Cluster Tree Based Self Organization of Virtual Sensor Networks

Transcription

1 Cluster Tree Based Self Organization of Virtual Sensor Networks H. M. N. Dilum Bandara, Anura P. Jayasumana Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, CO 80523, USA. Tissa H. Illangasekare Division of Environmental Science and Engineering, Colorado School of Mines, Golden, CO 80401, USA. Abstract With recent advances enabling the deployment of large-scale collaborative Wireless Sensor Networks, numerous factors point to the need for sensor network deployments where subsets of nodes cooperate on specific tasks or networks supporting multiple applications. Virtual Sensor Networks (VSNs) is a concept to provide such support for the formation, usage, adaptation, and maintenance of subsets of sensors collaborating on a specific task(s). We describe a cluster tree based mechanism to form VSNs, facilitate inter-vsn and intra-vsn communication, and perform other VSN support functions. Simulation based results are used to evaluate its performance. The results indicate that cluster-tree based approach is significantly more efficient and reliable compared to random-routing based protocols. Keywords- Cluster tree, self organization, virtual sensor networks. I. INTRODUCTION A. Motivation Recent advances in wireless communications and miniature, low power, and low cost sensors are enabling the deployment of large-scale and collaborative Wireless Sensor Networks (WSNs). These networks enhance the perception of our surrounding by sensing the physical world at a far greater temporal and spatial granularity than has been hitherto possible. Numerous WSN systems are being proposed and implemented leading to novel applications in areas such as habitat monitoring, disaster response, eldercare, and battlefield intelligence. In contrast to early sensor networks that were dedicated to a specific application, collaborative networks that perform different tasks and deployed in the same geographical region are emerging. e-sense [6] and U-City [12] are two such projects. These networks combine several heterogeneous sensor networks that are deployed in the same geographical region. Better resource efficiency can be achieved by allowing such multiple networks to collaborate and share resources with each other [10]. Certain sensing applications may also involve dynamically varying subset of sensor nodes [10] and/or users [6, 12]. Virtual Sensor Networks (VSNs) is an emerging concept that supports such collaborative, resource efficient, and multipurpose sensor networks that may involve dynamically varying subset of sensors and users [10]. VSNs are useful in three major classes of applications. Firstly, VSNs are useful in geo- This work is supported in part by a grant from Environmental Sciences Division of the U. S. Army Research Office (AMSRD-ARL-RO-EV). graphically overlapped applications, e.g., monitoring rockslides and animal crossing within a mountainous terrain. Different types of devices that detect these phenomena can relay each other for data transfer without having to deploy separate networks (Fig. 1). Secondly, VSNs are useful in logically separating multipurpose sensor networks, e.g., smart neighborhood systems with multifunctional sensor nodes. Thirdly, VSNs can be used to enhance efficiency of systems that track dynamic phenomena such as subsurface chemical plumes that migrate, split, or merge. Such networks may involve dynamically varying subsets of sensors. A VSN can be formed by providing logical connectivity among these collaborative sensors. Nodes can be grouped into different VSNs based on the phenomenon they track (e.g., rockslides vs. animal crossing) or the task they perform. VSNs are expected to provide the protocol support for formation, usage, adaptation, and maintenance of subset of sensors collaborating on a specific task(s). VSNs should make efficient use of intermediate nodes, networks, or other VSNs to deliver messages across members of a VSN (Fig. 1). This new concept opens up many new research directions. However, it is necessary to build algorithms and protocols that support the formation and maintenance of VSNs on resource constrained WSNs. Realization of VSNs require some structure within the sensor filed, many-to-many routing, and implementation of many VSN management functions. Formation of some structure within the sensor field can greatly simplify VSN formation, management, and communication. In contrast to some conventional WSNs that make use of many-to-one communication model (i.e., node-to-sink), VSNs require communication within and across VSNs (i.e., many-to-many, Fig 1). VSN management functions should be able to get new nodes into a VSN, remove nodes from a VSN, detect multiple VSNs, allow them to communicate with each other, etc. Figure 1. Two geographically overlapped VSNs.

