# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 4 Topology
### Location Based
By using HGR, both notable energy savings and a high delivery ratio are achieved. Additionally, reduction of the end to end delay is possible, but not guaranteed. Minimum Energy Communication NetworkMENN (MEN) [124] sets up and maintains a minimum energy network for wireless sensor nodes using low power GPS. Although this protocol is used for mobile networks too, it is best suited to WSNs without mobility. It assumes a master site as the information the base station has in sensor networks. MECN identifies a relay region for each node consisting of sensor nodes in a surrounding area where data transmission via those is more efficient that direct communication. Furthermore, MECN manages to find a subnet with a lower number of nodes that will need less energy to transmit between two specific nodes. With this method, it finds global minimum energy paths without the need to consider all the nodes of the entire network. This is achieved with the use of a localized search for every node considering its relay region. MECN has two phases:  
1. First phase: The construction of a sparse graph begins by taking positions of a two-dimensional plane, consisting of the enclosures of each transmit node in the graph. This construction process needs local computations in nodes and the graph contains globally energy efficient links. 2. Second phase: It finds the optimal links within the enclosure graph and uses the distributed Bellman-Ford shortest path algorithm with the cost metric of power consumption. In case of mobile nodes, the position is tracked using GPS. MECN achieves energy saving and fault tolerance. However, link maintenance consumes energy and adds overhead. _Small MECN_ (SMECN) [125] is an extension of MECN which considers obstacles between two nodes while the network is still assumed to be fully connected as in MECN. SMECN constructs a subnet like the one in MECN, which is smaller in terms of number of edges and uses the minimum energy property. According to this property for any pair of nodes in a graph associated with the sensor network, there is a path, named minimum energy-efficient path, that has the smallest cost regarding energy consumption over all possible paths between this pair of nodes. SMECN saves more energy than MECN. Moreover, both link maintenance cost and number of hops are reduced. Nevertheless, additional overhead is caused. _Geographic Adaptive Fidelity_ (GAF) [126] is a routing protocol primarily designed for mobile Ad Hoc networks but can be applied to WSNs too. It constructs a virtual grid with points that are associated with the GPS location of nodes. Nodes that are associated with the same point have equivalent energy cost for routing packets. Using this grid, unnecessary nodes can be turned off keeping the level of routing fidelity intact and increasing the network lifetime. GAF defines the following three states:  
1. Discovery state: to determine neighbor nodes in the grid. 2. Active state: reflecting participation in routing. 3. Sleep state: when radio circuits are turned off. Node states and the related parameters are application dependent. GAF can handle mobility nodes with every sensor node estimating the time it leaves the grid, sending this information to its neighbors. Sleeping nodes, to keep routing fidelity, can adjust sleeping times and wake up before the leaving time of active nodes expire. In this way, considerable energy efficiency is attained. Additionally, GAF supports both mobility and non-mobility nodes and keeps the network connected by keeping a representative node, in a region of virtual grid, active. On the other hand, nodes neither aggregate nor merge data. _Power Aware Scheduling and Clustering algorithm_ (PASC) [127] is a clustering protocol for homogeneous large scale WSNs with multi-hop routing that combines neighbor nodes in clusters or zones to reduce similar data sensed from neighboring regions and keeps only the cluster heads active by turning some sensor nodes off.