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* Department of Computer Science and Engineering, MaharishiMarkandeshwar University, Mullana (Ambala), Haryana, India, E-mail: [email protected], [email protected]

** Department of Computer Science and Engineering, Mody Institute ofTechnology and Science, Lakshmangarh, Rajasthan, India, E-mail:[email protected]

INTERNATIONAL JOURNAL OF COMPUTER SCIENCE & MANAGEMENT SYSTEMS3(1), June 2011, pp. 27-39I J C S M S

© Research Science Press

SEEAR-II: A System Model for Secure and Energy EfficientAdaptive Routing in Wireless Sensor Networks

D. Prasad*, Manik Gupta* and R. B. Patel**

Abstract: Wireless Sensor networks have become promising future to many applications. In the absence of adequate security, deploymentof sensor networks is vulnerable to a variety of attacks. A sensor node’s limitations and nature of wireless communication pose uniquesecurity challenges. Researchers have introduced secure communication using secure key management and secure routing techniquesto address the unique security needs in sensor networks.

In this paper, we have presented a system model for Secure and Energy Efficient Adaptive Routing (SEEAR-II) in WSNs, which anextension of SEEAR and provide ample energy wirelessly to the base station (BS) as well as to the sensor nodes (SNs) in the deploymentarea, at a very reasonable cost with the help of power beam. To incorporate the security, we are using three keys out of which one isstatic (i.e. ID) and remaining two are dynamic, which computed on fly and keep on changing each time when the network is synchronized.In SEEAR-II, the synchronization time less than the time required by an adversary to compromise any node, so that even if some nodesget compromised, the keying materials of the node have already been changed.

Keywords: Wireless sensor network (WSN), sensor node (SN), base station (BS), static keys, dynamic keys, logical neighbors (LN),physical neighbors (PN), actual neighbors (AN) and isolated nodes.

I. INTRODUCTION

Wireless sensor networks (WSNs) are built up of sensornodes (SNs), which consist of sensing, computing,communication, actuation and power components thatcooperatively perform the task of collecting relevantdata and monitor its surrounding for some change orevent to occur [1]. Thus, two types of architectures werestudied for WSNs. One for SNs itself and second fornetwork architecture required for communicationamong the SNs. WSNs has its own features that notonly differentiate it from other wireless networks butalso craft the scope of wireless application to disasterrelief, military surveillance, habitat monitoring, targettracking and in many civic, medical and securityapplications [2, 3]. Some features of WSNs that imposesome limitation on WSNs and were kept in mind beforedeveloping SEEAR-II are as following [11]:

A. Resource Constraints

SNs have limited resources, including lowcomputational capability, small memory, low wireless

communication bandwidth, and a limited powerbattery.

B. Traffic Characteristics

In WSNs, the primary traffic is in the upstreamdirection from the SNs to the sink node or BS, althoughthe BS or sink nodes may occasionally generate certaindownstream traffic for the purposes of query andcontrol. In the upstream, this is a many-to-one type ofcommunication. Depending on specific applications,the delivery of upstream traffic may be event-driven,continuous delivery, query-driven delivery, or hybriddelivery.

C. Small Message Size

Messages in sensor networks usually have a small sizecompared with the existing networks. As a result, thereis usually no concept of segmentation in mostapplications in WSNs.

D. Addressing Schemes

Due to relatively large number of SNs, it is not possibleto build global addressing schemes for the deploymentof a large number of SNs as overhead of identitymaintenance is high.

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28 International Journal of Computer Science & Management Systems (IJCSMS)

E. Sensor location and Redundancy of Data

Position awareness of sensor network is important,since data collection is normally based on location.Also, there may be common phenomena to collect data,so there is a high probability that this data has someredundancy. There are three criteria that drive thecommon design issues for large-scale sensor networks;scalability (these networks might involve thousands ofnodes), energy-efficiency (in particular, wirelesscommunication can incur significantly higher energycost than computation), and robustness (toenvironmental effects and node and link failures).

F. Network Lifetime

The time for which the network is operational or, putanother way, the time during which it is able to fulfillits tasks (starting from a given amount of storedenergy). It is not quite clear, however, when this timeends. Possible definitions are:

• Time to first node death: When does the firstnode in the network run out of energy or failand stop operating?

• Network half-life: When have 50% of thenodes run out of energy and stopped operating?

G. Time to Partition

When does the first partition of the network in two (ormore) disconnected parts occur?

H. Density of Nodes

In WSNs, the number of nodes per unit area i.e. thedensity of the network – can vary considerably.Different applications will have very different nodedensities. Even within a given application, density canvary over time and space because nodes fail or move;the density also does not have to be homogeneous inthe entire network (because of imperfect deployment)and the network should adapt to such variations.

