Taming the Underlying Taming the Underlying Challenges of Reliable Challenges of Reliable Multihop Routing in Multihop Routing in

Sensor NetworksSensor Networks

Key ObservationsKey Observations Many wireless links are lossyMany wireless links are lossy Loss rate may change dynamicallyLoss rate may change dynamically

EEnvironmental factorsnvironmental factors HHighly correlated behavior of an applicationighly correlated behavior of an application

Routing should consider these Routing should consider these underlying factorsunderlying factors A lot of existing work on routing are based A lot of existing work on routing are based

on abstract MAC & physical layer modelon abstract MAC & physical layer model Simply assume 802.11 takes care of MAC Simply assume 802.11 takes care of MAC

layer issueslayer issues

ContributionsContributions

Empirical link quality observation Empirical link quality observation Connectivity analysisConnectivity analysis

LLikelihood of the success of a communicationikelihood of the success of a communication Distance, residual energy, congestion, Distance, residual energy, congestion,

channel contention,channel contention,…… Link quality estimationLink quality estimation

Neighborhood managementNeighborhood management Routing for periodic data collection Routing for periodic data collection

applicationsapplications

Empirical Observation of Empirical Observation of Link CharacteristicsLink Characteristics

Measure loss rates between many Measure loss rates between many different pairs of nodes at different different pairs of nodes at different distancesdistances A sequence of linearly arranged sensor A sequence of linearly arranged sensor

nodes with a spacing of 2 feetnodes with a spacing of 2 feet One transmitter sends packets 200 One transmitter sends packets 200

packets at the rate of 8 packets/secpackets at the rate of 8 packets/sec Remaining nodes counts the number of Remaining nodes counts the number of

successfully received packets successfully received packets

Empirical ResultsEmpirical Results

A simple probabilistic means can be A simple probabilistic means can be used to capture the link behavior in used to capture the link behavior in simulationssimulations Connected regionConnected region Transitional region: link probability Transitional region: link probability

with mean & variance from the with mean & variance from the empirical data empirical data

Disconnected regionDisconnected region

SphericalSpherical radio range radio range assumptionassumption in current in current researchresearch Localization, Sensing Coverage, Topology ControlLocalization, Sensing Coverage, Topology Control

Radio IrregularityRadio Irregularity Deepak Ganesan, etc., Deepak Ganesan, etc., “Complex Behavior at Scale: An “Complex Behavior at Scale: An

Experimental Study of Low-Power Wireless Sensor Networks” , Experimental Study of Low-Power Wireless Sensor Networks” , UCLA/CSD-TR 02-0013, 2002UCLA/CSD-TR 02-0013, 2002

Alberto Cerpa, etc.,Alberto Cerpa, etc., “SCALE: A Tool for Simple Connectivity “SCALE: A Tool for Simple Connectivity Assessment in Lossy Environments”, Assessment in Lossy Environments”, CENS-TR 03-0021, 2003CENS-TR 03-0021, 2003

Jerry Y. Zhao, etc.,Jerry Y. Zhao, etc., “Understanding Packet Delivery “Understanding Packet Delivery Performance in Dense Wireless Sensor Network”, Performance in Dense Wireless Sensor Network”, ACM SenSys, ACM SenSys, 20032003

Alec Woo, etc.,Alec Woo, etc., “Taming the Underlying Challenges of Reliable “Taming the Underlying Challenges of Reliable Multihop Routing in Sensor Networks”, Multihop Routing in Sensor Networks”, ACM SenSys, 2003ACM SenSys, 2003

DOI Concept DOI Concept Tian He, etc., Tian He, etc., “Range-Free Localization Schemes in Large Scale “Range-Free Localization Schemes in Large Scale

Sensor Networks”, Sensor Networks”, MobiCom, 2003MobiCom, 2003

Link EstimationLink Estimation

Individual nodes estimate link quality by Individual nodes estimate link quality by observing packet success and loss eventsobserving packet success and loss events

Use the estimated link quality as the cost Use the estimated link quality as the cost metric for routingmetric for routing

Good estimator should: Good estimator should: React quickly to potentially large changes in React quickly to potentially large changes in

link quality link quality StableStable Small memory footprint Small memory footprint Simple, lightweight computationSimple, lightweight computation

WMEWMAWMEWMA

SnoopingSnooping Track the sequence numbers of the packets Track the sequence numbers of the packets

from each source to infer lossesfrom each source to infer losses Window mean with EWMAWindow mean with EWMA

WMEWMA(t, a) = (#packets received in t) / WMEWMA(t, a) = (#packets received in t) / max(#packets expected in t, packets received max(#packets expected in t, packets received in t)in t)

t, a: tuning parameterst, a: tuning parameters t: #message opportunitiest: #message opportunities

Take average in a windowTake average in a window Take EWMA of the averageTake EWMA of the average

