opnet-based modeling and simulation of mobile zigbee...
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Peer-to-Peer Netw. Appl.DOI 10.1007/s12083-015-0349-8
OPNET-based modeling and simulation of mobile Zigbeesensor networks
Xiaolong Li1 ·Meiping Peng2 · Jun Cai3 ·Changyan Yi3 ·Hong Zhang3
Received: 7 January 2015 / Accepted: 23 March 2015© Springer Science+Business Media New York 2015
Abstract Modeling and simulation can help to validateand evaluate the performance of wireless sensor networks(WSNs) within specific applications. In order to resolve theissue of the restriction on node mobility in existing Zig-bee WSN simulation models, this paper proposes a Zigbeecompliant new simulation model using the OPNET simu-lator. Based on the Zigbee MAC layer model in OPNETModeler, we develop a network layer model and propose animproved AODV routing algorithm to support node mobil-ity, both of which are compatible with Zigbee protocols.We further present in details the structure of the network
� Xiaolong [email protected]
Meiping [email protected]
Changyan [email protected].
Hong [email protected]
1 Guangxi Key Laboratory of Trusted Software,Guilin University of Electronic Technology,Guilin, China
2 School of Computer Science and Engineering,Guilin University of Electronic Technology,Guilin, China
3 Department of Electrical and Computer Engineering,University of Manitoba, Winnipeg, Manitoba, Canada
layer process model and the implementation procedures ofits kernel functions. Comprehensive performance compar-isons are performed between the proposed model and theZigbee model in OPNET standard libraries. In order to eval-uate the effectiveness of the proposed model in the aspectof node mobility support, time intervals between route fail-ure occurrence and route recovery are measured as well.The experimental results show that the proposed simulationmodel achieves better performance, compared to the orig-inal one. In addition, when node mobility causes routingfailures, alternative routes can be established quickly by theproposed model.
Keywords OPNET · Zigbee · IEEE802.15.4 · Mobility ·Simulation · Network layer
1 Introduction
Wireless sensor networks generally comprise a large num-ber of sensor nodes deployed in an area of interest to collectphysical or environmental conditions, such as temperature,humidity, pressure, etc. In wireless sensor networks, per-formance evaluation is critical to test the practicability ofnetwork architectures and protocol algorithms, and providesguidelines in performance optimization. Among differentcandidates, simulation offers a cost-effective way. Recently,researchers have developed many simulation models ondifferent simulation platforms, such as OPNET, NS-2,TOSSIM, EmStar, OMNeT++, J-Sim, ATEMU, and Avrora[1]. Compared with other simulators , OPNET is moresuitable to simulate behaviors of networks in the realworld. OPNET Modeler, as a network simulator, providesan industry-leading network technology development envi-ronment [2]. It can be used to design and study network
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modeling and simulation in applications, equipments, proto-cols and network communications, and show flexibility andintuition in designing practical systems.
Recently, Zigbee technology has been widely adopted todevelop wireless sensor network applications [3] by forminga wireless mesh network with low rate, low power con-sumption, and secure networking. In Zigbee protocol stack,the physical layer and the MAC layer protocols have beendefined by IEEE802.15.4 standard [4]. Its network layerbuilt upon both lower layers should be designed to enablea mesh networking, support node joining or leaving, assignnetwork addresses to devices, and perform routing. ZigbeeAlliance is working at providing a standardized base setof solutions for sensor networks [5]. In this paper, a net-work layer model is proposed for mobile sensor networksin order to accomplish all defined functions. The applica-tion layer aims at providing the services for an applicationprogram, consisting of application support sub-layer, appli-cation framework, and Zigbee device object. Since this layeris related to specific applications, and is not the main focusof this paper,the design of the application layer is omittedhere.
The simulation of Zigbee sensor networks withinOPNET simulator has been attracting interests fromresearchers. There are many research works on simula-tion modelling and evaluation of sensor nodes in OPNET[6, 7]. For example, Kucuk et al. [6] presented a detailedimplementation methodology for their proposed positioningalgorithm, called M-SSLE. Shrestha et al. [7] proposed asimulation model for new networking nodes equipped withmultiple radio technologies. However, few works focusedon the simulation model of mobile sensor networks in liter-ature. Device mobility is inevitable and must be conciliated[8, 9], where lack of the support for simulation on mobileZigbee sensor network is a major limitation in this field ofresearch, evaluation and development.
