Modified rts cts exchange mechanism for manet


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Modified rts cts exchange mechanism for manet

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME27MODIFIED RTS/CTS EXCHANGE MECHANISM FOR MANETWITH A MULTI LEVEL COLLISION DOMAIN ARCHITECTUREM ABDUL GAFURDept. of Computer Science & EngineeringJNTU, HyderabadAndra Pradesh, IndiaNIRAJ UPADHAYAYADept. of Computer Science & EngineeringJB Institute of Engineering & Technology, HyderabadAndra Pradesh, IndiaSYED ABDUL SATTARDept. of Computer Science & EngineeringRoyal Institute of Technology & Science, HyderabadAndra Pradesh, IndiaABSTRACTEfficient utilization of available capacity in mobile adhoc network is always achallenge. Capacity is wasted due to the hidden terminal problem and exposed terminalproblem. Classic method of virtual carrier sensing make use of an RTS/CTS exchange priorto data exchange depends merely on size of data. This method is too conservative to rely onin adhoc network which is usually applied in emergency situations because it degrades thethroughput performance of adhoc networks. Besides, it causes the unnecessary overhead onthe nodes and delay in the networks. In this paper we propose a modified RTS/CTS methodwhich overcomes the limitations of the standard method. Besides the size of packet we alsoconsider the traffic around the node to decide for an RTS/CTS exchange. The modifiedscheme monitors the traffic around every node and avoids the use of RTS/CTS exchangewherever the chance for hidden terminal problem is less irrespective of the packet size. Ourscheme is an optimized approach rather than conservative. We analyzed standard RTS/CTSscheme and other advanced proposals and compared with our proposal. We simulated thenetworks of various numbers of nodes with different traffic rate and analyzed theINTERNATIONAL JOURNAL OF ADVANCED RESEARCH INENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 4, Issue 4, May – June 2013, pp. 27-37© IAEME: Impact Factor (2013): 5.8376 (Calculated by GISI)www.jifactor.comIJARET© I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME28performance. Simulation result shows that our approach provides better performance inheavily loaded network as well as in lightly loaded network. Performance enhancement ofour proposed scheme is the result of a realistic assumption of less chance for hidden terminalsin a network part of low collision rate rather than pessimistic assumption of omnipresenthidden terminals all over the network.Keywords: Adhoc Networks, Collision, Contention Window, CTS, RTS1. INTRODUCTIONMobile Ad hoc Network (MANET) is a kind of decentralized wireless network ofindependent mobile nodes without having a central coordinator. MANETs are widelyapplied in potential crisis management services applications in civil and militaryenvironments, such as responses to hurricane, earthquake, tsunami, terrorism and battlefieldconditions where the entire communication infrastructure is destroyed and restoringcommunication quickly is crucial [1]. As the large scale disasters very frequently happen inthese days it is important to have efficient and durable disaster emergency communicationsystems like Mobile Adhoc networks.In Mobile ad hoc networks sharing of wireless bandwidth among ad hoc nodes mustbe organized in a decentralized manner as there is no central coordinator. Carrier SenseMultiple Access with Collision Avoidance (CSMA/CA) and its variants are widely used inad hoc networks. However, all these CSMA/CA based MAC protocols suffer from the wellknown “hidden terminal” problem [2]. The hidden terminal problem occurs when thesimultaneous transmissions of two transmitters that lie outside carrier-sense (CS) range causeinterference at one or both receivers and prevent successful reception. An example is shownin Fig 1. In Fig1 we have three nodes in which we assume A send data to B and C also needto send to B. Transmission range of A and C is shown. From the figure it is clear that node Aand node C cannot hear each other. By only sensing the medium, node C will not be able tohear transmissions by node A and start transmission and eventually leads to collision at B.Transmission range of A Transmission range of CFig 1: Hidden Terminal ProblemTo overcome the hidden terminal problem adhoc networks usually depends on avirtual sensing mechanism using a pre-data control information exchange. One such virtualsensing mechanism is the 802.11 Request to Send/Clear to Send (RTS/CTS) exchangeresulting in nodes getting exclusive access to the channel for a specific time period.Nevertheless, this mechanism causes some nodes who heard the RTS/CTS exchange toA B C
