

delays based on the locations of the virtual modems. Packetloss resulting from collisions and BER is also accuratelysimulated. Once the simulations are successful, the agents aresimply copied to the actual modem and tested in the field. Noporting effort is required between simulation and field testing.We use this simulation tool extensively in our own underwaternetworking research.

V. APPLICATIONS AND EXPERIMENTS

Next we present some application examples and experi-ments where the flexibility provided by the UNET-2 modemwas exercised.

A. Dynamic frequency band adjustmentDuring an experiment in Singapore waters, we had two

network nodes deployed at a depth of about 4 m. One ofthe node was moored, while the other slowly moved away.At short range the communication was very good, but as therange increased to more than 2 km, the nodes were unableto communicate. From the diagnostic information providedby the modem, the cause of the loss in communication wasidentified as very low signal-to-noise ratio (SNR) in the upperhalf of our frequency band (27–36 kHz). Although packetscould be detected at the receiver, error-free communicationwas not possible. With a simple change in a few physical layerparameters (carrier frequency and the number of null carriers),we were able to move our frequency band to 18–30 kHz andcommunication was restored.

As shown in Fig. 5, the UNET-2 modem is now usedby STARFISH autonomous underwater vehicles (AUVs) [16].After integrating the modem on the AUV, the communicationperformance was found to be limited by the self-noise of theAUV. The primary contribution to the noise was from thethruster, which generated a strong tonal between 19–20 kHzdepending on the thrust setting and some spectrally smoothnoise with a peak around 20 kHz. Again, with a simple change

in carrier frequency and the number of null carriers, we wereable to move our frequency band to 24–36 kHz to obtain goodcommunication performance.

B. Link tuning

The default control channel settings in the UNET-2 mo-dem offer Doppler-resilient robust communication at a datarate of about 400 bps. Although this is sufficient for manyapplications (e.g. sending commands to AUV, receiving statusinformation from AUV, ranging, etc), some applications (e.g.file/image transfer) require higher data rates. In an experimentin Singapore waters, we deployed two network nodes at arange of 200–400 m. The task was to manually tune themodulation and FEC parameters to obtain a high-rate commu-nication link. By using the phase-coherent modulation scheme,we were able to tune the link to yield data rates of up to 9 kbpsfor reliable communication within a few minutes. This high-rate link was then be used to transmit a large amount of datarapidly.

The task of an automated link tuning algorithm is totune the parameters of a link automatically while data isbeing communicated [6]. We have implemented a Link Tunernetwork stack agent in the UNET-2 modem that is able touse machine learning techniques to observe communicationperformance and adapt the data channel parameters in real-time. Preliminary tank testing shows that the parametersrapidly converge within a couple of minutes to yield error-free data rates of about 5 kbps. The agent is currently beingfield tested.

C. Cooperative positioning

When deployed in teams, the STARFISH AUVs are able touse range measurements between vehicles for position estima-tion and navigation [17], [18]. With the UNET-2 modems, theSTARFISH AUVs are now able to obtain range informationat regular intervals using OWTT measurements. As shownin Fig. 6, a STARFISH EComms (external communications)agent deployed on the UNET-2 modem coordinates the trans-mission of ranging beacons and status information, and routesrange information and incoming commands to the STARFISHcommand and control system. Ranging information fromAUVs with good navigation sensors enables AUVs with poornavigation sensors to accurately navigate and accomplish theirmissions.

Sender!

Receiver!

Sender!

Receiver!

One or more"data packets!

One or more"data packets!

ACK for packet(s)"in previous block!

ACK for packet(s)"in previous block!

Gap (≥ ACK duration)"for receiver to return ACK!

(a) Conventional ARQ!

(b) Juggling-like ARQ!Fig. 7. An illustration of juggling-like stop-and-wait scheme for half-duplexmodems in presence of long propagation delay (figure adapted from [20]).

Fig. 8. A UNET-PANDA node being prepared for deployment as a networknode in a multihop networking experiment.

The UNET-2 modem was recently successfully integratedand tested with autonomous surface crafts (ASCs) from theSingapore-MIT alliance for research and technology (SMART)to demonstrate cooperative positioning between ASCs andAUVs.

D. Juggling-like stop-and-wait protocolIn [19], a new transmission scheme, known as the juggling-

like stop-and-wait scheme, was proposed to improve thechannel utilization of point-to-point data transfer between half-duplex acoustic modems. This scheme is studied in detailin [20] and depicted in Fig. 7. As seen in the figure, the timingrequirements for transmission and reception are stringent; anyshift in timing (too early or too late) is unacceptable dueto the half-duplex nature of the modems [21]. This protocolhas been implemented and tested on the UNET-2 modem.Accurate control of packet transmission timing was criticalfor the successful implementation of this protocol. Since asmall tank does not provide the necessary propagation delaysfor such a protocol to work, the simulation tools were vitallyimportant during the implementation. Accurate timing modelsin the simulator allowed us to develop and test the protocol insimulation prior to field testing.

E. UNET-PANDA network nodesThe UNET-2 modem has been integrated into a UNET-

PANDA network node shown in Fig. 8. The UNET-PANDAis a rapidly deployable and self-recovering bottom-mountedsystem that can be used as a self-contained sensing andnetwork node. It contains a UNET-2 modem, additional SBC,GPS and batteries. Once deployed, the electronics packageof the UNET-PANDA is anchored and floats 1–2 m above theseabed. It can be later recovered via an acoustic command thattriggers the self-recovery unit of the UNET-PANDA to surface.The unit remains attached to the electronics package andanchor via a line. All three components (anchor, electronicspackage, self-recovery unit) of the UNET-PANDA are thenrecovered from a surface vessel. The UNET-PANDA node hasbeen used in several underwater networking experiments inSingapore [22].

VI. CONCLUSIONS

In this paper we described the key features of the UNET-2software-defined modem, particularly focusing on the flexi-bility and adaptability that it provides researchers with. Wedescribed the hardware and software architecture and thenetwork stack that the modem uses. We also illustrated theflexibility provided by the modem through some practicalapplication examples and experiments. We believe that theUNET-2 modem provides a powerful and extensible tool forunderwater network research, allowing a variety of novelprotocols to be easily implemented and tested. The networkstack in the UNET-2 modem is not strongly tied to the modem,and can be easily used with other modems and also as apowerful simulation tool. We have found that the ability ofthe network stack to simulate an underwater network in thelab with the exact implementation of network protocols to betested in the field is invaluable.

ACKNOWLEDGMENTS

We would like to thank several colleagues and collaboratorswho have been instrumental in making the UNET-2 modema success. Alan Low and Ang Yen Khim from ST Electron-ics (Info-Comm Systems) helped develop and test the dry-end electronics for the modem. Manu Ignatius developed asubstantial part of the DSP software. Many other colleaguessuch as Shankar Satish, William Tan, Rohit Bhatnagar andcollaborators such as Sultan Imenov (SMART), Tawfiq Taher(SMART), Marek Doniec (MIT), Haojie Zhuang (I2R) andMa Xiaoping (I2R) provided valuable feedback as users of themodem, and helped test the modems in a variety of applicationscenarios. Tan Soo Pieng and Mohan Panayamadam helpeddevelop the UNET-PANDA nodes which were used in manyof the tests. Profs. Barbastathis and Patrikalakis (MIT) kindlyprovided access to SMART ASCs for testing cooperativepositioning.

REFERENCES

[1] M. Chitre, S. Shahabudeen, and M. Stojanovic, “Underwater acousticcommunications and networking: Recent advances and future chal-lenges,” The Spring 2008 MTS Journal, ”The State of Technology in2008”, vol. 42, no. 1, pp. 103–116, 2008.

[2] M. Chitre, M. Motani, and S. Shahabudeen, “A scheduling algorithm forwireless networks with large propagation delays,” in IEEE OCEANS’10Sydney, Australia, May 2010.

[3] H.-H. Ng, W.-S. Soh, and M. Motani, “BiC-MAC: Bidirectional-concurrent MAC protocol with packet bursting for underwater acousticnetworks,” in OCEANS 2010, sept. 2010, pp. 1 –7.

