Empirical Study of Mobility effect on IEEE MAC protocol for Mobile Ad- Hoc Networks

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1 Empirical Study of Mobility effect on IEEE MAC protocol for Mobile Ad- Hoc Networks Mojtaba Razfar and Jane Dong mrazfar, Department of Electrical and computer Engineering California State University Los Angeles ABSTRACT To design an efficient and effective MAC layer protocol for Mobile Ad-Hoc networks is a challenging task. IEEE MAC protocol, which supports ad hoc network mode, provides a good reference for research work in this area. In the recent years, many researchers have investigated the performance of IEEE on MANET both theoretically and empirically. However, the impact of the mobility on MAC layer design has not been evaluated thoroughly. In our research, we used OPNET simulator to analyze the performance of IEEE MAC protocol under various mobility patterns for different network topologies. The findings revealed interesting correlation between the speed of movement/ the mobility pattern and key network performance parameters including delay and throughput. We also investigated the impact of mobility on fairness issues in media access. The empirical study presented in this paper will be useful to enhance the MAC design of MANET with median or high mobility nodes. Keywords: MANET (Mobile Ad-Hoc Networks), Mobility, (Medium Access Control) MAC, IEEE , OPNET 1. INTRODUCTION Mobile Ad Hoc Networks (MANET) are becoming more and more popular due to its ability to offer convenient, flexible and low cost network service for many nontraditional applications. Unlike the widely used Wi-Fi network which relies on the access point to attach to the existing networking infrastructure, MANET is infrastructure-less, where each node acts as a sender, receiver, and router. While the freedom to deploy a mobile ad hoc network at anytime anywhere is very attractive, to make such network function properly presents a lot of technical challenges. For MAC layer protocols, the wellknown challenges are imposed by hidden and exposed terminal problems, fairness access issues, limited bandwidth, limited power supply, as well as limited transmission range and mobility. IEEE802.11was primarily designed for WLAN, but it also supports ad hoc network mode. The MAC layer protocol in IEEE laid the foundation for many proposed MAC layer protocols for MANET. Therefore, it is worthwhile to evaluate the performance of IEEE on MANET to see how to improve the design. Many existing research [1-3] focused on the effectiveness on handling hidden and exposed terminal problem, and some addressed fairness issues. Mobility, although an important design factor of MANET, its impact on MAC layer performance has not been fully analyzed yet. Most of the current researches that investigated the mobility effect are focused on the network layer since it is a major concern in routing [4-5]. In [6], the authors briefly compared the performance IEEE and other MAC protocols under network scenarios with mobility. However, to develop a full understanding of the mobility effect on MAC layer performance including delay and throughput, fairness, collision probability, a more comprehensive and in-depth study is necessary. The objective of our research is to conduct such study using OPNET [7] to show how mobility impacts the key MAC layer performance parameters. In this paper, we will present our findings of the empirical study using OPNET simulation. Due to the nice property of OPNET, it is possible to set up different network scenarios with different mobility patterns, which allowed us to better study the impact of various factors including speed, transmission range, and moving trajectory. The network parameters that were taken into account in our study includes delay, throughput, collision count, overhead of control traffic, and backoff time. The documented results will be useful to enhance the MAC design of MANET with different mobility. The paper is organized as follows. Section 2 provides a brief overview of IEEE and highlights the important

2 design issues. The empirical study exploring the mobility effect using the OPNET software is presented in section 3. Experimental results are described in this section as well. Finally, we will conclude our findings in Section OVERVIEW OF IEEE MAC PROTOCOL IEEE MAC layer protocol is referred to as Distributed Coordination function DCF which was based on virtual carrier sensing and the physical carrier sensing [8]. IEEE DCF uses RTS/CTS/DATA/ACK when the size of the data frame is large enough; it may just use carrier sense or it may use both methods referred to as CSMA/CA with RTS/CTS as a MAC protocol. The three major issues related to MAC layer protocol over MANET are the ability to handle hidden and exposed terminal problems, the ability to ensure fair access of multiple stations, and the ability to cope with mobility Hidden and Exposed Terminal Problems These two problems have become a major issue in MANET. Hidden terminal problem [3] occurs when two stations are out of the range of each other and trying to send to the same receiver. As a result, the effect significantly decreases the throughput and makes the delay longer. Exposed terminal problem, on the other hand, is when a node is blocked from transmission to the other stations due to the transmission of the adjacent node. This will cause collision and bandwidth waste (less spatial reuse) and will bring up the starvation