INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

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1 INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN ISSN (Print) ISSN (Online) Volume 4, Issue 4, May June 2013, pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJARET I A E M E MODIFIED RTS/CTS EXCHANGE MECHANISM FOR MANET WITH A MULTI LEVEL COLLISION DOMAIN ARCHITECTURE M ABDUL GAFUR Dept. of Computer Science & Engineering JNTU, Hyderabad Andra Pradesh, India NIRAJ UPADHAYAYA Dept. of Computer Science & Engineering JB Institute of Engineering & Technology, Hyderabad Andra Pradesh, India SYED ABDUL SATTAR Dept. of Computer Science & Engineering Royal Institute of Technology & Science, Hyderabad Andra Pradesh, India ABSTRACT Efficient utilization of available capacity in mobile adhoc network is always a challenge. Capacity is wasted due to the hidden terminal problem and exposed terminal problem. Classic method of virtual carrier sensing make use of an RTS/CTS exchange prior to data exchange depends merely on size of data. This method is too conservative to rely on in adhoc network which is usually applied in emergency situations because it degrades the throughput performance of adhoc networks. Besides, it causes the unnecessary overhead on the nodes and delay in the networks. In this paper we propose a modified RTS/CTS method which overcomes the limitations of the standard method. Besides the size of packet we also consider the traffic around the node to decide for an RTS/CTS exchange. The modified scheme monitors the traffic around every node and avoids the use of RTS/CTS exchange wherever the chance for hidden terminal problem is less irrespective of the packet size. Our scheme is an optimized approach rather than conservative. We analyzed standard RTS/CTS scheme and other advanced proposals and compared with our proposal. We simulated the networks of various numbers of nodes with different traffic rate and analyzed the 27

2 performance. Simulation result shows that our approach provides better performance in heavily loaded network as well as in lightly loaded network. Performance enhancement of our proposed scheme is the result of a realistic assumption of less chance for hidden terminals in a network part of low collision rate rather than pessimistic assumption of omnipresent hidden terminals all over the network. Keywords: Adhoc Networks, Collision, Contention Window, CTS, RTS 1. INTRODUCTION Mobile Ad hoc Network (MANET) is a kind of decentralized wireless network of independent mobile nodes without having a central coordinator. MANETs are widely applied in potential crisis management services applications in civil and military environments, such as responses to hurricane, earthquake, tsunami, terrorism and battlefield conditions where the entire communication infrastructure is destroyed and restoring communication quickly is crucial [1]. As the large scale disasters very frequently happen in these days it is important to have efficient and durable disaster emergency communication systems like Mobile Adhoc networks. In Mobile ad hoc networks sharing of wireless bandwidth among ad hoc nodes must be organized in a decentralized manner as there is no central coordinator. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and its' variants are widely used in ad hoc networks. However, all these CSMA/CA based MAC protocols suffer from the well known hidden terminal problem [2]. The hidden terminal problem occurs when the simultaneous transmissions of two transmitters that lie outside carrier-sense (CS) range cause interference at one or both receivers and prevent successful reception. An example is shown in Fig 1. In Fig1 we have three nodes in which we assume A send data to B and C also need to send to B. Transmission range of A and C is shown. From the figure it is clear that node A and node C cannot hear each other. By only sensing the medium, node C will not be able to hear transmissions by node A and start transmission and eventually leads to collision at B. Transmission range of A Transmission range of C A B C Fig 1: Hidden Terminal Problem To overcome the hidden terminal problem adhoc networks usually depends on a virtual sensing mechanism using a pre-data control information exchange. One such virtual sensing mechanism is the Request to Send/Clear to Send (RTS/CTS) exchange resulting 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 to 28

