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UNIVERSITA' DEGLI STUDI DI PADOVA CORSO DI RETI WIRELESS A.A. 2011 - 2012 UNDERWATER SENSORS NETWORKS Alessandro Gonella

UNDERWATER SENSORS NETWORKS

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Presentazione sulle problematiche e sulle soluzioni nello sviluppo di reti wireless di sensori sottomarine

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UNIVERSITA' DEGLI STUDI DI PADOVA

CORSO DI RETI WIRELESSA.A. 2011 - 2012

UNDERWATER SENSORS NETWORKS

Alessandro Gonella

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Reasons● Ocean sampling networks● Environmental monitoring● Undersea exploration● Disaster prevention● Assisted navigation● Distributed tactical surveillance● Mine reconnaissance

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MAJOR CHALLENGES

● Radio waves are extremely strongly attenuated in salt water → use acoustic● The speed of sound underwater is approximately

2e5 times lower than the speed of light● Severely limited bandwidth

● The channel is severely impaired (multipath and fading)

● High propagation delay, extremely variable

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MAJOR CHALLENGES (2)● High bit error rates and temporary losses of

connectivity (shadow zones)● Battery power is limited● Underwater sensors are prone to failures

because of fouling and corrosion

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Differences with terrestrial sensor networks

● Cost● Deployment● More power requested● Memory for data caching● More spatial correlation● Channel highly temporally and spatially

variable. The horizontal channel is by far more rapidly varying than the vertical channel

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Factors that influence acoustic communications

● Transmission loss:● Attenuation: absorption due to conversion of

acoustic energy into heat ● Spreading of sound energy as a result of the

expansion of the wavefronts● Noise: Man made or ambient● Multi-path propagation generates intersymbol

interference● The very high delay variance prevents from accurately

estimating the round trip time (RTT)

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TOPOLOGY

The network topology is in general a crucial factor● Energy consumption● Capacity● Reliability

● 3 basic communication architectures

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2 dimensional UWSN

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3 dimensional UWSN

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UWSN with autonomous underwater vehicles

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MAC layer● Frequency division multiple access (FDMA):

● narrow bandwidth in UW-A channels and the vulnerability of limited band systems to fading and multipath

● Time division multiple access (TDMA):● long time guards required● syncronization

● Carrier sense multiple access (CSMA):● prevents collisions with the ongoing transmission at

the transmitter side● need a guard time between transmissions

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MAC layer (2)

Code division multiple access (CDMA):● it is able to distinguish among signals

simultaneously transmitted by multiple devices through codes that spread the user signal over the entire available band

● allows reducing the number of packet retransmissions

● DSSS leads to less error rate than FHSS [4]

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Open research issues• In case CDMA is adopted, it is necessary to design access codes with high auto-correlation and low cross-correlation properties to achieve minimum interference among users• Research on optimal data packet length is needed to maximize the network e ciencyffi• Distributed protocols should be devised to reduce the activity of a device when its battery is depleting without compromising on network availability

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Routing protocolsSome characteristics, such as the extremely long propagation delays, are better addressed at the network layerRouting protocols divided in 3 categories:• Proactive protocols: attempt to minimize the message latency induced by route discovery, by maintaining up-to-date routing information → Overhead

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Routing protocols (2)● Reactive protocols: A node initiates a route

discovery process only when a route to a destination is required● high latency in the establishment of paths● UW-ASNs is unlikely to vary dynamically on a

short-time scale● Geographical routing protocols: establish

source–destination paths by leveraging localization information● How to determine position?

