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Transport Protocol in Transport Protocol in Wireless Sensor Wireless Sensor
NetworksNetworks
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MotivationMotivation What is expected out of a “transport” protocol for What is expected out of a “transport” protocol for
sensor networks ? sensor networks ? Reliability, congestion control, mux/demux,……Reliability, congestion control, mux/demux,……
Why can’t we use the existing protocols ?Why can’t we use the existing protocols ?Resource constraints – power, storage, computation Resource constraints – power, storage, computation
complexity, data rates, …complexity, data rates, …
Are these constraints common for all sensor Are these constraints common for all sensor networks ? networks ?
No, they are application specific.No, they are application specific.
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Motivation ..contd.Motivation ..contd.
►Any application can have a Any application can have a unionunion of of the constraints that we know or yet the constraints that we know or yet to figure outto figure out
►Spectra for known constraints:Spectra for known constraints:Low data Rate High data Rate
Power limited Not Power limited
Storage limited Not Storage limited
Bursty samples Periodic samples
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Motivation ..contd.Motivation ..contd.
General notion for sensor networksGeneral notion for sensor networksLow data Rate High data Rate
Power limited Not power limited
Storage limited Not storage limited
Sink
user
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Motivation ..contd.Motivation ..contd.
Radar application:Radar application:
Range of Transport protocols is yet to be Range of Transport protocols is yet to be exploredexplored
ESRT, PSFQ, CODA …….……!!!!………..TRABOLESRT, PSFQ, CODA …….……!!!!………..TRABOL
Low data Rate High data Rate
Power limited Not Power limited
Storage limited Not storage limited
High data RateNot Power limited
Not Storage limited
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Two Issues in TransportTwo Issues in Transport
First look at First look at resource-constrainedresource-constrained sensors sensors
►What is the proper reliability notion for What is the proper reliability notion for wireless sensor networks?wireless sensor networks? ESRT as an exampleESRT as an example
►How to achieve reliable end-to-end How to achieve reliable end-to-end delivery in a less wasteful way? delivery in a less wasteful way? PSFQ as an examplePSFQ as an example
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Main Design GuidelinesMain Design Guidelines
► Application drivenApplication driven Application-oriented notions such as reliability, Application-oriented notions such as reliability,
loss recovery, delayloss recovery, delay►Filling in the “gray area”, Different from the InternetFilling in the “gray area”, Different from the Internet
Expose application semanticsExpose application semantics►E.g., the concept of “application level framing” hereE.g., the concept of “application level framing” here
► ““End to end” versus “region-based”End to end” versus “region-based” Other extreme: Hop by hop?Other extreme: Hop by hop?
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References for This PPTReferences for This PPT
►ESRT: Event-to-Sink Reliable Transport ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networksin Wireless Sensor Networks
►PSFQ: A Reliable Transport Protocol for PSFQ: A Reliable Transport Protocol for Wireless Sensor NetworksWireless Sensor Networks
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Event-to-Sink Reliable Transport Event-to-Sink Reliable Transport (ESRT) for Wireless Sensor (ESRT) for Wireless Sensor
NetworksNetworks
Features:Features:►Event-to-sink reliabilityEvent-to-sink reliability►Self-configurationSelf-configuration►Energy awareness [low power Energy awareness [low power
consumption requirement!]consumption requirement!]►Congestion ControlCongestion Control►Variation in complexity at source Variation in complexity at source
and sink. [computation complexity]and sink. [computation complexity]
S
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ESRT: OverviewESRT: Overview
► Focus on events, not individual pieces of Focus on events, not individual pieces of datadata Reflect application/user’s viewpointReflect application/user’s viewpoint
► Application-drivenApplication-driven Application defines what its desired event Application defines what its desired event
reporting rate should bereporting rate should be► Includes a congestion-control elementIncludes a congestion-control element► Runs mainly on the sinkRuns mainly on the sink►Main goal: Adjust reporting rate of sources Main goal: Adjust reporting rate of sources
to achieve optimal reliability requirementsto achieve optimal reliability requirements
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Problem DefinitionProblem Definition► Assumption:Assumption:
Detection of an event is related to the number of packets received during Detection of an event is related to the number of packets received during a specific intervala specific interval
► Reliability is measured in terms of the number of packets received. Or Reliability is measured in terms of the number of packets received. Or reporting frequency i.e., number of packets/decision interval.reporting frequency i.e., number of packets/decision interval.