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME29refrain from sending data even though they would not have interfered with any ongoingtransmission. This problem is termed as “exposed terminal problem” and Fig 2 illustratessuch a situation.In Fig 2 Node A wants to send to node B. Node A sends an RTS and waits for B tosend CTS. Assume node D wants to send to node C and D transmits an RTS to C just beforeA sends the RTS to B. C transmits a CTS as a response to RTS from D. This CTS is alsoheard by B and refrain from sending the CTS to A even though simultaneous datatransmission from A and D would not interfered each other. Exposed terminal problemoccurs when two transmitters lie within CS range and are prevented from transmittingsimultaneously, even though their transmissions do not mutually interfere [7].We have to deal with these two issues for achieving improved capacity utilization.Capacity is wasted because of the failed transmissions due to hidden terminals andunexploited transmission opportunities due to exposed terminals. The existence and intensityof hidden and exposed terminals in a given network depends on the topology and on the CSrange. In a mobile adhoc network topology cannot be predicted or controlled. We can onlycontrol the carrier-sense range. The number of Hidden terminals can be reduced by having alarger CS range, but results in more exposed terminals. On the other hand a smaller CS rangeresults in fewer exposed terminals but more hidden terminals. In short both hidden andexposed terminals cannot be simultaneously eliminated or reduced by adjusting the CS rangealone.Although several alternative proposals have been made to address the abovementioned shortcomings of standard RTS/CTS scheme, many of them are not satisfactorilyaddress the key issues of keeping the simplicity of the protocol and avoiding the overhead onthe nodes on duty in emergency situations where usually adhoc networks are applied. In thispaper we propose a modified RTS/CTS method which overcomes the limitations of thestandard method and other related proposals. The basic principle of our scheme is to identifythe areas in the network in which chance for the presence of “active” hidden terminals aremore and areas where the packets are smoothly flowed and regulate the use of RTS/CTScontrol packets accordingly. For this purpose we monitor the traffic around every node andavoid the use of RTS/CTS exchange wherever the chance for hidden terminal problem is lessirrespective of the packet size. Our scheme is an optimized approach rather than conservative.Transmission range of A Transmission range of DTransmission range of B and CFig 2: Exposed Terminal ProblemA B C D
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME302. DISCUSSION ON RELATED WORKIn [2] authors proposed a method of delayed response to RTS from destination nodes.If a node A wants to communicate with the destination node B, firstly A transmits an RTS toask for agreement. Once the destination node receives RTS, it does not reply CTSimmediately, but forwards the RTS to its neighbor nodes to inform the channel states in nextslot, then in the third slot replies CTS. Obviously this method makes too much delay in datatransmission and wastage of the channel bandwidth.In [3] it uses an approach of calculating the number of hidden nodes around thereceiver to decide on the use of RTS/CTS prior to data transmission. But this method ishaving the overhead of keeping the list of neighboring nodes and finding out number ofhidden terminals around the receiver. Since mobility is common in adhoc networks thisneighbors and hidden nodes will change frequently. Therefore this list has to be updated timeto time. This makes overhead in the network.Another common approach is to tune the CS threshold for having an improved spatialreuse [4, 5]. The main drawback of this approach is the poor fairness due to asymmetric CSranges. Another approach for better spatial reuse is to control the transmission power [6].However, hidden and exposed terminals cannot both be eliminated by either adjusting the CSthreshold or controlling the transmission power. MACA-P [2] enhances the RTS/CTSmechanism to increase concurrency. The RTS/CTS exchange between a pair of nodes isfollowed by a control gap during which another pair of nodes can also exchange RTS/CTSmessages and synchronize its data transmission with the first node pair. RTS/CTS messagesand control gap significantly increase the overhead per data transmission. In [7] The proposedsolution consists of two phases. In the first phase, exposed links in the mesh topology aredetected through an offline training process. Coordination of simultaneous transmissions overexposed links is then done in the second phase. But detection of exposed link is not easy tasksand causes for extreme