a fault-tolerant energy-efﬁcient clustering protocol of a wireless...

WIRELESS COMMUNICATIONS AND MOBILE COMPUTINGWirel. Commun. Mob. Comput. 2014; 14:175–185

Published online 26 January 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.1240

RESEARCH ARTICLE

A fault-tolerant energy-efficient clustering protocol ofa wireless sensor networkLutful Karim1*, Nidal Nasser1 and Tarek Sheltami2

1 School of Computer Science, University of Guelph, Guelph, Ontario, Canada2 Electrical & Computer Engineering Department, College of Engineering, Alfaisal University, Saudi Arabia

ABSTRACT

Energy efficiency in specific clustering protocols is highly desired in wireless sensor networks. Most existing clusteringprotocols periodically form clusters and statically assign cluster heads (CHs) and thus are not energy efficient. Everynon-CH node of these protocols sends data to the CH in every time slot of a frame allocated to them using the timedivision multiple access scheme, which is an energy-consuming process. Moreover, these protocols do not provide anyfault tolerance mechanism. Considering these limitations, we have proposed an efficient fault-tolerant and energy-efficientclustering protocol for a wireless sensor network. The performance of the proposed protocol was tested by means of asimulation and compared against the low energy adaptive clustering hierarchy and dynamic static clustering protocols.Simulation results showed that the fault-tolerant and energy-efficient clustering protocol has better performance than boththe low energy adaptive clustering hierarchy and dynamic static clustering protocols in terms of energy efficiency andreliability. Copyright © 2012 John Wiley & Sons, Ltd.

KEYWORDS

wireless sensor network; clustering protocol; DSC; LEACH; LESCS; FT-EEC; fault-tolerant

*Correspondence

Lutful Karim, School of Computer Science, University of Guelph, Guelph, Ontario, Canada.E-mail: [email protected]

1. INTRODUCTION

Designing energy-efficient and reliable routing protocols ishighly important in a resource-constrained wireless sensornetwork (WSN). Clustering protocols of WSNs are consid-ered very energy efficient, wherein several sensor nodes inthe communication range constitute a cluster. Each clusterhas a cluster head (CH), which coordinates all the nodesof a cluster. A number of base stations (BSs), also knownas ‘sinks’, connect the WSN to other networks. The CHaggregates the data received from the non-CH nodes andsends them to the BS. There also exist some gateway nodesin a cluster for intercluster communications. In clusteringprotocols, CHs produce limited useful information from alarge amount of raw data that are sensed by the membernodes of a cluster. Transmitting this precise and nonredun-dant information to the BS of the network consumes lessenergy [1,2]. Figure 1 illustrates different types of nodesand data transmissions from a CH to a BS through gatewayand intermediate nodes.

Because sensor nodes have limited bandwidth, energy,storage capacity, and processing speed, designing a fault-tolerant and energy-efficient clustering (FT-EEC) protocol

is an important research issue in WSNs. However, mostexisting clustering protocols of WSN are both not energyefficient and fault tolerant. For instance, the low energyadaptive clustering hierarchy (LEACH) and dynamic staticclustering (DSC) protocols, which are considered to beenergy-efficient protocols, do not provide any fault toler-ance mechanism. Hence, if a node or CH fails, it cannot bedetected by the CH or BS. Moreover, because the CH orBS does not subscribe to the non-CH nodes for the eventsof interest, it is assumed that the non-CH nodes send datato the CH in every time slot of a frame allocated to them.Hence, more energy is consumed. In this paper, we proposean FT-EEC protocol that provides reliability using a faulttolerance mechanism. Using this mechanism, the CH willbe able to detect the failure of non-CH nodes and the BSwill be able to detect the failure of CHs. Moreover, the CHand/or BS subscribe to the non-CH nodes of a cluster to benotified only when an event of interest occurs, and so thenon-CH nodes do not send data to CH in every time slot ofa frame allocated to them, which reduces energy consump-tion. Moreover, clusters are assumed to be homogeneous(equal size) and a set of minimum number of nodes areselected as active in each cluster. Variable-sized frames in

Copyright © 2012 John Wiley & Sons, Ltd. 175

Fault tolerant clustering protocol of WSN L. Karim, N. Nasser and T. Sheltami

Base Station

Cluster Head

Non-Cluster Head Node

Gateway

Cluster 1

Cluster 2

Cluster 3

Cluster 4

Figure 1. Clustering protocol of wireless sensor network.

different rounds will be chosen by BS for reducing theenergy consumption of the networks. We perform a sim-ulation using the C programming language to measure theperformance and reliability of the proposed FT-EEC pro-tocol and also compare it against the LEACH and DSCprotocols.

The remaining part of this paper is organized as follows:Section 2 briefly presents a few clustering protocols ofWSN. Section 3 discusses the proposed FT-EEC protocolwith its algorithm (pseudo-code). Section 4 analyzes theFT-EEC protocol and presents the simulation with perfor-mance comparison. Finally, Section 5 concludes the paperwith some future research directions.