WMEWA (t =30, a =0.6)WMEWA (t =30, a =0.6)

Neighborhood Neighborhood ManagementManagement

Neighborhood tableNeighborhood table Record information about nodes from which it Record information about nodes from which it

receives packetsreceives packets How does a node determine which nodes How does a node determine which nodes

it should keep in the table?it should keep in the table? Keep a sufficient number of good Keep a sufficient number of good

neighbors in the tableneighbors in the table Similar to cache managementSimilar to cache management

Management PoliciesManagement Policies InsertionInsertion

Heard from a non-resident sourceHeard from a non-resident source Adaptive down-sampling techniqueAdaptive down-sampling technique Probability of insertion = N/T = neighbor table Probability of insertion = N/T = neighbor table

size / #distinct neighborssize / #distinct neighbors At most N messages can be inserted for every T At most N messages can be inserted for every T

messagesmessages

EvictionEviction FIFO, Least-Recently Heard, CLOCK, FIFO, Least-Recently Heard, CLOCK,

FrequencyFrequency

#Good neighbors #Good neighbors maintainable (table size 40)maintainable (table size 40)

Cost-based routingCost-based routing

MMinimize #retransmissionsinimize #retransmissions AA longer path w/ fewer longer path w/ fewer

#retransmission could be better #retransmission could be better than a shorter path w/ more than a shorter path w/ more #retransmissions! #retransmissions!

Routing FrameworkRouting Framework

Other Routing IssuesOther Routing Issues

Parent selectionParent selection Rate of parent changeRate of parent change PParent snoopingarent snooping CCyclesycles DDuplicate packet eliminationuplicate packet elimination QQueue managementueue management Relation to link estimationRelation to link estimation

Cost metricCost metric

MT (Minimum Transmission) metric: MT (Minimum Transmission) metric: Expected number of transmissions along Expected number of transmissions along

the paththe path For each link, MT cost is estimated by For each link, MT cost is estimated by

1/(Forward link quality) * 1/(Backward 1/(Forward link quality) * 1/(Backward link quality).link quality).

Performance Evaluation: Performance Evaluation: Tested Routing AlgorithmsTested Routing Algorithms

Shortest PathShortest Path SP: A node is a neighbor if a packet is SP: A node is a neighbor if a packet is

received from itreceived from it SP(t): A node is a neighbor if its link SP(t): A node is a neighbor if its link

quality exceeds the threshold tquality exceeds the threshold t tt = 70%: only consider the links in the = 70%: only consider the links in the

effective regioneffective region t = 40%: also consider good links in the t = 40%: also consider good links in the

transitional region transitional region

Minimum Transmission (MT)Minimum Transmission (MT) Use the expected #transmissions as the cost metricUse the expected #transmissions as the cost metric

BroadcastBroadcast Periodic fPeriodic floodinglooding Choose a parent based on the source address of the 1Choose a parent based on the source address of the 1stst

flooding message in each epochflooding message in each epoch Destination Sequence Distance Vector (DSDV)Destination Sequence Distance Vector (DSDV)

Choose a parent based on the freshest sequence Choose a parent based on the freshest sequence number from the rootnumber from the root

Maintain a minimum hop count when possibleMaintain a minimum hop count when possible Ignore link quality Ignore link quality –– consider a node a neighbor once consider a node a neighbor once

heard from itheard from it Periodically reevaluatePeriodically reevaluate

Packet level simulationsPacket level simulations

BBuilt a discrete time, event-driven uilt a discrete time, event-driven simulator in Matlabsimulator in Matlab

Empirical study of a Empirical study of a sensor fieldsensor field

Evaluate SP(40%), SP(70%), MTEvaluate SP(40%), SP(70%), MT 50 Berkeley motes50 Berkeley motes 5 * 10 grid w/ 8 foot spacing5 * 10 grid w/ 8 foot spacing

90% link quality in 8 feet90% link quality in 8 feet 3 inches above the ground3 inches above the ground

Link Quality of MTHop Distribution

E2E success rateE2E success rate StabilityStability

Irregular Indoor NetworkIrregular Indoor Network

30 nodes scattered around an indoor 30 nodes scattered around an indoor office of 1000ftoffice of 1000ft22

E2E Success Rate Link Estimation

ConclusionsConclusions

Link quality estimation and neighborhood Link quality estimation and neighborhood management are essential to reliable management are essential to reliable routingrouting WMEWMA is a simple, memory efficient WMEWMA is a simple, memory efficient

estimator that reacts quickly yet relatively estimator that reacts quickly yet relatively stablestable

MT (Minimum Transmissions) is an MT (Minimum Transmissions) is an effective metric for cost-based routingeffective metric for cost-based routing

The combinations of these techniques can The combinations of these techniques can yield high E2E success rates yield high E2E success rates

Questions?Questions?