3 refrain from sending data even though they would not have interfered with any ongoing transmission. This problem is termed as exposed terminal problem and Fig 2 illustrates such a situation. In Fig 2 Node A wants to send to node B. Node A sends an RTS and waits for B to send CTS. Assume node D wants to send to node C and D transmits an RTS to C just before A sends the RTS to B. C transmits a CTS as a response to RTS from D. This CTS is also heard by B and refrain from sending the CTS to A even though simultaneous data transmission from A and D would not interfered each other. Exposed terminal problem occurs when two transmitters lie within CS range and are prevented from transmitting simultaneously, 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 and unexploited transmission opportunities due to exposed terminals. The existence and intensity of hidden and exposed terminals in a given network depends on the topology and on the CS range. In a mobile adhoc network topology cannot be predicted or controlled. We can only control the carrier-sense range. The number of Hidden terminals can be reduced by having a larger CS range, but results in more exposed terminals. On the other hand a smaller CS range results in fewer exposed terminals but more hidden terminals. In short both hidden and exposed terminals cannot be simultaneously eliminated or reduced by adjusting the CS range alone. Although several alternative proposals have been made to address the above mentioned shortcomings of standard RTS/CTS scheme, many of them are not satisfactorily address the key issues of keeping the simplicity of the protocol and avoiding the overhead on the nodes on duty in emergency situations where usually adhoc networks are applied. In this paper we propose a modified RTS/CTS method which overcomes the limitations of the standard method and other related proposals. The basic principle of our scheme is to identify the areas in the network in which chance for the presence of active hidden terminals are more and areas where the packets are smoothly flowed and regulate the use of RTS/CTS control packets accordingly. For this purpose we monitor the traffic around every node and avoid the use of RTS/CTS exchange wherever the chance for hidden terminal problem is less irrespective of the packet size. Our scheme is an optimized approach rather than conservative. Transmission range of A Transmission range of D A B C D Transmission range of B and C Fig 2: Exposed Terminal Problem 29

4 2. DISCUSSION ON RELATED WORK In [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 to ask for agreement. Once the destination node receives RTS, it does not reply CTS immediately, but forwards the RTS to its neighbor nodes to inform the channel states in next slot, then in the third slot replies CTS. Obviously this method makes too much delay in data transmission and wastage of the channel bandwidth. In [3] it uses an approach of calculating the number of hidden nodes around the receiver to decide on the use of RTS/CTS prior to data transmission. But this method is having the overhead of keeping the list of neighboring nodes and finding out number of hidden terminals around the receiver. Since mobility is common in adhoc networks this neighbors and hidden nodes will change frequently. Therefore this list has to be updated time to time. This makes overhead in the network. Another common approach is to tune the CS threshold for having an improved spatial reuse [4, 5]. The main drawback of this approach is the poor fairness due to asymmetric CS ranges. 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 CS threshold or controlling the transmission power. MACA-P [2] enhances the RTS/CTS mechanism to increase concurrency. The RTS/CTS exchange between a pair of nodes is followed by a control gap during which another pair of nodes can also exchange RTS/CTS messages and synchronize its data transmission with the first node pair. RTS/CTS messages and control gap significantly increase the overhead per data transmission. In [7] The proposed solution consists of two phases. In the first phase, exposed links in the mesh topology are detected through an offline training process. Coordination of simultaneous transmissions over exposed links is then done in the second phase. But detection of exposed link is not easy tasks and causes for extreme overhead. Moreover, as we mentioned before we have to do this detection frequently because of the mobility of nodes. 3. PROPOSED SCHEME The basic principal of our approach is to correctly identify the cases in which RTS/CTS exchange is necessary prior to data transmission and cases in which direct data transmission is desirable thereby to optimize the use of RTS/CTS exchange wherever it is possible without having a cost of high collision of packets due to hidden terminals and hence a performance degrade in the network. This method of optimized use of RTS/CTS yields two benefits simultaneously. It reduces unnecessary overhead and delay in the network and reduces the chance for exposed terminal problems. In other words performance enhancement of our approach is the direct impact of reduced overhead and delay in packet transmission and avoidance of the case of leaving the channel unused due to exposed terminals Categorizing the Network in to Multilevel Collision Domains According to our proposed scheme the decision on whether an RTS/CTS is to be used is taken adaptively taking in to account the current scenario of the shared medium and collisions encountered by the nodes. Based on the result of traffic analysis network is virtually divided into different collision domains CD 1, CD 2 CD n where n is the number of 30

