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Transport protocols in Wireless Sensor Networks
Congduc PHAMUniversity of PauLIUPPA laboratory
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Transport layer for WSN/WMSN
Reliability/loss recoveryCongestion control
Congestion detectionFairness issues
Higher semantic than packet levelMultipoint communicationData aggregation, data dissemination
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Congestion Control
Feedback should be frequent, but not too much otherwise there will be oscillationsCan not control the behavior with a time granularity less than the feedback period
feedback Closed-loop control
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TCP congestion control
cwnd grows exponentially (slow start), then linearly (congestion avoidance) with 1 more segment per RTT
If loss, divides threshold by 2 (multiplicative decrease) and restart with cwnd=1 packet
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Congestion windowdoubles every round-triptime
packetack
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WSN vs Internet
Small fraction of time dealing with impulses, but data of greatest importance!
As in wireless transmission, difficult to distinguish congestions from node failures or bad channel quality
Sensors have limited resourcesSimplicity in congestion detection and control
algorithmsGreat interest of in-network processing: hop-by-
hop CC more efficient than E2E?WSN are collaborative in nature. Fairness
issues less important?
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Reference architecture
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CC scenario in WSN
Densely deployed sensorsPersistent hotspotsCongestion occur near the sources
Sparsely deployed sensors, low rateTransient hotspotsCongestion anywhere but likely far from the
sources, towards the sinkSparsely deployed sensors, high rate
Both persistent and transient hotspotsHotspot distributed throughout the network
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Some ideas for CC in WSN
Congestion detectionMonitor buffer/queue sizeMonitor channel busy time, estimate channel’s loadMonitor the inter-packet arrival time (data, ctrl)
Congestion notificationExplicit congestion notification in packet header,
then broadcast (but then energy-consuming!)Congestion control
Dynamic reporting rate depending on congestion level
In-network data reduction techniques (agressive aggregation) on congestion
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TCP or UDP?
TCPConnection-oriented, 3-way
handshakeAssumes segment losses results from
congestionE2E reliability, congestion mechanismFairness as a function of RTT
UDPNo flow control nor congestion controlNo reliability
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TCP or specialized approach?
TCP with appropriate modifications is better than UDP is standardized protocols are to be used.
Specialized approaches allow for a specific preference
between reliability and congestion control
Application awareness is also possible
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Proposals for WSNs
ESRTCODAPSFQRMSTHDAGARUDASenTCPSTCP
See the reference section for complete references of the
papers
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Which CC for WMSN? (1)
WSN: scalar dataWireless Multimedia Sensor
Networks add video, audio forEnlarging the view
• Field of View of single camera is limited• Multiples camera overcome occlusion
effects
Enhancing the view• Can help disambiguate cluttered situations
Enabling multi-resolution views
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Which CC for WMSN? (2)
Reliability should be enforced at the packet levelSome packets are more important
than others in most of video coding schemes
Collaborative in-network processingReduce amap the amount of
(redundant) raw streams to the sink
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Related projects
SensEye, UMass http://sensors.cs.umass.edu/projects/senseye/
Cyclops, UCLAPanoptes, Portland State UniversityVSAM team at Carnegie Mellon
Univ.
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Test-case in TCAP
Lower priority, backup sensor?
Grid-based localization?
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Design issues & directions
Cross-layeringUse low-level information
Collaborative in-network processingDistributed algorithmsRouter-assisted, node-assisted, ad-
hocSimple congestion detection
mechanism