I. Maintainability

As both the environment of a WSNs and the WSNsitself change (depleted batteries, failing nodes, newtasks), the system has to adapt it by monitoring its ownhealth and status to change operational parameters orto choose different trade-offs (e.g. to increase theinterval of monitoring data and reduce quality whenenergy resource become scarce).

J. Node Deployment

Node deployment can be random, deterministic or selforganizing. For deterministic deployed networks theroutes are pre-determined, however for randomdeployed networks and self-organizing networks routedesignation have been a challenging subject.

K. Energy Consideration

Since the life-time of the WSNs depends on energyresources and their consumption by sensors, the energyconsideration has a great influence on route design.The power consumed during transmission is thegreatest portion of energy consumption of any node.Direct communication consumes more power thanmulti-hop communication; however the multi-hopcommunication introduces extra topology managementand medium access control.

L. Miscellaneous Applications

WSNs may be used in different environmentssupporting diverse applications, from habitatmonitoring and target tracking to security surveillanceand so on. These applications may be focused ondifferent sensory data and therefore impose differentrequirements in terms of quality of service (QoS) andreliability. Thus sensor networks are applicationspecific.

Now days, WSNs are not used only for security orsocial intention but also used for commercial purposes.Extensive research is going on in almost all fields ofsensor network, including sensor design,communication protocol stack design, and operatingsystem for sensors, security and management algorithmdesign. The design goals of WSNs are applicationspecific, but share some common attributes like energyefficiency, scalability, robustness, network life time,fault tolerance, self organization and data aggregation.Out of which, energy efficiency and security are moreimportant. In recent years, the availability of cheap andtiny micro-sensors and low power wirelesscommunication enabled the deployment of largequantity of wireless sensor nodes, which are scatteredin the interested area.

In WSNs, routing, security and networks lifetimeare seemed to be incompatible. But, in SEEAR-II, thebalance of energy consumption among all the sensornodes is maintained, in order to avoid the “hot spot”problem.

SEEAR-II: A System Model for Secure and Energy Efficient Adaptive Routing in Wireless Sensor Networks 29

The main emphasis while designing anddeveloping the protocol is uniform load distributionamong SNs, in order to increase the network lifetime.Though many existing approaches increase energyefficiency, but technique such as; dynamic routingwhere, data is forwarded to nodes with the highestresidual energy, may cause problem such as, unboundeddelays. However, SEEAR-II is efficient enough to solvethe security and energy issues, with dynamic routingand key management techniques that is not sufferedfrom the traditional problems of unbounded delays andeasy compromise of the nodes. Beside this, the networkacts more efficiently in terms of energy with the helpof wireless energy provided by the BS to thedeployment area.

SEEAR-II is scalable, secure and energy efficient.SEEAR-II enhances the security level of Q-compositekeys scheme by changing the keying material everytime, the network is synchronized. To increase thescalability and life of the network, SEEAR-IIintroducing one more parameter (i.e. distance betweenSNs) in the Q-composite key scheme, for theestablishment of communication link between SNs.

Rest of the paper is organized as follows. Section IIsummarizes the related works. In section III, Systemmodel and protocol description is presented. Section IVpresents an algorithm for protocol implementation. Insection V, we analyze SEEAR-II in respect to energy,security and life of the network. Issues in charging oflaser diode array, in the power supply model is discussedin section VI, followed by the results and discussions insection VII. Finally, we conclude SEEAR-II and discussthe scope of future work in section VIII.

II. RELATED WORKS

Security is a big issue, when WSNs are deployed in ahostile environment. Secret keys should be used toencrypt the exchanged data between communicatingparties. In the Internet or traditional wireless networks,such as, cellular networks, most security protocols arebased on asymmetric cryptography, such as; RSA orElliptic Curve Cryptography (ECC) [6, 7] are notapplicable, due to the high computational complexity,high-energy consumption and increased code storagerequirements. Furthermore, due to unpredictablenetwork topology and lack of infrastructure support,trusted-server based key distribution protocols are notsuitable for WSNs either [5]. Research shows that keypre-distribution mechanism could be a practical method

to solve the key distribution problem in WSNs. Thebasic idea of key pre-distribution scheme is preloadingsome secret keys into SNs, before they are deployed.After the deployment, the nodes discover shared keysfor secure communications. It is divided into 3 phases;i.e. Key distribution, Shared key discovery and Path-key establishment. During these phases, secret keysare generated, placed in sensor nodes, and each sensornode searches the area in its communication range, tofind another node to communicate. A secure link isestablished, when two nodes discover one or morecommon keys (this differs in each scheme), andcommunication is done on that link between those twonodes. For this purpose, various keying techniques arebeing used. Some of the common key managementschemes are as follows [8]:

1. Single Network-wide Key Establishment

2. Pair-wise Key Establishment

3. Dynamic Key Management

4. Public Key Schemes

5. Q-Composite Random Key Management

Each of the above WSN key management schemeconsists of three main components [9]:

(1) key establishment (2) key refreshment (3) keyrevocation

Key establishment is about creating a session keybetween the parties that need to communicate securelywith each other. Key refreshment prolongs the effectivelifetime of a cryptographic key, whereas, keyrevocation ensures that an evicted node is no longer toable to decipher the sensitive messages that aretransmitted in the network.