2 B. Contributions We propose a cluster tree based approach for forming VSNs. Consider an application such as plume tracking, where a subset of nodes organizes themselves to form a VSN to track a specific plume. Whenever a node detects a relevant event for the first time it sends a message towards the root of the cluster tree indicating that it is aware of the phenomenon and wants to collaborate with similar nodes. The node may join an existing VSN or makes it possible for other nodes that wish to form a VSN, to find it. Use of a cluster tree or a similar structure guarantees that two or more nodes observing the same phenomenon will discover each other. Simulation based results show that our approach is more efficient and reliable than Rumor Routing [4] and is able to combine all the nodes that collaborate on a specific task into a VSN. We further present a mechanism to deliver unicast and multicast messages within VSNs. Sections II provides a brief description of the proposed VSN formation approach, cluster tree formation process, and communication mechanism. Performance analysis is presented in Section III. Finally, concluding remarks and future work are presented in Section IV. II. VIRTUAL SENSOR NETWORKS A. Virtual Sensor Network Support Functions Formation, usage, adaptation, and maintenance of VSNs require implementation of many functions and protocols. These functions and protocols should be able to get new nodes into a VSN, remove nodes from a VSN, detect multiple VSNs, merge VSNs together (e.g., when two chemical plumes merge), split a VSN into multiple VSNs (e.g., when a chemical plume splits), and facilitate communication within and across VSNs. Self-organization of VSN members is the first step. Whenever a node detects a relevant event for the first time, it should send a VSN formation/discovery message within the network indicating that it is aware of the particular phenomenon and wants to collaborate with similar nodes. The node may join an existing VSN (if there is one) or makes it possible for other nodes that wish to form a VSN, to find it. Therefore, every node that detects a relevant event for the first time executes the following function and inform other nodes about its interest to form/discover a VSN. Form_Discover_VSN(msg) The message (msg) format should be similar to the following: struct msg: source //Node ID of the source node type //Type of phenomenon, VSN ID reading //Sensor reading(s) where phenomenon type indicate a particular VSN. These messages can be distributed within the network using a random routing scheme such as Rumor Routing [4], Zonal Rumor Routing [3], or Ant Routing [8]. Though these infrastructure-less approaches are relatively simple to implement, they incur significant overhead [3, 4] and do not guarantee that two nodes that detects the same phenomenon are going to identify each other [4, 5]. Alternatively, formation of some structure within the network can easily deliver these messages. Such an approach can significantly reduce the overhead and it will guarantee that two nodes that detect the same phenomenon are going to meet with each other. Intermediate nodes that relay VSN formation/discovery messages need to keep track of the following VSN routing data to facilitate communication within members of a VSN: struct VSN_table: neighbor //Sender of the VSN msg type //Type of phenomenon, VSN ID In multifunctional WSNs, a node may belong to multiple VSNs hence may keep track of different VSN types for the same neighbor. If a node no longer detects the phenomenon, it may unsubscribe from the VSN by sending an unsubscribe message to other VSN members: Unsubscribe_VSN(msg) Inter-VSN and intra-vsn communication models are application dependant. Unicast messages are required when a message needs to be send to a specific VSN member. For example, if a chemical plume is predicted to be moving towards a certain direction, node(s) in that region need to be informed. Multicast messages are useful in delivering messages to all the members of a VSN. For example, if each node independently calculates the average chemical concentration of a plume, each other s data needs to be shared. When VSNs merge or split, it may be required to inform all the members of existing VSNs, hence broadcast within all the VSNs is also important. Therefore, following functions are required while maintaining VSNs: Unicast_VSN(destination, type, data) Muticast_VSN(type, data) Broadcast_VSN(data) In addition to these functions, it is required to develop functions that are able to detect multiple VSNs and handle VSN dynamics such as migrating, merging, and splitting VSNs. However, for the time being we only present an algorithm to self-organize VSN members and allow them to communicate with each other. B. Cluster Tree Formation Imposing some structure within the network to effectively achieve the application objectives is an attractive option for the self-organization of large-scale WSNs. Cluster based organization, and arranging clusters in form of a tree simplifies many higher-level functions and distributed application deployments. We use our Generic Top-down Cluster and cluster tree formation (GTC) algorithm [2] to form a cluster tree within the network. The GTC algorithm is configurable, independent of network topology, and does not require a-priori neighborhood information, location awareness, or time synchronization. Hopahead Hierarchical Clustering (HHC), a special scheme of GTC, produces more uniform and circular clusters and a cluster tree with a lower depth [1, 2]. The HHC scheme is used to form set of clusters with low overlap. Fig. 2 illustrates the ideal circular packing that we would like to achieve. Let n i be the i-th node and C i be the i-th cluster. The first node that initiates the cluster formation

