Performance Improvement of Optimized Link State Routing (OLSR) Protocol Navaid Akhter 1, Ammar Masood 2, Irfan Laone 3 Institute of Avionics and Aeronautics/Department of Avionics, Air University, Islamabad, Pakistan 1,2,3 Abstract- OLSR, a leading proactive protocol of MANET maintains consistent and up to date network topology at all the times and has emerged as the choice for MANETs due to low latency for route determination. Hence, OLSR generate a large amount of control overhead in order to maintain an up-to-date routing table which consumes bandwidth that should have been employed by user data traffic instead. This paper addresses this issue by optimizing OLSR under specific network and mobility conditions which are actually more of practical interest and thereby, our work does have a valuable contribution to provide guidelines for large number of cases of general interest. The proposal has shown to consistently outperform the default implementation by reducing the routing overhead under specific network and mobility conditions considered at no extra cost. Other parameters like data traffic and end-to-end delay also improved with the approach presented in this study which shows the efficiency of the scheme selected. Keywords: MANETs, Routing Protocols, OLSR, Improvement in Control Messages Intervals, Optimality in performance 1 Introduction Research concerning MANETs is currently of great interest. The performance of MANET is related to the efficiency of the routing protocols in adapting to frequently changing network topology and link status [1]. Because of the importance of routing protocols in the dynamic multi hop networks, a number of routing protocols have been proposed in the last few years; concurrently, a great deal of research work is being undertaken by researchers to improve their performances. In OLSR (a leading MANET routing protocol), maintaining an up-to-date routing table for the entire network calls for excessive communication between the nodes as periodic control messages updates are flooded throughout the network. Hence OLSR generate a large amount of control overhead which consumes valuable bandwidth that should have been employed by user data traffic instead. Therefore, excessive control overhead in OLSR is detrimental to its overall performance in data forwarding, which has been analyzed for improvement in our research work. Other parameters like data traffic and end-to-end delay also improved with the approach presented in this study. 2 MANET Routing Protocols Mobile Adhoc Network (MANET) is an autonomous system of mobile nodes connected by wireless links [2]. MANET routing protocols are based on how routing information is acquired and maintained by the mobile nodes and thus, can be divided into proactive, reactive and hybrid routing protocols [3]. With proactive routing protocol, nodes in a MANET continuously evaluate routes to all the reachable nodes and attempt to maintain consistent, up-to-date routing information. On the other hand, in reactive routing protocols for MANETs (also called on-demand routing protocols), routing paths are explored only when needed. Hybrid routing protocols are proposed to combine the merits of both proactive and reactive routing protocols and to overcome their shortcomings. 3 Optimized Link State Routing (OLSR) Protocol The Optimized Link State Routing Protocol (OLSR) [4] is developed for MANETs and does not need central administrative system to handle its routing process. Because of its proactive characteristic, the protocol provides all the routing information to all participating hosts in the network at all times. However, as a drawback, OLSR protocol needs that each host periodically send the updated topology information throughout the entire network by flooding. This increases the protocol s bandwidth usage as the routing overhead is high. Although, flooding in OLSR is minimized by the Multi Point Relays (MPRs), which are the only nodes allowed to forward the topological messages [5,6], still the routing overhead is high as compared to reactive routing protocols. 3.1 Control Messages Intervals OLSR employs two types of control messages: Hello messages and TC messages.