overhead. Moreover, as we mentioned before we have to do thisdetection frequently because of the mobility of nodes.3. PROPOSED SCHEMEThe basic principal of our approach is to correctly identify the cases in whichRTS/CTS exchange is necessary prior to data transmission and cases in which direct datatransmission is desirable thereby to optimize the use of RTS/CTS exchange wherever it ispossible without having a cost of high collision of packets due to hidden terminals and hencea performance degrade in the network. This method of optimized use of RTS/CTS yields twobenefits simultaneously. It reduces unnecessary overhead and delay in the network andreduces the chance for exposed terminal problems. In other words performance enhancementof our approach is the direct impact of reduced overhead and delay in packet transmissionand avoidance of the case of leaving the channel unused due to exposed terminals.3.1. Categorizing the Network in to Multilevel Collision DomainsAccording to our proposed scheme the decision on whether an RTS/CTS is to be usedis taken adaptively taking in to account the current scenario of the shared medium andcollisions encountered by the nodes. Based on the result of traffic analysis network isvirtually divided into different collision domains CD1, CD2…CDn where n is the number ofcollision domains in the network. Rate of packet collision in each CDi (RCDi) is less than thatof CDi+1.
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME31i.e. RCD1<RCD2<RCD3…< RCDnInitially all the nodes in the network (say N) will be in lowest collision domain.i.e. N € CD1Each node keeps track of its success and failure rate of data transmission. Based onthis success and failure rate the node will move from one collision domain to another. Unlikeinitial case after some time of network activities total number of nodes will split into differentcollision domains depends on the success and failure rate of data transmission of each node.i.e. N1€ CD1, N2€ CD2 … Nn€CDn where Ni⊆N,Depends on the collision intensity of each collision domains we broadly classify theminto two zones namely Red zone and Green zone. Red zone contains the entire collisiondomain above a threshold called RTSthreshold by which we start using RTS/CTS prior to everydata transmission. On the other hand Green zone contains all the collision domain under theRTSthreshold where direct data transmission without a prior RTS/CTS exchange is possible.RTSthreshold by which we start using RTS/CTS virtual carrier sensing is defined in terms ofcollision intensity experienced by the nodes and packet size rather than mere packet size.When a data packet is ready to send, the node first check the intensity of trafficaround the node. If the intensity is below the pre-defined level the node can transmit dataimmediately without a preceding virtual carrier sensing using RTS/CTS irrespective of thesize of the data. Otherwise the node should perform the RTS/CTS exchange prior to datatransmission. Flow chart in Fig 3 explains the working of our proposed scheme. As we showin the flow chart if there is a collision experienced by the packet from the node after a directtransmission (without preceding RTS/CTS) due to low collision intensity around the node,collision counter is incremented. If the value of the collision counter crosses the pre-definedCollision Threshold (Colthreshold) the node will move to Red zone of higher collisionhenceforth the node start using virtual carrier sensing. On the other hand if there is nocollision even after a direct transmission collision counter will be decremented. If thecollision counter comes under the Colthreshold the node will move to Green zone hence forth noneed of RTS/CTS exchange prior to data transmission.3.2 Value addition of Contention WindowSince behavior of node on transmission fully depends on the collision domain inwhich node belongs, analyzing the collision intensity is most important in our scheme. Wecan introduce a counter associated with each node to count number of collisions occurredduring data transmission from each node. Based on the value of this counter we cancorrectly analyze the collision intensity around the nodes. But keeping a separate counterwith each node is highly inefficient especially in mobile devices. To solve this issue we usethe size of contention window as an indicator of the collision intensity around the node [8].This is only possible by a reasonably varying adjustment of contention window(CW) thatcorrectly reflect the collision intensity of the nodes. Therefore we cannot rely on standardcontention window adjust mechanism using Binary exponential Backoff (BEB) since CWadjustment(increment or decrement)in BEB is not in proportion of failure or success in datatransmission.
