A deployment procedure for wireless sensor networks

A deployment procedure for wireless sensor networks Tzu-Che Huang, Hung-Ren Lai and Cheng-Hsien Ku Networks and Multimedia Institute, Institute for Information Industry tzuche@nmi.iii.org.tw Abstract Since the wireless signal is invisible and unpredictable, the deployment issue for a wireless sensor network (WSN) becomes a critical task. In this paper, a procedure of deployment for a wireless sensor network in an indoor environment is proposed. The objective of the procedure is to provide a solution guide for people who don t have sufficient wireless communication knowledge and experience in how to correctly place the wireless sensor nodes or devices in the interesting indoor environment, especially in a large-scale deployment case, and then the desired communication performance of the wireless sensor network can be achieved. The addressed deployment procedure is divided into four sub-procedures and is discussed in detail in the following. A deployment plan for an office and the real deployment testing results are then proposed to demonstrate the efficiency of the proposed deployment procedure. 1. Introduction In recent years, based on the maturity of low-power, low-cost and small-scale circuit design technology, a newly application area of wireless sensor networks for monitoring and sensing environment, detecting the events, tracking targets in the given regions and providing personal network services, is arisen very quickly. Most of the services in this newly application area need certain infrastructure to achieve the data sensing, processing and communication tasks. That is why the deployment procedure is regarded as the most important issue in wireless sensor network [1]. The traditional way in the deployment of wireless sensor networks is mostly placing sensor nodes by random or ad hoc method which is convenient and suitable for outdoor applications [2-4]. In such cases, the sensor nodes are capable of self-configuration and self-discovery. To reduce power consumption, in [5], they established a topology that act as a communication backbone. In [6], they established a virtual network whose topology of a mesh of stars which also provides communication functionality. It is represented in [5] and [6] that the backbone network is essential to wireless sensor networks while it require more effort to be established. Since our use of wireless sensor networks lies on indoor applications, we consider that a reliable communication channel should be constructed before any application-oriented devices being put into action. When it comes to sensor network deployment, we put our attention on deploying the communication backbone of sensor network while fulfilling the connection quality and coverage constraints into target area. Furthermore, the deployment planning in the proposed method, a 3-D space and 3-D antenna radio pattern are employed to make the simulation results will be more approximated the real physical environment. After planning of deployment, we also suggest some efficient testing procedure to measure the communication performance of the established networks; these are the major differences between our method and others [6-8]. The rest of the paper is organized as follows: in Section 2, the proposed deployment procedure for wireless sensor networks is addressed. In Section 3, an office deployment demonstration is illustrated to show the efficiency of the proposed deployment method. Then, we make a conclusion in the last Section. 2. The deployment procedure for WSN The proposed procedure includes four sub-procedures, which are (1) Planning Procedure; (2) Device Configuration Procedure; (3) Network Verification Procedure and (4) Requirement Completion Procedure. The whole procedure is shown in Figure 1 and the details of these procedures are described below. 2.1. Planning Procedure In the construction of the backbone of a wireless sensor network, we take a few things into consideration: the network connectivity, the signal coverage in the area, and the number of devices used. The most important thing we care about is the network connectivity which stands for the ability whether the packets can be successfully sent to their destination through the established networks. It must be assured that each device in the backbone network is inter-connected, since it was essential to the whole network and the applications established above. After that, we take the signal coverage and the devices used into account in the next step. 1

As a goal, we would like to expand the signal coverage of the backbone network to cover the whole area. Generally speaking, expanding the coverage usually increases number of devices used, raises the cost of the sensor network at the same time. The Planning Procedure is proposed to find the suitable position of each device by minimizing the number of devices used while expanding the network coverage to the whole desired region. Figure 1. The proposed deployment procedure. Figure 2. 3-D space model fetched by the planning tool. We have built an automated application program which helps the users finding the exact position where the devices should be located and fulfill the coverage constraint in the target area while limiting the amount of devices required in a reasonable number. The required inputs of the application, given by the users, are: a 3-D Space Model of target region, the Antenna Models (3-D radio patterns) of devices used and the Signal Quality Threshold (sensitivity of employed antenna) of the receiver s antenna being considered to be connected. Some other constraints can be assigned to the application, including the redundancy of the devices, the density of the devices in certain area and the area should or should not be placed with sensor devices, etc. The 3-D Space Model we are using includes information about location and material of walls and doors in target area, along with the coverage requirements. We did some experiments for measuring the transmission attenuation of the received signal strength when the signal passes through the material such as cement, glass and woods etc. It is the effort that we try to make the proposed planning procedure as approximating the practical situation as possible; however, there must be some factors that can not be measured or considered in this procedure, they are arisen from the differences between antennas, the radio pattern measurement error, the interference from environment and so on. At least, the proposed planning tool can provide a useful tool for those who have no experience in how to deploy the wireless device into the interesting area. According to the results of simulations applied in the automated application software, we are able to obtain the locations and the antenna orientation of each sensor device of network in target region. The device information generated by the software includes the location, antenna orientation, the sensitivity and the transmission power of devices. 2.2. Device Configuration Procedure The program running on each device being planted are identical, while there are some individual configurations need to be applied to each device before placing them on the location decided in the Planning Procedure. The required configurations are: the power level of the antenna used by each device, the operation channel of the network, and the default topology of the backbone network. Since the topology of the wireless sensor network is formed as a tree, we can configure the default topology of the backbone network by retaining a default parent identity in each device. Therefore, the parent identity of a device should have been recognized before it being configured. We use a 64bits MAC address, which is pre-configured in each device, as the unique identity. After assigning the connected parent of the device, the topology of the backbone network can be easily established. We built another tool as an aid to help users performing the procedure in a convenient way. The planning result would be fetched by the device configuration tool and users are able to sketch the 2