In [10], the adequacy of current provisions for deal-ing with different mobility cases was assessed. Simulationresults demonstrated that the current model in OPNET stan-dard libraries is ineffective in dealing with nodal mobility.Since OPNET Modeler provides a comprehensive simu-lation environment for modeling distributed systems andcommunication networks, many simulation studies for Zig-bee sensor networks were performed in OPNET simulator[11–16]. According to the performance studies using theZigbee model within OPNET Modeler standard libraries(ZMOMSL), there are several disadvantages on this model.For example, its address assignment mechanism may wasteaddress space, the high communication overheads mayreduce network lifetime, and the network joining strategymay result in significant traffic collisions and jams [17, 18].Among all these disadvantages, the most critical issue is thatthe Zigbee model can not support the mobility of device
nodes. This motivated us to develop a new simulation modelbased on the OPNET simulator for mobile Zigbee sensornetworks.
The main contributions of this paper are summarized asfollows. 1) We adopt the OPNET simulation developmentplatform to design a mobile Zigbee sensor network simu-lation model compatible with Zigbee protocols, where thephysical layer and the MAC layer defined by IEEE 802.15.4are employed. 2) We provide a node level design of mobilesensor nodes, present a process level model of its networklayer model and the detailed implementation procedure ofthe key functions. 3) In order to further decrease the com-munication overhead of nodes, an improved AODV routingalgorithm is also proposed, which demonstrates superiorcapability in supporting node mobility.
The rest of this paper is organized as follows. InSection 2, we discuss the design of network process modelin details. In Section 3, we propose a new simulationmodel which enables mobile support for Zigbee devices.Section 4 presents our simulation results and demonstratesexperimental comparison between our proposed model andZMOMSL. Section 5 draws conclusions.
2 The design of simulation system model
2.1 Design of node model
As shown in Fig. 1, a Zigbee node model within OPNETModeler typically incorporates the physical layer, the MAClayer, the network layer and the application layer. The physi-cal layer comprises a transmitter module, a receiver module,and a wireless pipeline model. The wireless pipeline modelcan be configured to build a real radio environment. Inthe MAC layer, Carrier Sense Multiple Access with Col-lision Avoidance (CSMA/CA) protocol is used. For thenetwork layer, following services are provided: forminga network, nodes joining and leaving a network, networkaddress assignment, neighbor discovery, and route mainte-nance discovery. The application layer is responsible forproducing and processing sensing data. In the rest of thepaper, we will focus on the design of the network layermodel for mobile Zigbee sensor networks.
2.2 The design of network layer model
Three types of devices are defined in the Zigbee standardframework: coordinator, router, and end device. Coordina-tor is responsible for forming a new network, storing the keyparameters of the network and connecting to other networks.There is always a single coordinator in a Zigbee network.In Zigbee-based WSNs, sink node typically plays the roleof network coordinator. Router has the routing capability.
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Fig. 1 The developed node model
Specifically, it could allow other devices to join the net-work as its child nodes, and route data packets. End devicehas no routing capability, which relies only on its parents(the coordinator or a router) to route data packets. Com-pared with coordinator and router, end device has simplerhardware structure.
Each device node has a 16-bit short address for and a64-bit extended address in a Zigbee network. The 64-bitextended address is set by manufactures, similar to theMACaddress which is unique for each node. The 16-bit shortaddress is dynamically assigned to the node by its parentcoordinator or router when the node joins the network andit is similar to the IP address in the Internet network. Zigbeestandard uses a distributed address allocation mechanismfor assigning address to the node when it joins the network.Network address is determined by the following networkparameters which is provided by coordinator: *Cm the max-imum number of children allowed for each router, *Rm themaximum number of routers as subrouters linked to eachrouter, and *Lm the maximum depth of the whole network.The coordinator decides the depth of the whole network.The size of the address sub-block allocated by each parentat depth d , Cskip(d), could be described as:
Cskip(d) ={1 + Cm(Lm − d − 1) Rm = 11+Cm−Rm−CmR
Lm−d−1m
1−RmRm > 1
(1)
where Cskip(d) = 0 means that the node has no capability toaccept child nodes, and Cskip(d) > 0 otherwise. If networkaddress Ar(d + 1, m) is assigned to the m − th router child
andAe(d+1, m) is assigned to them−th end device child atdepth d+1, they can be obtained by the following equations:
Ar(d + 1, m) = Aparent + Cskip(d) × Ri (2)
Ae(d + 1, m) = Aparent + Cskip(d) × Rm + i (3)
where Ri ∈ [0, Rm], and Aparent represents the address ofthe parent, i ∈ [0, (Cm − Rm)].