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME32Fig 3. Flow chart of the Proposed SchemeIn the BEB scheme, each node doubles its contention window, CW, up to themaximum contention window (CWmax) after a collision occurs and resets its CW to theminimum value (CWmin) after a single successful transmission:CW = min(2.CW;CWmax) ; upon a collisionCW = CWmin ; upon a successWhere CWmax and CWmin are the maximum and minimum value of CW respectively.CWmax and CWmin are defined to avoid the contention window from growing too large andshrinking too small. The values of the CWmin and CWmax are pre-determined based on theexpected range of the number of active nodes and the traffic load of the network[9,10].Because of the exponential expansion of CW on collision and a sudden fall to the minimumvalue of CW on a single transmission success current CW size cannot be taken as an indicatorof the packet collision.To support our proposed scheme with a Contention Window which correctly reflectsthe collision intensity around the nodes we make use of our recently proposed backoffscheme called Collision Based Contention Protocol. As per this scheme if a collisionhappens, the rate of expansion of CW depends on the current size of CW. If the current CWsize is CWmin then it slightly increase its size. As the size at the time of collision increasesthe rate of expansion also increases. On the other hand, on a success of data transmission, asthe CW size at the time of transmission increases the rate of contraction of CW sizedecreases. This method of Contention Window adjustment mechanism not only solve thefairness issues and poor performance of standard BEB scheme but also provide a value addedContention Window which also can be used as a perfect indicator of collisions associatedwith each node. Interested readers will get more about this protocol in [8].3.3 Handling of Hidden terminals and exposed terminalsAs we discussed before two main problems in an adhoc network which degrades itsperformance are hidden terminal problem (HTP) and exposed terminal problem. Exposedterminal problem arises as a byproduct of the solution to hidden terminal problem. In otherwords, exposed terminal problem is the result of RTS/CTS exchange mechanism employed inMANETs. From this it is clear that exposed terminal problem can be mitigated by reducingRTS/CTS exchange. But this reduction of RTS/CTS exchange causes for the frequent hiddenterminal problem. Our approach successfully overcomes this issue. When the nodes are in thelow collision domain packets from them experience less number of collisions. From this we
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME33can reasonably assume that hidden terminal problem is very less for a node under lowcollision domain otherwise the node would have suffered lot of transmission failure andmoved to higher collision domain due to the above mentioned inherent property of ourapproach. Because of this less number of HTP or a best case of absence of HTP in lowcollision domain we completely avoid the RTS/CTS exchange in this region. This way weavoid unnecessary delay and overhead and effectively solve the issue of exposed terminalproblems. As a result better performance is obtained.Our approach also works well in a worst case of sudden topology changes in thenetwork due to node movement and several hidden terminals in the previously mentioned lowcollision domain. Because of the no RTS/CTS exchange surely packets will collide. But dueto this collision the respective nodes will immediately move to higher collision domain andstart sending RTS/CTS. In this way our scheme effectively deals with hidden terminalproblems and exposed terminal problem.4. PERFORMANCE EVALUATION AND RESULT4.1 Simulation AnalysisEvaluation and comparison of proposed scheme with standard scheme is made usingnetwork simulator Ns-2. Ns-2 is a powerful network simulator. Ns-2 is extensively used by thenetworking research community. It provides substantial support for simulation of TCP, routing,multicast protocols over wired and wireless (local