In this article, we proposed protocol SEEAR-IIwhich is scalable, secure and energy efficient. SEEAR-II enhances the security level of Q composite keysscheme, by changing the keying material, every timenetwork is synchronized. To increase the scalabilityand life of the network, SEEAR-II introduces one moreparameter (i.e. distance between SNs) in the Qcomposite key scheme for the establishment ofcommunication link between SNs. Beside the energyefficient method, SEEAR-II also introduces the conceptof power beam [12] to supply power wirelessly to theBS as well as to the network throughout the life of therechargeable batteries and the laser diode arrays andthus increasing the lifetime of the network almost toinfinity.


Due to the recent efforts of MIT and Intel Co. [14,15, 16], wireless electricity comes into revolutionaryphase, which motivate us to think upon and work overthe concept so as to make the WSN’s life never lasting.Though this concept is motivating to work upon futureapplications, but still it is facing problem related tothe transmission range, 3 to 8 meters. So, due to thisproblem as well as introduction of new hardwarestructure to the SNs, the current technology introducedby MIT and Intel Co. limits anyone to work upon WSN.But, if one thinks about the principle of powertransmission in space to the space vehicles or spacebased solar power satellites (SPS), as mentioned in [17,18] or to recharge batteries of satellites in geo-stationary orbits as in [19] made us to think beyondthe concept of simple wireless electricity, as discussedin [14-16]. Though the concept of SPS is capableenough to provide unlimited power to the SNs in thedeployment area as well as to the flying BS over it,but a large burden of extra cost is also involved in it asincludes a large number of SNs and the BS to getcharged through the satellites, which are governed byany third party and hence the point of security alsoarises. So, it would be more cost effective and moresecured, if we are able to apply the same principle fromthe land itself rather than space, at the deployment areaby the first party itself. This can be possible withMicrowave power transmission (MPT) or Laser PowerBeaming.

III. SYSTEM MODEL & PROTOCOLDESCRIPTION

We have divided the SEEAR-II into two phase i.e. pre-deployment phase and post-deployment phase.

I. Pre-Deployment Phase

In the pre-deployment phase, the network is deployedwith an assumption that it is free from adversarialattacks during the setup phase. The pre-deploymentphase deals with the following activities:

1. Delivering energy to the BS.

2. Cluster Formation among the sensor nodes inthe deployment area.

3. Generating a strong security model for thesensor nodes at their respective cluster ends.

Initially the power is supplied to the BS with thehelp of beam director (BD

2), as shown in Figure.1 [12],

so that BS can compute various operations at its own

end. Here, the BS is considered to be any flying objectlike UAVs, which remains over the top of thedeployment area. Though it may change its position,but it can compute the Localization ID (LID) of thesensor nodes in the deployment area with its ownreference [4] to incorporate more security with respectto the GPS system. However, the position of the BScan be tracked by beam director (BD

2) on the basis of

GPS system.

After the BS receives energy to the threshold levelby beam director (BD

2), the static sensor nodes are

randomly deployed from the BS in the deploymentarea. On the basis of the node Localization ID (LID),the BS divides the deployment area into variousclusters. After the successful completion of thedeployment phase of the SNs, the power supply phasein the deployment area begins after some time, sincethe SNs are initially completely charged beforedeployment. As per the principle to be followed inthe power supply model [13], the power is generatedfrom the prime power generator, which can be eitherin DC or in AC. This prime power generator thengenerates the laser via laser power supply associatedwith the laser cooler, which maintains the temperatureof the laser beam, so that the laser beam so generatedwill not damage any equipment on which it is to bedirected. The laser beam is then directed towards thelaser receiver with the help of beam director underthe control of tracking and safety sensors. Here, thesafety system ensures that the beam is unobstructedand is directed at the receiver properly; howeverfor tracking either an optical tracker is used, whichis straight forward or can be supplemented withGPS or Localization ID (LID) based techniquefor acquisition. The laser receiver used in thismodel contains an array of near-infrared laserdiodes [13].