2. RELATED WORKS

In this section, several existing clustering protocols ofWSN are described [1,3–13].

2.1. Low energy adaptive clusteringhierarchy protocol

The LEACH protocol works well for homogeneous net-works, where every node has the same initial energy. Thisprotocol works in rounds and each round is divided intotwo phases: cluster formation and steady phase. In thecluster formation phase, a cluster is formed and p � n sen-sor nodes are selected as CH for the proper utilization ofenergy where, n is the number of sensor nodes and p isthe desired percentage of CH. Otherwise, if only one nodeis selected as CH, it will fail because of the shortage of

energy. The decision to select a node as CH is based on athreshold value T .n/. If a random number (between 0 and1) chosen by a node A is less than T .n/; A is selected asa CH in the current round. Moreover, the suggested per-centage of CH of the networks and the number of time anode is selected as a CH are also decision criteria for anode to be a CH in the current round. The steady state isdivided into many frames and the CH assigns time slots foreach non-CH node using a time division multiple access(TDMA) scheme in each frame. At the end of each round,the CH collects and aggregates data and sends them to theBS. In LEACH, a new cluster formation is initiated in everyround, which is not energy efficient. Occasionally, all CHsexist in a close area (as CH rotates in a cluster) so non-CHnodes require more energy to communicate with the CHs.Moreover, cluster size in LEACH can be very large andvery small at the same time, which results again in imbal-anced energy consumption. Thus, the work carried out byKatiyar et al. [8] proposed the Far Zone-LEACH protocolthat eliminates the drawback of having variable-sized clus-ters by creating zones into the clusters that are large andfar from the BS. Zones are created based on the minimumreachability power (MRP). If the MRP of a node is greaterthan the average MRP, then the node is a zone member. Themaximum residual energy node is selected as a zone head.The zone head sends the collected data of zone membersto the CH.

The LEACH-centralized (LEACH-C) protocol differsfrom LEACH by using a central cluster formation algo-rithm in BS. The LEACH-fixed protocol uses the samecluster formation algorithm as LEACH, but CHs are fixedand selected once in the first setup phase; only theirposition rotates in the cluster.

176 Wirel. Commun. Mob. Comput. 2014; 14:175–185 © 2012 John Wiley & Sons, Ltd.DOI: 10.1002/wcm

L. Karim, N. Nasser and T. Sheltami Fault tolerant clustering protocol of WSN

2.2. Secured low energy adaptiveclustering hierarchy

In LEACH, security is obtained by providing CH authen-tications, data confidentially, etc. However, LEACH doesnot provide two-party data authentications. A new idea toprovide security in the LEACH protocol is to use a ran-dom predeployment key distributions scheme. This proto-col is known as secured LEACH (sec-LEACH). This keydistribution approach is designed only for a node to com-municate with a static set of neighboring nodes. In thisapproach, a key pool of size S is generated. A key ringof randomly selected m keys for the key pool is assignedto a node. A unique identification (ID), idx is assigned toa node X using a pseudo-random function. idx is used toseed a random number generator to generate m randomnumbers, Rx, which represent the key IDs assigned to thenode idx. Each node also has a key to share with the BS. Inthe advertisement phase, CHs broadcast their IDs so thateach node can use these IDs to produce a set of m keysof a CH using pseudo-random number generator. A nodejoins a cluster if the CH of that cluster and the node has acommon key. This condition might restrict a node to jointhe closest cluster and also create some orphan nodes (i.e.,nodes that reside outside of a cluster). Hence, orphan nodessend data directly to a BS, which can be far away. In thiscase, this protocol is expensive in terms of energy con-sumption. Moreover, nodes in a cluster might be far awayfrom the CH. Hence, this key distribution method is notenergy efficient. Thus, the authors in [14] proposed a grid-based secure LEACH protocol (GS-LEACH), which tradesoff between security and energy efficiency by providingrandom key distributions and a controlled deployment ofnodes in a grid.

2.3. Grid-based secure low energy adaptiveclustering hierarchy protocol

In the GS-LEACH protocol, n sensor nodes are deployedaround each point of the grids (square) in a region R. Sen-sors around each grid point forms a cluster and selects fewCHs to communicate with the BS. A BS resides outsideor far away from R and is assumed to have more energyand processing capabilities. CH rotates in the cluster fora proper distribution of energy and performs data fusion.Prior to a group deployment, each node is assigned to arandomly generated m keys from a key pool of size Sfor communicating with other nodes in a cluster/group.In addition to these keys, each node has a key to com-municate with the BS. During the setup phase, each nodeelects itself as a CH with some probability and sends ashort message and key information to other members inthe group. The nodes that have a common key with CHresponds to CH by sending a join request message and theID of the common key. The nodes in the group whose keyIDs do not match with the CH key ID remains as candi-dates for CH with a high probability in the next round.