3.1.1 Hello Interval This parameter represents the frequency of generating a Hello message. Hello Interval determines the time between successive Hello messages, which is set to 2 seconds by default. Hello messages are never forwarded. 3.1.2 TC Interval This parameter represents the frequency of generating a TC message. In OLSR, the rate of the topological state updates is the sending rate of TC messages. TC messages are broadcasted periodically within the TC interval, to other MPRs, which can further relay the information to further MPRs. TC messages are broadcasted once per refreshing period and the default value is 5 seconds. TC messages are one of the major sources of overhead in OLSR, as they are flooded throughout the network, but they are essential to maintain consistent connectivity knowledge of complete network. 3.2 Problem in OLSR One advantage of OLSR is that it provides lower route discovery latency than on-demand protocols because of its proactive nature. But the flip side is that OLSR generates a large amount of control overhead which consumes precious bandwidth. Since the resources in wireless networks are severely constrained, the increased channel contention could lead to network congestion resulting in significant lowering of network performance. Further, scalability issues arise in OLSR due to the excessive routing message overhead caused by the increased network population. The size of routing table grows non-linearly with the increase in number of nodes and the control messages can block the actual data packets. Hence, excessive control overhead in OLSR is detrimental to its overall performance in data forwarding and poses a research challenge that need to be addressed. 3.3 Proposed solution Optimization of local and global topology dissemination intervals (i.e. Hello and TC intervals respectively) is proposed under specific network and mobility conditions which are actually more of practical interest and thereby, our work does have a valuable contribution to provide guidelines for large number of cases of general interest that result in low routing overhead (as compared to the default settings) and thus beneficial for OLSR performance. This study targets on reduction in control overhead with improvement in performance of OLSR by optimizing control messages intervals. 3.3.1 Logical Reasoning of proposed solution Hello messages are broadcasted periodically for link sensing and neighbor detection. This is also required to complete the MPR selection process. After the MPR selection process is completed, TC messages are generated and are disseminated throughout the network. Subsequent to the receipt of these TC messages, the nodes calculate the routing table and the links are available for data communication. Further, MPRs broadcast the TC messages in the network to maintain a consistent and up-to-date view of complete network topology. In case of any topology changes, the MPR selection process is re-initiated and the routing table is re-computed by the nodes [8]. MANETs require minimum control overhead to reduce channel contention and battery consumption problems. TC messages share a large amount of overhead in OLSR because of its global dissemination nature. Decreasing the broadcast frequency of TC messages reduces the overall routing traffic sent while not incurring any degradation in throughput / end-to-end delay under specific network and mobility conditions as shown in this study. Further, due to frequent topology changes caused by high mobility, the routing information needs to be updated more frequently so as to update the topology changes and guarantee the correctness of route selection. This requires that nodes of OLSR employed MANET detect link changes more quickly and broadcast topology updates with lesser delay. This can be achieved by increasing the Hello messages sending rate for faster response to the link and neighbor changes (especially in case of high mobility scenarios); hence, providing better throughput as compared to the default Hello interval. The increase in routing overhead because of the increase in Hello sending rate above is compensated by the reduction in routing overhead due to the decrease in TC messages sending rate as mentioned earlier. 4 Related Work The authors of OLSR in RFC 3626 (OLSR) [4] pointed out that the nodes may send control messages at different rates, if beneficial for specific deployment. Many strategies have been proposed by OLSR researchers using different performance metrics to improve the performance of OLSR by varying control messages intervals [7, 8, 9, 10, 11, 12, and 13]. However, these works usually target to reduce control overhead while having certain deficiencies and implementation complexities. Our work, however, does not include any added complexity or depends upon any measurement of network parameters and provides improved performance of OLSR under specific network and mobility conditions by just modifying the OLSR control messages intervals. With our approach, now network is able to achieve an increase in data traffic
