A prioritized MAC protocol for multihop, event driven wireless sensor network

A PRIORITIZED MAC PROTOCOL FOR MULTIHOP, EVENT-DRIVEN WIRELESS SENSOR NETWORK NGUYEN TRUNG KIEN NATIONAL UNIVERSITY OF SINGAPORE 2006 A PRIORITIZED MAC PROTOCOL FOR MULTIHOP, EVENT-DRIVEN WIRELESS SENSOR NETWORK NGUYEN TRUNG KIEN (B.Eng Hons.) (Hanoi University of Technology, Vietnam) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2006 Acknowledgements I would like to express my sincere thanks to my main supervisor, A/P Chua Kee Chaing, for his valuable support I am very grateful for his constant guidance and encouragement I thank him for the time he has spent with me, reading, discussing and critiquing my work I would like to thank Dr Mehul Motani, my cosupervisor His advice and feedback about my research have greatly enhanced and strengthened the work I thank him for all the time and energy he has invested into my research I am indebted to my parents, for everything they have given to me They taught me the value of knowledge, the joy of love and the importance of family They have stood by me in everything I have done, providing constant support, encouragement, and love I would like to thank my two sisters and my wife for their endless love and support I am very happy to have such a wonderful family members Finally, I am grateful to NUS for giving me financial support Without it I would not have done this work -i- To my beloved family To whom I have learned - ii - Table of Contents Acknowledgements i Dedication ii Table of contents iii Summary vi List of tables ix List of figures .x Chapter Introduction 1.1 Wireless sensor networks 1.1.1 Event-driven wireless sensor networks 1.1.2 Multihop communication pattern 1.2 Motivation 1.3 Problem statement and proposed solutions 1.4 Key contributions 11 1.5 Outline of the thesis 11 Chaper Review of MAC protocol for wireless sensor networks .13 2.1 2.1.1 Background 13 Why existing MAC can not be used in wireless sensor networks? 14 - iii - 2.2 IEEE 802.11 Standard .15 2.3 Energy efficient MAC protocols for wireless sensor networks 18 2.3.1 Energy Conservation Principles 18 2.3.2 Sensor-MAC protocol 19 2.3.3 Timeout-MAC protocol 22 2.3.4 Energy and Rate based MAC protocol [31] 23 2.3.5 Traffic-Adaptive MAC Protocol (TRAMA) 24 2.3.6 Data-gathering MAC protocol 25 2.4 SIFT – A low latency MAC protocol for event-driven wireless sensor networks 27 2.4.1 SIFT design 27 2.4.2 Simulation results 29 2.4.3 Problem of SIFT and our proposed protocol - PSIFT 36 Chapter PSIFT – A Prioritized MAC Protocol for Multihop, Event-driven Wireless Sensor Networks 38 3.1 PSIFT design .38 3.2 PSIFT description .41 3.2.1 RTS/CTS hand-shaking 43 3.2.2 Suppressing reports using acknowledgement (explicit ACK) 44 3.3 Simulation results and analysis 46 3.3.1 Simulation topology 47 3.3.2 Simulation details 48 3.3.3 Latency experiments 49 3.3.4 Throughput experiment 60 3.3.5 Summary 63 - iv - Chapter PSIFT with overhearing mechanism 64 4.1 Suppression using overhearing (implicit ACK) 65 4.2 Simulation results and analysis 67 Chapter Conclusions and future work 72 5.1 Conclusions .72 5.2 Future work 73 References 74 -v- Summary Wireless sensor networks are characterized by large number of devices, unattended deployment, energy constraints, common device failures, frequent configuration changes, and a significant range of task dynamics In which, many sensor networks are event-driven Event-driven sensor networks operate under an idle or light load and then suddenly become active in response to an interested event The transport of event report is likely to lead to varying degrees of congestion in the network depending on the sensing application It is during these periods of event happen that the likelihood of congestion is greatest and the information in transit of most importance to users In additions, the reports generated by different nodes in vicinity of event position are correlated That is why not all the sensor nodes need to send their report, only a few of them is enough for the sink to identify the event Exploiting the node redundancy in dense wireless sensor network can improve its delay performance Furthermore, a sensor network is typically composed of a large number of small-size, low-cost, low power sensor nodes which are randomly deployed and communicated with one another to send data to the sink Because of limited - vi - capability of sensor node, the report packet can only be broadcast to the neighboring nodes, which then further relay the packet to its neighbor in the direction to the sink (controlled by routing protocol) Traversing hop by hop, the report finally reaches the sink to be analyzed and taken further action as required by the application In this multihop transmission scenario, data packet that traverses through longer route (through higher number of