A proper probability assignment ensures each node to bea CH and balances energy in n rounds. In the steadystate, CH uses the TDMA scheme and sends the time slotinformation to each node of the group (cluster). Everynode is allowed to sense, process and send encrypted datato CH only in its allowed time slot and thus, energy issaved. After getting encrypted data from each node, CHdecrypts it using the common key. CH aggregates dataand sends them to the BS using a unique BS encryptionkey.

In GS-LEACH, each node is allowed to transmit datain a small region (around a grid point), which ensures lesscommunication overhead. In sec-LEACH a node is allowedto communicate with any CH (based on the common keys)even if the CH is far away from the node. Therefore,sec-LEACH is not energy efficient.

2.4. Threshold-sensitive energy-efficientsensor networks protocol

Threshold-sensitive energy-efficient sensor networks(TEEN) is a hybrid of the data-centric and hierarchicalclustering protocols. It reduces the number of data trans-mission to BS by implementing two thresholds: soft andhard. Hence, the lifetime of a network increases. Initially,clusters are formed and CHs are selected by the BS. CHsbroadcast these two thresholds to all non-CH nodes. Thehard threshold is the minimum value of a sensed attribute.When a node senses an event (e.g., temperature) whosevalue is equal to or greater than the hard threshold, thenode turns on its radio to transmit data to the BS. After thehard threshold, a node will transmit only when the attributevalue is changed by the soft threshold.

To reduce the limitation of responsive data collection(only when threshold value is reached) in TEEN, Adap-tive threshold sensitive energy-efficient sensor networks(APTEEN) is proposed, which works both in periodic andresponsive data-collection modes. APTEEN has also thedrawback of engaging the BS in creating clusters andCHs. Moreover, clusters are formed here in multiple lev-els and require more communications (so it is not energyefficient).**

2.5. Geographic adaptive fidelity protocol

The geographic adaptive fidelity protocol [6] is a location-based protocol designed especially for ad hoc networks andalso works for WSN. Some nodes remain in sleeping modeto preserve energy without affecting the network connec-tivity. The geographic adaptive fidelity protocol forms avirtual grid (geographic area) for a set of nodes, which areequivalent in terms of their communication ability with thenodes of a neighboring grid. This protocol also representshierarchical clustering, where a grid/geographic area cor-responds to a cluster. However, because nodes use a globalpositioning system for locating positions (i.e., consumesmore energy), this protocol is expensive.

Wirel. Commun. Mob. Comput. 2014; 14:175–185 © 2012 John Wiley & Sons, Ltd. 177DOI: 10.1002/wcm


2.6. Low energy static clustering scheme

In static clustering protocols, CHs deplete energy fasterthan other sensor nodes and fail. This is known as a hotspotproblem. In [5], the authors proposed a low energy staticclustering scheme for WSN, which solves the hotspot prob-lem of static clustering by distributing the energy consump-tion equally over all nodes. Hence, this scheme increasesthe network lifetime.

The low energy static clustering scheme considers allsensor nodes homogeneous, energy-constrained, and fixed.Energy consumption is different for different types of sen-sors. The BS is fixed and located on the outside of theWSN, and the whole network fails because of the failureof a certain percentage of sensor nodes.

This protocol has three phases: (i) the centralist networkclustering calculation phase; (ii) cluster formation; and(iii) the intracluster scheduling phase. In the centralist net-work clustering phase, the BS divides the whole networkinto a few numbers of clusters and also selects CHs for eachcluster. Once clusters are formed and CHs are selected,they are fixed (never change) and the BS broadcasts thisinformation to the whole network. In the cluster formationphase, each non-CH node joins in a cluster whose centralpoint is closest to that node. The CH records the positionand energy information of each node in its routing tableand also assigns a ranking/sequence number 0, 1, 2, . . . ,k–1 to the sensor nodes. In the intracluster schedulingphase, CH will assign a gateway (a sensor node) for itscluster to communicate with neighboring clusters. Thegateway of one cluster sends information to the gatewayof a neighboring cluster for intercluster communication.A gateway performs data collection, fusion, and transmis-sions to the gateway of neighboring clusters using multi-hop data transmission. If a sensor permanently works as agateway, it depletes energy very quickly and fails (hotspotproblem). To avoid this problem, the most residual energynode is selected as a gateway in the next round.

2.7. Dynamic static clustering protocol

In [1], Bajaber and Awan proposed a DSC for WSNs andcompared its performance with the LEACH-C protocol.They found that the DSC had a better performance thanLEACH-C in terms of energy efficiency, network lifetime,and communication overhead.

The DSC protocol has dynamic and static cases. Thedynamic case is divided into two phases: setup and steadyphase. In the setup phase, the BS forms clusters and selectsa CH for each cluster based on the energy levels andpositions of the sensor nodes. Then, the BS broadcasts CHIDs to all nodes. A sensor node will be a CH if its IDmatches with the CH ID. In the steady phase, CH uses theTDMA scheme by dividing each frame into x number oftime slots, where x is the total number of non-CH nodesin that cluster. A non-CH node transmits data to the CHonly in the allocated time slots and saves energy by turningits radio off (sleep mode) in all other time slots. When around is completed, data transmitted by all non-CH nodesare aggregated and sent by the CHs to the BS. In the nextround, the current CH of a cluster selects a node as anew CH, which has the most remaining energy. Figure 2represents the working principle of the DSC protocol.

