Abstract: Topology maintenance is essential to the operation of wireless sensor networks. It aims to repair and reconstruct the current topology by rotating node roles, calling topology construction or maintenance algorithms to improve the life cycle of the network. Firstly, the topology maintenance is defined, the design goal of topology maintenance is described, and a general model of topology maintenance is designed. Then it elaborated the research progress of topology maintenance technology and compared and analyzed the representative algorithms. 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, biomedicine, emergency rescue and other fields due to their low power consumption, low cost, distributed and self-organizing characteristics. Generally, a wireless sensor network consists of hundreds or thousands of sensor nodes, each of which has the ability to perceive the current environment, communicate with neighboring nodes through broadcasting, and perform local calculations on the collected information. However, these capabilities are very limited for each node, especially the node's energy limitation severely limits the life cycle of the network, thereby affecting the network's service quality and further applications. Therefore, in recent years, many researchers have conducted 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 in wireless sensor networks, topology control usually changes, simplifies, or optimizes the topology of the network to save energy 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 usually regarded as a single process, which does not include the maintenance of network topology, which affects the classification of topology control algorithms. The current classification is limited to how to construct the network topology, and ignores the topology maintenance in topology control.
Although the topology maintenance is simply defined, and the topology optimization technology is divided into static, dynamic, and hybrid topology maintenance according to the target optimization topology construction time. However, there is no systematic elaboration of topology maintenance, and the definition of topology maintenance is not rigorous. The classification of topology maintenance technology is also inconsistent with the current research status, because there is basically no static and hybrid mentioned in the article in the existing research. Topology maintenance algorithm or protocol. Therefore, in order to study the topology maintenance technology in the wireless sensor network more deeply, this article has carried on the topology maintenance from the aspects of topology maintenance definition and model, topology maintenance design goals, and current research status and existing problems and development directions. set forth. Section 1 describes the basics of wireless sensor network topology maintenance, mainly gives a new definition of topology maintenance, and points out the design goals of topology maintenance. Section 2 designs a general model for topology maintenance, and describes the triggering standards and maintenance strategies in the model in detail. Section 3 summarizes the current research work on topology maintenance and makes a comparative analysis. Section 4 analyzes the shortcomings in the current research and points out the development direction of the topology maintenance technology. Finally, the full text is summarized.
1 Basics of topology maintenance
Wireless sensor network topology control consists of two parts, namely topology construction and topology maintenance. Once the initial network optimization topology is established, the network begins to perform the tasks it specifies. Since every behavior included in the network task, such as sensing, data processing, and transmission, consume energy, over time, the current network topology is no longer in the optimal operating state, so it needs to be maintained to maintain Remain optimal or close to optimal state.
1.1 Definition of topology maintenance
The topology control of the wireless sensor network can be seen as a repeated process, as shown in Figure 1. First, there is a topology initialization phase for all wireless sensor networks. At this stage, each node transmits with its maximum transmit power to establish the initial topology. After the initialization phase, the initial topology is optimized by running different algorithms or protocols, and finally an optimized topology is constructed. This phase is called topology construction. Once the optimized network topology is established in the topology construction phase, the topology maintenance phase must begin to work.
In the topology maintenance phase, the current topology status is monitored in real time, and the topology restoration or reconstruction process is triggered when appropriate. As can be seen from Figure 1, during the life cycle of the network, the topology maintenance cycle runs until the network dies. At present, there are few literatures on the definition of topology maintenance, and literature  has a simple definition of topology maintenance, pointing out that "topology maintenance refers to when the topology of the network currently working is not the most optimized topology, timely repair and switch Or reconstruct a new network topology, so that the network reaches a predetermined nature and extend the life of the network. "
The definition does not indicate the time and operation of the topology maintenance operation. In particular, the definition mentions that the topology is at or near optimal and reaches the predetermined nature, but it does not indicate which specific stage is optimal or nature. Because with the operation of the network, the optimal state and nature of the network are also changing. Therefore, this article has a strict definition of topology maintenance, that is, topology maintenance is a periodic process. In each cycle, it is triggered by different trigger criteria (such as time, energy, node failure, etc.). Rotate the node role or re-run the topology construction process or call a special 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 life cycle of the network .
1.2 Design goals
Topology maintenance is the same as other sensor network technologies, and its main purpose is to extend the life cycle of the network. In addition, sensor networks are constructed to perform certain tasks, such as performing sensing and transmitting sensory data, so one or more quality of service goals such as maintaining sensor coverage and maintaining network connectivity are also commonly considered.
Moreover, different applications of wireless sensor networks lead to different topological maintenance design goals or different priorities of the underlying network. Therefore, this article only introduces the main design goals for topology maintenance.
(1) Network life cycle
The network life cycle has been defined in different ways, such as based on the number of nodes, based on sensor coverage and network connectivity and scalable network life cycle.
Topology maintenance is a very effective technology to extend the network life cycle. 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 working state. However, maximizing the life cycle of the network is a very complex issue, and it has always been the main goal of topology maintenance research.
