## 4 Lifetime Improvement Mechanisms in Routing
### Mobile Sink Mechanism
#### 4.2.3 Controlled Sink Mobility  
Controlled sink mobility is another method for lifetime enhancement which is achieved from network feedbacks. The mechanism proposed by [PERSON] _et al._[10], for example, controls the multiple mobile sinks simultaneously by using a distributed heuristic approach applied in WSNs. At network set up, the entire field will be partitioned into several sites. After that, each sink node starts the training phase by flooding a request packet in which it asks all sensor nodes within the site to send back a test packet. It includes the information about the path between the sender node and the sink at corresponding site. Finally, they should share the collected information among themselves. By investigating the test packets, a sink node can estimate the incoming traffic to each relay node \(q\) when the source node \(p\) is generating data packets and sending them to the sink. On the other hand, the sensor nodes can also compute the distance from the sink and encapsulate this information in the test packet during the training phase. At the end of this phase, each trained sink selects a particular site and  
Figure 4: Joint routing mechanism and sink mobility [9]. announces its current position to all sensor nodes. In this way, the nodes can set up routes towards the sink. During data transmission, the nodes are responsible for reporting their residual energy to the sink. The data packets carry the information about nodes' power level to the closest sink by employing a piggyback mechanism. A sink is also able to calculate the data flow rate received from each sensor node and share the mentioned information with the other mobile sinks. Each sink periodically decides whether to relocate or not. In order to de-synchronize sink decisions, it waits for a random time before any movement. After this period of time, the sink calculates the expected lifetime improvement through relocating to an unoccupied site according to the available information about the condition of the network. If the sink node recognizes that repositioning to a new site can enhance the network lifetime more than a specific threshold, it informs the current source nodes that it is on the move and then shuts down the transmission. Then it selects the site, repositioning to which causes the maximum lifetime and informs the other sinks of its relocation decision. After that, it starts to move to the new site if the other sinks are static at the moment. As soon as it reaches the new site, it activates itself by sending a routing packet to the nearby sensor nodes in order to encourage them to construct new routes. The authors also proposed a stochastic mobility pattern in which the decision for site selection is completely random. One of the benefits of this algorithm is that the sensor nodes have at least one active sink as a destination for their packets at any time. Since the source nodes do not need to buffer packets during sink movement, the end-to-end delay will be decreased remarkably. However, the sensor nodes have to consume additional energy for receiving availability and unavailability message which are broadcasted by sink nodes [10]. The simulation results indicate that this distributed heuristic can significantly increase the lifetime, with a gain of +77.7% in comparison with random sink mobility and on the order of +382.4% compared to a static sink approach [10]. _Mobile Sink Based Routing Protocol (MSRP)_[11], which was proposed by [PERSON] and [PERSON], is an energy-efficient hierarchical protocol that prolongs the network lifetime by employing a controlled sink mobility mechanism. In this algorithm, the sensed data will be collected from the cluster heads (CHs) by a single mobile sink which moves in the vicinity of CHs. The information about the residual energy of the CHs is also collected by mobile sink during data gathering. Based on this information, the mobile sink moves to the CHs that have higher energy levels.