The static case has only the steady phase, which is sim-ilar to that of the dynamic case except that after a certainnumber of rounds (i.e., 10), a new cluster formation (setup)phase is initiated. However, the static case has a lessernumber of cluster formation phases as compared with thedynamic case and thus has less transmission overhead.

Among other clustering protocols, the energy-efficientprotocol with static clustering partitions the network intosome static clusters and uses the residual energy of sen-sor nodes to select CH. However, if CHs reside at theboundary of clusters, energy consumption for intraclustercommunication increases. Thus, the work carried out byChaurasiya et al. [3] proposed the enhanced efficient pro-tocol with static clustering, which also considers the spatialdistribution of sensor nodes along with residual energyto select CH. The adaptive clustering algorithm based onenergy restriction [12] is another clustering protocol thatselects CH based on residual energy and network coveragerate (i.e., the quality of communication). Thus, this proto-col is important for WSN having sensor nodes with dif-ferent communication ranges. Cluster formation for staticclustering protocols are carried out at the beginning of apredefined round even if the remaining energy of CH isat a critical stage. In [4], Elhawary and Hass proposed acooperative clustering protocol, where each node on thepath from the source node to the destination becomes aCH. Each CH recruits neighboring nodes to coordinate itstransmission towards the destination and thus forms a clus-ter to reduce packet delivery failure. In [11], the authorsproposed the hybrid clustering approach, where cluster

Figure 2. Cluster formation and steady state.



formation is carried out on-demand at the beginning of thenext round if the remaining energy of CH is lower than athreshold value. However, in these protocols, nodes do notcooperatively send data to the BS.

3. PROPOSED FAULT-TOLERANTAND ENERGY-EFFICIENTCLUSTERING PROTOCOL

Fault tolerance is the mechanism that provides the contin-uous operation of the network by detecting the failure ofsensor nodes and assigning the operation of failed nodesto other nodes. Thus, fault tolerance mechanisms providethe reliability of a clustering protocol of WSNs. How-ever, most existing clustering protocols do not provide anyfault tolerance mechanism. Moreover, in the LEACH andDSC protocols, each non-CH node A sends data to theCH in every time slot allocated to A using the TDMAscheme. If the CH subscribes events to non-CH nodes,the non-CH nodes could save much energy by sendingdata only in the time slot when a subscribed event occurs.To provide reliability and reduce energy consumption, wepropose an optimized FT-EEC protocol. The followingsubsections present the working principle and pseudo-codeof the FT-EEC protocol.

3.1. Working principle

We assume that all nodes have the same initial energy.They know their locations and send the location informa-tion through multihop to BS. Then, the FT-EEC protocolworks in two phases: setup phase and steady phase.

3.1.1. Setup phase.In this phase, BS selects a number of nodes as CHs based

on the following criteria: initially, BS selects a node A ran-domly as a CH. Then, BS selects another node B randomlyas a CH if nodeB is not within the communication range ofnode A. Whenever a node is selected as a CH, BS notifiesthat node. Then, CHs broadcast their IDs to the network.A non-CH node joins the cluster of a CH, from which itreceived the message, with the highest signal strength, thatis, the received signal strength indicator (RSSI).

In most clustering protocols of WSN, every non-CHnode of a cluster senses events. In the FT-EEC protocol,a set with a minimum number of active nodes are selectedby CH based on the sensing and communication range, Rsand Rc (this eliminates overlap of sensing and also avoidsa sensing hole). Each cluster is divided into several smallsquares. Each square has at least one active sensor node,which is selected based on the most residual energy. How-ever, even if this assumption does not hold, the region ofthat square is sensed by an active node of the neighboringsquares. This is because an active sensor node in a squarehas the sensing coverage of all the neighboring squares(Figure 3). Therefore, a very high probability of not havinga sensing hole in the network is achieved. As a result, fault

hh2h

Figure 3. Zone-based topology where the sensing range Rs D2p

2 h.

tolerance is also accomplished. For instance, whenever theactive sensor node of a square g1 fails, the network opera-tion (i.e., sending and receiving messages) proceeds. Thisis carried out without activating another sensor node forthat square or re-establishing the network topology. Thisis because the active node of the neighboring square of g1fully covers g1.

Using this topological structure, there is a high proba-bility that no sensing hole exists in the network (Figure 3).The terms h and Rs are the side of a square and the sens-ing range, respectively. Let us consider the following: theactive sensor node a1 of square g1 exists at the bottomleft corner and the active node a2 of square g2 exists atthe top right corner. If the node a2 fails, then the squareg2 can still be covered by the node a1 of square g1. Thisis even if no other neighboring square of g2 has an activenode. This is explained as follows: the farthest point p2of g2 is within the sensing range of a1. By following thePythagoras formula, the square coverage is possible whenRs D

p.2h/2C .2h/2 D

p8h2 D 2

p2 h. Figure 3 also

clarifies this relationship between Rs and h.