(2) Coverage and connectivity
Coverage and connectivity are the basic problems of wireless sensor network topology maintenance. During the process of restoration, switching or reconstruction of the original optimized topology, the topology maintenance must maintain the coverage or connectivity of the original topology.
(3) Safety and fault tolerance
During topology maintenance, some sensor nodes may fail or fail due to energy exhaustion, physical damage, or environmental interference. The failure of these sensor nodes does not affect the overall task of topology maintenance. As mentioned in , a fault-tolerant self-organizing method is proposed 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 . Therefore, security is also an important goal of topology maintenance.
(4) Energy efficiency and convergence time
As with other functions of wireless sensor networks, topology maintenance algorithms must be energy efficient. In other words, the topology maintenance algorithm should have low computational complexity and low message overhead. In addition, during the topology maintenance process, the current topology will be replaced by a new topology, so there is a transition time between the activation of the new topology, which should be as small as possible.
(5) Energy balance and scalability
The topology maintenance technology should try to evenly distribute energy consumption among all nodes of the network. In addition, sensor nodes deployed in areas of interest or target may be hundreds to thousands or even tens of thousands. The topology maintenance protocol or algorithm should be able to run in a network of nodes 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 and characteristics of topology maintenance, 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 with the current topology of the network, generates an optimized topology through the topology maintenance process, and runs periodically until the network dies.
As can be seen from the above figure, each topology maintenance cycle is via triggers and decision makers.
Among them, the trigger mainly triggers the topology maintenance process according to the designed trigger criteria such as time, energy or node failure. The decider is used to select the topology maintenance strategy.
Next, the model will be described in detail.
The trigger is responsible for periodically triggering the maintenance process of the current network topology, which has an important impact on the performance of topology maintenance. Because if triggered in advance, unnecessary energy will be consumed due to frequent running of topology maintenance protocols or algorithms, and delayed triggering will cause the network to run in a sub-optimal or even disconnected state, reducing or even failing to achieve the quality of service of the network. Common triggering standards are:
Time: Topology maintenance is triggered after the network runs for a period of time. The size of this time is usually fixed and predefined, and is usually completed by a timer.
SPAN triggers the update process of the coordinator nodes in the network based on time, so as to realize the topology maintenance of the backbone network.
Energy: Given the energy limitations of wireless sensor devices, it is necessary to trigger topology maintenance when the energy level of a node is below a certain threshold. In the LPH algorithm, when the node's remaining energy E (i) is lower than the average remaining energy Eavr, the topology maintenance process within the cluster is triggered. In the CLTC algorithm, when the energy of the cluster head node drops to the threshold value M, the topology maintenance process in 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: When one or some nodes in the network fail, topology maintenance is triggered. For example, in the SMSS algorithm, when node u finds that a certain node m fails, it will check whether m is its determined neighbor, and if so, re-run 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: Use the node degree of the network or the node degree of some important nodes to trigger the topology maintenance process. The adaptive energy-saving algorithm proposed by AFECA uses neighbor density to trigger the topology maintenance process.
In addition, these trigger conditions can also be used in any combination to trigger the topology maintenance process, such as energy and node failure, or time and energy. In addition, other network parameters can also be used as trigger criteria, such as link failure, frequent packet loss, and congestion and long routing paths.
(2) Decision maker
The decider mainly determines which strategy is adopted to maintain the current network topology, which is the core of topology maintenance. The topology maintenance strategy can be divided into two types. One is the topology maintenance strategy based on role rotation, which means that by switching the roles of nodes in the network-such as sleep / work, cluster head / non-cluster head, etc., to save energy and achieve extension The purpose of the network life cycle. The other is a topology maintenance strategy based on topology reconstruction. Its essence is to run the algorithm in the topology construction stage or special topology maintenance algorithms and protocols to maintain the network topology.
In the topology maintenance strategy based on role rotation, we must first clarify the role that each node in the network can play. The role migration of each node is closely related to the characteristics and design of the topology maintenance protocol or algorithm. The factors that determine the role of the node include node density, location, communication traffic, packet loss rate, time, and external environmental conditions. If the node is currently in role 1, when an event occurs, the node performs a corresponding test to decide whether to enter role 2 or continue to be in role 1.
In the topology maintenance strategy based on topology reconstruction, the algorithm in the topology construction stage or the special topology maintenance algorithm is called again. Therefore, the frequency of calling the algorithm is the key. Once the trigger triggers the topology maintenance process, the topology maintenance strategy should comprehensively consider the related performance of the network and decide whether to call related algorithms or protocols to balance the network energy consumption and ultimately extend the network life cycle.
In addition, the decider can also use different maintenance strategies to maintain the current network topology at different stages according to the network operation. Whether it is a topology maintenance technology based on role switching or topology reconstruction, the decision maker is also responsible for monitoring the life cycle. That is to say, in the life cycle of the network, the decider periodically maintains the network topology according to the maintenance strategy, and once the life cycle of the network ends, the decider stops the maintenance process and declares the network dead.
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