topology of the network by the information included in. Other than default topology, the operation channel of the network is also determined by users. As for the power level of each device, they are already determined in planning procedure and are also included in the planning results. After fetching the information required, the tool is switched to the configuration mode. The devices are configured one-by-one and should be in the sequence arranged by the device configuration tool in order to make sure that the parent identity of the device under configuration already existed. After all the devices are configured, the users may plant the devices into target area. 2.3. Network Verification Procedure We have provided yet another procedure for users to verify the performance of the deployed wireless sensor network backbone. There s a command entry on each device of which users can make use. With a hand-held mobile device attached to a network compatible component, users may examine the network with some predefined tests. We have defined the following tests to verify the validity of the network: parent connectivity, network coverage and device coverage. For a wireless backbone, the first thing we care about is the interconnection status between devices. As mentioned above, the network topology is formed as a tree, so by ensuring that each edge connection in the tree is valid, we can be sure that the network interconnection is valid. It means that if all the devices are connected to their parent, they are connected to all the devices in the network. The purpose of parent connectivity testing is to retrieve the connection status between a device and its parent. When a device received a test parent connection command from a hand-held mobile device, it will: send a number of packets to its parent, accumulate the packet lost rate, calculate the average link quality and then report the testing results back to the user for further estimation. The user will have to test the devices one-by-one. After all the devices are tested with parent connectivity test, user can get an overall connectivity status of each path in the tree-form network. Network coverage was used to verify the coverage of the network in the target region. The purpose of the network coverage test is to make sure that all packets from the target region can be successfully sent to their destination. Therefore, after the backbone network has been formed, users can discretionarily add, change and remove any end device beyond the placed nodes or can freely use mobile devices in the target region. Users are able know that which of the network devices are being sensed, along with a link quality reference value and may be connected in a particular location. The underlying operations were periodically scanning the channel being used and return the results back to user. To verify the signal coverage of the whole network, the users are recommended to traverse the whole region and make sure that all the locations of interest are covered by the network. We offered device coverage test for users to check the connection quality from a specific network device to some locations of interest. After receiving a device roar command, it will start sending packets to users mobile device with a period of ten packets per second until a device silence command is received or no acknowledge received for one minute. Users may use this test function to analyze the packet lost rate and the link quality between a network device and a specific location where they place their hand-held devices in. 2.4. Requirement Completion Procedure The wireless sensor network deployed based on the planning result might not be as capable as needed. It means that the network connectivity or the coverage of the network is not valid to the applications going to be applied upon the network structure. There are many reasons that would cause the incapability, like: incapable space model, invalid antenna modal, theoretic and realistic mismatch, inaccurate device planting location etc. To fix this problem, we provided the requirement completion procedure as assistance to help the users solving the problem of the network. The method provided was to insert additional devices into the network. Users are able to identify the invalid connection of the network from the results provided in network verification procedure. After identifying the invalidity connection, the next step is to choose a location to place the new additional device. The users use the network coverage test to find out a location which is able to connect both ends of the invalid connection. The location found is where the new device can be placed to improve the insufficiency of the original network. The coverage problem can also be solved by inserting a device between the nearest network device and the location of interest. If the test result obtained from network verification procedure reveals that the network was too far from being capable, the users should reverse back to Planning Procedure, modify some constraints or the Space Model, and do the first three procedures all over again. 3. Deployment for an office case We applied the proposed procedure on the wireless sensor network deployment in our office. We listed some of our testing environment below. The protocol of the wireless sensor network we used was based on IEEE 802.15.4 standard. The testing platform was employed a Chipcon CC2420 RF transmitter and Atmel ATmega128 microcontroller. A simulated annealing like searching algorithm is utilized to locate all devices in wireless sensor network in the planning procedure. The device RF settings: transmitter power is 0 dbm and the sensitivity of receiver antenna is -94 dbm. The floor plan of the office is in Figure 3. It took us about 30 minutes to sketch the floor plan and assign 3

