## 11 Summary  
In WSNs, data are routed from one node to another and to the base station by using different routing protocols. There are numerous data routing protocols designed for WSNs. According to the underlying network structure, data routing protocols in WSNs fall into three broad classes known as data-centric, hierarchical and location-based. In data-centric networks, each node has the same role and the duty of each node is transmitting the data packet toward the base station. In hierarchical networks, all the nodes are divided into several groups (clusters) and they have different responsibilities. The low level nodes as cluster members sense and collect data from the surrounding environment and the high level  
Figure 12: Jumping Transmission in DMRF. nodes as cluster heads manage and aggregate the data of their clusters. Location-based protocols using the positional information of the nodes to relay data to some desired regions. Such information can be achieved by some hardware devices such as GPS or by exchanging information between the nodes. On the other hand, most of the data routing protocols in WSNs are designed for a specific goal and aim to achieve one main goal. Also, some of them aim to achieve two or three goals together by considering a trade-off between the goals which can be considered as a new class of data routing as QoS-aware protocols. The main goals of data routing protocols in WSNs can be categorized as energy conservation, fast delivery and fault-tolerance. Each of these goals is addressed with different attitudes. Normally, energy efficiency can be improved by balancing the energy among the nodes or reducing the energy consumption of the nodes. Delay-awareness is provided with two attitudes: hard delay awareness in which on time data delivery has the highest priority or soft delay awareness which aims to route data through the shortest paths. Also, fault tolerant data routing is obtained in two forms: fault prevention to decrease the rate of fault occurrence and fault recovery to repair the failures. Basically, these goals and attitudes are satisfied by proposing various methods or introducing a set of heuristics into the routing algorithms; for example, energy conservation in energy-efficient data routing approaches is achieved by considering the residual energy of the nodes during routing mechanism, selecting the least energy consuming paths through considering some metrics such as minimum number of hops, controlling the transmission ratio, reducing number of packet transmission or by managing the duty cycle carefully; Fast delivery is provided typically by delay-based scheduling of node transmission queues and selecting the routes with minimum delay; Fault tolerance is addressed in the reviewed algorithms by maintaining multiple routing paths and using an available one on-demand, retransmissions or switching the next hops in case of failures. Moreover, each of the data routing protocols in WSNs has some operational characteristics in their routing mechanism which are used to achieve the desired goal. Here, we briefly introduce some of the main operational factors which are effective on data routing mechanisms:  
_Mobility_: In some applications, sensor nodes or the sink(s) can be mobile. When sensor nodes are mobile, they can configure their position to contribute in balancing energy consumption in some regions that contains high traffic load and make the network partitioning easier [76, 77]. Also, when the sink is mobile [51, 66], it can approach the nodes and gather the information of the nodes for balancing energy consumption among the nodes. In this condition, data can be sent periodically or it can be delayed until the sink changes its position to be in a shorter distance with the node; hence, shortening the transmission distance reduces the energy consumption. Of course, it should be noted that deploying mobile sink and sensor nodes increase the deployment of WSN.