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Some proposed protocols● Vector-Based Forwarding Protocol for Underwater

Sensor Networks [5]:● The forwarding path is specified by the the routing

vector, a vector that connects source and destination

● Each packet carries the positions of the sender, the destination and the forwarder

● Upon receiving a packet, a node computes its position relative to the forwarder by measuring its distance to the forwarder and the angle of arrival of the signal

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Some proposed protocols (2)

● Recursively, every node calculates its position● If a node determines that it is close enough to the

routing vector it includes its own position in the packet and forwards it

● Otherwise, it discards the packet● Redundant and interleaved paths from source to

destination● Energy and bandwidth waste

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Some proposed protocols (3)● A Resilient Routing Algorithm for Long-term

applications in Underwater Sensor Networks [6]● Virtual circuit: multihop connections established a

priori● Each packet associated with a particular connection

follows the same path● Centralized coordination → less flexible architecture● Aim to achieve optimal performance at the network

layer with minimum signaling overhead

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Some proposed protocols (4)● Two-phase approach

● The network manager determines optimal primary and backup multihop data paths such that the energy consumption of the nodes is minimized

● An on-line distributed solution guarantees survivability of the network, by locally repairing paths in case of disconnections or failures, or by switching the data traffic on the backup paths in case of severe failures

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Open research issues● Need to develop algorithms to provide strict or loose latency

bounds for time critical applications● Need to develop mechanisms to handle loss of connectivity

without provoking immediate retransmissions● Devise routing algorithms that are robust with respect to the

intermittent connectivity of acoustic channels● Accurate modeling is needed to better understand the

dynamics of data transmission at the network layer. Moreover, credible simulation models and tools need to be developed

● Algorithms and protocols need to be developed that detect and deal with disconnections due to failures, unforeseen mobility of nodes or battery

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Transport layer● Achieve reliable collective transport of event features and

perform flow control and congestion control● High (and high variable) RTT:

● a ect the throughput of most TCP implementationsff● make it hard to e ectively set the timeout of the ff

window-based mechanism that most current TCP implementations adopt

● Rate-based transport protocols are promising but they rely on feedback control messages sent back by the destination to dynamically adapt the transmission rate

● It is important to discriminate losses due to impairments of the channel from those caused by congestion

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Design principles● The transport layer should handle shadow zones● Minimize energy consumption● Rate-based transmission of packets● Intermediate nodes should be capable of determining

and reacting immediately to local congestion● Losses should trigger the protocol to take appropriate

actions, supported by the information from lower layers

● There should be mechanism to guarantee the end-to-end reliability

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Segmented Data Reliable Transport (SDRT) [7]

● Uses error correction codes to recover errored packets to reduce retransmissions

● Transmits the packets within the window quickly, and the remaining packets at a lower rate

● Encoding and decoding codes are computation-intensive operations

● No mechanism to guarantee the end-to-end reliability (hop-by-hop)

● The total computation overhead will be too high for the network

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Open research issues● New flow control strategies to tackle the high delays of

the control messages● New e ective mechanisms for e ciently infer the ff ffi

cause of packet losses● The effects of multiple concurrent events on the

reliability and network performance requirements must be studied

● It is necessary to devise solutions to handle the effects of losses of connectivity caused by shadow zones

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Bibliography● [1] - A Survey of Practical Issues in Underwater Networks - Jim Partan, Jim

Kurose, and Brian Neil Levine● [2] - Underwater Acoustic Networks - Ethem M. Sozer, Milica Stojanovic, and

John G. Proakis● [3] - State-of-the-Art in Protocol Research for Underwater Acoustic Sensor

Networks - Ian F. Akyildiz, Dario Pompili, and Tommaso Melodia● [4] - Analysis of Channel Effects on Direct-Sequence and Frequency-

Hopped Spread-Spectrum Acoustic Communication - Lee Freitag, Milica Stojanovic, Sandipa Singh, and Mark Johnson

● [5] - VBF: Vector-Based Forwarding Protocol for Underwater Sensor Network - P. Xie, J.-H. Cui, and L. Lao

● [6] - A Resilient Routing Algorithm for Long-term applications in Underwater Sensor Networks - Pompili, Melodia, I. F. Akyildiz

● [7] - SDRT: A Reliable Data Transport Protocol for Underwater Sensor Networks - P. Xie and J.-H. Cui

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