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Topologies Different topologies may be implemented according to the application (resemble network topologies) Topologies that make this discipline a new originate from the need of building a cost effective application and managing the sensor network Each topology has its own benefits, challenges, and tradeoffs Choosing proper topology to suit the application is the key for the wireless sensor network Some topology examples are shown in the following slides Pritee Parwekar 1 Star Topology Pritee Parwekar 2 Mesh Topology Pritee Parwekar 3 Hybrid Star/Mesh Topology Pritee Parwekar 4 Wireless Sensor Networks Topology There are a number of different topologies for radio communications networks. A brief discussion of the network topologies that apply to wireless sensor networks are outlined below. 5 Star Network (Single Point-to-Multipoint) A star network is a communications topology where a single basestation can send and/or receive a message to a number of remote nodes. The remote nodes can only send or receive a message from the single basestation, they are not permitted to send messages to each other. 6 Star Network The advantage of this type of network for wireless sensor networks is in its simplicity and the ability to keep the remote node’s power consumption to a minimum. It also allows for low latency communications between the remote node and the basestation. The disadvantage of such a network is that the basestation must be within radio transmission range of all the individual nodes and is not as robust as other networks due to its dependency on a single node to manage the network. Star Network Topology 8 Star Topology A classical star topology of WSANs was studied in Korber et al. (2007).In the star topology, the BS serves as a network controller and as a gateway to upper layers. The BS may have a wired bus and a wireless radio interface. The TDMA (time division multiple access) technique is employed in the MAC layer. Each sensor is integrated into an actuator to form a sensor-actuator module. These modules are able to reach the BS in one-hop 9 An example in military applications. 10 Mesh Network A mesh network allows for any node in the network to transmit to any other node in the network that is within its radio transmission range. This allows for what is known as multihop communications; that is, if a node wants to send a message to another node that is out of radio communications range, it can use an intermediate node to forward the message to the desired node. This network 11 Mesh Topology Advantage of redundancy and scalability. If an individual node fails, a remote node still can communicate to any other node in its range, which in turn, can forward the message to the desired location. In addition, the range of the network is not necessarily limited by the range in between single nodes, it can simply be extended by adding more nodes to the system. Mesh Topology Disadvantage of this type of network is in power consumption for the nodes that implement the multihop communications are generally higher than for the nodes that don’t have this capability,often limiting the battery life. Additionally, as the number of communication hops to a destination increases,the time to deliver the message also increases, especially if low power operation of the nodes is a requirement. Mesh Topology 14 Hybrid Star – Mesh Network A hybrid between the star and mesh network provides for a robust and versatile communications network, while maintaining the ability to keep the wireless sensor nodes power consumption to a minimum. In this network topology, the lowest power sensor nodes are not enabled with the ability to forward messages. This allows for minimal power consumption to be maintained. However, other nodes on the network are enabled with multihop capability, allowing Shubhalaxmi Kher, Arkansas State University 15 Hybrid Topology Generally, the nodes with the multihop capability are higher power, and if possible, are often plugged into the electrical mains line. This is the topology implemented by the up and coming mesh networking standard known as ZigBee. Hybrid Topology 17 Crossbow Software Framework Pritee Parwekar 18 Full power network topology Reduced network topology via Minimal Spanning Tree (Change in Tx Range) Reduced network topology via Connected Dominating Set (Select a subset of nodes that cover all the network and turn off non-selected nodes) Topology maintenance algorithms In the same manner as topology construction, there are many ways to perform topology maintenance: Global Vs. Local Dynamic Vs. Static Vs. Hybrid Triggered by time, energy, density, random, etc. Global DGTRec (Dynamic Global Topology Recreation): Periodically, wake up all inactive nodes, reset the existing reduced topology in the network and apply a topology construction protocol. SGTRot (Static Global Topology Rotation): Initially, the topology construction protocol must create more than one reduced topology (hopefully as disjoint as possible). Then, periodically, wake up all inactive nodes, and change the current active reduced topology to the next. HGTRotRec (Hybrid Global Topology Rotation and Recreation) Work as the SGTRot, but when the current active reduced topology detects a certain level of disconnection, reset the reduced topology and invoke the topology construction protocol to recreate that particular reduced topology. Local DL-DSR (Dynamic Local DSR-based TM) This protocol, based on the Dynamic Source Routing (DSR) routing algorithm, recreates the paths of disconnected nodes when a node fails. Topology construction algorithms There are many ways to perform topology construction: Change the transmission range of the nodes Turn off nodes from the network Create a communication backbone Clustering, etc. Some examples of topology construction algorithms are: Tx range-based Geometry-based: Gabriel graph (GG), Relative neighborhood graph (RNG) Spanning Tree Based Direction Based: Nearest neighbor graph, Cone Based Topology Control (CBTC), Distributed RNG Neighbor based Routing based: Hierarchical CDS-based(connected dominating set) Cluster-based: Low Energy Adaptive Clustering Hierarchy Topology construction and maintenance Topology control have been divided into two subproblems: topology construction, in charge of the initial reduction, and topology maintenance, in charge of the maintenance of the reduced topology so characteristics like connectivity and coverage are preserved. Topology construction and maintenance This is the first stage of a topology control protocol. Once the initial topology is deployed, specially when the location of the nodes is random, the administrator has no control over the design of the network; for example, some areas may be very dense, showing a high number of redundant nodes, which will increase the number of message collisions and will provide several copies of the same information from similarly located nodes. However, the administrator has control over some parameters of the network: transmission power of the nodes, state of the nodes (active or sleeping), role of the nodes (Clusterhead, gateway, regular), etc. By modifying this parameters, the topology of the network can change. construction and maintenance After some time being active, some nodes will start to run out of energy. Especially in wireless sensor networks with multihoping, it is a fact that nodes that are closer to the sink spend higher amounts of energy that those farther away due to packet forwarding. The network must restore the reduce network periodically in order to preserve connectivity, coverage, density the application requires. construction and maintenance Topology control is a technique used mainly in wireless ad hoc and sensor networks to reduce the initial topology of the network in order to save energy, cut down interference and extend the lifetime of the network. The main goal is to reduce the number of active nodes and active links, preserving the saved resources for future maintenance. The minimum spanning tree of a planar graph. Each edge is labeled with its weight, which here is roughly proportional to its length. The number and nature of links that are to be used among all potentially available links can be controlled. Topology control can be achieved by modifying the transmission radii, selecting a given subset of the links, or moving some nodes (if such functionality is available). A general approach employed in data gathering and data aggregation is to construct a spanning tree which is rooted at the sink and connects all sensor nodes in the network. If one node fails, the topology will be reorganized into a Current problems at the network layer can be classified into three categories: topology control, routing, and coordination. A well-organized network topology can not only prolong the lifetime of a network, but also enhance data communications. Topology control problems can be subdivided into neighbor discovery problems and network organization problems. Neighbor discovery problems are defined as problems in detecting and discovering neighbors which are located within the transmission range. 33 34