3 Figure 2. Physical shape of ideal single-hop clusters. process is referred as the root node (n 1 ). The root node can be one of the sensor nodes or it can be a resourceful base station. It sends a cluster formation broadcast and allows all the neighbors to join its cluster C 1. Each of those neighbors then forwards the broadcast to the next set of neighbors. This process continues until TTL, i.e., number of hops to forward a message, expires. For example, after hearing the broadcast from n 1, n 2 joins the cluster. The broadcast is then forwarded to n 3 and from n 3 to n 4. Finally, n 4 is 3-hops away from the (Cluster Head) CH and it is a candidate to be selected as a new child CH. The root node can select several such nodes as CHs of new child clusters. At the root node, six nodes have to be selected as child CHs. For all the other levels, it is sufficient that each parent CH selects only three nodes as child CHs (e.g., only C 8, C 9, and C 10 are formed by n 4 ). This process continues until the entire network is covered. Cluster tree is rooted at the root node and formed by each CH keeping track of its parent and child CHs. Although it is better to select the next set of CHs to be separated far apart, i.e., in different directions, in the absence of location information, nodes have to be picked randomly. This results in a solution that is far from ideal. However, our scheme is able to form uniform and more circular clusters, and a cluster tree with lower depth [2]. Refer [1] and [2] for the pseudo code of the GTC algorithm and extensive performance analysis. C. Cluster Tree Based Virtual Sensor Network Formation We form a VSN by connecting nodes observing the same phenomenon through a virtual tree. We make use of the cluster tree formed by our HHC scheme to deliver VSN formation/discovery messages and to communicate within and across VSNs. The algorithm given in Fig. 3 is used to form such a virtual tree. Whenever a node detects a relevant event for the first time it sends a VSN formation/discovery message (using the Form_Discover_VSN(msg) function) towards the root node of the cluster tree, indicating that it is aware of the phenomenon and wants to collaborate with similar nodes. Intermediate CHs need to keep track of the following information: struct my_data cluster_members //List of cluster members n //No of VSNs VSNs //Array of VSN IDs m //No of VSN routing entries VSN_table //VSN routing table A CH executes the Handle_VSN_Message function, whenever a VSN formation/discovery message is received. If the message is from one of its cluster members, the CH checks its array of VSN entries to determine whether it is already aware of the VSN (line 2). If not, the CH marks itself as being part of the VSN (line 3), i.e., its cluster is detecting the phenomenon. Handle_VSN_Message(msg) //Initially n = 0, m = 0 IF msg.source my.cluster_members IF(msg.type my_data.vsns) my_data.vsns(n) msg.type n n + 1 Forward_To_Parent_CH(msg, my.parent_ch) my_data.vsn_table(m) (msg.source, msg.type) m m + 1 ELSE IF(msg.type my_data.vsn_table) Forward_To_Parent_CH(msg, my_data.parent_ch) my_data.vsn_table(m) (child_ch, msg.type) m m + 1 Figure 3. VSN formation algorithm. Note that, it is possible for a CH to be part of multiple VSNs, e.g., multifunctional sensor nodes. It then forwards the message to its parent CH using the Forward_To_Parent_CH function. The CH further adds the cluster member to its VSN routing table (line 6), regardless of it being previously aware of the VSN or not. These routing entries are useful when delivering messages to members of a VSN. If the received message is from one of the child CHs, the parent CH checks its VSN routing table to see whether it is already aware of the VSN (line 9). If not, the message is forwarded to its parent CH so that the parent can also keep track of the VSN (line 10). Similarly, the message is forwarded up to the root node. The virtual tree is formed by all the CHs along the path towards the root node keeping track of the VSN, i.e., by adding the child CH to their VSN routing tables (line 11). Only CHs that are in the phenomenon mark themselves as part of the VSN and other CHs only support their communication. For example, let us analyze how a sensor network that tracks subsurface chemical plumes forms a VSN (Fig. 4). Two chemical plumes are located around