5 collision domains in the network. Rate of packet collision in each CD i (RCD i ) is less than that of CD i +1. i.e. R CD1 <R CD2 <R CD3 < R CDn Initially all the nodes in the network (say N) will be in lowest collision domain. i.e. N CD 1 Each node keeps track of its success and failure rate of data transmission. Based on this success and failure rate the node will move from one collision domain to another. Unlike initial case after some time of network activities total number of nodes will split into different collision domains depends on the success and failure rate of data transmission of each node. i.e. N 1 CD 1, N 2 CD 2 N n CD n where N i N, Depends on the collision intensity of each collision domains we broadly classify them into two zones namely Red zone and Green zone. Red zone contains the entire collision domain above a threshold called RTS threshold by which we start using RTS/CTS prior to every data transmission. On the other hand Green zone contains all the collision domain under the RTS threshold where direct data transmission without a prior RTS/CTS exchange is possible. RTS threshold by which we start using RTS/CTS virtual carrier sensing is defined in terms of collision 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 traffic around the node. If the intensity is below the pre-defined level the node can transmit data immediately without a preceding virtual carrier sensing using RTS/CTS irrespective of the size of the data. Otherwise the node should perform the RTS/CTS exchange prior to data transmission. Flow chart in Fig 3 explains the working of our proposed scheme. As we show in the flow chart if there is a collision experienced by the packet from the node after a direct transmission (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-defined Collision Threshold (Col threshold ) the node will move to Red zone of higher collision henceforth the node start using virtual carrier sensing. On the other hand if there is no collision even after a direct transmission collision counter will be decremented. If the collision counter comes under the Col threshold the node will move to Green zone hence forth no need of RTS/CTS exchange prior to data transmission. 3.2 Value addition of Contention Window Since behavior of node on transmission fully depends on the collision domain in which node belongs, analyzing the collision intensity is most important in our scheme. We can introduce a counter associated with each node to count number of collisions occurred during data transmission from each node. Based on the value of this counter we can correctly analyze the collision intensity around the nodes. But keeping a separate counter with each node is highly inefficient especially in mobile devices. To solve this issue we use the 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) that correctly reflect the collision intensity of the nodes. Therefore we cannot rely on standard 31

6 contention window adjust mechanism using Binary exponential Backoff (BEB) since CW adjustment(increment or decrement)in BEB is not in proportion of failure or success in data transmission. Fig 3. Flow chart of the Proposed Scheme In the BEB scheme, each node doubles its contention window, CW, up to the maximum contention window (CW max ) after a collision occurs and resets its CW to the minimum value (CW min ) after a single successful transmission: CW = min(2.cw;cw max ) ; upon a collision CW = CW min ; upon a success Where CW max and CW min are the maximum and minimum value of CW respectively. CW max and CWmin are defined to avoid the contention window from growing too large and shrinking too small. The values of the CW min and CW max are pre-determined based on the expected 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 minimum value of CW on a single transmission success current CW size cannot be taken as an indicator of the packet collision. To support our proposed scheme with a Contention Window which correctly reflects the collision intensity around the nodes we make use of our recently proposed backoff scheme called Collision Based Contention Protocol. As per this scheme if a collision happens, the rate of expansion of CW depends on the current size of CW. If the current CW size is CWmin then it slightly increase its size. As the size at the time of collision increases the rate of expansion also increases. On the other hand, on a success of data transmission, as the CW size at the time of transmission increases the rate of contraction of CW size decreases. This method of Contention Window adjustment mechanism not only solve the fairness issues and poor performance of standard BEB scheme but also provide a value added Contention Window which also can be used as a perfect indicator of collisions associated with each node. Interested readers will get more about this protocol in [8]. 3.3 Handling of Hidden terminals and exposed terminals As we discussed before two main problems in an adhoc network which degrades its performance are hidden terminal problem (HTP) and exposed terminal problem. Exposed terminal problem arises as a byproduct of the solution to hidden terminal problem. In other 32

