Energy-saving design of wireless sensor network routing protocol

Many people believe that the importance of wireless sensor networks is comparable to that of the Internet: just as the Internet allows computers to access various digital information regardless of where they are stored, sensor networks will expand the ability of people to interact remotely with the real world. . It is even known as a brand new type of computer system, because it is different from the dispersible characteristics of the hardware in the past and the ability of collective analysis. However, in many ways, the current wireless sensor network is like the Internet as far back as 1970, when the Internet was only connected to less than 200 universities and military laboratories, and researchers are still experimenting with various communication protocols and addressing Program. Now, most sensor networks only connect less than 100 nodes, and more nodes and communication lines will make it very complicated and difficult to work properly.

Many types of sensors in wireless sensor networks can detect various phenomena in the surrounding environment including earthquakes, electromagnetics, temperature, humidity, noise, light intensity, pressure, soil composition, size, speed and direction of moving objects. MEMS-based micro-sensing technology and wireless networking technology have given broad application prospects to wireless sensor networks. These potential application areas can be summarized as: military, aviation, anti-terrorism, explosion-proof, disaster relief, environment, medical, health care, home furnishing, industrial, commercial and other fields. The wireless sensor network is a brand-new information acquisition platform, which can monitor and collect information of various detection objects in the network distribution area in real time, and send the information to the gateway node to achieve complex target detection and tracking within the specified range. It has the characteristics of rapid deployment and strong invulnerability, and has broad application prospects.

1 Low power routing protocol

1.1 LEACH Agreement

The full name of LEACH is "Low Energy AdapTIve CluSTering Hierarchy". The basic idea of ​​the algorithm is: randomly select cluster head nodes in a cyclic manner, and distribute the energy load of the entire network to each sensor node evenly, so as to achieve the purpose of reducing network energy consumption and improving the overall network survival time. Simulation shows that, compared with the general planar multi-hop routing protocol and static layered algorithm, LEACH can extend the network life cycle by 15%. LEACH continuously executes the cluster reconstruction process in the loop during the operation, each cluster reconstruction process Can be described by the concept of rounds. Each round can be divided into two phases: the establishment phase of the cluster and the stable phase of transmitting data. In order to save resource overhead, the duration of the stabilization phase is greater than the duration of the setup phase. The process of cluster establishment can be divided into four stages: the selection of cluster head nodes, the broadcast of cluster head nodes, the establishment of cluster head nodes and the generation of scheduling mechanisms.

The initialization phase is the cluster formation phase. In the initial stage of each round, each sensor node must decide whether to act as a cluster head node. This decision mainly depends on the number of cluster head nodes required in the network (set at initialization) and the number of times the node has become a cluster head node so far. The cluster head nodes must be selected from those that have not been cluster head nodes until all the nodes in the network have been cluster head nodes, and then re-election, all nodes get the chance to become cluster head again. The selection method of the cluster head node is: each sensor node randomly selects a value between 0 and 1. If the selected value is less than a certain threshold T (n), then this node becomes a cluster head node. The calculation method of T (n) value is as follows:

Among them, p is the percentage of the number of cluster head nodes in the network, r is the current round number, G is a set, and the nodes in the set are the nodes that have not served as cluster head nodes in the previous 1 / p round. Using this threshold, each node will act as a cluster head node in a 1 / p round operation, and the symbol mod is the modulo operation symbol.

At the 0th round (r = 0), the probability of each node acting as a cluster head node is p, and the node acting as a cluster head node at the 0th round cannot be used as a cluster head node again in the subsequent 1 / p round. In this way, the number of remaining nodes becomes smaller, so the probability of being able to serve as a cluster head node must be increased to ensure that the number of clusters in each round remains balanced. After 1 / p-1 round, T = 1, at this time, any node that has not been a cluster head node in the past 1 / p can become a cluster head node, because the flag values ​​of all nodes are Asked between 0 ~ 1. After 1 / p round, all nodes can act as cluster head nodes again.

Once the cluster head nodes are selected, they use the same energy to broadcast an advertising packet to other nodes in the network. In this process, the receivers of other non-cluster head nodes have been in working state in order to receive advertising packets from different cluster heads. According to the principle of minimum communication energy, they select the source node of the advertising packet with the strongest signal as its own cluster The head node and send a message to its cluster head node to tell the cluster head node that it has joined the cluster.