clusters E, F, J, K, N, O, and R. Assume that a node in cluster J first detects the plume. It sends a VSN formation/discovery message towards the root node. Firstly, the message is forwarded to its own CH J. This is the first time that J is receiving this information therefore; it marks its self as detecting the phenomenon (indicated by the circle in Fig. 4(b)). It then forwards the message to its parent CH H. H caches the information about the new VSN (i.e., add child CH J to its VSN routing table) and forwards the message to its parent CH B. Finally, the message is forwarded to the root node (A). Both the root node and B also cache the information about the new VSN. Suppose that another node in cluster K detects the same phenomenon therefore forwards its message to the root node. It will follow the path K C A. K, C, and A cache the information about the VSN. However, the root node is already aware of the phenomenon. Therefore, it does not need to caches the new information. However, the root node keeps track of the branch (i.e., child CH C) that the message was received from. Keeping track of such branches enable routing among nodes that tracks the same phenomenon. Meanwhile other nodes in the event region will also detect the plume. When nodes in cluster R detect a plume, they will also send messages towards the root node. When the first message is received from one of the cluster members, CH of R also marks itself as detecting the phenomenon. The message is then

4 Figure 4. A hypothetical sensor field that tracks chemical plumes: (a) Clusters detecting two chemical plumes, (b) Virtual trees that connect VSN members. forwarded to its parent CH H. However, H is already aware of the phenomenon and has already informed its parent CH B. Therefore, H will not forward the message any further. H keeps track that R is also in the phenomenon. The same occurs when cluster members of O detect the plume. Nodes in clusters J, K, R, and O may need to communicate with each other to detect migrating, splitting, and merging plumes. Therefore, form a virtual network, hereafter referred as the virtual tree, on top of the cluster tree. The virtual tree connects all the CHs that are detecting the same phenomenon (Fig. 4(b)). These clusters make use of other CHs in the virtual tree to communicate with each other. Similarly, clusters E, F, and N can form another logical tree. These two trees belong to the same VSN if they monitor the same phenomenon. If not, they can be considered as two separate VSNs. Our approach can also enable multiple VSNs to communicate with each other because each virtual tree is guaranteed to meet at the root node. Sending data about an event towards the root node guarantees that two or more nodes observing the same phenomenon will identify each other. Though random routing algorithms such as Rumor Routing [4], Zonal Rumor Routing [3], and Ant Routing [8] can deliver such messages without any infrastructure, they do not guarantee that two nodes observing similar events will find about each other [4, 5]. Many long lived (i.e., significantly higher TTL values) agents are required, even to achieve moderate probability of successful delivery [4, 5]. These agents need to keep track of the visited nodes to reconstruct the routing path; therefore size of a message increases as an agent travels [4, 5]. In our approach, a VSN formation/discovery message needs to travel only up to the root node hence require significantly lower TTL than random routing schemes. Routing path (virtual tree) is constructed by adding entries to each CH s VSN routing table hence size of a VSN formation/discovery message is fixed. Therefore, our approach has a much lower overhead and it guarantees to form a VSN by connecting all the nodes that observe the phenomenon. D. Communicating Within And Across VSNs VSN communication models may need to support the unicast, multicast, and broadcast functions defined in Section II.A. While the virtual tree is formed, each CH keeps track of the child CHs (i.e., branches of the cluster tree) that are related to a VSN. This information is useful in delivering multicast and broadcast messages without any addressing scheme. To facilitate unicast messages within a VSN, CHs needs to know about other CHs that are detecting the same phenomenon (e.g., CHs E, F, J, K, N, O, and R in Fig. 4). Therefore, each CH needs to inform its address to all the member CHs of the VSN. It is not necessary to keep track of individual cluster members as far as a CH can represent all its cluster members. Address sharing can be accomplished in several ways. One of the easiest solutions is to store all the CH addresses at the root node and use it as a lookup table (similar to the DNS). This approach requires extra control messages and the root node becomes a single point of failure. Instead, a CH may send a broadcast within the VSN whenever it forms/discovers a new VSN, allowing other member CHs to cache the new address. However, the new CH does not know any addresses of the other CHs that are already in the VSN. Therefore, it has