received (vis-à-vis the payload with default control messages intervals) while routing traffic and end-to-end delay are both reduced. We have constructed fairly robust scenarios for experiments to investigate the effect of control messages intervals on the routing overhead of OLSR. Table 2 SIMULATION PARAMETERS 5 Performance Evaluation Because of the unavailability of wide range of real MANETs, the performance analysis of wireless applications or protocols in the context of MANETs often require to be evaluated through simulation studies [14]. The performance analysis on a real network (if available) can be rather tedious if large networks are considered (typically hundreds of nodes). This is why simulation is an important tool in the sense that it can often help to improve or validate protocols [15]. OPNet Modeler 14.5 network simulator was used for analysing the performance of OLSR in this study. 5.1 Choice of Network and Mobility Conditions The MANET routing protocols perform differently under different network & environmental factors like node mobility, number of nodes, number of source-destination pairs, traffic type, traffic intensity, propagation models etc. For the purpose of performance analysis in this study, we selected two factors: Node mobility and Number of nodes, because of their major impact on the mechanics of the protocol vis-à-vis routing overhead. We started with a carefully designed network scenario for all the experiments and varied one parameter at a time and thus stressed the network in different axis as shown in Table 1. Table 1 Network & Mobility Factors As mobility has the most significant impact on MANET routing protocols [16], scenarios have been constructed to evaluate the proposed solution against varying nodes speed (from 5 mps to 40 mps) while keeping the number of nodes fixed. The speed range has been selected with keeping in view the speed selected for extreme practical scenarios (low to high) and most of the other research work in this area. Table 2 depicts the parameters selected for the Scenarios. 5.2 Performance Metrics The performance metrics investigated during this study were the data traffic received and routing overhead in OLSR protocol vis-à-vis its improvement by optimizing the OLSR control messages interval under various network and mobility conditions. However, end-to-end packets delay was also kept under-check so as to ensure that the optimization of control messages for improvement in routing overhead do not degrade this parameter. The definition of improved performance is that the routing protocol must provide applications with high data traffic received, minimal routing overhead and low end-to-end delay. 6 Results 6.1 Experiments The simulation study adopts a step-by-step performance optimization approach. Firstly, the impact of each control messages interval of OLSR has been analyzed distinctly on the data traffic received, routing traffic overhead and the end-to-end packets delay by stressing the mobility factor. The outcome of the results has been analyzed further to see how these control messages interval can be optimized simultaneously so as to efficiently maximize the data traffic received (payload) while minimizing the routing overhead and end-to-end packets delay. The steps are depicted in Figure 1.
Hence, from Experiment No 1, it is concluded that an improvement in data traffic received by increasing the Hello messages sending rate is at the expense of increased routing traffic overhead. 6.2 Experiment No 1 Figure 1: Simulation study steps In experiment No 1, the Hello message update rate has been increased from default value (2 seconds) to various values like 1.9, 1.8, 1.7, 1.6, 1.5 seconds etc in order to facilitate the routing protocol to speed up the adaptation to neighbor changes while keeping the TC interval at 5 seconds (default value). The value of the state holding timer interval (neighbor hold time) was adjusted accordingly. After various iterations, it was found that Hello interval of 1.8 seconds (TC at default value) provides the best balance between data traffic and routing traffic and the results are shown in Figure 2. 