These arrays composed of the photovoltaic cellsthat match with the intensity of the beam andwavelength of the laser and are efficient, compact,robust, reliable and relatively inexpensive. However,for greater range or low power applications, other laserslike diode-pumped fiber lasers can be provided thatare relatively expensive to that we are using in thismodel. With current laser cells, the deliverable poweris limited mainly by cell cooling and can easily exceed6 KW/m2 or about 1HP per square foot. Thearchitecture of the power supply model of SEEAR-IIis as shown in the Figure 1.


Figure 1: Power Supply Architecture of SEEAR-II

In the Figure 1, first of all the power is supplied soas to generate a laser beam from BD

2, which directs

the laser beam to one of the laser receiver with thehelp of optical tracking and safety sensors. Thephotovoltaic cells in the laser receiver provides powerto the motor of flying BS, so that it communicates withthe deployment area continuously by omitting the needof providing extra fuel filling station to the flying BSor some other fuel related problems at the BS end thatmay cause its dislocation from the network for fuelfilling purposes.

After the successful deployment of the SNs in thedeployment area, the working of BD

1 will begin. This

may be placed either at ground with some AC line supplyor may be embedded with some high power militaryvehicle. BD

2 will also direct laser beam to the second

laser receiver at the BS. The photovoltaic cells in secondreceiver will now act as prime power to the laser beamdirector associated with LID based tracking and safetysensor, which will help in directing laser to the SNs,

containing small arrays of photovoltaic cells as receiverthat are sufficient to supply power to the SNs for someperiod of time. The laser is directed to all the clustersformed by the SNs in the deployment area either onrotation basis or on priority basis; the priority may beassociated with the level of energy or even more numberof directors can be provided in some ratio to the SNs inthe deployment area. However, instead of using BD

3

only, more number of directors can also be used forcontinuous power beaming; one for each cluster, or onemay think that rather than using LID based tracking andsafety sensor even ordinary tracking and safety sensorcan be used here in a way to direct the beam throughoutthe deployment area continuously, this can be a goodoption in case of SNs which senses data periodicallyand consumes least energy but not in case of SNs wherethe energy consumption is high. But, here most of theenergy gets wasted with decreased intensity of the beamand even if one tries to increase the intensity, then eitherthe safety sensor will not allow or it may cause harm tothe beam director due to high temperature.


Besides, regular power supply to the deploymentend, the base station also generates key sets from thekey pool, for each node. By using the concepts of Q-composite keys, the nodes needs to have at least Qnumber of keys in common to establish acommunication link rather than only one key, whichenhances the security level of SEEAR-II. But, thedrawback with Q-composite is that if the Q number ofkeys is common between two nodes, which are faraway from each other, then to establish communicationlink between such nodes is a bad idea, because to makecommunication between these nodes is very energyconsuming. Keeping this in mind, SEEAR-II imposesa new constraint of distance on Q-composite concept.In SEEAR-II, communication link between such nodeswill be established, only if the distance between suchnodes is less than or equal to some threshold value D

0.

The value of D0is guided by the deployment area and

the density of nodes within that area.

node under consideration, it store the ID of that nodein the ‘LNbr’ list. The BS finds all logical neighborsfor each node and stores them in list ‘LNbr’, as shownin Figure 3(a). If any entry in the list ‘LNbr’ remainsempty then BS assign new key set to the correspondingnodes and repeat the process.

B. Finding Physical Neighbor (PN)

To find the physical neighbors of any node, BScompute the distance of the node under considerationwith all its logical neighbors and store the ID of allthose node, which distance is less than or equal to D

0

in ‘PNbr’ list as shown in Figure 3(b).

C. Finding Actual Neighbor (AN)

With the help of these two lists, for each nodes BSfinds all those nodes, which falls within distance D

0

and having at least Q keys in common and store themin ‘Nbr’ list, as shown in Figure 3(c). Any emptylocation in list ‘Nbr’ indicate that the correspondingnode falls far away from the remaining node, whichchance is very rare, because of dense deployment, andeven if it happened, we can ignore it, because suchnodes are very few in numbers.

In this way, all the exhaustive operations aremanaged by the BS itself rather than the SNs in thedeployment area. Hence, we can achieve more securityeven in large sized network without any loss of energyas in the conventional scheme of the Q-Compositescheme.

Figure 2: Data Structure Representing Node Information

In SEEAR-II, the BS generates N key sets of Kkeys in each, from the key pool, for the node to bedeployed in the deployment area, and maintains a listcontaining node ID, node Localization ID (LID) andkey sets assign to the node, as shown in Figure 2. InSEEAR-II, nodes having Q keys in common, are knownas logical neighbors, nodes having distance less thanor equal to D

0 between them, are known as physical

neighbors and nodes satisfying both criteria, are knownas actual neighbors.