to request those addresses from a CH along the path towards the root node. If the parent CH is aware of the VSN, it may provide those addresses. If not, the parent CH can request its own parent CH to provide the addresses. If none of the CHs along the path towards the root node is aware of any other CH, the root node can provide those addresses. Similarly, addresses of multiple VSNs can be shared through the root node. We make use of a hierarchical addressing scheme that reflects the parentchild CH relationship in the cluster tree [1], while delivering unicast messages. III. PERFORMANCE ANALYSIS A. Simulator A discrete event simulator was developed using C [1] nodes are randomly placed on a circular region with a radius of 500m. The root node is placed in the middle of the sensor field. Inter-cluster communication is single-hop. Three different simulation scenarios are considered. In the first scenario, the phenomenon is localized within a single region. Three different event regions are considered in the second scenario. The third scenario randomly distributes the phenomenon around the entire sensor field. Randomly picked nodes from those regions are assumed to detect the event. All the event regions are considered to be parts of the same VSN. Except where noted the simulation results presented assume 500 nodes detect the same phenomenon in all three scenarios. An energy model similar to the one in [7] is used with a multipath fading factor of 2.2 [11]. Each node has 2J of energy. Size of a form/discover VSN mes-

5 sage is assumed to be 120-bits and a data packets is assumed to be 800-bits. The results are based on 100 samples and the random function is initialized with a different seed based on time. B. Results Fig. 5 shows two virtual cluster trees that are formed in scenarios one and two. In the first scenario, all the nodes that detect the phenomenon are localized hence only a few branches are required to form the virtual tree (Fig. 5.(a)). Many branches are required to form the virtual tree when the phenomenon is distributed in three regions (Fig. 5(b)). Different branches of the cluster tree meet either at the root node or at other CHs. Fig. 6 shows the total number of hops travelled by all the VSN formation/discovery messages. First few messages that propagate towards the root node have to travel large number of hops. However, they contribute to the formation of most of the branches in the virtual tree. When the phenomenon is localized within a single region, most of the new nodes that detect the phenomenon do not need to send their VSN formation/discovery messages all the way up to the root node. They will get to know about other VSN members from CHs that are along its path. Therefore, the number of hops travel by a VSN formation/discovery message reduces as more and more nodes detect the phenomenon. When the phenomenon is distributed within three regions, not many CHs along the path of the VSN formation/discovery message are aware of the VSN. Therefore, these messages need to be forwarded several more hops. Most of the paths towards the root node are different, when the phenomenon is fully distributed. As a result, these messages need to travel a higher number of hops. We further simulate rumor routing to compare the probability of two VSN members discovering each other. We considered the second scenario (Fig. 5(b)) and randomly selected Total number of hops Scenario 1 Scenario 2 Scenario Number of event nodes added over the time Figure 6. Hops travelled by VSN formation messages. P T = -20dBm. node pairs spanning different event regions. Each node then sends a single agent and tries to discover the other. With rumor routing, when TTL is 350, two nodes are able to figure out each other with a probability of The probability is increased to 0.84 when TTL is 600 and it was further increased to 0.91 when TTL is These values are comparable with the data given in [4]. However, to form VSNs we may need all the nodes to figure out each other therefore actual overhead would be significantly higher. Alternatively, as seen in Fig. 6, our scheme requires only 553-hops (896-hops if inter-cluster communication is multi-hop) and it guarantees that all the nodes will identify each other. Overhead of the HHC cluster tree formation is 4-5 messages per node [1]. Therefore, our approach is much more efficient than Rumor Routing, even with the overhead of cluster tree formation. Fig. 7 shows the number of unicast messages that can be exchanged within a VSN before battery runs out in any of the nodes. First scenario delivers the highest number of messages while the distributed scenario delivers the lowest number of messages. In the first scenario, communication occurs within a single region therefore the virtual tree uses only a subset of Figure 5. Virtual tree formed by nodes detecting the same phenomenon: (a) Single event region, (b) Three event regions. Dark squares indicate CHs, dark dots indicate the nodes that detect the event, and dark lines indicate the virtual tree that connects members of the VSN. P T = -12dBm.