6.3 Experiment No 2 In experiment No 2, the TC interval has been decreased from default value (5 seconds) to various values like 6, 7, 7.5 seconds etc in order to reduce the routing overhead while keeping the Hello interval at 2 seconds (default value). The value of state holding timer interval (topology hold time) was adjusted accordingly. Since the Hello interval is kept constant, the reduction in overall routing overhead is the result of decrease in TC messages overhead. The TC interval of 7.5 seconds was found to be the optimized value under the considered mobility conditions that provide a decrease in routing traffic overhead while having almost no effect on data traffic and the results are as shown in Figure 3. Figure 3: Results of data traffic and routing traffic vs Speed with change in TC interval Figure 2: Results of data traffic and routing traffic vs speed with change in Hello interval In Figure 2, the data traffic and routing traffic are plotted on Y-axis and variation in speed is plotted on X- axis. In all the simulations across specific range of nodes speed while changing the Hello interval and keeping the TC interval fixed, it can be appreciated that increase in Hello sending rate (i.e. Hello 1.8 secs) from the default value (Hello 2 secs) improves the data traffic received as it helps the routing protocol to quickly adapts the changes in neighbors and update the routing tables accordingly. On the other hand, the routing traffic overhead also increases with the increase in Hello sending rate which clearly depict that although fast Hello messages improves the protocol reactivity to link failures; however this is at the cost of increased routing overhead. In Figure 3, the routing traffic and data traffic are plotted on Y-axis and variation in speed is plotted on X- axis. Firstly observing the routing traffic behavior with default values of OLSR, it is revealed that as the speed increases, the routing traffic decreases. This is because of the reason that as mobility increases, link breakages increases and therefore TC messages are either not generated or if they are generated than they are not forwarded to the entire network. Now with the modified settings, it is evident that as TC messages sending rate is reduced from 5 seconds (default) to 7.5 seconds, the routing traffic is less as compared to the routing traffic at default TC interval. Further, it is observed that decreasing the TC sending rates from default value to 7.5 secs although reduces large routing overhead; brings no significant change in data traffic received. This is because of the fact that repetitive TC messages are broadcasted throughout the network to maintain the network topology. Lowering down the sending rate of these TC messages
under specific mobility conditions although reduces large routing overhead, brings no significant change in data traffic received at the nodes which is more sensitive to the change in Hello interval than the TC interval. 6.4 Outcome of Experiment No 1 and Experiment No 2 Through simulations it has been explored that TC messages generate more overhead than Hello messages because TC messages are forwarded globally to each node in the network while Hello messages are only exchanged locally between neighboring nodes. Increasing the Hello messages sending rate helps the routing protocol to quickly adapt the changes in neighbors and update the routing tables accordingly. Hello interval rate has been increased from default value (2 secs) to 1.8 secs in order to speed up the adaptation to neighbor changes and thus achieving higher data traffic received than what it is achieved at the default value. This is particularly to cater the declined performance of OLSR under high mobility scenarios. Decreasing TC messages sending rate leads to significant reduction in control overhead but do not downgrade the data traffic received under specific mobility conditions considered in the experiments. 6.5 Experiment No 3 Experiments 1 and 2 provides a comprehensive understanding of OLSR s control timers behavior vis-à-vis performance metrics and gives insightful guidance in optimizing these timers for an improved performance in data traffic received while introducing low routing overhead as compared to the default values. The results have been exploited further in experiment 3 to formulate that how these two timers can be optimized simultaneously under considered network and mobility conditions so as to efficiently minimize the routing overhead while achieving maximum data traffic received without compromising on end-to-end delay. The OLSR control messages intervals Hello 1.8 secs and TC 7.5 secs (as discussed in experiments 1 and 2) were selected and compared against the default intervals to see if there is any improvement as stated above. Figure 4: Result of Routing traffic vs Speed with change in HELLO and TC intervals First observing the routing traffic behavior with default values of OLSR (i.e. Hello def_tc def), it is revealed that as speed increases the routing traffic decreases. This is because of the reason that as mobility increases, link breakages increases and therefore TC messages are either not generated or if they are generated than they are not forwarded to the entire network. This results into decrease in routing traffic as the speed increases and vice versa. Now with the modified intervals (i.e. Hello 1.8_TC 7.5), the similar behavior of decrease in routing overhead with increase in mobility is observed as stated above. Further, as the TC messages sending rate is reduced from 5 seconds (default) to 7.5 seconds, the routing traffic is noticeably reduced as compared to the routing traffic at default TC interval. Also due to the increase in Hello sending rate from 2 seconds (default) to 1.8 seconds, the routing traffic would have increased (as observed in experiment 1). However, this has been compensated with the reduction of large routing overhead due to the decrease in TC interval. Hence, the overall result is the reduction of routing overhead as compared to the routing overhead with default Hello and TC values. The average reduction in routing overhead achieved with the modified Hello and TC intervals is 14.06 %. 