3.1.2. Steady phase.In the steady phase, a number of frames constitute a

round, where each node has a time slot allocated to it in aframe using the TDMA scheme. In the steady phase, eachactive nodeA that is selected in the setup phase sends eitherthe data that is sensed (i.e., subscribed events) or a spe-cial packet to the CH in its allocation timeslot using theTDMA scheme. All other nodes remain in sleep mode byturning the radio off. Special packets inform the CH thatA is still alive in the case when no subscribed event occursin A, to notify the CH in that time slot. If the CH does notreceive any data or a special packet from A, the CH willassume that A has failed. Then the CH will exclude thefailure node from the allocated time slot (fault detection).The size of the special packet is much smaller than that



of the sent event or data. Hence, sending a special packetconsumes less energy compared with that of a data packet.

At the end of a frame, if BS does not receive any datafrom a CH, the BS sets a timer and sends a ‘hello’ messageto that CH. If the BS does not hear any ‘ACK’ messagefrom the CH before the timer expires, the BS assumes thatthe CH has failed. Then the BS assigns the node with themost residual energy of the cluster as a new CH. Usually,at the end of each round, the CH assigns the node with themost remaining energy as a new CH. If the BS is not awareof the failure of a CH, the new CH assignment at the endof a round will not be performed.

At the end of each round, the CH and active nodes willbe reselected based on the residual energy, number of sleeprounds, free buffer spaces of nodes, etc. For instance, hav-ing the same residual energy in nodes A and B , node Awill be selected as either the CH or the active node if nodeA has more remaining storage space than that of node B .Further ties are broken by comparing the number of sleeprounds of a node. A node is given priority with the high-est number of rounds in which it was in sleep mode. Thisreselection of CH and active nodes at the end of each roundbalances the energy consumption of individual nodes andnetwork.

Dynamism is achieved again by using a variable num-ber of frames to form a round based on the energy statusof the nodes of the networks. The BS selects the num-ber of frames that constitute a round and broadcasts thisnumber to the networks. Moreover, the cluster formationphase is initiated at a certain number of rounds, Rn, whichalso depends on the energy status of sensor nodes and thenetwork. This balances the energy consumption of the net-work. The following formula can be used to recalculateRn.

Rn DPrev No.OfRounds

Prev NetEnergy�Current Net Energy (1)

Each node keeps track of the number of messages trans-mitted and sends this information along with the datapacket to CH. The BS estimates the total network energyconsumption whenever it receives data packets from CH.

3.2. Algorithm

The pseudo-code of the FT-EEC protocol is presented asfollows:

Initial State (Cluster formation, CH selection)

1. BS selects a node randomly as CH[0] //initial CHselection

2. for i 1 to MaxNoOfCHs dodo while CH[i] is not selected

NodeID[i ] random( )for j 0 to i � 1 do

if NodeIDŒi �D CHŒj � then exit

end ifif Rc of NodeID[i ] intersect with Rc of CH[j ]

then exitend if

end forCH[i ] NodeID[i ]

end whileend for

3. fori 1 to MaxNoOfCHs doCH[i ] broadcast its ID to the networkfor each node j that receives the message do

if Node[j ] is not a CH thenNode[j ] stores RSSIOfCH[i ]

end ifend for

end for4. for each non-CH node j in the network do

if RSSIOfCH[i ] is maximum thenNode[j ] joins to cluster iCH[i] subscribes to Node[j ] for event of interest

end ifend for

/�. . . . . . . Active Node Selection . . . . . . . . . . . . .�/5. for each cluster i do

do divide ClusterArea[i ] into a square grid[i ,j ]SideOfSquare Rs

2p2

ActiveNode[i , j ] Node[1]for k 2 to NoOfNodesInGrid[i , j ] do

if ResidualEnergyNode[k] > ActiveNode[i ,j ]then ActiveNode[i , j ] Node[k]

else if ResidualEnergyNode[k]D ActiveNode[i ,j ] then

if bufferSpaceNode[k] >bufferSpaceNode[kC 1] thenActive Node[i ,j ] Node[k]

end ifend if

end forwhile cluster is not fully divided

end for

Steps 1 and 2 present the CH selection process. One CHis selected randomly in Step 1. Step 2 presents how theremaining CHs are selected so that their communicationranges do not overlap. In Steps 3 and 4, nodes join a CHbased on the RSSI value. In Step 5, each cluster is dividedinto a number of small squares to select the minimumactive nodes in a cluster.

