## 4 Lifetime Improvement Mechanisms in Routing
### Power Control Mechanism  
Since sensor nodes are power-constrained and data transmission consumes a significant amount of energy, power control mechanisms can provide an energy efficient scheme with high link quality for wireless sensor networks [19]. Although the ability to vary the transmission power is in the hands of the physical layer, it is a key tool in combination with the upper layer protocols such as MAC and routing algorithms to prolong sensors' lifetime [18]. Some power control techniques proposed for routings in WSNs are discussed as follows. [PERSON] _et al._[17] presented an adaptive transmitter power approach in order to minimize the overall energy needed for sensor-to-sink communications. Their solution is based on a _Broadcast-On-Update_ (BOU) method. According to this novel model, a configuration packet is used to adjust in-network nodes' transmitter power level. This packet presents the cost, the identity and the location of the node that sends it. First of all, a configuration packet is sent by the sink with a cost value of zero. Each node that receives the packet should update its node cost parameter and rebroadcast it with the new value of node cost. Then it has to calculate its own transmitter power so that it will be able to reach the neighboring node where it recently receives the configuration packet. After computing the edge cost between this node and its neighbor, the sum of the neighboring node cost and the mentioned edge cost totally in the name of path cost should be compared with the current node cost. If it is smaller, it should be assigned to the current node as its new node cost. Otherwise, the packet is dropped. At the end, the node rebroadcast the configuration packet which includes the updated node cost and its location. Since explosive broadcasting is the main drawback of the BOU protocol, the authors presented the _BOU-WA_ scheme to overcome this problem. Based on this method, the node should wait before broadcasting according to a proportional probability in order to find a better path to the sink from the energy-efficiency point of view. The simulation results [17] indicate that when the waiting time before broadcasting is increased up to a specific threshold, the energy consumption of BOU-WA will be improved by a rate of 350% more than the BOU scheme. However, the execution cost due to the complexity of computing the waiting time is the main drawback of BOU-WA algorithm. _Adaptive Transmission Power Control (ATPC)_[18] is another energy-efficient algorithm which is proposed to apply in several routing schemes. In this protocol, a model is built for all neighbors by each node that describes the correlation between link quality and power transmission. In order to maintain individual link quality over time dynamically, the proposed model applies a feedback-based transmission power control scheme. The authors believe that the Received Signal Strength Indicator (RSSI) and Link Quality Indicator (LQI) [44] are two suitable parameters in order to calculate the optimal transmission power level. According to this algorithm, each node proceeds to broadcast a beacon in the initialization phase but at different transmission power levels. After that, the neighboring nodes should calculate RSSI/LQI values related to these beacons and a notification packet has to send these values back to the beaconing node. Now it can use the least square approximation technique in order to specify the optimal transmission power level. Based on the feedback mechanism, the transmission power level is adjusted in the runtime tuning phase. Although ATPC could predict the proper transmission power level accurately and reach an acceptable link quality over time, it still suffers from the initialization phase overhead. One of the other drawbacks of ATPC is that the performance of the algorithm may negatively affected by conflicting interferences and transmissions [18]. Furthermore, this protocol has the execution cost for calculating the RSSI/LQI parameters.