problem of the unlucky node. IEEE DCF proposed the RTS/CTS handshaking method in order to alleviate the negative impact of these issues on the whole network. In the literature, several schemes have been proposed to solve the hidden and exposed terminal problems using different mechanism. In [9], the authors explored the IEEE MAC protocol with and without using RTS/CTS handshaking method. The total WLAN retransmissions, data traffic sent/received, WLAN Delay factors of the whole network was investigated using both methods. They demonstrated that in the scenarios that the Hidden terminal problem exists, it will be a good idea to use this option as it decreases the delay of the network dramatically. They also mentioned that this handshaking method is not necessary to be used where the hidden nodes are not present due to the overhead that it adds to the network. The mobility factor was investigated when the hidden nodes exist. However, the speed of the nodes and the location of them have not been studied in this paper. MACA [10] on the other hand, did not use the carrier sensing option and instead, it used the RTS/CTS/DATA handshake to reserve and use the channel. Although this protocol was a simple design, the control channel collisions made the scheme not effective in the MAC layer. Moreover, in these papers, they never spoke of the effect of the Mobility of the performance of the whole network Fairness issue Another important factor that should be considered in designing MAC protocols is to make sure that all nodes have fair access to the channel in order to transmit their data. So far most of the single channel MAC schemes rely on the back-off procedure. Upon collision, the mobile nodes will go through the back-off procedure and will try to retransmit after a certain amount of time. Because the backoff time is different for different nodes, some nodes may have more chance to transmit than the others and they are favored in data transmission. This will result in starving problem of the unlucky nodes with long contention window size. Therefore, designing good strategies for back-off procedures and providing fair chances among nodes to access the channel is one of the important aspects in MANET. MACAW [11] for wireless LANs is another single channel schemes which tried to improve the performance of MACA protocol. A five handshake RTS/CTS/DS/DATA/ACK has been used in this protocol which leads to alleviation of the hidden and exposed terminal problem and better fairness among nodes. By using a different back-off approach (MILD), this protocol allowed the nodes to access the channel in a fair manner which is more desirable in ad hoc networks. However, the effect of mobility on fairness issue using this protocol has not been investigated Mobility issue For the infrastructure-based networks, the access point has the major influence on the delivery of the data to the destination. Within a Basic Service Set (BSS), the stations have to share information using the access point and therefore their position towards each other is not that important. Hence, the mobility of nodes does not have a major effect on the MAC layer protocol [9]. For infrastructure-less network as MANET, the mobile nodes are in direct contact with each other. Since they can be sender, receiver and router, mobility has a significant impact on the performance of their data delivery. One may wonder what influence may the mobility of the nodes cause on the performance of the network. Will mobile nodes be treated the same way as they move? Will the efficiency of the transmission stays the same as the mobility varies? How will the delay and overall throughput be affected via different mobility pattern? Can we enhance the network performance using the handshaking RTS/CTS method under high mobility? Most of the questions do not have a solid answer yet. In our research, we will explore the relationship between mobility and all these factors using OPENT simulation. The results presented in this paper will shed

3 some light to answer some of the questions related to the impact of the mobility on MANET networks. 3. EMPIRICAL STUDY USING OPNET 3.1. OPNET Simulator OPNET modeler is one of the powerful simulation software allowing the users to implement different network topologies using a friendly graphic user interface. As lots of research papers in networking field used NS-2 simulator [12], OPNET makes it easier to use as it provides ready-touse components without the need of writing codes to create real time network simulations. It also provides the flexibility for advanced users to create their own network node and link by hard coding. For our research, OPNET is selected since its Wireless Modeler includes a rich library of detailed mobile protocols and application models that can be utilized to create MANET with various mobility patterns MANET and Mobility in OPNET OPNET [1] uses the IEEE MAC protocol with DCF for Mobile Ad-Hoc Networks. RTS/CTS handshaking option is also included in case a user decides to implement it. The software has different objects for MANET networks such as the MANET station, MANET work station, and Mobility configuration options in order to set up the movement of the nodes. In fact, Mobility is one of the most valuable options that are included in the simulator so that the users can easily define the way the stations move. The speed of the stations can also be easily defined for various applications. This option makes it simpler in real time simulations in comparison to the other simulators where the mobility is a difficult task to define and implement. Figure 1 illustrates a MANET scenario with pre-defined node mobility. The statistics that are related to this work are explained briefly as follows: 1) MANET delay: the end to end delay of MANET packets for the whole network (seconds). 