Xiuzhen Cheng [e-mail]

1
Xiuzhen Cheng
cheng_at_gwu.edu
Csci332 MAS Networks Challenges and
State-of-the-Art Research Underwater Sensor
Networks
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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

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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

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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

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Two-dimensional Sensor Networks
RF
Two acoustic radios
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Three-dimensional sensor networks
floating
buoy
anchor
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Challenges to enable 3D monitoring

• Sensing coverage
• Need collaborative regulation on sensor depth
• Communication coverage
• Connectivity requirement

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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?)

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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

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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

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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 lt1 Long 10100 25 Medium
1-10 around 10 Short 0.11 2050 Very
short lt0.1 gt100
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Physical Layer
New development needed for inexpensive
transceiver modems, filters, etc.
Evolution of modulation technique Type Year
Rate kbps Band kHz Range kma FSK 1984
1.2 5 3s PSK 1989 500 125 0.06d
FSK 1991 1.25 10 2d PSK 1993 0.30.5
0.31 200d90s PSK 1994 0.02 20 0.9s
FSK 1997 0.62.4 5 10d5s DPSK 1997 20
10 1d PSK 1998 1.676.7 210 4d2s
16-QAM 2001 40 10 0.3s a The subscripts
d and s stand for deep (gt100m) and shallow water
(lt100)
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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.

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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

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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!

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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

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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.