Chapter 2 Background
2.3 Related Work
2.3.1 LEACH
LEACH is a self-organizing, adaptive clustering protocol that uses randomization to distribute the energy load among the sensor in the WSN. The nodes organize themselves into clusters, with one cluster head to manage the cluster. The operation of LEACH is broken up into rounds. Each round begins with set-up phase for clustering the nodes and the following steady-state phase for data transferring to the base station.
During set-up phase of LEACH, each node decides whether or not to become a cluster-head node for this current round. The decision of cluster head is based on the percentage of cluster heads for the network and the number of times the node has been a cluster head so far. The node chooses a random number between 0 and 1. If the number is less than the threshold T(n), the node becomes a cluster head for the current round. The threshold is set in Equation:
𝑇(𝑛) = {
𝑃
1−𝑃×(𝑟 𝑚𝑜𝑑1𝑝) 𝑖𝑓 𝑛 ∈ 𝐺 0 𝑜𝑡ℎ𝑒𝑟𝑤𝑖𝑠𝑒
where P is the desired percentage of cluster heads, r is the current round, and G is the set of nodes that have not been cluster heads in the previous 1
𝑃 rounds. The nodes that are cluster heads in round 0 cannot be cluster heads for the next 1
𝑃 rounds.
Each node that has elected itself a cluster head for the current round broadcasts an advertisement message to the non-cluster-head nodes. The non-cluster-head nodes make the decisions of selecting cluster head with the RSSIs of the received advertisements. And then the non-cluster-head nodes decide which cluster it belongs and send join request to its own cluster head. When cluster head receives join request,
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it starts to create TDMA schedule according to the received join requests. Then, the cluster-head node sends the TDMA schedule to tell its members when they can send their data individually. Once the cluster is created and TDMA schedule is fixed. Then the operation of steady-state phase begins. The non-cluster-head nodes start to transmit the sensed data to its belonging cluster head. Then the cluster head aggregates all the data it received and transmits the aggregated data to the base station.
But in the method of LEACH, there are three problems, one is the random cluster head election and the other one is the number of cluster in each round and the last one is the issue about bandwidth. Because the cluster election doesn’t take the residual of energy into account, the distribution of energy consuming is not evenly while the node acts as cluster head again and again. Moreover, the unfixed number of cluster heads affects the energy consumption while too many or too little cluster heads is not good for WSN. Finally, choose the node with higher bandwidth will increase the system throughput.
2.3.2 LEACH-C
LEACH-centralized (LEACH-C) is a protocol that uses a centralized clustering algorithm and the same steady-state phase as LEACH. LEACH-C uses a central among all the nodes. To complete this job, the base station computes the average node energy, and whichever nodes have energy below this average energy can’t be cluster
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heads for the current round. LEACH-C attempts to minimize the amount of energy for the non-cluster-head nodes to transmit their data to the cluster head, by minimizing the total sum of squared distances between all the non-cluster-head nodes and the closest cluster head. Once the cluster heads and associated clusters are found, the base station broadcasts a message containing the cluster head ID and corresponding TDMA schedule for each node. The steady-state phase of LEACH-C is identical to LEACH.
To avoid reusing the node as cluster head frequently, LEACH-C considers the remaining energy in each node and selects the nodes that its energy level is bigger than the threshold. LEACH-C takes a global view of whole WSN, so every node needs to communicate with the base station. But the initial energy of each node is different in heterogeneous WSN. If focusing on residual energy and not considering the original energy, the performance of energy balancing may not be better. So in our work, we will add the consideration of original energy of a node to balance the load of energy.
But there are some shortcomings for LEACH-C, the cost of communicating with the base station for each round is high where the base station is far from the WSN,
SEP (Stable Election Protocol) is improved from LEACH and implemented for heterogeneous WSN. SEP is proposed for a two-level heterogeneous WSN, which contains two types of nodes according to the initial energy, i.e., advance nodes and
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normal nodes. In SEP, each advance node is equipped with α times additional energy than normal nodes and the number of advance node occupies m fraction of the total number of nodes. SEP assigns two different weighted election probabilities, one for normal and other for advance nodes to select cluster heads. Cluster heads selecting probability of normal nodes and advance nodes are shown in Equation 2.5 and Equation 2.6, respectively. The two equations are shown below,
𝑝𝑛𝑟𝑚 = 𝑝𝑜𝑝𝑡
1+𝛼∙𝑚
(2.5)
𝑝𝑎𝑑𝑣 = 𝑝𝑜𝑝𝑡
1+𝛼∙𝑚 × (1 + 𝛼) (2.6)
Popt is the optimal probability of each node to become cluster head. The probabilities of cluster head in LEACH are replaced by Pnrm and Padv. The idea is that the advance nodes have to become the cluster heads more often than normal nodes. Cluster heads election in SEP is an extension of LEACH. SEP selects the probability of being cluser heads from initial energy value of normal and advance nodes. But it is obvious that after few rounds, a normal node might have more energy than the advance nodes.
2.3.4 EDCS
EDCS (Efficient and Dynamic Clustering Scheme) focuses on the heterogeneous multi-level WSN. EDCS assumes that each sensor node is equipped with a different initial energy. The process of cluster head election for a given multi-level heterogeneous WSN in EDCS, it determines the probability of node to be a cluster head through average network residual energy estimation in next round by average energy consumption forecast in ideal state and references the values of historical energy consumption simultaneously. The probability for cluster head election is related to the initial energy, current remaining energy, and average energy of nodes.
EDCS is proposed for guaranteeing that high-energy nodes have more chances to be cluster head than low-energy nodes, so it keeps energy consumption of each node
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balanced as much as possible.
2.3.5 EEPCA
EECAP is an Effective Energy Prediction Clustering Algorithm for heterogeneous WSN. In EECPA, each node independently selects itself as the cluster head node based on energy factor and communication cost factor, which leads to the probability of cluster head election related to node’s current residual energy and average communication cost after being selected, i.e., the probability of cluster head election is directly related energy and communication cost. In order to save energy consumed by broadcasting energy information in each round of nodes clustering, EECPA is established for nodes whose data collection is of regularity in time interval and message length. Considering the changes in networks environment and errors between calculated and actual node energy consumption, if the difference between the residual energy and the predicted value is within a certain range, the node doesn’t need to broadcast its energy information.
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