2) Throughput: the total number MANET traffic which is received in bits per second by all the MANET receivers. 3) Media Access delay: The global statistic for the total of queuing and contention delays of the data, management, delayed block-ack and Block-ACK frames transmitted by all WLAN MACs in the network (seconds). 4) Back off slots: the number of slots that a stations needs to back off before transmission while contenting for the medium, and the number of slots in the contention window after the successful transmission of the station. 5) Retransmission attempts: the total number of retransmissions by all the WLAN MACs in the whole network until the delivery of the packet or being discarded as a result of reaching the short or long retry limits. We used this factor to study the impact of mobility on the collision counts as well as the effectiveness of RTS/CTS handshake. Fig.1. Mobility of the Mobile Ad-Hoc Networks Simulation environment In our study, two different network topologies were created and analyzed to evaluate the mobility effect on the node s behavior and the network performance. Different settings have been applied to the two topologies based on the needs of the network simulations. In the first topology, the relationship between mobility, transmission range and the overall throughput and delay of the network is investigated. As Figure 2 illustrates, one subnet consists of eight nodes around each other. Another single node is approaching this sub network with a constant speed. To study the impact of mobility among the nodes inside the sub network, different scenarios were created to compare the delay and throughput where the nodes are either static or moving randomly. We also changed the speed of the nodes in different steps to see the influence of this factor on the network. To evaluate the effect of the transmission range, we also varied the transmission range in different scenarios according to the distance and the area that were used in the simulation. In addition, the mobility impact on this network with different traffic loads was also studied. Table 1 shows the setting used for the first topology. Fig.2. First topology where a node is approaching a static network

4 Attribute Transmission power (W) Varies per scenario Data Rate (bps) 11 Mbps Physical Layer Method Direct Sequence Buffer Size (bits) Packet Size (bits) Exponential (1024) Traffic generated per node Varies per scenario Node s Speed Varies per scenario Nodes movement method Defined/Vector trajectory Simulation time (min) 60 Table.1. Topology 1 configurations For the second topology, two similar subnets are created and each consists of 7 nodes. One of the nodes is static and it transmits to the other static node in the second subnet. The other nodes inside the subnet are either static or moving while trying to transmit data to the static station inside their subnet. The two subnets are moving towards each other with a constant speed. The internal nodes are located with different distances from the static receiver in order to study the effect of different movement trajectories on the fairness among nodes. Note that the internal nodes are in the transmission range of each other. That is, each subnet allows their nodes to transmit inside the region of the subnet. The transmission range for the static receiver is higher than the others due to the fact that the receiver will need to transmit to the other static receiver located at the second subnet. In this topology, we not only look into the delay and throughput factors, but also check the fairness among nodes and the effect of RTS/CTS. Table 2 shows the setting we used for the second topology. Attribute Transmission power of the static receiver (W) Mobile Nodes Transmission power (W) Data Rate (bps) 11 Mbps Physical Layer Method Direct Sequence Buffer Size (bits) Packet Size (bits) Varies per scenario Traffic generated per node Varies per scenario Internal Node s Speed (m/s) 0.2 Subnet speed (m/s) 1 Nodes movement method Defined trajectory Simulation time (min) Experimental results Table.2. Topology 2 configurations Impact of mobility on delay and throughput A) The impact of mobility with lower transmission range To evaluate the impact of mobility with lower transmission range, three scenarios were created under the first topology (figure 2). The transmission range and traffic load for these scenarios are the same, while the mobility inside the subnet is different: 1) Scenario 1: inside nodes are static 2) Scenario 2: inside nodes move with speed 0.2m/s 3) Scenario 3: inside nodes move with speed 1m/s For all these scenarios, the single node in approaching the sub-network with a constant speed 0.2m/s. Table 3 shows the configuration of transmission power and traffic load of these scenarios. The Domain which covers an area of square meter allows the nodes to move inside this region. A lower transmission range is defined so that the nodes can sense each other at a maximum of 80 meters distance. That is, the nodes will not be able to sense each other at some parts of the Domain. This will allow us to see the effect of lower transmission region on the networks using the mobility feature. Attribute Transmission power (W) 3E-005 Packet Size (bits) Exponential (1024) External Node s Speed (m/s) 0.2 Traffic generated per node Exponential (0.1) Fig.3. Second topology where the mobile nodes are moving around the static receiver inside the two subnets Table 3: Common parameters