Since the tree address allocation mechanism can providea simple and reliable routing method for the entire Zigbeenetwork, it is employed in our proposed simulation modelto assign a network address to a device node (a router or anend device) when it joins the network at the first time. Inorder to build a model of the IEEE 802.15.4/Zigbee proto-cols supporting node mobility, we propose a network layerprocess based on OPNET simulation platform as shown inFig. 2. In the network process module, by moving into theinit state and forcedly traversing to state wait, all types ofdevices begin executing initialization procedure.
1) If the node is a coordinator, it will first setupa network. By executing the transition functionwpan execute scan(), the coordinator will scan allchannels to select an unoccupied channel. After that,it will invoke the setnetwork state’s Enter execswpan zigbee setnetwork() to choose a network IDand configure other network parameters. Then it willmove into the active state to deal with network join-ing and leaving requests, or routing messages. Amongall functions associated with the active state, func-tion wpan handle mac pk() will be executed whenreceiving data packets from the MAC layer, whilefunction aodv rte rrep hello message send()
achieves the routing functionality. By periodicallybroadcasting hello data packets and receiving hello
packets from neighbors, the nodes (except the enddevices) will update their routing tables and neighbortables. For end devices, they will choose one propernode from candidate routers within its neighborhoodas its parent.
2) If the node is not the coordinator, it will scan all chan-nels by executing function wpan execute scan(). If itfinds an available channel in which a network is oper-ating, it will transit to Join network state and beginto execute function wpan zigbee join network(),where it will send a JOINRequest packet to the rel-evant router or coordinator asking to join the networkand then wait for the response to its request. Afterreceiving the corresponding JoinResponse packet,the node will enter the active state.
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2.3 Network join
As shown in Fig. 2, all deployed nodes begin at the init
state. Each node is categorized as coordinator, router, or enddevice. MAC layer channel scan is immediately called bythe coordinator after the process of network initialization,and its status would be changed to set network. When theMAC layer channel scan is completed, coordinator will lookfor an appropriate channel for establishing a new Zigbeenetwork. After the suitable radio channel is found, the coor-dinator will assign a network identifier to the new network,which does not conflict with other existing networks. Thenit will assign a network address to itself. Its status would beswitched to the active state. After the coordinator finishesthe above operations, the network is formed. Since then,other nodes will have the opportunity to join the network.The details of network join implementation procedure aredescribed below and are illustrated in Fig. 3.
When a child node A wants to send the join request, A
first carries out the channel scan procedure at theMAC layerand then broadcasts a beacon request frame. The processstatus is then transferred to join network. Meanwhile, atimer of channel sensing duration is started. In our model,the initial value is chosen in the interval [0.2s, 0.4s], wherein
0.2s is extremely adequate for awaiting and receiving replyframe from its neighboring nodes, even considering the pro-cession delay, the transmission delay and the propagationdelay. During this period of time, after receiving beaconframes, node A stores the neighbor information in its neigh-bor table. At the end of the timer, the MAC layer schedulesa remote interrupt to notice the network layer. Then, nodeA selects one node (coordinator or router) with the smallesthop count from itself as its potential parent node, denotedby B, which has both the capability and permit to acceptnew child nodes. Then, node A sends a join request frame tonode B, and starts a timer to wait for the corresponding joinresponse frame. The initial value of the timer is set as 0.2s.If it receives node B’s joining response frame before time-out, nodeA joins the network successfully. Its process statusmoves into active state for data communications. Other-wise, node A selects a new appropriate parent node in itsneighbor table and sends a new join request frame. How-ever, if there is no appropriate parent node in the neighbortable, it will call for MAC layer channel scan again. Theprocess status will stay in join network until node A joinsa network successfully.