7 words, exposed terminal problem is the result of RTS/CTS exchange mechanism employed in MANETs. From this it is clear that exposed terminal problem can be mitigated by reducing RTS/CTS exchange. But this reduction of RTS/CTS exchange causes for the frequent hidden terminal problem. Our approach successfully overcomes this issue. When the nodes are in the low collision domain packets from them experience less number of collisions. From this we can reasonably assume that hidden terminal problem is very less for a node under low collision domain otherwise the node would have suffered lot of transmission failure and moved to higher collision domain due to the above mentioned inherent property of our approach. Because of this less number of HTP or a best case of absence of HTP in low collision domain we completely avoid the RTS/CTS exchange in this region. This way we avoid unnecessary delay and overhead and effectively solve the issue of exposed terminal problems. As a result better performance is obtained. Our approach also works well in a worst case of sudden topology changes in the network due to node movement and several hidden terminals in the previously mentioned low collision domain. Because of the no RTS/CTS exchange surely packets will collide. But due to this collision the respective nodes will immediately move to higher collision domain and start sending RTS/CTS. In this way our scheme effectively deals with hidden terminal problems and exposed terminal problem. 4. PERFORMANCE EVALUATION AND RESULT 4.1 Simulation Analysis Evaluation and comparison of proposed scheme with standard scheme is made using network simulator Ns-2. Ns-2 is a powerful network simulator. Ns-2 is extensively used by the networking research community. It provides substantial support for simulation of TCP, routing, multicast protocols over wired and wireless (local and satellite) networks, etc. The simulator suite also includes a graphical visualizer called network animator (nam) to assist the users get more insights about their simulation by visualizing packet trace data. AWK, a text processing utility has been used to extract desired information from ns trace file. We have used Linux Operating System to run our simulation code. We choose networks of varying number of nodes in an area 500m x 500m. The random way point motion pattern is adopted. We used the CBR traffic and packet size of 1500 bytes. The performance is evaluated by adding new nodes in the network as time varies or expedited arrival of several nodes simultaneously. Sufficient time is given for running the simulation in order to get chances for every node to participate in the network activity of transmission or reception. 4.2 Performance Metrics Firstly we measured the Packet Delivery Ratio (PDR). It is the ratio of the number of delivered data packet to the data packet actually sent to the destination. ie. Number of packet receive/ Number of packet send Fig 3 shows that PDR from our novel scheme is higher than the standard scheme in lightly loaded network as well as heavy one. This performance gain is resulted from the relatively congestion free network due to less control overhead. Then we measured throughput at various instant in the networks of different number of nodes. This metric is helpful to analyze 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 the 33

8 nodes simultaneously. Fig 4 and Fig 5 show the Throughput performance of our protocol compared to standard scheme in lightly loaded network with 20 nodes and heavily loaded network with 80 numbers of nodes respectively. Table 1 Simulation Parameters Parameters Values Number of nodes Varying Simulation time Varying Simulation 500x500 Area(m) Random Topology wireless Phy 1500 Packet size 500 Queue length 10µs SIFS 50µs DIFS Free Space ProType Omni Antenna type directional CW min 15 or 31 CW max 1023 Fig 4. Packet Delivery Ratio Fig 5. Throughput Comparison with 20 nodes 34

