Evaluation of Metadata-Based Data Aggregation Scheme in Clustering Wireless Sensor Networks Yoshitsugu Obashi1, Tomohiro Kokogawa1, Yi Zheng2, Huifang Chen2, Hiroshi Mineno2, and Tadanori Mizuno2 1
NTT Service Integration Laboratories, 3-9-11 Midori-Cho Musashino-Shi Tokyo 180-8585 Japan
[email protected] 2 Shizuoka University, 3-5-1 Johoku Naka-Ku Hamamatsu-Shi Shizuoka 431-8011 Japan
Abstract. In a wireless sensor network (WSN) system, an important problem for the special application, in which the battery of a sensor node cannot be exchanged, is how to extend the life time of the entire WSN system. In the past we proposed an improvement to the LEACH protocol to lengthen the lifetime of a WSN system and showed the superiority of our proposal by simulation. In this paper we set up an experimental WSN system using sensor nodes called “MICAz”. We try to measure the lifetime of the WSN systems based on the protocol of our proposal and to compare with the simulation result. We confirm by experiment that the lifetime of our proposal is up to 1.2 times longer than that of the LEACH protocol at one sensor node. The protocol of our proposal is sufficiently effective though the effect of the simulation cannot be achieved. Keywords: wireless sensor network, energy efficiency, power consumption.
1 Introduction A wireless sensor network (WSN) system has many sensors that can measure various factors. The measurement data is collected from the sensors through a wireless network, enabling the monitoring system to obtain an overall view of the conditions in the whole area. Such a WSN system has become more realistic. In such WSN systems, one key issue is to reduce battery power consumption. How to extend the lifetime of the entire WSN system has been closed up as a problem. There have been several studies on reducing battery power consumption and extending the lifetime of the entire WSN system. These aimed to achieve battery lifetimes that decrease the power consumption of each sensor node by utilizing a sleep mode and improve the exchange of data between sensor nodes to lengthen the lifetime of the WSN system. Our previous paper [1] proposed an improvement to the LEACH protocol [2] to lengthen the lifetime of a WSN system and showed the superiority of our proposal by simulation. In this paper, we set up an experimental WSN system using sensor nodes called “MICAz” [3], which are produced by Crossbow Technology Inc., to measure the lifetime of WSN systems based on proposed protocol and compare them with the simulation results. B. Apolloni et al. (Eds.): KES 2007/ WIRN 2007, Part III, LNAI 4694, pp. 477–483, 2007. © Springer-Verlag Berlin Heidelberg 2007
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2 Targeted Applications In general, the architecture of a WSN system depends on the target application, and there is a wide range of different applications. We assume an application that gives priority to detecting a target event during a long period, so sensors are scattered in large quantities; that is, there is a lot of overlapping among the detection ranges of the sensors. For example, a lot of soundsensors might be distributed to search for survivors buried in rubble after an earthquake. In this case, a WSN system that can search with minimal battery consumption by each distributed sensor over several days is required. For this example the following system characteristics are important. (1) Each sensor node gathers all sounds that might contain voices and sends them to the center server, which can identify human voices, because the processing power of a sensor node is extremely small. (2) When a sound occurs, it will almost always be detected by more than one sensor node. (3) It is necessary to suppress the battery power consumption of the of each sensor node because the battery of a sensor node cannot be exchanged. It is also necessary for the total system to continue working after some sensor nodes stop working because of their batteries have run out of power.
3 Modified LEACH Protocol The LEACH protocol is typical of protocols proposed for making WSN systems longlived. Our previous paper [1] proposed an improvement to the LEACH protocol based on using a cluster head (CH) to receive the first communication as metadata from two or more detecting sensor nodes. After CH has received the metadata, the real measurement data is obtained from only one sensor node. This decreases the total amount of data transferred, which has the largest influence on the power consumption of the sensor node. The difference between the LEACH protocol and the modified LEACH protocol is in the steady-state phase. The LEACH protocol When an event takes place somewhere in the detected area, the sensor nodes within sensing range of this event sense it and generate a sensing information. Each sensor node transfers the sensing information to a corresponding cluster head (CH), which collects it and sends the aggregated data to a sink node. Modified LEACH protocol When the sensor nodes within sensing range of the event generate the sensing information, they also generate the metadata of the sensing information. Every sensor node transfers metadata to the corresponding CH. The CH collects the metadata and recognizes that they are the same because all sensor nodes within the sensing range of
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(3) (2)
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Fig. 1. Data aggregation process in single-hop clustering WSN system using the modified LEACH protocol
the same event generate the same metadata. CH selects only one sensor node to transmit the sensing information. Then, CH sends the sensing information to the sink node (Fig. 1). The motivation behind the modified LEACH protocol is that the length of the metadata is always smaller than that of the sensing information and total data length between all sensor nodes within the sensing range of the same event and the sink node via CH is smaller. Since more energy is consumed for data transfer than for CPU operation for making metadata, the total energy consumption of sensor nodes and CH is smaller when the modified LEACH protocol is used instead of the LEACH protocol.
4 Evaluation of the Modified LEACH Protocol Our previous paper [1] showed the superiority of the modified LEACH protocol by simulation using a model that described the power consumption using mainly data transmission, reception, and processing at each sensor node. In this paper we try to measure the lifetime of the WSN systems based on both protocols and to compare with the simulation result, and to clarify that the modified LEACH protocol is a realistic and effective proposal in the future. 4.1 Simulation Using some parameters in Table 1, our simulation results suggest that the lifetime of the WSN system using the modified LEACH protocol is about 1.5 times longer than that of the WSN system using the LEACH protocol (Fig. 2).
