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

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

2. Reasons
● Ocean sampling networks
● Environmental monitoring
● Undersea exploration
● Disaster prevention
● Assisted navigation
● Distributed tactical surveillance
● Mine reconnaissance
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3. 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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4. 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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5. 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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6. 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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7. TOPOLOGY
The network topology is in general a crucial factor
● Energy consumption
● Capacity
● Reliability
● 3 basic communication architectures
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8. 2 dimensional UWSN
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9. 3 dimensional UWSN
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10. UWSN with autonomous underwater
vehicles
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11. 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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12. 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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13. 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 efficiency
• 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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14. Routing protocols
Some characteristics, such as the extremely long
propagation delays, are better addressed at the
network layer
Routing 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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15. 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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16. 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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17. 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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18. 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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19. 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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20. 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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21. Transport layer
● Achieve reliable collective transport of event features and
perform flow control and congestion control
● High (and high variable) RTT:
● affect the throughput of most TCP implementations
● make it hard to effectively set the timeout of the
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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22. 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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23. 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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24. Open research issues
● New flow control strategies to tackle the high delays of
the control messages
● New effective mechanisms for efficiently infer the
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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25. 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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