A. Finding Logical Neighbor (LN)

The method of finding Logical neighbors isstraightforward. To find the Logical neighbors of anynode, BS compare all the keys in the key set, assign tothe node under consideration, with all the keys in thekey sets assign to other nodes one by one, and wheneverBS find any node having Q keys in common with the

Figure 3: Representing List of Neighbors

II. Post-Deployment Phase

Once the BS obtains the actual neighbors for all nodes,it constructs a network graph where, edges represents


secure link and nodes represents sensor nodes in thenetwork. From the network graph, BS obtain minimumspanning tree as shown in the Figure 4, and designateone of the node as cluster head and set it tocommunicate with base station. This delegation mustbe on rotation basis, otherwise the energy of the nodecommunicating continuously with the BS will bedepleted soon and the whole network will bedisconnected, even due to presence of some externalpower source like that by beam director (BD

3), as

shown in Figure 1. To rotate the delegation, BS canchoose any scheduling scheme; SEEAR-II using thescheme presented in GANM [10]. BS computes linkkeys between a node and all of its neighbors byapplying some hash function, as shown in thealgorithm. It also computes a timer value, as shown inthe algorithm, to synchronize the network.

synchronized. Once the network is synchronized, nodeswithin the cluster start sensing the surrounding andsend the sensed data, in the following format to itsparents, where, these data are aggregated and thisaggregated data is forwarded to their parent; thisprocess is continued and finally aggregated datareaches to the node communicating with BS throughwhich data is reached to the BS.

Source ID Neighbors Link Keys Data

Data Packet

Timer Value

This whole process of resynchronization isrepeated after the regular interval of time in order toenhance the security level of each cluster within thenetwork, by generating unpredictable key values, inthe least possible interval. To enhance the security levelfurther, link keys and timer values may be encryptedbefore their transmission in the data packet.

IV. IMPLEMENTATION OF SYSTEM MODEL

In our network model, we have assumed that all sensorsare distributed in an evenly randomized manner in apolygon region, and the network has the followingproperties:

1. There exists a unique BS, located at the top ofthe network with a maximum height of 1 miledue to the power constraint of power beam[13].

2. Each SN has a unique identity.

3. All sensors cannot move after being deployed.

4. Network is homogeneous i.e. all sensor nodesare equivalent, having the same energy,computing and communication capacity.

5. Location of nodes is obtained using virtual co-ordinate system as in [4].

6. The transmitter can adjust its amplifier powerbased on the transmission distance.

7. Each sensor node consists of photo-voltaiccells for charging the power both from the sunas well as from the beam director embeddedat the BS.

Algorithm

List ‘LNbr’ is an array of pointer, in which locationsare pointing to the link list of ID of logical neighborsof the node under consideration (i.e. nodes with Q keys

Figure 4: Communication Network of Our Protocol

Once the link keys and timer value are computed,BS constructs N packets; one for each node, containingnode ID, set of link keys for that node and timer value,as shown:

Synchronization Packet

Node ID Neighbors Link Keys Timer ValueCluster ID

BS broadcast these packets in the network. Nodesin the area receive only the packet meant for them,store this information within its memory, and ignoreother packets. On receiving the above packet, timerwithin the node is triggered and the whole network gets


in common, can be represented as CKeysI,J

for nodes Iand J), ‘PNbr’ is an array of pointer, in which locationsare pointing to the link list of ID of those nodes whosephysical distance is less than D

o, ‘Nbr’ is an array of

pointer, in which locations are pointing to the nodesallowed to communicate with each other, according toSEEAR-II as well as node under consideration and ‘Iso’is list of all those nodes, which are isolated from thenetwork.

The function SECURE_LINK(Iso[I]) is used toestablish a secure link among the isolated nodes presentin the list ‘Iso[I]’. However, the functionRESYNCHRONIZE ( ) is used to resynchronize theentire network in the synchronization time, ‘T

sync’ so

that, each delegate node, ‘Dlgt’ in the list communicateswith the BS on round robin basis. The synchronizationtime, ‘T

sync’ must be less than the threshold time, ‘T

0’

(by some tolerance value ε), which is the time takenby an adversary to capture any node in the network.Moreover, T

BSand timer[I] are the timers maintained

at the Base Station and at each node in the network;acting as their respective dynamic keys.

For each cluster in the deployment area, thefollowing algorithm will be executed concurrently, soas to find the dynamic cluster head that will act asdelegate node to communicate with the BS.

1. While ((Nbr[1])||( Nbr[2])||( Nbr[3])||…||(N br[N])= NULL) repeat steps 2 to 5

2. Initialize C :=1.

3. For I:=1 to N

If (Nbr[I] = NULL) add its ID to Iso[C];C:=C+1.