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50 nJ/bit 125 nJ/bit 5 nJ/bit/message
Fig. 2. Results of simulation of network lifetime. The LEACH protocol (LEACH-traditional) versus the modified LEACH protocol (LEACH-meta). The length of the metadata and the sensing data is 8 bytes and 64 bytes, respectively.
4.2 Experiment Each sensor node of a WSN system consists of four components: sensor, data processor, transmitter/receiver (radio transceiver), and battery. The total power consumption of a sensor node is obtained by adding the power consumption of (1-1) (4) as follows. (1-1) sensor is in active-status. (This part is always active.) (2-1) data processor is in active-status. (2-2) data processor is in idle-status. (3-1) radio transceiver is in active-status. (3-2) radio transceiver is in idle-status. (4) transition-status of each component is caused by protocol overhead. The assumption of the simulation of 4.1 is that power consumption of (2-1) and (3-1) is extremely large compared with that of other components. We try to determine the correct assumption through several experiments using MICAz. Measuring the power
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consumption of each sensor node directly is difficult, so we monitor decrease of power voltage of each sensor node’s battery. For our experiment we have developed software on TinyOS that monitors the amount of transmitting/receiving data and power voltage at each sensor node automatically. The results of the experiments are as follows. (1-1); Power consumption of the sensor is considerably small compared with the processor and the transceiver, while the sensor is always active. (2-2); TinyOS of MICAz is a simple structured operating system, so making the idle-status of the processor by stopping the battery is difficult. The idle-status is now implemented by the programming-loop status, so the power consumption of the idlestatus of the processor is almost the same as that of the active-status. Experimental results suggest that the power consumption in both statuses of the processor is about four times larger than that of the transceiver (in active-status). (3-2); Power consumption of the transceiver, which is in idle-status (not transmitting/receiving), is almost the same as that in active-status only if the transceiver is powered-on. In our implementation, in which no power is consumed the idle-status of the transceiver is changed to the power-off status. However, the problem with this implementation is that some time is taken to change from the idle-status to the active-status because of the protocol overhead to operate sensor nodes. (4); The CH needs to prevent the competition of data transmitting/receiving between many sensor nodes by the time-sharing control [4]. The overhead with power consumption is caused by such controls. Protocol overhead described in (3-2) also wastes battery power. There are several restrictions in our experiment; the shortage of sensor nodes and lack of functions of TinyOS of MICAz. Therefore we set up the WSN system, which consists of 16 sensor nodes attached to light sensors and 1 CH. Then we measured the amount of transmitting/receiving data and power consumption of each sensor node while some luminous sources sometimes flash intermittently. We cannot implement the time sharing control because emulating the operation of the LEACH protocol and the modified LEACH protocol perfectly is difficult. However comparing the modified LEACH protocol using metadata whose amount is one-fifth that of sensing data obtained by the LEACH protocol, we can monitor the power consumption of one sensor node in the experiment. The lifetime of the modified LEACH protocol is up to 1.2 times longer than that of the LEACH protocol at one sensor node, as shown in Fig. 3.
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3400 )v m ( 3200 e d3000 o n r o s 2800 n e s 2600 f o e ga 2400 tl o v 2200 r e w2000 o p
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Fig. 3. Power voltage of a sensor node decreases with the passage of time. Compare the modified LEACH protocol using metadata whose amount is one-fifth that of sensing data obtained by the LEACH protocol.
5 Conclusion The simulation using ideal sensor nodes demonstrates that the lifetime of the WSN system using the modified LEACH protocol is about 1.5 times longer than that of the WSN system using the LEACH protocol. However, in this paper, the experiment using MICAz demonstrates several differences because of the preconditions and the results of the simulation as follows. (A) The idle-status of the MICAz processor is now implemented by the programming-loop status, so the power consumption of the idle-status of the processor is almost the same as that of the active-status. Experimental results suggest that the power consumption of both statuses of the processor are about four times larger than that of the transceiver (in active-status). These findings indicate a decreased advantage of the modified LEACH protocol. However, in the future, the function of the OS of the sensor node will be enhanced, and the processor will consume less power, so this disadvantage will be removed. (B) By our implementation the idle-status of the transceiver is changed to the poweroff status, which consumes no power. However, this implementation causes a problem in which some time is taken to change from the idle-status to the activestatus because of the protocol overhead between sensor nodes. This problem wastes battery power.
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We need to add a function to the sensor node that enables the transceiver to be in the transmit/receive-ready status with low power consumption like in the sleep mode. [5] (C) We confirmed by experiment that the lifetime of the modified LEACH protocol is up to 1.2 times longer than that of the LEACH protocol at one sensor node. The modified LEACH protocol is sufficiently effective though the effect of the simulation cannot be achieved.
References 1. Chen, H., Mineno, H., Mizuno, T.: A Meta-Data-Based Data Aggregation Scheme in Clustering Wireless Sensor Networks. In: MLASN, May 2006, pp. 43–49 (2006) 2. Heinzelman, W.R, Chandrakasan, A., Balakrishnan, H.: An Application-Specific Protocol Architecture for Wireless Microsensor Networks. IEEE Trans. on Wireless Communications 1(4), 660–667 (2002) 3. Crossbow Technology Inc. MPR2400J/420/520 MIB User’s Manual (July 2006) 4. EI-Hoiydi, A.: Aloha with Preamble Sampling for Sporadic Traffic in Ad Hoc Wireless Sensor Networks. In: Proc. IEEE Int’l Conf. Comm (ICC), IEEE, Los Alamitos (2002) 5. Shi, X., Stromberg, G.: SyncWUF:An Ultra Low-Power MAC Protocol for Wireless Sensor Networks. IEEE Trans. on mobile computing 6(1), 115–125 (2007)