and satellite) networks, etc. The simulator suitealso includes a graphical visualizer called network animator (nam) to assist the users get moreinsights about their simulation by visualizing packet trace data. AWK, a text processing utility hasbeen used to extract desired information from ns trace file. We have used Linux Operating Systemto run our simulation code. We choose networks of varying number of nodes in an area 500m x500m. The random way point motion pattern is adopted. We used the CBR traffic and packet sizeof 1500 bytes. The performance is evaluated by adding new nodes in the network as time variesor expedited arrival of several nodes simultaneously. Sufficient time is given for running thesimulation in order to get chances for every node to participate in the network activity oftransmission or reception.4.2 Performance MetricsFirstly we measured the Packet Delivery Ratio (PDR). It is the ratio of the number ofdelivered data packet to the data packet actually sent to the . Number of packet receive/ Number of packet sendFig 3 shows that PDR from our novel scheme is higher than the standard scheme in lightlyloaded network as well as heavy one. This performance gain is resulted from the relativelycongestion free network due to less control overhead. Then we measured throughput atvarious instant in the networks of different number of nodes. This metric is helpful toanalyze the behavior of our protocol when new nodes are entering the network as time goes.For this we made a simulation with nodes entering the network one by one rather than all thenodes simultaneously. Fig 4 and Fig 5 show the Throughput performance of our protocolcompared to standard scheme in lightly loaded network with 20 nodes and heavily loadednetwork with 80 numbers of nodes respectively.
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME34Table 1Simulation ParametersParameters ValuesNumber of nodesSimulation timeSimulationArea(m)TopologyPhyPacket sizeQueue lengthSIFSDIFSProTypeAntenna typeCWminCWmaxVaryingVarying500x500Randomwireless150050010µs50µsFree SpaceOmnidirectional15 or 311023Fig 4. Packet Delivery RatioFig 5. Throughput Comparison with 20 nodes
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME35Fig 6. Throughput Comparison with 80 nodesFrom the above two graphs it is clear that our protocol provide better throughput inthe network of small number of nodes as well as large number of nodes. Performanceenhancement is more in lightly loaded network. In lightly loaded network chance for hiddenterminals are less compared to heavy loaded one. Therefore we can avoid or reduce the use ofRTS/CTS exchange in this case without having fear of packet collision. This is effectivelydone in our protocol and hence we get a best result in lightly loaded network.Besides, when more and more nodes enter to the network our protocol starts usingRTS/CTS exchange and avoids collisions due to hidden terminals. Therefore our protocolalso provides higher aggregate throughput both in the case of smaller and larger networks.Fig 6 shows a comparison of aggregate throughput of our protocol with standard protocol fornetworks of various numbers of nodes.As we mentioned before this throughput gain is achieved with a controlled use ofRTS/CTS rather than a conservative use with a pessimistic assumption of hidden terminalsanywhere anytime. Because of the conservative approach against collision in the standardscheme collisions are less compared to our scheme. But this will not result in gain inthroughput because of the bandwidth wastage due to exposed terminal issues and over usageof control message for virtual carrier sensing. Our approach of controlled use of RTS/CTSresults in a slight increase in collision in the low collision domain as a cost for reduced use ofvirtual carrier sensing. But this creates better utilization of available bandwidth by preventingexposed terminal issues and reduced use of control message. In other words, our approachprovides a considerable gain in aggregate throughput with a cost of slight increase in collisiononly on low collision domain. Fig 7 shows a comparison of gain in throughput againstincrease in collision. It is clear that increment in collision is negligible to the resultedthroughput gain.Fig 7. Aggregate Throughput Comparison for different networks