Steady State (TDMA scheme, Data/Special Packetsending)

6. for each round i dofor each node j of a Cluster k do

nodeTimeSlot[k, j ] timeslot t



if node[k][j ] has a sensed event thennode[k][j ] sends data to CHcalculate dataEnergyConsumption[k][j ],ReceiveEnergyofCH[k]

else //no event to send, notify CH that node isstill alive

node[k][j ] sends a special (small) packetto CHcalculate SpecialEnergyConsumption[k][j ],ReceiveEnergyofCH[k]

end ifend for

/� A round is finished �/CH[k] aggregates Data and Sends to BScalculate CHEnergyConsumption[k] foraggregation and sending to BSCH[k] selects new CH and Active nodes basedon Residual energy, number of sleep rounds,buffer space (Step 5)calculate CHEnergyConsumption[k] for a newCH selection

end for7. Iterate Step 6 for a certain number of rounds Rn

and then go back to cluster formation phase. Rn isrecalculated at the beginning of steady phase usingEquation (1).

Steps 6 and 7 present the steady state, where each sen-sor sends data to CH using the TDMA scheme. If a sensorhas no sensed event or data, it sends a small special packetto CH to notify the CH that the node is still functioning.These functionalities detect the failure of a node that is apart of a fault tolerance process.

3.3. Energy model

The energy required for transmitting a packet of size n overdistance d is given by

ETX D n�"elecC n�"�aird˛ (2)

The energy required for receiving a packet is

ERX D n�"elecC n�"�aird˛ (3)

where, "elec and "air in Equations (2) and (3) represent theenergy spent in the transmitter electronics circuitry and theenergy spent in Radio Frequency (RF) amplifier for prop-agation loss, respectively. The constant (propagation lossexponent) ˛ is dependent on the surrounding environment.For free space ˛ D 2.

4. ANALYSIS AND SIMULATION

The following subsections present the mathematical analy-sis, simulation setup, and performance analysis of the exist-ing and proposed FT-EEC protocol based on the simulationresults [1].

4.1. Analysis

The energy consumption of a node for sending a datapacket of size ndata bytes to the BS at distance d is

Edata.ndata; d /D ndata � "elecC ndata � d2 � "air (4)

The energy consumption of a node for sending a specialpacket of size nspecial bytes to the BS at distance d is

Espec.nspec; d /D nspec � "elecC nspec � d2 � "air (5)

We assume that the probabilities of a node to send the datapacket and special packet are Pdata and Pspec, respectively.Hence, the total energy consumption of a node, consideringthat each node will either send the subscribed event/datapacket (only when the event occurs) or a special packet (tonotify the node is still alive) to the BS is

ETX D Pdata�Edata.ndata; d /CPspec�Espec.nspec; d / (6)

Similarly, the energy consumption of a CH from the receiptof a data packet or special packet is

ERX D Pdata � ndata"elecCPspec � nspec"elec (7)

(There is no energy consumption for receiving data in freespace).

If the distance of the BS from a CH is dBSDist and thesize of an aggregated data is nAggData, the energy consump-tion of a CH to aggregate and send data to the BS at the endof each round is

EAggregate TX D nAggData�"elecCnAggData� d2BSDist�"air(8)

As the size of the special packet (nspec) is much smallerthan that of the data packet (ndata), we assume that

ndata D k � nspec;wherekis a constant: (9)

In simulation, we assume that the value of k is atleast 32.

Moreover, the probability of sending data packets andspecial packets has the following relationship:

Pdata D l �Pspec where 0 < l � 1 (10)



By placing Equations (9) and (10) into Equation (6),we get

ETX DPdata �Edata.ndata; d /CPspec �Espec.nspec; d /

DPdata �Edata.ndata; d /CPdata

l� 1k�Edata.ndata; d /

DEdata.ndata; d /�Pdata C

Pdata

lk

�(11)

From Equation (11), the transmission energy of a nodein the DSC protocol can be calculated and related to theFT-EEC as follows:

Edata.ndata; d /DETX

.Pdata C Pdatalk /Dm�ETX)EDSC_TXDm�EFT�EEC_TX

(12)

mD 1.Pdata C Pdatalk /

(13)

where EDSC_TX and EFT�EEC_TX represent the energyconsumption of a node for transmitting data in the DSCand FT-EEC protocol.

Considering the values of k and l in Equations (9)and (10), respectively, the value of the denominator ofEquation (13) will always be less than 1. Thus, the valueof m in Equation (13) will be m > 1. For instance, ifPdata D 0:9, then Pspec D 0:1 and l D 0:9=0:1 D 9 (usingEquation (10)). Using Equation (12) and k D 16 we findmD 1=.0:9C .0:9=.9� 16//D 1:1034 > 1.

Again, if Pdata D 0:1, then Pspec D 0:9 and l D0:1=0:9 D 9 (using Equation (10)). Using Equation (13)and k D 16 we find mD 1

0:1C 0:10:11�16D 6:37 > 1.

Hence, Equation (12) implies that the DSC protocol con-sumes more energy in data transmissions than the FT-EECprotocol.

Similarly, we can relate the DSC protocol with theFT-EEC protocol in terms of the receiving energy con-sumption of CH. From Equation (7), we can easily find thefollowing relationship:

EDSC_RX Dm�EFT�EEC_RX (14)

where EDSC_RX and EFT�EEC_RX represent the energyconsumptions of a CH for receiving data in the DSC andFT-DSC protocols. From this analysis, it can be concludedthat our proposed FT-EEC protocol is more energy efficientthan the traditional DSC protocol.

