Load-Balancing Clusters in Wireless Sensor Networks - ResearchGate

S j ∈ RSet Gi ⇔[( R Gi > d Sj->Gi ) Λ (R Sj,max > d Sj-->Gi )]Where, R Gi is the range of gateway G i , R Sj,max is themaximum range of sensor S j and d Sj->Gi is the distancebetween sensor S j and Gateway G i . A final set (FSet) isconstructed based on the minimum communication costbetween sensors and gateways [8]. For the purpose ofrecovery each gateway constructs another set containingnodes that do not belong to the cluster of the gateway butare included in its RSet. This set is called a Backup set(BSet). Each node only belongs to a single FSet but can bepart of many BSets. The definition of BSet is defined as:S j ∈ BSet Gi ⇔[( S j ∈ RSet Gi ) Λ (S j ∉ FSet Gi )]When a sensor has to be recovered all the gateways checktheir own BSets for the sensor. The sensor is recovered if itis present in the BSet of the gateway. If a sensor is presentin multiple BSets, it is accommodated by the gateway,which has the minimum communication cost with the sensorother then one failed. Once the sensor is associated with thebackup gateway, it is removed from the BSet of the backupgateway as well as the RSet of the faulty gateway.Due to previous schedule the receivers of the sensor areturned during the route update slot to receive the new updatefrom the gateway. Therefore, the backup gateway informsthe sensor about the new association in the same slot. NewTDMA schedules are given to the sensor according to thecardinality of the new gateway and the sensor becomes apart of the backup cluster.5. Experimental ValidationThe effectiveness of our recovery approach is validatedthrough simulation. This section describes simulationenvironment, fault injection technique and validation of theprotocol.5.1 Environmental SetupExperiments are performed on simulations with 1000sensors and 3 gateways uniformly distributed in a 10 × 10square kilometer area. Each sensor is assumed to have aninitial energy of 5 joules. A node is considered nonfunctionalif its energy level reaches 0 joules. The maximumrange of the sensors is set to 0.5 times the maximumdistance between two nodes in the system. Initial range ofthe gateways is considered enough to cover the whole area.It is assumed that the channel is collision free and packetsare not dropped in the medium. Sensors are given IDs inrandom fashion. Sensors are informed about the first TDMAschedules by their respective gateways. Schedule for firstinter-gateway communication is decided duringbootstrapping. Nodes switch on their transmitter if neededand receiver circuitry only during their allocated slots.Fault injection is used to test the robustness andbehavior of the sensor network. Fault injection allowsstudying the effectiveness fault detection and recoverycapabilities of our system. We use a simulation-based faultinjection methodology to inject communication faults in thegateways. We created a fault library of possible link, rangeand complete failures. We then created a timely orderedfailure list using Poisson distribution for occurrence offaults. Faults are picked from the library based on a Uniformdistribution for the type of failure and Normal distributionfor the location of the fault.We implement a fault-injector module to trigger thefaults as events. The fault-injector keeps a check on thesystem time and compares it with the timestamp of the nextentry in the failure list. Whenever the system time equals afault event time, the fault-injector selects the fault from thelibrary, reads the fault destination (gateway id) and insertsthe fault in the event queue of the gateway. When a gatewayencounters the fault in the event queue, it simulates thefault. The consequences of a fault experienced by thesystem are based on its type.G1RouteRFDataDataStatus:G2RouteDataDataCF:Fault InjectorG3DataStatusRouteDataRF-G1 CF-G2 LF-G3 LF-G1 RF-G3 RF-G3 …Fig 5. Design of the fault injection environmentFig 5 describes the design of the fault-injectionenvironment. It shows a link fault being injected by thefault-injector in gateway G3. Previously, gateway G2 stopsall operation due to complete failure. Also, gateway G1 issuffering from a range fault. In order to measure theperformance of our approach we calculate the coverage ofthe algorithm. Coverage is the ability of the system to detectand recover from the occurrence of a fault during normalsystem’s operation [17].Coverage = Probability [system recovers/fault occurs]We have injected 1000 faults in order to measure thecoverage of our algorithm. Since, complete failures are lesscommon compared to other faults, we have inserted themwith lower frequency than range and link failures. AllLF:5

complete failures are detected instantaneously during thestatus period. We injected faults to decrease the range ofgateway G1 by 2% every 15 min of operation. On alloccasion our detection mechanism detects the faults andidentifies the sensors that have gone out of range from thegateways. Sensors are successfully recovered to othergateways till the range of G1 drops below a threshold andonly the sensors very close to G1 are left in the cluster.After every subsequent range fault in G1 coverage decreasesbecause the algorithm fails to find any gateway toaccommodate the sensors.Link failures are injected in gateway G3 to study theimpact on number of status messages in the system. Due tolink failures status messages are not received by othergateways and forwarding scheme is activated. Forwardingincreases the number of status message by (N-2) 2 , where Nis the number of gateways in the system. The period ofstatus updates is controlled by MILD algorithm until weinject a burst of faults making the recovery more frequent.The coverage of link failure has been observed to be 100%throughout the simulation until all the incoming/outgoinglinks from a gateway fails. After all link fails, any fault onG3 are detected as a complete failure by other gateways.Recovery of sensors in the cluster of G3 is only done on thefirst complete failure.The results of the fault injection experiments clearlydemonstrate that the system is resilient to communicationfaults and recovers efficiently without re-configurations ormanual repairs.6. Conclusions and future work:High-energy gateway node acts as a centralized manager tohandle the sensors and serves as a hop to relay data fromsensors to a distant command node. In this paper we haveintroduced a two phase; detect and recover fault-toleranceapproach to recover sensors from the failed gatewayswithout shutting down or re-clustering the system.Gateways can suffer from complete, link or range failurescaused due to software or hardware faults. Our approachenables fault-tolerance in the system by performing periodicchecks on the status of the gateways. Sensors managed by afaulty gateway are recovered by re-associating them to otherclusters based on backup information created during thetime of clustering.Our future plan includes extending the clustering modelto allow gateway mobility. Also, we plan to integratebootstrapping and energy-aware routing to our approach.References[1] R. 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