6.5.2 Data traffic vs Speed In Figure 5, the data traffic is plotted on Y-axis and variation in speed is plotted on X-axis. 6.5.1 Routing traffic vs Speed: In Figure 4, the routing traffic is plotted on Y- axis and variation in speed is plotted on X-axis. Figure 5: Result of Data traffic vs Speed with change in HELLO and TC intervals
Firstly, observing the data traffic behavior with default values of OLSR (i.e. Hello def_tc def); it is revealed that as speed increases the data traffic decreases. This is because of the reason that as mobility increases, link breakages increases and therefore the nodes are unable to forward the data to the required destination which resulted into dropping of data packets before reaching to the destinations. Now with the modified interval i.e. Hello 1.8_TC 7.5, similar behavior of decrease in data traffic is observed with the increase in node s mobility (due to the same reason as mentioned above). However the data traffic is now improved than what it is achieved with the default OLSR intervals (i.e. Hello def_tc def). This is because of the increase in Hello messages sending rate which speed up the routing protocol s adaptation to neighbor changes and route maintenance and thus resulting into less data drop and increase in data traffic received at the destinations. Further, it is observed that both the curves are tending to converge at very low speeds and at very high speeds. It is because of the reason that at very low speeds, there are no significant changes in neighbors so the default interval as well as modified Hello interval works almost the same manner and the change in Hello interval does not make any difference. Similarly at very high speeds, the topology changes might be too dynamic to be captured by the periodic updates of OLSR with default as well as with the modified settings so the change in Hello interval does not make any significant impact in this regime also. The average increase in data traffic achieved with the modified values of Hello and TC intervals is 6.19% 6.5.3 End to End packets delay vs Speed: In Figure 6, the delay is plotted on Y-axis and variation in speed is plotted on X-axis. Firstly, observing the end to end packets delay behavior with default values of OLSR (i.e. Hello def_tc def); it is revealed that as speed increases, the end to end packets delay decreases. Figure 6: Result of End to End packets delay vs Speed with change in HELLO and TC intervals This is because of the reason that as mobility increases, link breakages increases and therefore less number of source-destination pairs are now available at high speeds as compared to the scenarios at low speeds. This results into increase in channel capacity because of the occupation of same number of available channels now with less number of source destination pairs. Hence, the packets reach to the destination with lesser problems of channel contention and therefore end to end packets delay decreases. The similar behavior is observed with the modified intervals of OLSR because of the same reason as mentioned above. However, now with the modified intervals, the end to end packets delay is less as compared to the default settings. This is because of the increase in hello sending rate which increases the routing protocol s adaptation to neighbor changes and route maintenance that decreases the overall end to end packets delay. 6.6 Summary of Experiment No 3 The simulation results demonstrated that by optimizing the Hello and TC intervals, optimality in the routing protocol performance is achieved under specific mobility factors considered. Through simulations it has been explored that increasing rate of hello update leads to improvement in link establishment and node status maintenance. Further, decreasing rate of TC updates leads to significant reduction in control overhead but do not downgrade the data traffic received under specific mobility conditions considered. Hello interval has been slightly decreased from default value (2 secs) to 1.8 secs in order to alleviate the degraded performance of OLSR under high mobility scenarios thus achieving higher data traffic received than the default value. Increase in routing traffic due to increase in Hello interval has been compensated by decreasing the TC sending rate from 5 to 7.5 secs which drastically reduced the overall routing overhead while not posing any significant impact on data traffic received. This also resolves the problem of high routing overhead of OLSR (generated due to its proactive nature) under the specific mobility conditions considered. In the proposed Hello and TC intervals, OLSR is now able to sustain an increased data traffic received compared to the default values of Hello and TC intervals and at the same time, both the routing traffic and end-to-end packet delay are also reduced. 