Topology maintenance of wireless sensor networks

Abstract: Topology maintenance is critical to the operation of wireless sensor networks. It aims to improve and rebuild the current topology by rotating node roles, invoking topology construction or maintenance algorithms to improve the network life cycle. Firstly, the topology maintenance is defined, the design goal of topology maintenance is described, and a general model of topology maintenance is designed. Then the research progress of topology maintenance technology is expounded, and the representative algorithms are compared and analyzed. Finally, the problems and development trends in the current topology maintenance research are pointed out.

Wireless sensor networks have been widely used in military defense, industrial and agricultural control, environmental monitoring, biomedical and disaster relief due to their low power consumption, low cost, and distributed and self-organizing features. Typically, a wireless sensor network consists of hundreds of sensor nodes, each with the ability to perceive the current environment, communicate with neighboring nodes via broadcast, and perform local computations on the collected information. However, these capabilities are limited for each node, especially the energy limitation of the node severely limits the life cycle of the network, thus affecting the quality of service and further application of the network. Therefore, in recent years, many researchers have done a lot of research on the energy-saving aspects of wireless sensor networks, from congestion control to data compression, from sleep scheduling to topology control. The goal is to save as much energy as possible and maximize the network life cycle.

As a key energy-saving technology for wireless sensor networks, topology control usually saves energy by changing, simplifying or optimizing the topology of the network while maintaining important network characteristics such as connectivity and coverage. Moreover, a good network topology formed by topology control can improve the efficiency of routing protocols and MAC protocols. However, topology control is often viewed as a single process that does not include maintenance of the network topology, which affects the classification of topology control algorithms. The current classification is limited to how to build the topology of the network, while ignoring topology maintenance in topology control.

Although the topology maintenance is simply defined, and the topology optimization time is optimized according to the target, the topology maintenance technology is divided into static, dynamic, and hybrid topology maintenance. However, the topology maintenance is not systematically elaborated, but the definition of topology maintenance is not strict. The classification of topology maintenance technology is also inconsistent with the current research status, because there is basically no static and mixed in the existing research. Topology maintenance algorithm or protocol. Therefore, in order to study the topology maintenance technology in wireless sensor networks more deeply, this paper carries out topology maintenance from the aspects of topology maintenance definition and model, topology maintenance design goals, current research status and existing problems and development directions. Explain. Section 1 describes the basics of wireless sensor network topology maintenance, mainly giving a new definition of topology maintenance and pointing out the topology maintenance design goals. Section 2 designs a general model for topology maintenance and describes the triggering criteria and maintenance strategies in the model in detail. Section 3 summarizes the current research on topology maintenance and conducts comparative analysis. Section 4 analyzes the shortcomings of the current research and points out the direction of topology maintenance technology. Finally, the full text is summarized.

1 Topology Maintenance Basics

Wireless sensor network topology control consists of two parts, topology construction and topology maintenance. Once the initial network optimization topology is established, the network begins performing the tasks it specifies. Since every behavior included in a network task, such as sensing, data processing, and transmission, consumes energy, the current network topology is no longer in optimal operation over time, so it needs to be maintained. Re-optimal or near optimal.

1.1 Topology Maintenance Definition

Topology control of wireless sensor networks can be seen as a repetitive process, as shown in Figure 1. First, there is a topology initialization phase for all wireless sensor networks. At this stage, each node uses its maximum transmit power transmission to establish an initial topology. After the initialization phase, the initial topology is optimized by running different algorithms or protocols, and finally an optimized topology is constructed, which is called topology construction. Once the topology construction phase is established to optimize the network topology, the topology maintenance phase must begin to work.

In the topology maintenance phase, the current topology state is monitored in real time, and the topology recovery or reconstruction process is triggered when appropriate. As can be seen from Figure 1, during the life of the network, the topology maintenance cycle runs until the network dies. At present, there is very little literature on the definition of topology maintenance. The literature [8] has a simple definition of topology maintenance. It indicates that "topology maintenance refers to timely repair and switching when the topology of the current working topology of the network is not optimized. Or reconfigure a new network topology to bring the network to a pre-set nature and extend the life of the network."

This definition does not indicate the time of topology maintenance operation, the maintenance method adopted, especially the definition that makes the topology reach or near optimal and achieves the pre-set properties, but does not indicate which phase is optimal or nature. Because as the network operates, the optimal state and nature of the network are also changing. Therefore, this paper has a more rigorous definition of topology maintenance, that is, topology maintenance is a cyclical process. In each cycle, it is triggered by different triggering criteria (such as time, energy, node failure, etc.). Rotate the node role or re-run the topology build process or invoke a dedicated maintenance algorithm to repair or reconstruct the network topology, balance the network energy consumption, make the new topology the current optimal or close to the current optimal state, and ultimately extend the network life cycle .

1.2 Design goals

Topology maintenance, like other sensor network technologies, is primarily designed to extend the lifecycle of the network. In addition, sensor networks are built to perform certain tasks, such as performing sensing and transmitting sensory data, so one or more quality of service objectives such as maintaining sensor coverage and maintaining network connectivity are also generally considered.

Moreover, the application of wireless sensor networks results in different topological maintenance design goals or different target priorities. Therefore, this article will only introduce the design goals that are primarily considered for topology maintenance.

(1) Network life cycle

The network lifecycle has been defined in different ways, such as based on the number of nodes, based on sensor coverage and network connectivity, and a scalable network lifecycle.

Topology maintenance is a very effective technique to extend the life cycle of a network. For example, the topology maintenance protocols SPAN and CCP improve the life cycle of wireless sensor networks by shutting down redundant nodes and maintaining a subset of nodes in operation. However, maximizing the network life cycle is a very complex issue that has been the primary goal of topology maintenance research.