When the cluster head node receives a "report" message from a member node, it generates a TDMA time slot table based on the number of member nodes, telling the member when it can send data. This table will reach the member nodes through broadcasting. Due to the formation of the cluster structure, the member nodes only communicate with their own cluster head nodes. If they receive messages from other nodes, they will be automatically shielded. Therefore, there is no need to worry that the time slot list of the cluster head node is mistakenly received by members of other clusters. When the cluster in the network has been formed, and the TD-MA time slot table is also determined, data transmission begins. Member nodes can only communicate with cluster head nodes in the time slots allocated to them by the TDMA time slot table. Assuming that the sensor node always has data to send, in its own time slot, the member node will send the data to its own cluster head node. In the sending phase, when their own time slot does not arrive, member nodes can turn off their transceivers to save energy. The cluster head node must always keep its receiver on to receive data from different member nodes. When a round of data transmission is completed, the cluster head node will perform the necessary data fusion processing, fuse multiple data into one data, and then send it to the base station. After a period of time, the network begins to enter the next round of work cycle.

The LEACH protocol uses data compression technology and layered dynamic routing technology to improve the scalability and robustness of the network through local joint work, reduce the amount of data sent through data fusion, and randomly select cluster head nodes to reach the network The purpose of internal load balancing, thereby greatly saving energy.

Although the LEACH protocol has the above advantages, there are some shortcomings.

(1) Because the LEACH algorithm assumes that all nodes can directly communicate with the sink node, and each node has the computing power to support different MAC protocols, this protocol is not suitable for large-scale wireless sensor networks.

(2) The LEACH algorithm allows self-organized clusters to form in the network. Since cluster head nodes are randomly generated, this cannot guarantee a reasonable distribution of cluster head nodes. Therefore, it is very likely that the selected cluster head nodes are concentrated in a certain area in the network, so that there will be no clusters around some nodes.

(3) The LEACH algorithm ignores the imbalance of node energy loss caused by the distribution status of the selected cluster heads in the network and the different communication distances between nodes.

1.2 PEGASIS agreement

This protocol is an improvement of LEACH. The idea is: in order to extend the life cycle of the network, the nodes only need to communicate with their nearest neighbors. The communication process between the nodes and the convergence point is carried out in turn. When all nodes communicate with the convergence point, a new round of rotation is carried out between the nodes. Because this round-robin communication mechanism enables the energy consumption to be uniformly distributed to each node, the energy consumed for the entire transmission is reduced. Unlike LEACH's multi-core structure, PEGASIS (Power-Efficient GAthering in Sensor InformaTION Systems) protocol uses a chain structure in the sensor node to link. When running the PEGASIS protocol, each node first uses the signal strength to measure the distance between all its neighbors. While determining its nearest neighbor, it adjusts the strength of the transmitted signal so that only this neighbor can hear it. Secondly, each node in the chain sends data to the neighboring node to accept it suddenly, and only selects one node as the head of the chain to transmit data to the sink node. The collected data is transmitted and merged in a point-to-point manner and finally sent to the sink node

In the PEGASIS algorithm, the formation of the "chain" is the key to the entire communication. The method used to form the "chain" is: the node sends a test signal of decreasing energy to determine the nearest neighbor to itself by monitoring the response. In this way, all nodes in the network can understand each other's positional relationship and find their own neighbor nodes. In each round of communication, the nodes only need to communicate with their neighbor nodes. To ensure that each node has its neighbors, start from the node furthest away from the base station, the distance of neighbor nodes in the chain will gradually increase, because the nodes already in the chain cannot be accessed again. In turn, a chain containing all the nodes in the network is finally formed.

After the node chain is formed and the leader node is elected, the data transmission process begins. The data transmission in PEGASIS uses a Token mechanism, as shown in Figure 1. Token is very small, so it consumes less energy. In a round of communication, the leader node uses Token to control data transmission from the end of the chain. In the picture, C2 is the leader node, passing Token to C0 along the chain, Co transmitting data to C1, C1 fuse C0 data and its own data to form a data packet of the same length, and then pass it to C2. Then, C2 will Token is passed to C4, C2 receives data from C3, C4 in the same way. After these data are merged at C2, they are sent to the base station.


PEGASIS is a routing protocol established on the basis of LEACH. PEGASIS saves energy compared to LEACH mainly in the following aspects:

(1) In the local data aggregation stage, each node in the PEGASIS algorithm only communicates with its nearest neighbors, instead of communicating with cluster head nodes like the LEACH algorithm, the PEGAS-IS algorithm greatly reduces each round The communication distance of each node in communication reduces the energy consumed by each node in each round of communication.

(2) In the LEACH algorithm, a cluster head needs to receive data sent by multiple cluster member nodes, while in the PEGASIS algorithm, a leader node only needs to receive data packets sent by two nodes at most.

(3) In each round of communication, the PEGASIS algorithm has only one leader node communicating with the base station, while in LEACH there are multiple cluster head nodes communicating with the base station. PEGASIS also has some shortcomings: the node maintains location information (equivalent to the topology information of the traditional network) requires additional resources, which is not suitable when the global network information is difficult to obtain, and the uniqueness of the leader node makes it a The bottleneck of the entire communication process.

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