# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 4 Topology
### Location Based
During initialization, all nodes are homogeneous with a unique ID and Global Positioning System (GPS) location information, while the network is divided into zones. Then, during cluster head election, which is repeated in every round, nodes wait for a random time to receive messages and, in case they do not, they send a message to their neighbors requesting to be elected as cluster heads. This message contains their zone number and location information. After that, nodes that receive the request compare them with their own information and discard it, if it contains a number of a different zone or else they send a response with their residual energy. Candidate cluster heads wait for a predefined time (T) to gather all messages and are elected as cluster heads. To prevent increased energy consumption during data transmission, the multi-hop routes are optimized, while cluster heads choose the node with the highest residual energy for their next hop. In this way, reduction of energy consumption is achieved. The disadvantage of PASC is that nodes closer to the base station die earlier. _PASC-Ant Colony Optimization_ (PASC-ACO) [128] is an improved version of PASC protocol that forms clusters with the same mechanism as in PASC and utilizes the Ant Colony Optimization (ACO) algorithm, to route data to the base station, through the optimal path, during packet transmission. The use of PASC-ACO prolongs network lifetime and achieves higher efficiency in dense network areas. On the other hand, the use of GPS increases energy consumption. _PASC-Adaptive Redeployment_ (PASC-AR) [129] is another improved version of PASC protocol, which supports the same clustering mechanism that is used in PASC combined with Adaptive Redeployment (AR). During data transmission, the redundant mobile nodes are moved via AR in a sequential pattern from dense to sparse zones, in order to prevent nodes to sense similar areas. By using PASC-AR, network lifetime is prolonged and energy holes are covered. However, the use of mobility increases energy consumption. _Dynamic Energy Efficient Latency Improving Protocol_ (DEELIP) [130] is a protocol that utilizes Distance Metric (DM) to measure nodes distance and divides the network in parent-child hops in order to reduce transmission delay. DEELIP does not form conventional clusters. Instead, every node has parent and children nodes, in the next and the previous hops, respectively. During data transmission, the DM is used to measure the actual air distance for the next hop nodes instead of counting hops, while parent nodes are selected based on their least distance to the sending nodes. The base station broadcasts only once a packet containing its location information, so that receiving nodes can compare their distance from it. The use of DEELIP extends network lifetime, and achieves both low latency and improved stability. Yet, nodes are considered to be static during data gathering. _Distance Routing Effect Algorithm for Mobility_ (DREAM) [131] is protocol with mobile nodes that maintain a location table for every node in the network. Every node transmits its location frequently to nearby nodes and less frequently to faraway nodes in order to save resources. When a node needs to transmit a data packet to a faraway node, it first sends the packet to a nearby node which is located at the direction of destination node. Next, the receiving node forwards the message to a nearby node which is located at the direction of the destination. This procedure is repeated until destination node receives the packet. By using DREAM, efficient transmission of data packets is achieved and the end to end delay is kept small. The disadvantage of DREAM is that there is waste of network bandwidth due to node location transmission. A synopsis of the main characteristics of the aforementioned topology protocols belonging to the location based subcategory is presented in Table 10.