hops) is more important than the packet with shorter route Then the relay node must treat them differently according to their importance level In other word, the route-through traffic is more important than the originated traffic In this thesis, we propose a new Prioritized medium access control protocol (PSIFT) that utilized the redundancy characteristics of sensor nodes in event-driven wireless sensor networks In which many sensor nodes in vicinity sense the same event and contend to transmit report about it However, not all these reports are required by the sink to identify the event By suppressing unnecessary reports on the same event, PSIFT lowers the offer load of the network, which results in low collision probability and improvement in delay performance of the network PSIFT is a CSMA-based MAC protocol, using fixed-size contention window with a geographically increasing probability distribution of selecting transmission slot within the window Furthermore, to provide services compatible with the packet priority levels which increases each time packet travel one more hop, PSIFT uses different DCF inter frame space (DIFS) and contention window size (CW) for each traffic class PSIFT can differentiate service to each packet since the priority information is packed into the packet We also proposed the new report suppression mechanism based on the broadcast nature of wireless transmission This mechanism works very efficiently in large-scale, event-driven wireless sensor network - vii - The simulation results show that the new PSIFT MAC protocol can bring significant improvement in delay performance of the network When the sensing range grow up to transmission range, PSIFT gains up to 3-times latency reduction in delivering reports compared to the existing IEEE802.11 MAC protocol, while maintaining the correctness in reporting event - viii - successfully receives this report However, node A can hear this report too Thus, A can suppress its report based on the overhearing information right after node R received the report Using the implicit acknowledgement mechanism (overhearing by broadcast nature of wireless medium), contender nodes can suppression in the 1st hop transmission Therefore, the number of report can be severely limited from the 2nd hop and so on Moreover, overhearing mechanism (implicit ACK) can suppress event report much earlier than using explicit ACK since the contender nodes don’t need to wait until the report packet is successfully transmitted With explicit ACK, contender nodes only can suppress their report after the sending node does This may result in higher latency To account for transmission error, we also use the explicit ACK to control the retransmission of data packet in PSIFT The integration of both explicit and implicit ACK brings excellent performance to our proposed PSIFT protocol We present the numerical results obtained by simulation of PSIFT in the next chapter 4.2 Simulation results and analysis In this experiment, we compare performance of PSIFT under report suppression mechanisms We create the simulation scenario in which 50 static sensor nodes are randomly deployed in a [150m × 150m] flat area The sink is located at the right edge We limit the transmission range of all nodes to 50m When an event happen in the simulation area, those sensor nodes within d meters (d is sensing range of the sensor) from location of the event send a report packet to the sink using AODV routing protocol and suppress their report using ACK and - 67 - overhearing mechanism respectively We then measure the end-to-end delay and the delay in transmitting in each hop at the sink as the function of sensor’s sensing range Simulation results are plotted in Figure 4.2 and 4.3 120 802.11 Psift, suppress report using Acknowledgement 100 Psift, suppress report using Overhearing Average delay (ms) 80 60 40 20 10 15 20 25 30 Sensing range (m) 35 40 45 Figure 4.2 Average end-to-end delay in real adhoc routing scenario(AODV) when one report required We can see that PSIFT outperforms IEEE 802.11 in term of latency Consider PSIFT only, overhearing performs better than ACK mechanism Moreover, the gap is larger with higher sensing range (PSIFT with overhearing gains times delay reduction as compare to IEEE 802.11 at sensing range of 45m) With real adhoc routing scenario, the delay performance is affect a lot by routing overhead The longer sensing range is, the higher number of routing overheads is generated (because of higher number of contenders) Which then increase the contention level in the network That is why all the end-to-end delay curves tend to increase as - 68 - sensing range increases However, at short sensing range, the delay of PSIFT with overhearing suppression mechanism keeps rather flat while ACK mechanism and IEEE 802.11 keep increasing It only starts growing