6 Number of messages Transmission power (dbm) Figure 7. Number of unicast messages. Scenario 1 Scenario 2 Scenario 3 Figure 8. Number of multicast messages Transmission power (dbm) Number of multicast messages. Scenario 1 Scenario 2 Scenario 3 branches that initiates from the root node. Because events are localized, some of the source and destination nodes lie on the same branch, therefore most message do not require going through the root node. The network can deliver more and more messages as the workload of the root node reduces. The virtual tree formed by the third scenario span across most of the branches of the cluster tree. Communication within those distributed VSN members require most of the messages to be relayed through the root node. Therefore, deliver lower number of messages. The second scenario utilizes a subset of the branches of the cluster tree therefore delivers more messages than the third scenario. However, communication across three different regions increase the workload of the root node hence it cannot deliver as many messages as the first scenario. Energy requires to send a message significantly increases with the transmission power (P T ) therefore; number of messages that the network can deliver reduces with increasing P T. Fig. 8 shows the number of multicast messages that can be delivered within a VSN before battery runs out in any node. Multicasts are forwarded to all the members of a VSN. At each CH, a multicast messages need to be forwarded to all the branches that are in the virtual tree. For each branch, a separate copy of the message is forwarded. This significantly increases the workload of a CH and drains its energy much faster. Therefore, number of multicast messages delivered by the network significantly reduces. When phenomenon is localized within a single region, CHs around the event region have to handle many cluster members and child CHs. For each of them, a separate copy of the message needs to be send. This increase the workload on those CHs therefore reduces their lifetime. As a result, the first scenario delivers a lower number of multicast messages. Because many distributed CHs are involved in the third scenario, workload on a single CH is lower, i.e., lower number of VSN members per CH. Therefore, the third scenario delivers the highest number of multicast messages. Because of the extensive overhead, number of multicast messages that were delivered is much lower than the unicast messages. It was further observed that number of unicast messages that can be relayed within a VSN does not vary with the number of nodes that are observing the phenomenon. However, number of nodes in the phenomenon significantly affects the multicast messages. With increasing number of VSN members, the multicast messages need to be forwarded to many nodes. This significantly increases the workload of CHs therefore reducing the network capacity. In all three scenarios, the root node becomes a bottleneck while delivering unicast and multicast messages. We further developed two routing extensions that make use of cross-links within the cluster tree while delivering messages. By doing so, we were able to deliver twice as many messages [1]. IV. CONCLUSIONS AND FUTURE WORK Virtual sensor networks is an emerging concept that supports collaborative, resource efficient, and multipurpose sensor networks. We proposed a scheme for formation of VSN using a cluster tree. The nodes observing the same phenomenon form a virtual tree that connects one or more VSNs. Our scheme is much more efficient and reliable than a scheme based on Rumor Routing [1]. The logical tree can facilitate both inter-vsn and intra-vsn communication. We are current working on VSN management functions such as detecting multiple VSNs, merging two VSNs, and splitting a VSN into multiple VSNs. REFERENCES [1] H. M. N. D. Bandara, Top-down clustering based self-organization of collaborative wireless sensor networks, Master s Thesis, Department of Electrical and Computer Engineering, Colorado State University, [2] H. M. N. D. Bandara and A. P. Jayasumana, An enhanced top-down cluster and cluster tree formation algorithm for wireless sensor networks, In Proc. ICIIS 2007, Sri Lanka, Aug. 2007, pp [3] T. Banka, G. Tandon, and A. P. Jayasumana, Zonal rumor routing for wireless sensor networks, In Proc. International Conference on Information Technology: Coding and Computing, vol. 02, 2005, pp [4] D. Braginsky and D. Estrin, Rumor routing algorithm for sensor networks, In Proc. 1 st ACM International Workshop on Wireless Sensor Networks and Applications, Atlanta, Sep. 2002, pp [5] D.C. Dhanapala and A. P. 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