4. For I:=1 to C

Generate new key set and replace the key setin KSets[I] corresponding to node Iso[I] bynew set.

5. Call SECURE_LINK (Iso[I]).

6. Establish two way communication links by the linkkey as:

K:= HASH {k1||k

2||……||k

Q}.

7. Call MINIMUM_SPANNING_TREE for the graphobtained in step 6.

8. Traverse the Tree constructed in step 7 and storethe nodes in Dlgt[I].

9. Initialize I:=1.

10. while (I>0) repeat step 11 to 15

11. Temp:= Tsync

:= T0 - ε

12. while(Temp > 0)

(i) Delegate node Dlgt[I] to communicate with BS.

(ii) Temp: = Temp -1.

13. Call RESYNCHRONIZE ( ).

14. I:=I+1; Temp:= Tsync

15. If I==N then Set I:=1.

RESYNCHRONIZE ( )

1. Temp:= Tsync

2. while (Temp >= 0) repeat steps 3 to 6

3. If (Temp == Tsync

)

(i) Generate two way communication linksbetween each pair of nodes by the link keyas: K:= ((HASH {k

1||k

2||……||k

k}+

LID[Dlgt[I]])*TBS

)

(ii) x:= (int | (T0-HASH{n||LID[Dlgt[I]]})/2|).

(iii) Set TBS

:= x.

(iv) for I:=1 to NSet timer[I]:= x.

4. TBS

:= TBS

+ 1.

5. timer[I]:= timer[I] + 1.

6. Temp:= Temp-1.

SECURE_LINK (Iso[I])

1. for I:=1 to N repeat steps 2 to 3.

2. for J:=1 to N repeat step 3.

3. If (I!=J)

If ((CKeys I,J

>= Q) && (|LID I -LID

J| <= D

0))

Add the IDs of the nodes in the neighbor list(Nbr[I]).

V. ANALYSIS OF THE SYSTEM MODEL

In WSN routing, energy and security are the threeprimary factors that should be kept in mind, beforedesigning any protocol. It is a general myth thatefficient routing, security and networks lifetime areseemed to be incompatible, but SEEAR-II trying tobalance all these parameters. All these aspects areconsidered in development of SEEAR-II.

We are using three keys for communication; outof which one is static (i.e., ID of node) and remainingtwo are dynamic, which are computed by applying hashfunction; as given in the algorithm. These two dynamic


keys are changed every time, when the network getsresynchronized. So, in SEEAR-II, if some node getscompromised, it will be identified in the nextsynchronization. SEEAR-II resynchronize entirenetwork in the time less than T

0, where, T

0is the time

required to compromise any node by an adversary.Shorter is the value of T

0, higher is the security level

of the network.

In some protocols, highest residual energy nodesare identified within the network and all data to theBS are routed through that node, which may causesproblem, such as, unbounded delays. However, ratherthan checking nodes with highest residual energy,SEEAR-II delegate a node to communicate with BSon rotation basis, which is selected based on GANM[10], and we kept this rotation time less than T

0, so

that, even if somehow an adversary is able to captureit, its effect could be minimized.

Energy is considered to be most important factorto enhance the life of the network. In SEEAR-II,wireless energy is provided to both the BS as well asto the SNs in the deployment area; moreover,communication link between two nodes is establishedonly if the distance between these two nodes is lessthan D

0and they satisfied the key criteria of Q

composite keys. In SEEAR-II, algorithms to set up thenetwork are running at the BS, which saves energy ofSNs a lot. In Q-composite scheme, there is norestriction of distance between two nodes and ifcommunication link is established between two nodes,which are far away with each other, then much moreenergy is required to communicate with each other, ascompared to SEEAR-II. Also, by increasing the valueof Q in Q-composite scheme, more energy getsdissipated to match more number of keys, but SEEAR-II make it possible to enhance security with largervalues of Q and match the keys at the BS itself ratherthan at the deployment area, as in regular fashion.

VI. ISSUES IN CHARGING OF LASER DIODEARRAYS IN THE POWER SUPPLYMODEL

In SEEAR-II, the power is supplied to the BS as wellas SNs in the deployment area. Though a continuousbeam is directed to one of the laser diode array forsupplying power to the BS continuously and the otherbeam is directed continuously to the other laser diodearray which is connected to the beam director to furthersupply power to the SNs, hence it is not a matter of