when sensing range is 25m, which equal to half of transmission range This is because when the sensing range is less than half of transmission range, all contender nodes can hear others and can suppression perfectly 18 Psift, Overhearing Psift, Acknowledgement 16 Average delay (ms) 14 12 10 2 Hop number (a) Sensing range of 15m - 69 - 18 Psift, Overhearing Psift, Acknowledgement 16 Average delay (ms) 14 12 10 2 Hop number (b) Sensing range of 25m 18 Psift, Overhearing Psift, Acknowledgement 16 Average delay (ms) 14 12 10 2 Hop number (c) Sensing range of 35m Figure 4.3 Delay distribution at the sink as a function of sensing range of two suppression mechanisms with one report required When we consider performance of PSIFT at a certain node (the sink), performance of overhearing mechanism is worse as the sensing range increases but it - 70 - still lower than ACK mechanism The differences are clearly seen at the first hop latency From second hop onward, there is only slightly difference between two suppression mechanisms At short sensing range, performance of ACK mechanism is not too bad as compare with overhearing since ACK packet can cover almost all the contender nodes (Figure 4.3a) As the sensing range increases, first hop latency of overhearing mechanism is much better than ACK mechanism (Figure 4.3c) Because overhearing mechanism can suppress much more report packet than ACK mechanism at the first hop In summary, PSIFT with overhearing mechanism is better than ACK mechanism in real adhoc routing scenario, especially at long sensing range - 71 - Chapter 5 Conclusions and future work 5.1 Conclusions In this thesis, we presented PSIFT MAC protocol that brings significant improvement to delay performance of multihop, event-driven wireless sensor network The key idea in PSIFT is to utilize the node redundancy characteristics of event-driven sensor network, where many sensor nodes within the same area have correlated data about the same event and contend to send them Only a few of them is enough for the sink to identify the event By suppressing the unnecessary data, PSIFT can reduce the congestion level of the network, achieve low latency in delivering data to the sink and achieve energy savings as well We develop a novel mechanism to suppress the report message by utilizing the broadcast nature of wireless transmission (overhearing) as an implicit acknowledgement This mechanism works extremely well in multihop adhoc transmission scenario, can suppress almost all unnecessary reports at the first hop when the sensing range is small as compare to the transmission range - 72 - We also introduce and attach the priority level to each packet so that the relay node can handle differently based on its priority The priority level of each data packet increases as it traverses to the sink We assign different DIFS values together with different contention window sizes to different traffic class (based on packet priority level) to differentiate services provide to each data flow With this scheme, PSIFT can differentiate service to packet level This can further improve the delay performance and the correctness of data transmission We obtained experiment results to show that PSIFT can reduce the latency in delivering the first report up to three times compared with existing IEEE 802.11 when the sensing range increases to reach the transmission range, while maintaining the correctness in reporting event 5.2 Future work There is a limitation of our proposed protocol that we have to pre-set the DIFS values that assign to each packet based on the maximum number of priority levels that PSIFT design for In future, we plan to improve PSIFT so that it can adaptively assign the DIFS value to each priority class using heuristic algorithm, in order to be better in channel utilization and more flexible in implementation in real sensor network - 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80 - [48] Wei Liu; Wenjing Lou; Xiang Chen; Yuguang Fang, “A QoS-enabled MAC architecture for prioritized service in IEEE 802.11 WLANs”, Global Telecommunications Conference, 2003 (GLOBECOM '03), IEEE, Volume 7, Page(s):3802 – 3807, 1-5 Dec 2003 [49] NS-2 Network Simulator, http://www.isi.edu/nsnam/ns - 81 - ... designed and tested ER -MAC on low node’s density (100 nodes over a 1000m*1000m area), ER -MAC may have problem on large scale network 2.3.5 Traffic-Adaptive MAC Protocol (TRAMA) TRAMA [7] is a TDMA-based... T van Dam and K Langendoen propose an adaptive energy efficient MAC protocol (T -MAC) which automatically adapts the duty cycle to the network traffic As with S -MAC [28], nodes form a virtual... protocols treat all packets indifferently, that may lead to the degradation in delay performance and quality of service of the network Therefore, it is a need to develop a MAC protocol that can handle