UnderWater Acoustic Sensor Networks (UW-ASN) -Xiong Junjie

Underwater applications Seismic monitoring. Pollution monitoring Ocean currents monitoring Equipment monitoring and control Autonomous Underwater Vehicles (AUV) To make these applications viable, there is a need to enable underwater communications among underwater devices -> Wireless underwater networking Use sound as the wireless communication medium.

Why using sound as communication medium in UW-ASN? Radio waves propagate at long distances through conductive sea water only at extra low frequencies ( Hz), which require large antennae and high transmission power. Optical waves do not suffer from such high attenuation but are affected by scattering. Moreover, transmission of optical signals requires high precision in pointing the narrow laser beams.

Traditional approach for ocean-bottom monitoring Deploy underwater sensors to record data during the monitoring mission, and then recover the instruments. This approach has the following disadvantages: Real time monitoring is not possible. No interaction is possible between onshore control systems and the monitoring instruments. If failures or misconfigurations occur, it may not be possible to detect them before the instruments are recovered. The amount of data that can be recorded during the monitoring mission by every sensor is limited by the capacity of the onboard storage devices (memories, hard disks, etc).

UW-ASN 2D Architecture for ocean bottom monitoring

UW-ASN 3D Architecture for ocean bottom monitoring

UW-ASN 3D Architecture with AUVs

Differences with Terrestrial Sensor Networks Higher Power Consumption. Larger Memory. Higher Cost. Longer latency. Sparser Deployment. Few Spatial Correlation.

Challenges Battery power is limited and usually batteries can not be recharged because solar energy cannot be exploited. The available bandwidth is severely limited. Channel characteristics, including long and variable propagation delays, multi-path and fading problems. High bit error rates. Underwater sensors are prone to failures because of fouling, corrosion, etc. A unique feature of underwater networks is that the environment is constantly mobile, naturally causing the node passive mobility. The ocean can be as deep as 10 km.

Current research result: multi-hop 06 “Research Challenges and Applications for underwater sensor networking” suggests to focus on short-range communication to avoid the many challenges of long-range transfer. Mobicom workshop WuWNet07 “A delay-reliability analysis for multi-hop underwater acoustic communication” proves that multi-hop is very useful in shallow underwater acoustic networks

Current research result: cross-layer design WuWNet07 “State-of-the-Art in Protocol Research for Underwater Sensor Networks” believes that the underwater environment particularly requires cross-layer design solutions that enable a more efficient use of the scarce available resources

Current research result: drifter model 07 “A drift-tolerant model for data management in ocean sensor networks” uses real experiments to prove that a fleet of drifters monitoring model is practical as long as the deployment locations, deployment periods, initial drifter location are well designed.

UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol The ratio of propagation delay to transmission delay is high Channel utilization: T data /(T data +T prop )

UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol Data packet delay:1

UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol TDMA-like No precise clock synchronization No knowledge of propagation delays Completely decentralized operation

Utilizing acoustic propagation delay to design MAC protocols for underwater wireless sensor networks This paper also utilizes the acoustic propagation delay, but it has no schedule, and its ideas lie in nodes deployment to avoid collision which are really simple.

My idea In contrast with “UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol”, I tentative improvement might be: TDMA, synchronization, propagation is easy to calculate Centralized or distributed (according to network size), get a more compact schedule, reduce idle listening, add sleeping period at the end of each TDMA Cross-layer, handle data direction, congestion Model or algorithm