1. KAIS T A Survey of Practical Issues in Underwater Networks Jim Partan, Jim Kurose, and Brian Neil Levine, WUWNet’06 2007. 9. 20 Kim Taesung

2. Contents Introduction Underwater network operating regimes Physical layer MAC Protocols Mobility and sparsity Energy efficiency Conclusions

3. A Secure Group Key Management Scheme for Wireless Cellular Network Introduction Highlight differences between terrestrial radio sensor networks an d underwater acoustic sensor networks Characteristics of Underwater networks More expensive equipment, higher mobility, sparer deployments, different energy regimes.

4. Application of Underwater Networks Environmental Monitoring pollution monitoring (chemical, biological, etc.), monitoring of ocean currents and winds, improved weather forecast, detecting climate change. Disaster Prevention measure seismic activity from remote locations and provide tsunami warnings to coastal areas. Assisted Navigation Sensors can be used to locate dangerous rocks or shoals Distributed Tactical Surveillance. Mine Reconnaissance

5. Off-the-shelf oceanographic sensors Conductivity, Temperature, and Depth (CTD): $3k-$12k Acoustic Doppler Current Profiler (ADCP): $25k Seismometer: $10k

6. Autonomous Underwater Vehicles Anchored sensors AUV

7. Physical Layer Underwater communication uses acoustics 10kHz ~ 1MHz Speed of sound underwater is 1500 m/s Large propagation delays Multipath interference is common Frequency-dependent is generally time-varying, causing fading. Shadow zones where almost no acoustic signal exists This effect cause network connectivity dropouts.

8. Technological Limitations Communication is always half-duplex Acoustic transducers cannot simultaneously transmit and receive. AUVs can transmit at high data rates but harder for them to receive at high rates. High data rate is 5k bits/sec at a range of 2Km Low data rate is 80 bits/sec Main reasons are the propulsion noise and difficulties in mounting receiver arrays. The asymmetry in send and receive data rates Star topologies with base stations.

9. MAC Protocols J.Rice describe Seaweb Seaweb have used hybrid TDMA-CDMA Deployment takes more than a day. Covering region of over 100km 2 Freitag describe Mine Countermeasures(MCM) A single hop, star-topology 1 hour deployment, 5km 2 TDMA with low rate command and high rate data Smith describe ad hoc network protocol based on CSMA/CD Hidden terminal problem can be solved by MACA, MACAW and FAMA Several people adopted these protocol in underwater networks Park and Rodoplu adapt energy efficient protocol like S-MAC

10. CDMA in underwater networks CDMA is a conflict-free multiple access Each user is assigned a different spreading code Users can transmit packets without considering other are doing This solve many of MAC problems Near-far problem Received power for each users was equal. Closed-loop power control update in CDMA-based cell phone. Power control is a difficult and open problem Underwater networks have a time-varying, half-duplex channel with a low propagation speed.

11. Mobility and Sparsity Terrestrial sensor networks assume Fairly dense, continuously connected coverage of an area using inexpensive, stationary nodes Costs of underwater networks Fabrication An acoustic modem costs roughly $3K without sensors The rugged construction required to survive storms. Deployment Research ships cost from $5k/day for a small coastal ship to $25k/day for a large ocean-going ship Recovery Since nodes are not disposable, recovery will remain a costly operation. AUVs are a key element in most underwater network architetures Due to the economics and flexibility

12. Contention between navigation and data signals Autonomous mobile vehicle need navigation information This cannot be supplied by GPS It is supplied by acoustic transponders Need to share the channel between network communication and navigation signals. Freitag described results from passive navigation systems. Ouimet described broadcast ping packet ICoN prioritorize navigation and communication packet AUVs receive adequate navigation information, yet are still responsive to command.

13. Energy Efficiency Energy is limited in both terrestrial and underwater sensor networks Communication energy costs Transmit power dominate 100 times more than receive power 50W for transmitting 0.2W for listening and 2W for decoding Range of 2 – 3Km at a 25kHz, 80bit/sec to 5kbits/sec AUV Energy cost Propulsion power dominates network communication power As an example, REMUS AUV 30W for hotel power load : non-propulsion 15W – 110W for propulsion power: 1.5m/s – 2.9m/s For high speed AUV, communication energy can be neglected

14. Conclusions Underwater network will be mobile and sparse than terrestrial network Due to different energy and economic considerations

15. A Secure Group Key Management Scheme for Wireless Cellular Network Time for Any questions? Thank you for listening !