For a parent node, once it receives the beacon requestframe, it will broadcast a new beacon frame. When it
Fig. 2 Zigbee network layermodel in OPNET
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Fig. 3 Procedure for nodesjoining a network
receives a join request frame, it will use the distributedaddress allocation mechanism to judge whether it has abilityto adopt a child node. If available, it will assign a networkaddress and send a join response frame to the child node.Otherwise, it will discard the join request frame. Comparingwith Zigbee standard network layer protocol, the proposedprotocol has the following differences in network join pro-cedure. 1) In Zigbee standard network layer protocol, evenif the required parent node has no capacity to adopt the childnode, it is still required to send a feedback message. 2) Oncethe node is rejected to join, it will carry out the channel scanin the next round. However, this action is neither necessarynor energy-efficient because it is still possible that a propernode in its neighbor table can be its parent node. In addition,these actions will increase the traffic load and cause moretransmission conflicts. In Section 4, the experimental resultsdemonstrate that our proposed protocol can save more than30% communication overheads for network join procedure.
2.4 Route discovery and maintenance
AODV [19] routing algorithm is an on-demand algorithm,which builds up routes between source nodes and destina-tion nodes only when it desires to. It uses sequence numbersto avoid occurring routing loops.
In this paper, we propose an improved AODV rout-ing algorithm. Similar to AODV, the proposed algorithm
consists of two parts: route discovery and route mainte-nance. In route discovery, source node first broadcasts aroute request (RREQ) packet across the network via flood-ing. Once neighboring nodes receive the packet, each ofthem judges whether the destination address of the packetis its network address. 1) If two addresses match with eachother, the node will add this route to its routing table andestablishes the reversed pointer to the source in its rout-ing table entries. Next, it sends a route reply (RREP) to thesource node along the reversed direction. 2) If two addressesare not matched, then the node searches its routing table tofind a possible route to the destination. If the route exists,the node sends a join response frame to the source node,and sends a message to the destination. Otherwise, the nodeestablishes the reversed pointer to the source in its routingtable entries and then continues to flood the RREQ packet.Note that device nodes employ the destination addressand broadcast serial number of the source node as theunique identifier to avoid repeatedly broadcasting RREQpackets.
In route maintenance, every router node needs to main-tain its own routes to guarantee their validness after they areestablished. Thus, every node should periodically broadcasta hello message to determine whether the current routes arevalid. If a route becomes invalid, it will broadcast a routeerror (RERR) message to inform the source that the route isnow unreachable to destination(s).
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The routing process of the improved AODV routing algo-rithm in our proposed model is described as follows. If anend device intends to send a data packet, it sends the datapacket directly to its parent node. Note that the parent nodemust be a router or the coordinator. For a router or a coordi-nator, it sends the data packet directly to the next hop node ifit has a route to the destination. Otherwise, it initiates a routediscovery. The overall flow chart of the proposed routingalgorithm is given in Fig. 4.
3 Mobility support in the Zigbee-based WSNs
3.1 Mobility support for Zigbee router
In mobile Zigbee-based WSNs, routers (or the coordina-tor ) do not need to dynamically change network addresses,because they have the capability to maintain and repair theirroute tables. By adopting the improved AODV algorithm,routers may reduce the negative effects of node mobil-ity. When a router fails in the network, it is not requiredto change the network address of the router after beingassigned an initial network address.
3.2 Mobility support for end devices
For mobile WSNs, when end devices move outside theirparent nodes’ communication range, according to Zig-bee/IEEE802.15.4 Standard, they are required to find newparent nodes and change their current network addresses.In order to solve this drawback of the current Standard, an
Fig. 5 A diagram of mobility support for end devices
adaptive routing strategy is proposed for end devices. Forimplementing the function of neighbor discovery, a routeris required to periodically broadcast hello packets. Accord-ing to these received packets, end devices can generate andmaintain their own neighbor tables, and obtain the infor-mation of routers within its neighborhood. Obviously, allrouters can act as its potential parent node. When the enddevice leaves its parent node, it selects one from thesecandidates as its parent node to forward its data packets.
Figure 5 illustrates the procedure to support the nodemobility for end devices. In Fig. 5, node D is a child of nodeA in area 1 at the beginning. If node D moves to area 2, itwill become out of the communication range of the node A.In area 2, node D can receive HELLO messages from nodeC to update its neighbor table. Therefore, it can select nodeC as its parent node. Following this procedure, end devicedoes not need to search a new parent node in order to get anew network address.