discussion about the factors for the BS power supply.However, a number of factors are involved in supplyingpower to the SNs in the deployment area. They are:(1) the rated capacity of the photovoltaic cells used inthe laser diode arrays, (2) the run time of the SNs, (3)zenith distance i.e. the angular distance from theposition of the laser beam above the deployment areaof SNs, (4) the probability that recovery would occurfrom the discharged state or recovered state to fullycharged state (5) capacity fading rate of the laser diodearray that affects its life time. Beside all these factors,there is one most important factor involved in supplyingpower to the SNs, which is solar energy; as thephotovoltaic cells within in the SNs get charged withpower beam on rotation basis in addition to theexposure to continuous solar energy present in theenvironment during day time. The intensity of powerbeam can be adjusted by considering the factors likeweather, time intervals between charging. This givesimmense benefits in case of cloudy or rainy weather,hence ensuring almost 100% energy uptime of SNs atthe deployment end. Depleted State: When all thecharges near the anode of the PV cells within laserdiode array gets depleted and stops the flow ofelectrons from the battery then the PV cells enter intothe depleted state and the concerned SN will enter intothe sleep state.

The capacity fading rate of the laser diode arraywas given by Peukert Effect and can be described byEq. (1) as:

C = InT (1)

where, C - Rated capacity of the laser diode array.

I - Discharge current.

T - Runtime of the laser diode array.

n - Peukert’s exponent.The probability that a recovery would occur is givenby [13] as follows:

( / ) ;

0 ; 0

k N q N

r

e if q NP

if q or q N

− − ≠= = =

(2)

where, q - fraction of discharge of PV cells in laserdiode array.

N - total charge capacity of the laser diode array.

N-q/N - fraction of capacity remained in PV cells inlaser diode array.

k- constant dependent upon the amount of PV cells inlaser diode array used.


The intensity of power beam applied to the laserdiode arrays at various ends in this model depends uponvarious parameters:

(1) The environmental temperature or temperatureof the solar energy. Since, in the open airdeployment areas like military applications,farm fields, etc. exposure to sunlight is anatural phenomenon whose intensity cannot bechanged manually. The temperature of the laserbeam directed, which can be controlled by thesafety sensors at the beam directors. Moreover,this temperature can be programmed to varyaccording to the environmental temperatureand the density of SNs in the deployment area;as in large number of SNs the intensity of thebeam needs to be increased to decrease thetime slots among the charging of several nodes.

(2) Relation between the time and the amount ofenergy provided by the PV cells within thelaser diode array. For this we describe the fluxintensity of the PV cells involved which isgiven by [14] can be modified according tothe proposed model in SEEAR-II as in thefollowing equation:

I(z) = e-c(sec z1)^s I0 + e-c(sec z2)^s (3)

where, I(z) - total flux intensity in KW/m2 and isdefined as the total energy either by power beam orsolar energy or both absorbed by each PV cell involved.

I0- solar flux intensity outside the earth’s atmosphere

i.e. exoatmospheric solar flux (1.353 KW/m2).

c - empirical data numerical constant

s - empirical data numerical constant

z1 - zenith distance i.e. the angular or line of sight

distance from the position of sun directly above theSNs.

z2 - zenith distance i.e. the angular or line of sight

distance from the position of GPS based beam directorin the flying BS above the SNs.

1

01

ln

sec

sI

Iz

c

− =

(4)

1

2

lnsec

sIz

c

− = (5)

Figure 5 shows the zenith distances z1 and z

2 from

the SN to the sun and the power beam generatorrespectively.

Figure 5: Zenith Distance of a SN

Here, z1 is dependent on the time of the day,

whereas z2 is independent of time as the power beam

is directed continuously over the SNs on rotation basis,irrespective of time. If λ is the latitude of the SN and δis the solar declination, then z

1 can be calculated by

Eq. (6) [14] as:

z1 = cos-1(sin λ sin δ + cos λ cos δ cos t) (6)

where, t = (360/24)T (7)

T - number of hours from the highest point of the sun.

λ - latitude of SN in deployment area.

δ – solar declination angle i.e. the angle between earth-sun line and the equatorial plane.

However, z2can be calculated as in Eq. (8):

z2 = (co-ordinate position of the beam generator at the

BS) – (co-ordinate position of the SN) (8)

A photovoltaic cell involved in the laser diode arrayexists in one of the four states:

(1) Fully Charged State: The charges inside thephotovoltaic cells are full to their maximumcapacity.

(2) Discharging State: The charges near the anodeflows out of the battery in a rate faster thanthe internal diffusion inside the batterycomposition.

(3) Recovered State: Due to exposure to thesunlight or laser beam, internal diffusionoccurs among the electrons inside the


electrolyte so as to equalize the concentrationof charges within photovoltaic cells insidelaser diode array.

