# Comparative Study of Energy Efficient Routing Techniques in Wireless Sensor Networks
## 3 The Proposed Taxonomy
### Network Topology (Structure)
#### 3.6.1 Hierarchical (Cluster-Based)  
Hierarchical schemas define a specifically structured topology. They divide the sensor nodes into many groups called clusters, with a specially selected node in each group called the cluster head (CH). Those CHs coordinate and communicate directly between them or with the BS, as shown in Figure 3. Hierarchical routing protocols use several strategies to select the CH, e.g., the highest energy node or the one having the maximum number of neighbors in a cluster can be selected as CH. Hierarchical routing is an energyefficient method aiming to maximize the network lifespan and to ensure network scalability through the hierarchical structure. These protocols can be classified into block, grid and chain cluster-based protocols. Block Cluster-Based  
These protocols divide and arrange nodes in a virtual block, then setup a networking strategy to manage communication in and between blocks. Among the representative protocols of this class, we find:  
* **Chain Cluster-based Mixed (CCM)** CCM takes advantage of LEACH [61] and PEGASIS [62] on which it is based. It uses a CH selection algorithm based on the remaining energy without considering its distance to the BS. This protocol organizes the sensor into a vertical cluster and horizontal chains and transmits packets in the two stages, namely chain routing and then cluster routing. * **LEACH with Virtual Force (LEACH-VF)** LEACH-VF combines the LEACH algorithm with the two types of virtual forces, which are the attractive force, used to move the nodes towards the cluster head, and the repulsive force used to move the overlapping nodes away from each other [63]. Simulations show that this strategy maximizes the coverage area and solves the difficulty of sensing holes, but energy efficiency remains very limited. * **Hierarchical Clustering-based routing algorithm with Two cluster heads in each cluster for Energy balancing (HCTE)** HCTE selects two CHs in each cluster and routes data packets from the CHs to the BS using a multi-hop mechanism to enhance the network lifespan. Each CH is responsible for different tasks in the cluster. Simulations show that HCTE avoids unbalanced energy consumption, but it does not achieve energy efficiency [64]. Grid Cluster-Based  
These protocols divide the network into grids and arrange nodes accordingly. Then, they setup their own networking strategy to manage network communications. Among the representative protocols of this class, we quote:  
* **Hierarchical Geographic Multicast Routing (HGMR)** This protocol is a location-based multicast scheme [65]. It takes advantages of the geographic multicast routing (GMR) [66] to route packets along the multicast tree in one transmission, and it uses the hierarchical rendezvous point multicast (HRPM) [67] to decompose the multicast group into subgroups, which easily overcomes the problem of network scalability, but research shows that the used network distribution can affect the  
Figure 3: Hierarchical routing model. selection of the best routing paths which minimizes the effectiveness of routing task of HGMR. * **SLGC** SLGC is designed as a grid clustering algorithm. The CH of each cluster uses a single-method SLGC algorithm to send data queries to the BS. Research shows that this routing strategy makes it highly energy-efficient, but it overloads the network and incurs extra overhead in various cases such as complex data transmission [68]. These protocols build chains of nodes with a well-defined strategy to manage communication between sensor nodes. Among the representative protocols of chain cluster-based class, we quote:  
* **Concentric Clustering Scheme (CCS)** CCS represents an extension of PEGASIS [69]. In fact, it reduces energy consumption and takes into account the BS location in order to improve the network lifespan and improve its performance. The CCS divides the sensing area into concentric circular tracks assigned with various levels and constructs chains inside the track similar to PEGASIS.