(2) Coverage and connectivity

Coverage and connectivity are the basic problems of topology maintenance of wireless sensor networks. In the process of restoring, switching, or reconfiguring the original optimized topology, topology maintenance must maintain coverage or connectivity of the original topology.

(3) Safety and fault tolerance

During the topology maintenance process, some sensor nodes may fail or fail due to energy exhaustion, physical damage or environmental interference, and the failure of these sensor nodes does not affect the overall task of topology maintenance. A fault-tolerant self-organizing method is proposed in [12] to maintain a covered and connected backbone network. In addition, there are various types of malicious behaviors and attacks in the practical application of wireless sensors [13], so security is also an important goal of topology maintenance.

(4) Energy efficiency and convergence time

Like other functions of wireless sensor networks, topology maintenance algorithms must be energy efficient. That is to say, the topology maintenance algorithm should have low computational complexity and low message overhead. In addition, during topology maintenance, the current topology will be replaced by a new topology, so there is a transition time between when the new topology is activated, which should be as small as possible.

(5) Energy balance and scalability

Topology maintenance techniques should try to distribute energy consumption evenly across all nodes in the network. In addition, sensor nodes deployed in interest or target areas may be hundreds or even tens of thousands. The topology maintenance protocol or algorithm should be able to run in networks of different orders of magnitude.

2 topology maintenance model

At present, there is no literature describing the topology maintenance model. In order to better understand the operation process of topology maintenance and its characteristics, this paper designs a general topology maintenance model, as shown in Figure 2. As can be seen from the figure, topology maintenance is a periodic process. Each cycle starts from the current topology of the network and generates an optimized topology through the topology maintenance process, which runs periodically until the network dies.

As you can see from the figure above, each topology maintenance cycle is via triggers and decision makers.

The trigger triggers the topology maintenance process mainly according to the designed triggering criteria such as time, energy or node failure. The decision maker is used to select the topology maintenance strategy.

The model is described in detail below.

(1) trigger

The trigger is responsible for periodically triggering the maintenance process of the current network topology, which has an important impact on the performance of the topology maintenance. Because if it is triggered in advance, unnecessary energy is consumed due to frequent running of the topology maintenance protocol or algorithm, and the lag triggering will cause the network to operate in a sub-optimal or even non-connected state, reducing or even failing to achieve the quality of service of the network. Common triggering criteria are:

Time: The topology maintenance is triggered after the network runs for a period of time. The size of the time is usually fixed and predefined, usually by a timer.

SPAN triggers the update process of the coordinator nodes in the network based on time, thereby implementing topology maintenance of the backbone network.

Energy: Given the energy limitations of wireless sensor devices, it is necessary to trigger topology maintenance when the node's energy level is below a certain threshold. In the LPH algorithm, when the residual energy E(i) of the node is lower than the average residual energy Eavr, the intra-cluster topology maintenance process is triggered. In the CLTC algorithm, when the energy of the cluster head node falls to the threshold M, the topology maintenance process within the cluster is triggered. In the Poly algorithm, the topology maintenance process is triggered when the overall energy of the network is reduced by 10%.

Node failure: Trigger topology maintenance when one or some nodes in the network fail. For example, in the SMSS algorithm, when node u finds a node m failure, it checks whether m is the neighbor determined for it, and if so, re-runs the topology construction algorithm to maintain the network topology. In the EETMS algorithm, once the network finds a faulty node, the local topology maintenance process is triggered.

Network Density: The topology maintenance process is triggered by the node degree of the network or the node degree of some important nodes. The adaptive precision energy-saving algorithm proposed by AFECA uses the neighbor density to trigger the topology maintenance process.

In addition, these trigger conditions can be used in any combination to trigger topology maintenance processes such as energy and node failure, or time and energy. In addition, other network parameters can also be used as triggering criteria, such as link failure, frequent packet loss, and congestion and long routing paths.

(2) Decision maker

The decision maker mainly determines which strategy is used to maintain the current network topology, which is the core of topology maintenance. The topology maintenance policy can be divided into two types. One is a topology maintenance policy based on role rotation. That is to say, energy is saved by switching the roles of nodes in the network, such as sleep/work, cluster head/non-cluster head, etc. The purpose of the network life cycle. The other is a topology maintenance strategy based on topology reconstruction. The essence is to run the algorithm in the topology construction stage or a special topology maintenance algorithm and protocol to maintain the network topology.

In the topology maintenance strategy based on role rotation, it is first necessary to clarify the role that each node in the network can play. The role migration of each node is closely related to the topology maintenance protocol or algorithm features and design. The factors determining the role of the node include node density, location, communication traffic, packet loss rate, time, and external environmental conditions. If the node is currently role 1, when an event occurs, the node tests accordingly to determine whether to enter role 2 or continue to be in role 1.

In the topology maintenance strategy based on topology reconstruction, the algorithm of the topology construction phase or the special topology maintenance algorithm is mainly called. Therefore, the frequency of calling the algorithm is key. Once the trigger triggers the topology maintenance process, the topology maintenance strategy should consider the relevant performance of the network and decide whether to invoke the relevant algorithm or protocol to balance the network energy consumption and ultimately extend the network life cycle.

In addition, the decision maker can maintain different current network topologies by adopting different maintenance strategies at different stages according to network operation. Whether it is based on role transformation or topology-based topology maintenance technology, the decision maker is also responsible for monitoring the life cycle. That is to say, during the life cycle of the network, the decision maker periodically maintains the network topology according to the maintenance policy, and once the life cycle of the network ends, the decision maker stops the maintenance process and announces the network death.

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