Fig. 4 Flow chart of theimproved AODV algorithm
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Compared to the traditional AODV protocol, our pro-posed routing protocol is superior in following two aspects.1), the traditional AODV protocol does not distinguish dif-ferent types of devices, so as to result in broadcastingredundant RREQ packets for end devices during route dis-covery phase and aggravating network congestion. 2), theconventional AODV protocol can not provide any supportfor the mobility of three types of nodes.
4 Simulation and analysis
In this section, we conduct a comparative performanceevaluation between our proposed Zigbee compliant simula-tion model using the OPNET simulator (ZCNSMOS) andZMOMSL, in terms of the number of essential routers,networking overhead, and network join delay. In order toassess validity of the proposed model for supporting nodemobility, we conduct experiments to measure the time takenby nodes they establish new routes after previous onesfail. All experiments are completed in OPNET Modeler14.5. In experiments, all device nodes are uniformly scat-tered over a 100 × 100 m2 square area, and the number ofdevice nodes varies from 20 to 100. The commonly usedRandom Waypoint mobility model is employed and themaximum velocity is set at 3m/s. Environmental variablesapplied to performance evaluation are shown in Table 1. Foreach experiment, we randomly generate network topologies,repeat 20 runs, and calculate the average value.
We first test the number of essential routers of two mod-els for different network size so that all nodes can jointhe network. The results are displayed in Table 2. We canobserve that with the increase of the network size, denotedby n, the number of essential routers of both ZCNSMOSand ZMOMSL increases, however, the value of ZCNSMOSis always lower than that of ZMOMSL. When n=20, ZCNS-MOS needs less than half essential routers compared to
Table 1 Environmental variables
Deployment area 100 × 100 m2
Network size 20∼ 100
Transmission range 30m
Transmission power 0.05W
Packet size 512bytes
Packet interval Poisson(10)s
Simulation duration 600s
Mobility model Random waypoint
maximum velocity 3m/s
Packet start time 20s
The maximum number of child nodes 7
The maximum number of routers 5
The maximum depth 5
Table 2 The number of essential routers
Network size ZMOMSL ZCNSMOS
20 6.15 2.75
40 13.35 5.65
60 19.50 10.45
80 25.90 16.35
100 33.85 24.05
ZMOMSL. When n increases to 100, this ratio can be fur-ther reduced to nearly 1
4 . Since in general router is a fullfunction device with routing capability and is much moreexpensive than end device, our proposed ZCNSMOS has aclear advantage in terms of networking cost.
In order to evaluate the energy efficiency of two modelsin networking, we demonstrate the communication over-head required for all nodes joining a network. For compar-ison purpose, we use the number of ACK frames includingreceiving joining request packets and joining response pack-ets as a performance metric, since such number can effec-tively reflect the total communication overhead. Figure 6illustrates the relationship between network join overheadand network size. As expected, ZCNSMOS has lower net-working overhead than ZMOMSL for all scenarios underconsideration, it is because ZCNSMOS is designated tomake network join procedure more simplified.
Figure 7 illustrates the runtime with respect the numberof nodes attached to the network. The network size n is fixedat 80. For different values of n, similar conclusions can beobtained. From this figure, we can see that in ZCNSMOS,all device nodes can quickly join the network once theyare deployed. While for ZMOMSL, device node attachmentnetwork happens after nearly 6s. It is because in ZMOMSL,a large number of broadcast packets, which are generatedby all un-joined nodes, lead to traffic congestion and high
Fig. 6 The communication overhead of both ZCNSMOS andZMOMSL in networking
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Fig. 7 Comparison of ZMOMSL and ZCNSMOS in terms of networkjoin delay
packet loss in the network. Then, according to theMAC pro-tocol, a random waiting time is required for device nodes toretransmit data packets. For any device node, ZCNSMOSnot only makes it more rapidly to join and leave a net-work, but also gives mobility support. After the node movesto a new area and is required to rejoin a network, shorternetwork join delay can help device nodes participate in anetwork task more timely. As shown in above experimentalresults, it is apparent that ZCNSMOS enhances the perfor-mance significantly compared to ZMOMSL for the functionof network join of the network layer.