  10. 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME36Fig 8. Throughput gain v/s Increase in collision5. CONCLUSIONIn this paper, we proposed a modified pre-data handshake mechanism with acontrolled exchange of RTS/CTS in Mobile Adhoc Networks. We have analyzed variousproposals presented to overcome the drawback of the standard pre-data control scheme. Oursolution presents a better approach for getting an increased throughput with an improvedspatial reuse and reduced occurrence of exposed terminal problem. According to our schemeinstead of having a conservative approach of sending RTS and CTS prior to every datatransmission an optimized approach is adopted on pre-data control messages. The proposedscheme monitors the traffic around every node and categorizes them into different collisiondomains depends on the collision rate on the packets from each node. The very purpose ofthis dynamic grouping of nodes is to avoid the use of RTS/CTS exchange wherever thechance for hidden terminal problem is less irrespective of the packet size. Performance of thescheme is evaluated using the Ns-2.33 network simulator. The simulation result shows thatour proposed scheme outperforms the standard method of pre-data handshake mechanism innetwork of small number of nodes and large number of nodes. Performance enhancement ismore in lightly loaded network because in lightly loaded network chance for hidden terminalsare less compared to heavy loaded one and reduced use of pre-data control message will notcreate packet collision.REFERENCE[1] M.B Yassein, S. Manseer, A.A Hassan, Z.A Taye, “A New Probabilistic LinearExponential Backoff Scheme for MANETs,” IEEE International Symposium onParallel & Distributed Processing 2010, pp 1-6[2] N. Zhang, N. Liu, Q. Yu, “ Improved RTS-CTS Algorithm to Solve Mobile HiddenStation Problem in MANET,”Cross Strait Quad- Regional Radio Science and WirelessTechnology Conference (CSQRWC), 2011, pp 812 – 815[3] T.Shigeyasu, M.Akimoto, H. Matsuno, “Throughput Improvement ofIEEE802.11DCF with Adaptive RTS/CTS Control On the Basis of Existence of HiddenTerminals,” International Conference on Complex, Intelligent, and Software IntensiveSystems 2011, pp 46 –52.[4] H. Zhai, Y. Wang, “Physical Carrier Sensing and Spatial Reuse in Multirate andMultihop Wireless Ad Hoc Networks,” In Infocom, Barcelona, Spain, Apr 2006.
  11. 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME37[5] J. Zhu, B. Metzler, X. Guo, Y. Liu, “Adaptive CSMA for Scalable Network Capacityin High-Density WLAN a Hardware Prototyping Approach,” In Infocom, Barcelona,Spain, Apr 2006.[6] M. Cesana, D. Maniezzo, P. Bergamo, M. Gerla, “Interference Aware (IA) MAC: anEnhancement to IEEE 802.11b DCF,” In Vehicular Technology Conference (VTC),Orlando, FL, Oct 2003.[7] K. Mittal, E.M. Belding, “RTSS/CTSS: Mitigation of Exposed Terminals in Static802.11-Based Mesh Networks,” 2nd IEEE Workshop on Wireless Mesh Networks,2006, pp 3 – 12[8] M.A. Gafur, N. Upadhayaya, S.A Sattar, “Achieving Enhanced Throughput In MobileAdhoc Network using Collision Aware Mac Protocol,” International Journal of Ad hoc,Sensor & Ubiquitous Computing (IJASUC) Vol.2, No.1, March 2011[9] D. Jium, H.C. Chao, H.H Chen, “Slow Start Backoff Algorithm for Ad-Hoc WirelessNetworks,” IEEE Global Telecommunications Conference, 2010, pp 1-5.[10] L.P.K. Wong, D.J.Yin, “Throuhput Analysis of CSMA Protocol with Exponentialbackoff,” In in the proc. of Conference on Wireless and Optical Communications, 2010,pages 1—5.[11] Rambabu.V, Dr. A.N. Gaikwad and Bhooshan Humane, “Throughput Improvement inMedical Ad-Hoc Sensor Networks: A Review, Challenges and Future Scope forResearch”, International Journal of Electronics and Communication Engineering &Technology (IJECET), Volume 3, Issue 1, 2012, pp. 23 - 28, ISSN Print: 0976- 6464,ISSN Online: 0976 –6472.