1

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