To evaluate the effectiveness of our proposed modelZCNSMOS in terms of node mobility support, we mea-sure the length of intervals between route failure occurrenceand route recovery. In this experiment, the number of enddevices is fixed at 20, and the number of routers, denotedby λ, varies from 6 to 21. For different amounts of routers,the average values of time interval lengthes, the high val-ues and the low values are presented in Fig. 8. It is obviousthat for any values of λ, all time intervals have lengthes lessthan 0.1s. Compared to sensing intervals in conventionalapplications, which is 10s in our experiments, such timelengths are rather trivial and can be neglected. These resultsclearly demonstrate that ZCNSMOS can quickly establishnew routes for device nodes when their old routes can notbe maintained due to node mobility. With the increase ofλ, new route discovery time decreases. When λ becomes16, the average value is close to 0. This is because for enddevices, statistically, the increase of λ means that its neigh-bor table is large so that there are more routers within itsneighborhood. For routers, the increase of λ means the aris-ing of the possibility that they have routes to destinations.
5 Conclusion
In this paper, an OPNET-based simulation model, calledZigbee compliant new simulation model using the OPNETsimulator (ZCNSMOS), is proposed to achieve node mobil-ity support for Zigbee sensor networks. As the MAC layerof Zigbee networks has been defined by IEEE 802.15.4
Fig. 8 Time interval of newroute discovery in different sizeof sensor networks
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Standard, we focus our work on the framework designand the implementation of the network layer. The imple-mentation procedure of the key functions including net-work join, routing discovery and maintenance are inves-tigated in details. After that, we investigate the networkrouting protocols for routers and end devices, as to sup-port node mobility. Our experimental results show thatthe proposed model can achieve significant performanceimprovement in networking, routing, and node mobilitysupport.
Acknowledgments This work was supported by the National Nat-ural Science Foundation of China (Grant Nos. 61462021, 61262074),Opening Project of Guangxi Key Laboratory of Trusted Software(Grant No. PF130549), the Natural Science Foundation of Guangxi(Grant No. 2012GXNSFAA053224) and the Nature Science and Engi-neering Research Council of Canada (NSERC) Discovery Grant.
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Xiaolong Li received the B.E.degree from the Harbin Insti-tute of Technology, Harbin,China in 2003 and the Ph.D.degree from the Hunan Uni-versity, Changsha in 2008.Since July 2008, he has beenwith the Guilin Universityof Electronic Technology,Guilin China. In 2013, he wonthe best paper award at theChinaCom conference. Heis currently a Professor. Hisresearch interests include sen-sor networks, M2M networks,and ad hoc networks.
Meiping Peng received theM.E. degree in gui lin Uni-versity of electronic tech-nology, China, Gui lin, in2013. He will pursue a Ph.D.at the School of Electronicsand Information at the North-western Polytechnical Univer-sity, Xi’an, China. His currentresearch interests lay in wire-less sensor network, wirelesscommunication.
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Jun Cai received the B.S. andM.S. degrees from Xi’an Jiao-tong University, Xi’an, China,in 1996 and 1999, respec-tively, and the Ph.D. degreefrom the University of Water-loo, Waterloo, ON, Canada,in 2004, all in electrical engi-neering. From June 2004 toApril 2006, he was with Mc-Master University, Hamilton,On, as a Natural Sciences andEnginnering Research Councilof Canada Postdoctoral Fel-low. Since July 206, he hasbeen with the Department of
Electrical and Computer Engineering, University of Manitoba, Win-nipeg, MB, Canda, where he is an Associate Professor. His currentresearch interests include multimedia communication systems, mobil-ity and resource management in beyond-third-generation wirelesscommunication networks, and ad hoc and mesh networks.
Changyan Yi He is cur-rently a M.Sc. student inelectrical and computerenginnering, Univeristy ofManitoba, Winnipeg, MB,Canada. He received the B.S.degree from Guilin Universityof Electronic Technology,Guilin, China, in 2012. Hisresearch interests includeradio resource management,algorithmic game theory, auc-tion theory and optimizationin wireless CommunicationNetworks.
Hong Zhang received B.S.degree in computer sciencefrom JILIN University, China,in 2007. And he receivedM.S. degree in informationtechnology and telecommuni-cations from INHA Univer-sity, Korea, in 2010. He iscurrently a Ph.D. candidatewith the Department of Elec-trical and Computer Engineer-ing at University of Manitoba,Canada. His research interestsinclude green communication,heterogeneous networks andcognitive radio in wirelessnetworks.