Similarly, we can show that FT-EEC is more energy effi-cient than LEACH because a cluster formation phase isinitiated at the end of each round in LEACH, whereas aftera certain number of rounds, a cluster formation phase isinitiated in FT-EEC. Moreover, in the FT-EEC protocol, anon-CH node sends a special packet to the CH to notifythat it is still alive. This provides fault tolerance in theFT-EEC protocol.

4.2. Simulation setup

We simulate the proposed FT-EEC protocol using the Cprogramming language. In this simulation, the networksize is assumed to be 100 � 100 m and the position ofthe BS is at 90 � 170. The following table shows themain network parameters and their respective values usedin the simulation.

Users are allowed to input the number of nodes (maxi-mum 200), number of clusters (maximum 16), number ofrounds, and number of cluster formation phases in our sim-ulator. For networks with a fixed number of clusters andnodes, we change the number of rounds at different runs ofthe simulation.

4.3. Simulation results andperformance analysis

On the basis of the parameters and their values presentedin Table I, we perform a simulation of the LEACH, DSC,and FT-EEC protocols and measure their performances interms of energy consumption, number of communicationsand network lifetime (or energy dissipation). We run thesimulation 15 times at each round and take the average of15 values.

Simulation results show that the energy consumption ofthe LEACH and DSC protocols are much higher than thatof the FT-EEC protocol (Figures 4 and 5). The energyconsumption include consumption of non-CH nodes fortransmitting data and special packets to CH, consumptionof CH nodes for receiving data and special packets, aggre-gating data packets and sending to the BS, selecting newCH in the next round, etc. Student’s t -test also revealsthe same phenomenon that energy consumption of sensornodes over time are much higher in LEACH and DSC thanin the FT-EEC protocol.

Figures 6 and 7 show the number of communicationsover a number of rounds that include the number of dataand special packets transmission, message transmission foractive nodes selection in the FT-EEC protocol, message

Table I. Simulation parameters and their values.

Parameter Value

Network size 100 � 100Number of nodes Maximum 200Number of clusters Maximum 16Base station position (90 170)Data packet size 512 bitsSpecial packet size 16 bitsEnergy consumption for 40 nJoule/bit

sending data packetsEnergy consumption in 0.01 nJoule/bit/m2

free space/airInitial node energy 2 JoulesCluster head probability 3%



0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5 10 15 20 25 30 35 40 45 50Number of Rounds

Ene

rgy

Con

sum

ptio

ns (

mJo

ule)

FT-EEC

DSC

LEACH

Figure 4. Energy consumptions over the number of rounds.

0

1000

2000

3000

4000

5000

6000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33Number of Simulations

Ene

rgy

Con

sum

ptio

ns (

mJo

ule) FT-EEC

DSC

LEACH

Figure 5. Energy consumption over a number of simulations.

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

5 10 15 20 25 30 35 40 45 50Number of Rounds

Num

ber

of C

omm

unic

atio

ns FT-EECDSC

LEACH

Figure 6. Number of communications over a number rounds.

transmissions for a new CH, and cluster selection. It isobserved that the communication overhead is higher in theLEACH and DSC protocols than in the FT-EEC proto-col because the number of active nodes is much less inthe FT-EEC protocol than in the LEACH and DSC pro-tocols. Moreover, in the FT-EEC protocol, when there isno event or data packet to be sent, a non-CH transmitsa special packet to the CH to notify that it is still alive(hence, the fault tolerance is achieved). In this case, even

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Number of Simulations

Num

ber

of C

omm

unic

atio

ns

FT-EEC

DSC

LEACH

Figure 7. Number of communications over a number ofsimulations.

if both protocols have the same number of transmissions,the size of the special packet is much smaller than thatof the data packet (Table I). Thus, a special packet con-sumes less energy and has less communication overheadas well.

192

193

194

195

196

197

198

199

200

201

5 10 15 20 25 30 35 40 45 50

Number of Rounds

Rem

aini

ng N

etw

ork

Ene

rgy

(Jou

le)

FT-EEC

DSC

LEACH

Figure 8. Network lifetime over rounds.

192

193

194

195

196

197

198

199

200

201

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Number of Simulations

Rem

inin

g E

nerg

y (J

oule

FT-EEC

DSC

LEACH

Figure 9. Network lifetime over simulations.



Table II. Comparison among LEACH, DSC, and FT-EEC protocols.