7. Conclusion MANET is an autonomous system of mobile nodes connected by wireless links. The performance of MANET is related to the efficiency of the routing protocols. OLSR, a well known proactive protocol has emerged as the choice for MANETs (especially for delay sensitive applications) due to low latency for route determination. But at the same, time associated high routing overhead (due to proactive nature) has emerged as
a major performance issue in OLSR. In this study, we have addressed this issue by optimizing the OLSR under specific mobility conditions which are actually more of practical interest and thereby, our work does have a valuable contribution to provide guidelines for large number of cases of general interest. The default parameters of Hello and TC intervals of OLSR are selected such that the network performance is improved. The behavior of the routing protocol is tested based on the influence of node mobility using various performance metrics. From the results of simulations, it is concluded that the optimization of OLSR control messages intervals has shown to consistently outperform the default implementation of OLSR under specific mobility conditions considered during this study. We envisage undertaking research to analyze the scalability of OLSR protocol vis-à-vis control messages intervals with number of nodes and to set the boundary limits through detailed simulation studies. Furthermore, the performance analysis of OLSR protocol must be analyzed with realistic mobility models [17] so as to finalize realistic protocol performance. 8. References [1] M. Abolhasan, T. Wysocki, and E. Dutkiewicz, A review of routing protocols for mobile adhoc networks, Elsevier Journal of Ad Hoc Networks, 2004. [2] The Book of Visions 2000, Visions of the Wireless World, IST WSI Project, November 2000. [3] S. Corson, J. Macker, MANET RFC 2501, MANET: Routing Protocol Performance Issues and Evaluation Characteristics, January 1999. [4] C Adjih, T. Clausen, P. Jacquet, A. Laouiti, P. Minet, P. Muhlethaler, A. Qayyum, L. Viennot: Optimized Link State Routing Protocol, RFC3626, IETF, October 2003. [5] A. Qayyum, L. Viennot, A. Laouiti: Multipoint relaying: An efficient technique for flooding in mobile wireless networks, INRIA research report N 3898, March 2000, INRIA Rocquencourt, France. http://www.inria.fr/rrrt/rr-3898.html. [6] Jerome Harri, Christian Bonnet and Fethi Filali, OLSR and MPR: Mutual Dependences and Performances, EURECOM research report RR-05-138, 2005. [8] C. Gomez, D. Garcia, J. Paradells Improving Performance of a Real Ad-hoc Network by Tuning OLSR Parameters, 10th IEEE Symposium on Computers and Communications, ISCC 2005. [9] Carlos Miguel Tavares Calafate, Roman Garcia, Pietro Manzoni, Optimizing the implementation of a MANET routing protocol in a heterogeneous environment, Proceedings of Eighth IEEE International Symposium on Computers and Communications, ISCC '03. [10] Mounir Benzaid, Pascale Minet, Khaldoun Al Agha, Integrating fast mobility in the OLSR routing protocol, Mobile and Wireless Communication Networks, MWCN 2002. [11] P. Samar and Z. Haas, Strategies for Broadcasting Updates by Proactive Routing Protocols in Mobile Ad hoc Networks, Proceedings of the IEEE Military Communications Conference (MILCOM), Anaheim, California, USA, October 2002. [12] Pedro E. Villanueva-Peña, Thomas Kunz, Pramod Dhakal, Extending Network Knowledge: Making OLSR a Quality of Service Conducive Protocol, IWCMC 2006. [13] F. Bai, N. Sadagopan, A. Helmy, IMPORTANT: A framework to systematically analyze the Impact of Mobility on Performance of Routing protocols for Adhoc NeTworks, IEEE INFOCOM, 2003. [14] S. Kurkowski, T. Camp, and M. Colagrosso, "MANET Simulation Studies: The Incredibles," ACM SIGMOBILE Mobile Computing and Communications Review (MC2R), pp. 50-61, October, 2005. [15] OPNet tutorial Modeling concepts reference manual. [16] S Gowrishankar, T G Basavaraju, S. K. Sarka, Effect of Random Mobility Models Pattern in Mobile Ad hoc Networks, International Journal of Computer Science and Network Security, VOL.7 No.6, June 2007. [17] Jungkeun Yoon, Mingyan Liu, Brian Noble, Random Waypoint Considered Harmful, IEEE INFOCOM 2003. [7] Yang Cheng Huang, Saleem Bhatti, Daryl Parker, TUNING OLSR, The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC 06.