VII. RESULTS AND DISCUSSION

Simulation is done using Matlab as the plotting software,as well as the calculation engine to plot results for theenergy consumed by SNs with the increasing thresholddistance value required to identify the physicalneighbors, the effect of key set size on the time taken toestablish a secure link, comparison between SEEAR andQ-Composite random key pre distribution and finallywe plot the change in total flux intensity with respect totime, considering the total flux, considering both the fluxdue to solar energy and power beam..

The following are the simulation parametersconsidered for the implementation of the developedscheme:

• The distance between the BS and the networkis taken as 125m.

• Size of message is 80 bytes.

• Free space attenuation coefficient (Efs) is 10

pJ/bit/m2.

• Multipath attenuation coefficient (Emp

) is0.0013 pJ/bit/m4.

• Electronic power (Eelec

) is 50 nJ/bit.

• Size of committing value 8 bytes.

• Size of node ID 4 bytes.

• Size of MAC 8 bytes.

• Number of SNs within cluster is 11.

• Average distance between SNs within clusteris assumed to be 7.5m.

• Cluster area 10m2.

For realistic, our simulation uses the first orderradio model as the communication model. Equation(9) and (10) represent the energy dissipation, when aSN sends or receives an l-bit message.

Erecieve

= l + Eelec

(9)

2( ),

4( ),

E fal E E d ifdelec fs Emp

Etrans E fs

l E E d ifdelec mp Emp

× + × ≤=

× + × >

(10)

Figure 6 shows that the times taken for theestablishment of secure link increases with theincreasing size of the key set, assigned to the sensornodes.

It is observed that the lifetime of a SN decreasesas the distance between two nodes increases in Figure7 and finally, Figure 8 shows the comparison betweenthe traditional Q-composite random key pre-distribution technique and SEEAR-II. It can beobserved that the energy consumption for the securelink establishment in Q-composite random key pre-distribution scheme gets increased with the increasingsize of Q. However, in SEEAR, the energyconsumption remains constant since all the exhaustivetasks are managed by the BS rather than SN itself.

Here, the change in total flux intensity with respectto time, considering the total flux i.e. both the flux dueto solar energy and power beam. Following data valuesare assumed before the simulation of the model.

Empirical data numerical constant (c) = 0.357.

Empirical data numerical constant (s) = 0.678.

Exoatmospheric solar flux (I0) = 1.353 KW/m2.

Zenith Distance (z2) = 1 mile.

Latitude of SN in deployment area (ë), involved forthe calculation of z

1 = 60°.

According to the University of SouthernMississippi in [14] the value of solar declinationchanges throughout the year as shown below, due totilting of earth’s equatorial line by 23.45° with respectto its orbit.

Solar Declination Angle (δ), involved for thecalculation of

z1 =

23.50 ;

0 ;

23.50 ;

During Summer

During Equinox

DuringWinters

−

Result of simulation is given in Figure 9. Here,the plot describes total flux intensity with respect totime for different seasons: summer (yellow), equinox(pink), winters (blue), on the basis of Eq. (3). Figure 9represents the time cycle of 24 hours, where, it can beobserved that at different levels of total flux intensity,the laser diode can provide certain amount of currentat constant voltage per time slot, which leads us to thinkupon the idea that the laser diodes embedded on SNscan provide a discrete amount of charges depending


upon the value of a charge unit generated by the totalpower including solar power and laser beam. The totalflux intensity is maximum during noon, when both thesolar power and laser beam are available but, in night,due to supply of power beam, this flux intensity getsdecreased, as only the laser beam is available, but stillthe laser diodes on SNs are capable to work above thethreshold power, due to the energy so available by thelaser diode array.

VIII. CONCLUSION AND FUTURE WORK

In this article, we have presented a system model(SEEAR-II) for WSNs. The design of SEEAR-II ismotivated by the observation of Q- composite scheme.To enhance the security, SEEAR-II keeps on changingkeying materials every time network getsresynchronized.

Some of the advantages of SEEAR-II are asfollows:

1. SEEAR-II ensures high energy at the BS end,rather than any assumption in fictions.

2. SEEAR-II provides ample amount of energyto the BS as well as to the SNs with a verylow investment at the corporate end.

3. High energy by SEEAR-II may also ensurecontinuous sensing of data rather than periodicsensing as in general techniques of WSN.

As a future work, one can work upon some goodrobotics technology to introduce Mobile Sensor Nodesby taking advantage of high energy and homogeneous

Figure 6: Effect of Key Set Size on Secure LinkEstablishment

Figure 7: Effect of Distance between the Neighbors on theLifetime of Sensor Nodes

Figure 8: Comparison of Q-Composite Scheme withSEEAR-II

Figure 9: Total Flux Intensity with Respect to Time


distribution of SNs even though they have been spreadrandomly from the BS as well as fault revoking can bedone easily at the deployment end.

References

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