5 Fig.4. Comparison of average Delay and Throughput for static and moving inside nodes with speed 0.2m/s with lower transmission range Figure 4 compares the results for scenario 1(static) and 2 (node moving with low speed 0.2/m). Results show that when the nodes are not moving inside the domain, the network has higher throughput and lower delay. This seems be to due to the fact that when the nodes move around, the transmission range decreases and the connection establishment among nodes becomes weaker. Hence, the overall throughput decreases coming up with higher delay. Figure 5 compares the results for scenarios 2 (low movement speed) and 3 (high movement speed. Results demonstrate a higher delay and lower traffic being received as a result of an increase in the speed of the mobile nodes. Higher speed will make the node to move further from the receiver in a shorter amount of time and therefore, less chance to deliver their data to the destination. Higher delay is due to the fact the nodes are having more problem in delivering their data to the receiver and experiencing a higher back off time and retransmissions of the data. It also contributes to the internal collision or packet loss which prevents the delivery of the data. Fig.5. Comparison of average Delay and Throughput for the networks with low movement speed (0.2m/s) and high movement speed (1 m/s) with low transmission range. B) The impact of mobility with higher transmission range Attribute Transmission power (W) Packet Size (bits) Exponential (1024) Internal Node s Speed (m/s) 1 Traffic generated per node Exponential (0.005) Table 4: Common parameters In this case, we increased the transmission range ( W) so that the range covers the whole area of the movement. We also increased the Traffic generated by each node to see how the mobile stations behave while generating more traffic. Figure 6 shows the comparison results for the two network scenario with static inside nodes and moving inside notes (speed 0.2m/s). It is interesting to see that in this case, the mobility will have a positive impact which leads to a little higher throughput and lower delay for the entire network.

6 Attribute Transmission power (W) Packet Size (bits) Exponential (8192) Internal Node s Speed (m/s) 0.2 Traffic generated per node Exponential (0.0008) Table 5: Common parameters In this scenario, we investigated the effect of mobility on a network with stations generating a relatively larger traffic inside the network. We also increased the packet size per station. Similar to the first topology, results demonstrate a better performance of the network with mobility in comparison to the static one when the transmission ranges covers the movement paths. Fig.6. Comparison of average Delay and Throughput for static and moving inside nodes with speed 0.2m/s with higher transmission range We also found out that increasing the speed of a network with high transmission range will slightly improve the performance of the network in case of throughput and delay. The reason may be due to the fact that all nodes are within the transmission range of each other no matter how they move. Therefore, the random movement pattern of the inside nodes may lead to a more even distribution of the nodes that helps with channel access. C) Impact of group mobility on delay and throughput Starting from this subsection, we will describe our findings on the impact of group mobility pattern. The simulations were created using the second topology as discussed earlier (Figure 3). The two subnets are moving towards each other with a speed of 1 m/s. The internal nodes inside each subnet are moving with the speed of 0.2 m/s. The nodes have different distances to the fixed receiver. In the case where the nodes are moving, they move around the receiver based on their location and distance with regards to the receiver. Besides the delay and throughput factors, the fairness among nodes and the effect of RTS/CST method is investigated in the following section. Fig.7. Comparison of average Delay and Throughput for static and moving inside nodes with speed 0.2m/s for group mobility Impact of mobility on Fairness A) The fairness issue without RTS/CTS

7 To evaluate the fairness of IEEE MAC layer protocol for mobile network, we used the back off slot time which is the number of slots that a stations needs to back off before transmission while contenting for the medium, and the contention window size after the successful transmission of the station as the measurements.. Moreover, the retransmission attempt is a good factor to analyze the delivery efficiency of the packets per node when the stations are moving around the receiver. This factor reflects the impact of both the internal collision and the transmission errors including the loss of acknowledgment or an error occurred in the packet. The effect of the amount of traffic generated by each node on the fairness issues has also been investigated in this part. Node 3 and Node 6 are selected for our results. As mentioned before, the reason for this selection is the distance difference between the nodes and the receiver