the material of the instances within. After sketching the floor plan, we masked out the regions doesn t need to be covered by the network, as in Figure 4. Few more information is needed by the planning tool, like antenna models and signal quality threshold mentioned above. It took 40 seconds for the Planning Tool to find a deployment plan on our Pentium4 3.2GHz personal computer. The result is shown in Figure 5, and the dots indicate where the devices should be placed. Figure 6 shows the signal coverage of the network simulated by the Planning Tool software. the order of devices being configured. The whole device configuration procedure took us about 6 minutes by sketching the topology and configuring the devices. The devices are placed in the respective locations after configuration. The procedure after device placement was the network verification procedure. We used the provided tool to verify the Network Validity which takes about 20 seconds for each device and about 5 minutes to verify the whole network. Figure 3. Floor plan of the office under deployment. Figure 6. The simulated signal coverage. Figure 4. Mask out the area that needs no signal. Figure 7. The network topology and configuration order. Figure 5. The deployment plan. We used the device configuration tool to form our network into a tree as in Figure 7. The number indicates Figure 8. The test result of the locations selected. 4

We selected twelve locations where application-oriented devices will be placed as the locations of interest. In each location, we scanned for available network devices and choose one of them as the attachment to the network. We used device coverage test which would provide us the packet lost rate and link quality analysis result at each location of interest. The test locations and their respective results are shown in Figure 8. The whole test procedure for all twelve locations took us about 10 minutes to accomplish. According to the result of the tests, the network behaves as we expected and no Requirement Completion Procedure needed. 4. Conclusion Maria Serna Efficient and Reliable High Level Communication in Randomly Deployed Wireless Sensor Networks, Proceedings of the second international workshop on Mobility management & wireless access protocols, pp. 106-110, 2004. [7] Y.C. Wang, C.C. Hu and Y.C. Tseng, Efficient deployment algorithms for ensuring coverage and connectivity of wireless sensor networks, First International Conference on Wireless Internet, pp. 114-121, 2005. [8] Yung-Tsung Hou, Tzu-Chen Lee, Chia-Mei Chen and Bingchiang Jeng, Node Placement for Optimal Coverage in Sensor Networks, IEEE International Conference on Sensor Networks, Ubiquitous, and Trustworthy Computing, vol. 1, pp. 352-357, 2006. In the newly arisen applications of wireless sensor networks, rapid and massive sensor nodes deployment tool or solution is exigent. In this paper, a procedure of deployment for a wireless sensor network is addressed to guide users to complete the deployment tasks systematically. The procedure is designed mostly according to the experience of the wireless sensor network deployment task for an 8000m 2 and 3-floor exhibition in a museum. The application scenarios of our assumption are the wireless sensing applications and the mobile communication services, therefore, the connectivity between devices and signal coverage in the interesting region are such important to be regarded as the wireless communication performance index for the deployment procedure. Furthermore, other essential performance indices of the wireless sensor network can be included to evaluate the efficiency of the established networks dependence on the demand in the application. However, the proposed procedure is the main idea to help users to finish the deployment tasks. According to the results we presented above, the tool and procedures can really help us reach the goal of the application in a wireless sensor network we set up in the beginning. 5. References [1] Holger Karl and Andreas Willig, A short survey of wireless sensor networks, TKN Technical Report TKN-03-018, Technical University Berlin, October 2003. [2] K. Sohrabi et al. Protocols for self-organization of a wireless sensor network, IEEE Personal Communications, vol. 7, No. 5, pp.16-27, 2000. [3] W. Heinzelman, A. Chandrakasan and H. Balakrishnan, Energy-efficient communication protocol for wireless sensor networks, Proceeding of the Hawaii International Conference System Sciences, 2000. [4] M. Younis, M. Youssef, K. Arisha, Energy-aware routing in cluster-based sensor networks, Proceedings of the 10th IEEE/ACM International Symposium on Modeling, Analysis and Simulation of Computer and elecommunication Systems, 2002. [5] Alberto Cerpa and Deborah Estrin, ASCENT: Adaptive Self-Configuring sensor Networks Topologies, IEEE Transactions on Mobile Computing, pp.272-285, 2004. [6] Carme Àlvarez, Josep Díaz, Jordi Petit, José Rolim and 5