9 Fig 6. Throughput Comparison with 80 nodes From the above two graphs it is clear that our protocol provide better throughput in the network of small number of nodes as well as large number of nodes. Performance enhancement is more in lightly loaded network. In lightly loaded network chance for hidden terminals are less compared to heavy loaded one. Therefore we can avoid or reduce the use of RTS/CTS exchange in this case without having fear of packet collision. This is effectively done 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 using RTS/CTS exchange and avoids collisions due to hidden terminals. Therefore our protocol also 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 for networks of various numbers of nodes. As we mentioned before this throughput gain is achieved with a controlled use of RTS/CTS rather than a conservative use with a pessimistic assumption of hidden terminals anywhere anytime. Because of the conservative approach against collision in the standard scheme collisions are less compared to our scheme. But this will not result in gain in throughput because of the bandwidth wastage due to exposed terminal issues and over usage of control message for virtual carrier sensing. Our approach of controlled use of RTS/CTS results in a slight increase in collision in the low collision domain as a cost for reduced use of virtual carrier sensing. But this creates better utilization of available bandwidth by preventing exposed terminal issues and reduced use of control message. In other words, our approach provides a considerable gain in aggregate throughput with a cost of slight increase in collision only on low collision domain. Fig 7 shows a comparison of gain in throughput against increase in collision. It is clear that increment in collision is negligible to the resulted throughput gain. Fig 7. Aggregate Throughput Comparison for different networks 35

10 5. CONCLUSION Fig 8. Throughput gain v/s Increase in collision In this paper, we proposed a modified pre-data handshake mechanism with a controlled exchange of RTS/CTS in Mobile Adhoc Networks. We have analyzed various proposals presented to overcome the drawback of the standard pre-data control scheme. Our solution presents a better approach for getting an increased throughput with an improved spatial reuse and reduced occurrence of exposed terminal problem. According to our scheme instead of having a conservative approach of sending RTS and CTS prior to every data transmission an optimized approach is adopted on pre-data control messages. The proposed scheme monitors the traffic around every node and categorizes them into different collision domains depends on the collision rate on the packets from each node. The very purpose of this dynamic grouping of nodes is to avoid the use of RTS/CTS exchange wherever the chance for hidden terminal problem is less irrespective of the packet size. Performance of the scheme is evaluated using the Ns-2.33 network simulator. The simulation result shows that our proposed scheme outperforms the standard method of pre-data handshake mechanism in network of small number of nodes and large number of nodes. Performance enhancement is more in lightly loaded network because in lightly loaded network chance for hidden terminals are less compared to heavy loaded one and reduced use of pre-data control message will not create packet collision. REFERENCE [1] M.B Yassein, S. Manseer, A.A Hassan, Z.A Taye, A New Probabilistic Linear Exponential Backoff Scheme for MANETs, IEEE International Symposium on Parallel & Distributed Processing 2010, pp 1-6 [2] N. Zhang, N. Liu, Q. Yu, Improved RTS-CTS Algorithm to Solve Mobile Hidden Station Problem in MANET, Cross Strait Quad- Regional Radio Science and Wireless Technology Conference (CSQRWC), 2011, pp [3] T.Shigeyasu, M.Akimoto, H. Matsuno, Throughput Improvement of IEEE802.11DCF with Adaptive RTS/CTS Control On the Basis of Existence of Hidden Terminals, International Conference on Complex, Intelligent, and Software Intensive Systems 2011, pp

11 [4] H. Zhai, Y. Wang, Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks, In Infocom, Barcelona, Spain, Apr [5] J. Zhu, B. Metzler, X. Guo, Y. Liu, Adaptive CSMA for Scalable Network Capacity in High-Density WLAN a Hardware Prototyping Approach, In Infocom, Barcelona, Spain, Apr [6] M. Cesana, D. Maniezzo, P. Bergamo, M. Gerla, Interference Aware (IA) MAC: an Enhancement to IEEE b DCF, In Vehicular Technology Conference (VTC), Orlando, FL, Oct [7] K. Mittal, E.M. Belding, RTSS/CTSS: Mitigation of Exposed Terminals in Static 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 Mobile Adhoc 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 Wireless Networks, IEEE Global Telecommunications Conference, 2010, pp 1-5. [10] L.P.K. Wong, D.J.Yin, Throuhput Analysis of CSMA Protocol with Exponential backoff, 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 in Medical Ad-Hoc Sensor Networks: A Review, Challenges and Future Scope for Research, International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 1, 2012, pp , ISSN Print: , ISSN Online: