Xiuzhen Cheng cheng@gwu

1. Xiuzhen Chengcheng@gwu.edu Csci332MAS Networks – Challenges and State-of-the-Art Research – Underwater Sensor Networks

2. Introduction • Underwater acoustic sensor networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. • Acoustic communications are the typical physical layer technology • Radios propagate to long distance only at extra low frequencies, with a large antennae and high transmission power • Mica mote can transmit to 120cm at 433MHz in underwater

3. Applications • Ocean Sampling Networks • Environmental (chemical, biological, and nuclear) monitoring • Water quality in situ analysis • Undersea explorations (for oilfields, minerals, reservoirs, for determining routes for laying undersea cables, etc.) • Disaster prevention (earthquakes, etc.) • Assisted navigation • Distributed tactical surveillance • Mine reconnaissance

4. Challenges • Severely limited bandwidth • Severely impaired channel • Propagation delay is 5 times longer • High bit error rates, intermittent connectivity • Battery power • Underwater sensors are error-prone due to fouling and corrosion

5. Two-dimensional Sensor Networks RF Two acoustic radios

6. Three-dimensional sensor networks floating buoy anchor

7. Challenges to enable 3D monitoring • Sensing coverage • Need collaborative regulation on sensor depth • Communication coverage • Connectivity requirement

8. Autonomous Underwater Vehicles • Can reach any depth in the ocean • The integration of fixed sensor networks and AUVs is an almost unexplored research area • Adaptive sampling (where to place sensors?) • Self-configuration (where there is a failure?)

9. Design Challenges (1/2) • Difference with terrestrial sensor networks • Cost (more due to complex transceivers and hardware protection), deployment (sparser due to cost), power (higher due to long transmission range and complex DSP), memory (larger due to intermittent connectivity), spatial correlation (less likely to happen due to sparser deployment) • Underwater sensors • Protecting frames, many underwater sensors exist • New design: • Develop less expensive, robust, “nano-sensors” • Devise periodical cleaning mechanisms against corrosion and fouling • Design robust, stable sensors on a high range of temperatures • Design integrated sensors for synoptic sampling of physical, chemical, and biological parameters

10. Design Challenges (2/2) • A cross-layer protocol stack • All the layers in the TCP/IP model • Need a power management plane, a coordination plane, and a localization plane • Real-time vs. delay-tolerant networking • Application driven

11. Basics of Acoustic Propagation Underwater acoustic communications are mainly influenced by path loss, noise, multipath, Doppler spread, and high and variable propagation delay Available bandwidth for different ranges in UW-A channels Range [km] Bandwidth [kHz] Very long 1000 <1 Long 10–100 2–5 Medium 1-10 around 10 Short 0.1–1 20–50 Very short <0.1 >100

12. Physical Layer New development needed for inexpensive transceiver modems, filters, etc. Evolution of modulation technique Type Year Rate [kbps] Band [kHz] Range [km]a FSK 1984 1.2 5 3s PSK 1989 500 125 0.06d FSK 1991 1.25 10 2d PSK 1993 0.3–0.5 0.3–1 200d–90s PSK 1994 0.02 20 0.9s FSK 1997 0.6–2.4 5 10d–5s DPSK 1997 20 10 1d PSK 1998 1.67–6.7 2–10 4d–2s 16-QAM 2001 40 10 0.3s a The subscripts d and s stand for deep (>=100m) and shallow water (<100)

13. Data Link Layer • Challenges: low bandwidth and high/variable delay • FDMA is not suitable due to low bandwidth • TDMA is not suitable due to the variable delay (long-term guards) • CSMA is not efficient since it only prevents collision at the transmitter side • Contention-based schemes that rely on RTS/CTS are not practical due to the long/variable delay • CDMA is promising due to its robustness again fading and Doppler spreading especially in shallow water • Challenges: Error control functionalities are needed • ARQ, FEC, etc. • Open research issues • Optimal data packet length for network efficiency optimization • CDMA code, encoders and decoders, etc.

14. Network Layer • From sensors to surface stations • 3D routing • Existing routing schemes (proactive, reactive, and geographical routing schemes) may be tailored for underwater sensor networks • Challenges • Long/variable delay • Intermittent connectivity • Accurate modeling of the dynamics of the data transmission • Route optimization • The integration of AUV and sensors • Location discovery techniques for geographical routing protocols

15. Transport Layer • Totally unexplored area • Underwater sensor networks necessitate a new event transport reliability notion • Traditionally transport layer provides robust end-to-end approach • Challenges: long/variable delay • Needs flow control and congestion control • Most existing TCP implementations are unsuited due to the window-based flow/congestion control mechanisms (RTT is needed) • Rate-based transport protocols may not work due to the dependency on feedback control messages • Packet loss caused by high bit error rate • New strategies may be needed! • Open research issues: • Abundant!

16. Application Layer • Largely unexplored • Purposes • To provide a network management protocol • To provide a language for query the sensor networks • To assign tasks and to advertise events/data

17. Experimental Implementations • The Front-Resolving Observational Network with Telemetry (FRONT) project at u Connecticut • Sensors, repeaters, and gateways • Sensors are connected to acoustic modems • Repeaters are acoustic modems to relay data • Gateways are surface buoys • Experiment conducted: 20 sensors and repeaters are deployed in shallow water • AOSN program at the Monterey Bay Aquarium Research Institute • To study the upwelling of cold, nutrient-rich water in the Monterey Bay.