► Observed event reliabilityObserved event reliability r rii:: # of packets received in decision interval I# of packets received in decision interval I
► Desired event reliabilityDesired event reliability R: R: # of packets required for reliable event detection# of packets required for reliable event detection Application-specificApplication-specific
► Normalized reliability = observed/desired.Normalized reliability = observed/desired.
► Goal: configure the reporting rate of nodesGoal: configure the reporting rate of nodes Achieve required event detectionAchieve required event detection Minimize energy consumptionMinimize energy consumption
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Reliability vs Reporting Reliability vs Reporting frequencyfrequency
► Initially, reliability increases linearly with reporting frequencyInitially, reliability increases linearly with reporting frequency► There is an optimal reporting frequency (fThere is an optimal reporting frequency (fmaxmax), after which ), after which
congestion occurscongestion occurs► FFmaxmax decreases when the # of nodes increases decreases when the # of nodes increases
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Characteristic RegionsCharacteristic Regions► n: normalized reliability indicatorn: normalized reliability indicator
► (NC,LR): No congestion, Low reliability(NC,LR): No congestion, Low reliability f < fmax, n < 1-ef < fmax, n < 1-e
► (NC, HR): No congestion, High reliability(NC, HR): No congestion, High reliability f <= fmax, n < 1+ef <= fmax, n < 1+e
► (C, HR): Congestion, High reliability(C, HR): Congestion, High reliability f > fmax, n > 1f > fmax, n > 1
► (C, LR): Congestion, Low reliability(C, LR): Congestion, Low reliability f < fmax, n <= 1f < fmax, n <= 1
► OOR: Optimal Operating RegionOOR: Optimal Operating Region f < fmax, 1-e <= n <= 1+ef < fmax, 1-e <= n <= 1+e
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Characteristic RegionsCharacteristic Regions
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ESRT RequirementsESRT Requirements
►Sink is powerful enough to reach all Sink is powerful enough to reach all source nodessource nodes
►Nodes must listen to the sink Nodes must listen to the sink broadcast at the end of each decision broadcast at the end of each decision interval and update their reporting interval and update their reporting ratesrates
►A congestion-detection mechanism is A congestion-detection mechanism is requiredrequired
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Congestion Detection and Congestion Detection and Reliability LevelReliability Level
►Both done at the sinkBoth done at the sink
►Congestion:Congestion: Nodes monitor their buffer queues and Nodes monitor their buffer queues and
inform the sink if overflow occursinform the sink if overflow occurs
►Reliability LevelReliability Level Calculated by the sink at the end of each Calculated by the sink at the end of each
interval based on packets receivedinterval based on packets received
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Algorithm for ESRTAlgorithm for ESRT
► If congestion and low reliability: decrease reporting If congestion and low reliability: decrease reporting frequency aggressively. (exponential decrease)frequency aggressively. (exponential decrease)
► If congestion and high reliability: decrease reporting If congestion and high reliability: decrease reporting to relieve congestion. No compromise on reliability to relieve congestion. No compromise on reliability (multiplicative decrease)(multiplicative decrease)
► If no congestion and low reliability: increase reporting If no congestion and low reliability: increase reporting frequency aggressively (multiplicative increase)frequency aggressively (multiplicative increase)
► If no congestion and high reliability: decrease If no congestion and high reliability: decrease reporting slowing (half the slope)reporting slowing (half the slope)
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Components of ESRTComponents of ESRT
► In sink:In sink: Normalized reliability computationNormalized reliability computation A congestion detection mechanismA congestion detection mechanism
► In source:In source: Listen to sink broadcastListen to sink broadcast Overhead free local congestion detection Overhead free local congestion detection
mechanismmechanism
E.g., buffer level monitoring, CN – E.g., buffer level monitoring, CN – Congestion NotificationCongestion Notification
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ESRT Protocol OperationESRT Protocol Operation
►(NC, LR):(NC, LR):►(NC, HR):(NC, HR):►(C, HR): (C, HR): ►(C, LR): (C, LR):
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ESRT SummaryESRT Summary
►Reliability notion is application-basedReliability notion is application-based No delivery guarantees for individual No delivery guarantees for individual
packetspackets
►Reliability and congestion control Reliability and congestion control achieved by changing the reporting achieved by changing the reporting rate of nodesrate of nodes
►Pushes all complexity to the sinkPushes all complexity to the sink►Single-hop operation onlySingle-hop operation only
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PSFQ: OverviewPSFQ: Overview► Key ideasKey ideas