Features LEACH DSC FT-EEC

BS forms clustersp p p

BS subscribes for events of interest to CHs and non-CH nodes X Xp

CH subscribes for events of interest to non-CH nodes X Xp

Non-CH nodes send data to CH in every time slot of a framep p

X(using TDMA scheme)

Non-CH nodes send special packets to notify that they X Xp

are still aliveBS sends ‘Hello’ message to a CH if the CH does not send X X

p

aggregated data to BS at the end of a round Fault tolerance X Xp

Variable number of frames in a round X Xp

Avoid redundant coverage and data X Xp

Figures 8 and 9 present the network lifetime in terms ofthe remaining energy of the network (of all nodes) over anumber of rounds and simulations. We find that the energydissipation in the LEACH and DSC protocols over a num-ber of rounds is much greater than the FT-EEC protocol,hence, the LEACH and DSC protocols have shorter net-work lifetimes than the FT-EEC protocol. Student’s t -testalso reveals the same phenomenon.

Moreover, in the simulation results, we find the numberof CH or non-CH node failures, which represent the faulttolerance mechanism in the proposed FT-EEC protocol.FT-EEC can detect the failure of nodes not only for energyshortage but also for other reasons, including interference,deployment of nodes in cold or harsh conditions, etc. Theexisting DSC can only detect failure of nodes because ofenergy dissipation. This also represents robustness of theFT-EEC protocol.

Finally, in Table II, we briefly present a comparisonamong the LEACH, DSC, and FT-EEC protocols on someimportant features.

5. CONCLUSION ANDFUTURE WORK

In this paper, an FT-EEC protocol has been proposed. Themathematical analysis and simulation results showed thatthe FT-EEC protocol is more energy-aware than eitherthe LEACH or DSC protocol, which means the FT-EECprotocol lasts longer than the LEACH and DSC proto-cols. Moreover, the FT-EEC protocol requires a lessernumber of control packets than the DSC protocol. Inaddition, it can detect the failure of CH and non-CHnodes and hence, provides more reliability than the DSCprotocol.

In this paper, we compared the performance of theFT-EEC protocol only with the LEACH and DSC pro-tocols. In the future, we plan to compare the perfor-mance of the FT-EEC protocol against other fault-tolerantclustering protocols and consider/implement other factorsto get better performance and reliability.

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AUTHORS’ BIOGRAPHIES

Lutful Karim is currently a Ph.D.Candidate at the School of Com-puter Science, University of Guelph,Ontario, Canada. He worked as a fac-ulty member of computer science forabout 6 years. He has authored sev-eral refereed conference publicationsand journal publications, and been amember of organization committees

and technical program committees in several internationalconferences. His research interest includes wireless andmobile sensor networks, mobile and wireless computing,ubiquitous and pervasive computing, communication pro-tocols and algorithms, fault tolerant computing systems,and combinatorial optimizations.

Nidal Nasser received his B.Sc.and M.Sc. degrees with Honors inComputer Engineering from KuwaitUniversity, State of Kuwait, in 1996and 1999, respectively. He completedhis Ph.D. at the School of Comput-ing at Queen’s University, Kingston,Ontario, Canada, in 2004. He is

currently an Associate Professor and Chairman of the Elec-trical and Computer Engineering Department at AlfaisalUniversity, Saudi Arabia. He worked at the School of Com-puter Science at University of Guelph, Guelph, Ontario,Canada (2004–2011). Dr. Nasser is the founder and Direc-tor of the Wireless Networking and Mobile Comput-ing Research Lab @ Guelph (WiNG: http://wing.socs.uoguelph.ca). He has authored 129 journal publications,refereed conference publications and book chapters in thearea of wireless communication networks and systems.He has also given tutorials in major international con-ferences. Dr. Nasser is currently serving as an associateeditor of Wiley’s International Journal of Wireless Com-munications and Mobile Computing, Wiley’s InternationalJournal on Communication Systems, Wiley’s Security andCommunication Networks Journal and International Jour-nal of Ad Hoc & Sensor Wireless Networks. He has been amember of the technical program and organizing commit-tees of several international IEEE conferences and work-shops. Dr. Nasser is a member of several IEEE techni-cal committees. He received the Fund for Scholarly andProfessional Development Award in 2004 from Queen’sUniversity. He received the Computing Faculty Appre-ciation Award from the University of Guelph-Humber.He received the Best Research Paper Award at theACS/IEEE International Conference on Computer Systemsand Applications (AICCSA’08), at the International Wire-less Communications and Mobile Computing Conference(IWCMC’09) and at the International Wireless Communi-cations and Mobile Computing Conference (IWCMC’11).

Tarek Sheltami received his Ph.D.in Electrical and Computer Engineer-ing from the Electrical and ComputerEngineering Department at Queen’sUniversity, Kingston, Ontario, Canadaon April 2003. Dr. Sheltami is cur-rently an associate professor at theComputer Engineering Department atKing Fahd University of Petroleum

and Minerals (KFUPM) Dhahran, Kingdom of SaudiArabia. Before joining the KFUPM, Dr. Sheltami was aresearch associate professor at the School of InformationTechnology and Engineering (SITE), University of Ottawa,Ontario, Canada. He worked at GamaEng Inc. as a consul-tant on Wireless Networks (2002–2004). He also workedin several joint projects with Nortel Network Corporation.Dr. Sheltami has been a member of the technical pro-gram and organizing committees of several internationalIEEE conferences.


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