## 3 Multipath Routing in Wireless Sensor Networks
### Basic Principals in Designing Multipath Routing Protocols
Path length, packet loss rate, delay and residual battery level of the sensor nodes are among the basic components of the routing cost functions utilized by the existing multipath routing protocols. _Path Selection and Traffic Distribution:_ After construction of multiple paths, another important issue that should be addressed is the selection of an adequate number of paths for data transmission purposes. According to the main intention of designing each multipath routing protocol, a certain number of paths should be selected to meet the performance demands of the intended application. Therefore, proposing a perfect path selection mechanism to choose a sufficient number of paths is the most important part of designing a high-performance multipath routing protocol. One protocol may decide to use only the best path for data transmission and keep additional paths as the backup paths for fault-tolerance purposes [12, 32, 55]. In contrast, another multipath routing protocol may utilize several  
Figure 1: Various types of path disjointedness. **(a)** Node-Disjoint Paths; **(b)** Link-Disjoint Paths; **(c)** Partially Disjoint Paths. paths concurrently to provide reliable data transmission or even traffic distribution [56; 57; 58; 59; 60]. Still, the number of selected paths plays an important role to improve different performance parameters. In fact, due to the wireless interference between nearby nodes, using all the constructed paths in single-channel wireless networks does not necessarily provide higher data transmission capacity. Nevertheless, data transmission through a few numbers of paths may not efficiently utilize the underlying resources of dense wireless sensor networks. Once a set of paths are selected among the discovered paths, the multipath routing protocol should determine how to distribute network traffic over the selected paths. Based on the primary motivation behind the design of different multipath routing protocols they may utilize various traffic allocation mechanisms. For instance, transmission reliability can be guaranteed by introducing a certain degree of data redundancy in the data delivery process based on the reliability requirement of the underlying application. After that, the source node will utilize several paths to forward generated network traffic towards the sink node [57]. If the key idea is to improve performance demands such as throughput, data delivery ratio, delay, and lifetime, an efficient load distribution mechanism can be employed to distribute the overall network traffic over the constructed paths [41; 45]. Furthermore, to improve resource utilization over individual paths, the injected traffic rate over each path should be calculated according to the path capacity. _Path Maintenance_: Due to the resource constrains of sensor nodes and high dynamics of low-power wireless links, paths are highly error prone. Therefore, path reconstruction should be provided to reduce performance degradation. This is the main task of the path maintenance phase in multipath routing protocols. Path rediscovery process can be initiated in three different situations: (1) when an active path has failed, (2) when all the active paths have failed or, (3) when a certain number of active paths has failed. Since, the frequency of initiating route rediscovery process in the first approach is higher than for the two other approaches, using this strategy imposes a high overhead. Nevertheless, performing a route rediscovery process after the failure of all the active paths may significantly reduce network performance. Therefore, it seems that the third approach may represent a trade-off between the advantages and disadvantages of the first two approaches.