Slow data distribution (pump slowly) Slow data distribution (pump slowly) Quick error recovery (fetch quickly)Quick error recovery (fetch quickly) NACK-basedNACK-based Data caching guarantees ordered deliveryData caching guarantees ordered delivery Assumption: no congestion, losses due only to poor link qualityAssumption: no congestion, losses due only to poor link quality
► GoalsGoals Ensure data delivery in poor link quality caseEnsure data delivery in poor link quality case Minimize signaling overhead for detection/recovery operationsMinimize signaling overhead for detection/recovery operations Provide loose delay bounds for data delivery to all intended Provide loose delay bounds for data delivery to all intended
receiversreceivers
► OperationsOperations PumpPump FetchFetch ReportReport
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Probability of successful Probability of successful delivery using End to End delivery using End to End
Model Model
1
2
n-1
n
(1-p)
(1-p)n-1
(1-p)n
p is the error rate of wireless link between two hops
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End-to-end considered harmful ?End-to-end considered harmful ?
► Probability of reception degrades exponentially Probability of reception degrades exponentially over multiple hopsover multiple hops Not an issue in the Internet Not an issue in the Internet Serious problem if error rates are considerableSerious problem if error rates are considerable
► ACKs/NACKs are also affectedACKs/NACKs are also affected
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Proposed solution: Hop-by-Hop Proposed solution: Hop-by-Hop error recoveryerror recovery
► Intermediate nodes now responsible for Intermediate nodes now responsible for error detection and recoveryerror detection and recovery NACK-based loss detection probability is now NACK-based loss detection probability is now
constantconstant►Not affected by network size (scalability)Not affected by network size (scalability)►Exponential decrease in end-to-endExponential decrease in end-to-end
► Cost: Keeping state on each nodeCost: Keeping state on each node Potentially not as bad as it sounds!Potentially not as bad as it sounds!
►Cluster/group based communicationCluster/group based communication► Intermediates are usually receivers as wellIntermediates are usually receivers as well
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Multi-Hop Packet Multi-Hop Packet Forwarding Forwarding
1 2 3 4
1
11
22 2
33 3
When No Link Loss – Multi-Hop Forwarding takes place
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Issue: Recovering from Issue: Recovering from Errors Errors
2 431
2 lost
1 1 1
33
3
Recover 2
Recover 2
Recover 2
Error Recovery Control Messages are wasted
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How PSFQ Recovers from How PSFQ Recovers from ErrorsErrors
“Store and Forward”“Store and Forward”2 31 4
2 lost
Recover 2
1
22
3
11
33 2
2
No wastage of the Error Recovery control messages
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Pump operationPump operation
► Node broadcasts a packet to its neighbors every Node broadcasts a packet to its neighbors every TminTmin Data cache used for duplicate suppressionData cache used for duplicate suppression
► Receiver checks for gaps in sequence numbersReceiver checks for gaps in sequence numbers
► If all is fine, it decrements TTL and schedules a If all is fine, it decrements TTL and schedules a transmissiontransmission Tmin < Ttransmit < TmaxTmin < Ttransmit < Tmax By delaying transmission, quick fetch operations are By delaying transmission, quick fetch operations are
possiblepossible Reduce redundant transmissions (don’t transmit if 4 or Reduce redundant transmissions (don’t transmit if 4 or
more nodes have forwarded the packet already)more nodes have forwarded the packet already) Tmax can provide a loose delay bound for the last hopTmax can provide a loose delay bound for the last hop
► D(n)=Tmax * (# of fragments) * (# of hops) D(n)=Tmax * (# of fragments) * (# of hops)
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PSFQ Pump SchedulePSFQ Pump Schedule
If not duplicate and in-order and TTL not 0 Cache and Schedule for Forwarding at time t (Tmin<t<Tmax)
Tmin
TmaxTmin
Tmax
21
1
1
1
t
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Fetch operationFetch operation
► Sequence number gap is detectedSequence number gap is detected Node will send a NACK message upstreamNode will send a NACK message upstream
► ‘‘Window’ specifies range of sequence numbers missingWindow’ specifies range of sequence numbers missing► NACK receivers will randomize their transmissions to NACK receivers will randomize their transmissions to
reduce redundancyreduce redundancy It will NOT forward any packets downstream It will NOT forward any packets downstream NACK scope is 1 hopNACK scope is 1 hop NACKs are generated every Tr if there are still gapsNACKs are generated every Tr if there are still gaps
► Tr < TminTr < Tmin This is the pump/fetch ratioThis is the pump/fetch ratio
► NACKs can be cancelled if neighbors have sent similar NACKs can be cancelled if neighbors have sent similar NACKsNACKs
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““Fetch Quickly” Fetch Quickly” OperationOperation
21
11
2 lost2
3
Tmin
Tmax
TrRecover 2Tr 2
2
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Proactive FetchProactive Fetch
► Last segments of a file can get lostLast segments of a file can get lost Loss detection impossible; no ‘next’ segment exists!Loss detection impossible; no ‘next’ segment exists!