on their moving paths. Therefore, we can clearly see the effect of the different distances caused by mobility on the fairness among these nodes. hand, the retransmission attempts per node are much more less for the closer node to the receiver (Node 3). This might be due to the internal collision or the error inside the packets resulting in the failure of the delivery of the packets. We repeated the same procedure but changing the amount of traffic generated by each station to a higher level (exponential (0.0008)). We also increase the packet size up to eight times (exponential (8192)). Fig.9. Average Back off slot time and retransmission attempts for the two selected nodes with higher traffic Fig.8. Average Back off slot time and retransmission attempts for the two selected nodes with low traffic load Figures 8 and 9 present our simulation results for CSMA/CA without RTS/CTS. For the low load generated by each station, the two nodes have almost the same back off slot time (as shown in Figure 8) demonstrating that they have the same chance to access the channel. On the other As shown in Figure 9, the results illustrate that the back off slot time has increased dramatically for both nodes and that the difference becomes obvious as the distance to the receiver increases. This can be due to higher collision and more competition for accessing the channel resulting in higher back off s and retransmission attempts for both nodes. The closer the node is to the receiver, the higher chance it has to access the channel while moving around the receiver. B) The fairness issue using RTS/CTS Attribute Transmission power (W) Packet Size (bits) Exponential (1024) Internal Node s Speed (m/s) 0.2

8 Traffic generated per node Exponential (0.1) RTS threshold (bytes) 256 Internal Node s Speed (m/s) 1 Subnet speed (m/s) 1 Table 6: Common parameters In this case, we added the RTS/CTS option to each node to see the efficiency of this method on the network. Low traffic has been used in this scenario. From Figure 10, we can see that the retransmission attempts have been decreased for Node 3, which demonstrated the effectiveness of the handshaking method. Same results occurred for other nodes inside the network showing the good efficiency of the handshaking method. 4. CONCLUSION In this paper, the performance of the Mobile Ad-hoc networks is investigated using the IEEE MAC protocol. We have shown that Mobility can affect the network based on different factors. We studied the effect of varying the speed of the nodes, and their location on the network. We have also studied the fairness and the effect of the RTS/CTS handshaking process on the performance of the nodes inside the network. Our results show that the performance of the network varies as the mobile nodes move inside the network. We illustrated that the performance of the network improves as the traffic increases when a sufficient transmission ranges of nodes is provided. We have also shown that Mobility will cause the nodes to have longer back off times and retransmission attempts in order to deliver their information to the destination. The RTS/CTS handshaking method demonstrated its efficiency on the mobile nodes when the number of collisions becomes more and more. 5. REFERENCES Fig.10. Comparison of the average retransmission attempts for Node 3 using CSMA/CA only (RED); and using CSMA/CA with RTS/CTS (BLUE) C) The network performance using RTS/CTS We also studied the performance of whole network using the RTS/CTS access mechanism. The same configurations were used as the above simulation. Results depict that the delay of the whole network decreases due to the prevention of the collisions and allowing the nodes to have their data delivered in a shorter amount of time. Fig.11. Comparison of average network delay: Red plot without RTS/CTS; Blue plot-- with RTS/CTS [1] K. Xu, M. Gerla, and S. Bae, Effectiveness of RTS/CTS handshake in IEEE based ad hoc networks, Ad Hoc Networks, Elsevier, vol. 1, no. 1, pp , [2] T. S. Ho and K. C. Chen, Performance Analysis of IEEE CSMA/CA Medium AccessControl Protocol, in Proc. IEEE PIMRC 96, pp , [3] Khurana, S.; Kahol, A.; Jayasumana, A.P, Effect of Hidden Terminals on the Performance of IEEE MAC Protocol, LCN 98 proceedings, pp 12-20, [4] Samir R. Das, Robert Castañeda, Jiangtao Yan, Rimli Sengupta, Comparative performance evaluation of routing protocols for mobile, ad hoc networks. In 7th Int. Conf. on Computer Communications and Networks (IC3N), pages , October [5] Das, S.R, Perkins C.E, Royer, E.M., Performance Comparison of Two On-Demand Routing Protocols for Ad Hoc Networks in IEEE Proceedings, pp 3 12, [6] Jagadeesan S, Manoj BS, Murthy CSR. Interleaved carrier sense multiple access: an efficient MAC protocol for ad hoc wireless networks. Proceedings of IEEE ICC 03, May [7] Online Documentation, OPNET Modeler, Date visited: January [8] IEEE Working Group, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification, [9] H. Jasani, N. Alaraje, Evaluating the Performance of IEEE Network using RTS/CTS Mechanism, in IEEE EIT 2007 Proceedings. [10] P. Karn, MACA a new channel access method for packet radio, in: Proceedings of the ARRL/CRRL Amateur Radio 9 th Computer Networking Conference September 22, [11] V. Bhargavan, A. Demers, S. Shenker, L. Zhang, MACAW A Media Access protocol for wireless Lans, in: Proceedings of the ACM SIGCOMM, 1994, pp [12] Online Documentation, The Network Simulator - ns-2, Date visited: January 2011.