Xiuzhen ChengXiuzhen Cheng cheng@gwu.edu

CsciCsci332332 MAS Networks – Challenges MAS Networks – Challenges and State-of-the-Art Research and State-of-the-Art Research – – Underwater Sensor NetworksUnderwater Sensor Networks

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

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

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

Two-dimensional Sensor Networks T

wo

acou

stic

rad

ios

RF

Three-dimensional sensor networks

floating

anchorbuoy

Challenges to enable 3D monitoring

Sensing coverageNeed collaborative regulation on sensor depth

Communication coverageConnectivity requirement

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?)

Design Challenges (1/2)

Difference with terrestrial sensor networksCost (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 sensorsProtecting 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

Design Challenges (2/2)

A cross-layer protocol stackAll the layers in the TCP/IP model

Need a power management plane, a coordination plane, and a localization plane

Real-time vs. delay-tolerant networkingApplication driven

Basics of Acoustic Propagation

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 10Short 0.1–1 20–50 Very short <0.1 >100

Underwater acoustic communications are mainly influenced by path loss, noise, multipath, Doppler spread, and high and variable propagation delay

Physical Layer

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)

New development needed for inexpensive transceiver modems, filters, etc.

Data Link Layer

Challenges: low bandwidth and high/variable delayFDMA 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 neededARQ, FEC, etc.

Open research issuesOptimal data packet length for network efficiency optimization

CDMA code, encoders and decoders, etc.

Network Layer

From sensors to surface stations

3D routingExisting routing schemes (proactive, reactive, and geographical routing schemes) may be tailored for underwater sensor networks

ChallengesLong/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

Transport LayerTotally unexplored area

Underwater sensor networks necessitate a new event transport reliability notion

Traditionally transport layer provides robust end-to-end approach

Challenges: long/variable delayNeeds 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!

Application Layer

Largely unexplored

PurposesTo provide a network management protocol

To provide a language for query the sensor networks

To assign tasks and to advertise events/data

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.