► Solution: timeouts Solution: timeouts Node enters ‘proactive fetch’ mode if last segment Node enters ‘proactive fetch’ mode if last segment
hasn’t been received and no packet has been delivered hasn’t been received and no packet has been delivered after Tproafter Tpro
Timing must be rightTiming must be right► Too early: wasted control messagesToo early: wasted control messages► Too late: increased delivery latency for the entire fileToo late: increased delivery latency for the entire file
Tpro = a * (Smax - Smin) * TmaxTpro = a * (Smax - Smin) * Tmax► A node will wait long enough until all upstream nodes have A node will wait long enough until all upstream nodes have
received all segmentsreceived all segments If data cache isn’t infiniteIf data cache isn’t infinite
► Tpro = a * k * TmaxTpro = a * k * Tmax (Tpro is proportional to cache size)(Tpro is proportional to cache size)
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““Proactive Fetch”Proactive Fetch”
Tproc
1 2
last-1
last
last
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Report OperationReport Operation
► Used as a feedback/monitoring mechanismUsed as a feedback/monitoring mechanism► Only the last hop will respond immediately Only the last hop will respond immediately
(create a new packet)(create a new packet) Other nodes will piggyback their state info when they Other nodes will piggyback their state info when they
receive the report replyreceive the report reply If there is no space left in the message, a new one If there is no space left in the message, a new one
will be createdwill be created
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Experimental resultsExperimental results
► TmaxTmax = 0.3s, Tr = 0.1s= 0.3s, Tr = 0.1s
► 100 30-byte packets sent100 30-byte packets sent► Exponential increase in delay happens at 11% loss rate or higherExponential increase in delay happens at 11% loss rate or higher
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PSFQ SummaryPSFQ Summary►Slow data dissemination, fast data recoverySlow data dissemination, fast data recovery
All transmissions are broadcastAll transmissions are broadcast►NACK-based, hop-by-hop recoveryNACK-based, hop-by-hop recovery
End-to-end behaves poorly in lossy environmentsEnd-to-end behaves poorly in lossy environments NACKs are superior to ACKs in terms of energy NACKs are superior to ACKs in terms of energy
savingssavings►No out-of-order delivery allowedNo out-of-order delivery allowed
Uses data caching extensivelyUses data caching extensively►Several timers and duplicate suppression Several timers and duplicate suppression
mechanismsmechanisms Implementing any of those on motes is Implementing any of those on motes is
challenging (non-preemptive FIFO scheduler)challenging (non-preemptive FIFO scheduler)
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What more can be done at What more can be done at “transport”“transport”
► Robust “transport”Robust “transport” Now rely on end to end based approach: slowNow rely on end to end based approach: slow Possible new way: Location-based, rather than Possible new way: Location-based, rather than
node-based designnode-based design
► Application perspectiveApplication perspective Support aggregationSupport aggregation
► reliability & rate control & semantic aggregationreliability & rate control & semantic aggregation
Look at what application you haveLook at what application you have►Tradeoff of different metrics & requirementsTradeoff of different metrics & requirements