[4] K. Kredo, P. Djukic, and P. Mohapatra, “STUMP: exploiting positiondiversity in the staggered TDMA underwater MAC protocol,” in INFO-COM 2009, IEEE, 2009, pp. 2961–2965.

[5] M. A. Chitre, J. R. Potter, and S. H. Ong, “Optimal and near-optimalsignal detection in snapping shrimp dominated ambient noise,” OceanicEngineering, IEEE Journal of, vol. 31, no. 2, pp. 497–503, 2006.

[6] S. Shankar, M. Chitre, and M. Jayasuriya, “Data driven algorithms totune physical layer parameters of an underwater communication link,”in OCEANS’10 Sydney, Australia, May 2010.

[7] M. Chitre, S. H. Ong, and J. Potter, “Performance of coded ofdm invery shallow water channels and snapping shrimp noise,” in OCEANS,2005 MTS/IEEE, 2005, pp. 996–1001 Vol. 2.

[8] M. Chitre, J. R. Potter, and S. H. Ong, “Viterbi decoding of convolutionalcodes in symmetric α-stable noise,” Communications, IEEE Transac-tions on, vol. 55, no. 12, 2007.

[9] L. Freitag, M. Grund, J. Partan, K. Ball, S. Singh, and P. Koski, “Multi-band acoustic modem for the communications and navigation aid auv,”in OCEANS, 2005. Proceedings of MTS/IEEE. IEEE, 2005, pp. 1080–1085.

[10] S. Shahabudeen, M. Chitre, and M. Motani, “A multi-channel MACprotocol for AUV networks,” in Oceans 2007-Europe. IEEE, 2007, pp.1–6.

[11] (2012) Java Agent Development Framework (JADE). [Online].Available: http://jade.tilab.com/

[12] M. Chitre, L. Freitag, E. Sozer, S. Shahabudeen, M. Stojanovic, andJ. Potter, “An Architecture for Underwater Networks,” in OCEANS 2006- Asia Pacific, May 2006, pp. 1–5.

[13] S. Shahabudeen, M. A. Chitre, M. Motani, and Y. S. Low, “UnifiedSimulation and Implementation Software Framework for UnderwaterMAC Protocol Development,” in IEEE/MTS Oceans’09 Conference,Biloxi, US, October 2009.

[14] (2012) The Foundation for Intelligent Physical Agents (FIPA). [Online].Available: http://www.fipa.org/

[15] F. Durda IV. (1993) The AT command set reference. [Online]. Avail-able: http://nemesis.lonestar.org/reference/telecom/modems/at/summary-at.html

[16] T. B. Koay, Y. T. Tan, Y. H. Eng, R. Gao, M. Chitre, J. L. Chew,N. Chandhavarkar, R. Khan, T. Taher, and J. Koh, “STARFISH - A SmallTeam of Autonomous Robotics Fish,” in 3rd International Conferenceon Underwater System Technology: Theory and Applications 2010,Cyberjaya, Malaysia, Nov 2010.

[17] R. Gao and M. Chitre, “Cooperative positioning using range-only mea-surements between two AUVs,” in IEEE OCEANS’10 Sydney, Australia,May 2010.

[18] Y. T. Tan and M. Chitre, “Single beacon cooperative path planning usingcross-entropy method,” in IEEE/MTS Oceans’11 Conference, Hawaii,USA, Kona, Hawaii, USA, September 2011.

[19] M. Gao, W.-S. Soh, and M. Tao, “A transmission scheme for continuousARQ protocols over underwater acoustic channels,” in Proceedings ofIEEE ICC, Dresden, Germany, June 2009.

[20] M. Chitre and W.-S. Soh, “Reliable point-to-point underwater acousticdata transfer: To juggle or not to juggle?” IEEE/ACM Transactions onNetworking (TON), 2011 (under review).

[21] R. Bhatnagar, “Implementation and testing of ARQ protocols for un-derwater networks,” National University of Singapore, EE5003 ProjectReport, 2012.

[22] M. Xiaoping, A. Valera, and H. P. Tan, “Final report for robust mul-tihop underwater network to support long-range sensing applications,”Institute for Infocomm Research (I2R), Tech. Rep., 2011.