## 3 Data Routing Approaches in WSNs
### Energy-Efficient Approaches
#### 3.1.2 Energy Consumption Reduction
For performing an efficient data delivery, authors considered different costs for the paths; thus, the proposed routing approach selects the least cost path for data transmission from a node to the gateway. Furthermore, the gateway always monitors the energy level of the nodes and when it finds a node with insufficient energy to participate in routing operation, it immediately reroute the packet. The aim of the authors in Hierarchical Geographic Multicast Routing (HGMR) for wireless sensor networks [62] is enhancing efficient data forwarding and increasing the scalability to a large-scale network. HGMR almost incorporates the key design concepts of the Geographic Multicast Routing (GMR) [46] and Hierarchical Rendezvous Point Multicast (HRPM) protocols [63] and optimizes these two routing protocols in the wireless sensor network environment. HGMR, based on GMR, reduces the number of multicast packets from nodes to destination to conserve more energy. Moreover, it reduces the byte overhead of the packet which increases the average energy consumption in GMR; particularly, when the number of multicast members is high. For addressing this problem, it uses the idea of HRMP and divides the large groups of multicast members into multiple subgroups. On the other hand, HRMP is not efficient in packet transmission as it unicasts a same data packet to multiple sub-trees which consumes the energy of the nodes and overloads the bandwidth. Therefore, HRMP uses GMR's cost over progress optimizing broadcast approach in choosing the next relay nodes at each hop. In the proposed approach, based on HRMP idea, the multicast group is divided into some subgroups. Then, the sensing area is again divided into several cells where in each cell there is an Access Point (AP) for managing the members of the cell. Also, there is a Rendezvous Point (RP) which is responsible to all Aps. Thus, when a node decides to transmit its data, it uses HRPM unicast-based forwarding mechanism to transmit its data to its AP along with the GMR idea in selecting the next hop node. The GEAR (Geographic Energy Aware Routing) approach [1] replaces the network communications with limited geographical communications. The main idea of this routing approach is reducing the number of transmissions rather than broadcasting the packets to all the nodes. Also, data transmission toward the destination is performed by considering the cost of data transmission which is based on the energy level and distance to destination. In the proposed scheme, when users propagate their queries to the network, the queries which relates to a specific part of the network can be propagated to that area directly by GEAR algorithm. Thus, instead of broadcasting the message to the whole network, the queries are broadcasted only in their interested area. Therefore, this algorithm leads to an optimal propagation in applications using queries in geographic area level, Figure 4.  
Data forwarding in this approach includes two levels:  
1. Packet transmission to a specific area. In this level, each node upon receiving a packet, forwards the packet to the neighbor which is the nearest one to the destination. In some cases, where all its neighbors are further than itself (which is called hole), the neighbor with the best learning cost will be selected as the next hop. Learning cost is a proportion between remaining energy and distance to target which is considered in GEAR. 2. Second level is broadcasting the packets in the target area. Two modes of broadcasting are considered in this area. Restricted geographic flooding for scattered deployments and recursive flooding which acts by frequent splitting of the area for dense deployments. The goal of authors in [20] when proposing the GAF (Geographical Adaptive Fidelity) approach is reducing the energy consumption by limiting the number of nodes in the routing process. In this approach, a virtual grid is considered in the covered area and each node is assigned a point in the grid structure by using a GPS receiver.