## 2 Routing in Wireless Sensor Networks
Several routing protocols such as Threshold-Sensitive Energy-Efficient Sensor Network Protocol (TEEN) [19], Adaptive Periodic Threshold-Sensitive Energy-Efficient Sensor Network Protocol (APTEEN) [20], Low-Energy Adaptive Clustering Hierarchy (LEACH) [21], Power-Efficient Gathering in Sensor Information Systems (PEGASIS) [22] and Two Tire Data Dissemination (TTDD) [23] fall in this category. Routing protocols in the last group utilize the exact location of the sensor nodes to make routing decisions [24,25]. The geographic locations of sensor nodes can be obtained directly using Global Positioning System (GPS) devices or indirectly by exchanging some information regarding to the signal strengths received at each node. However, since localization support requires specific hardware components and imposes significant computational overhead to the sensor nodes, this approach cannot be easily used in resource-constrained wireless sensor networks. Geographic and Energy-Aware Routing (GEAR) [24] and Geographic Adaptive Fidelity (GAF) [26] can be referred as the geographic routing protocols. From the protocol operation perspective, all the existing routing protocols in the aforementioned categories can be further classified into negotiation-based, query-based, QoS-based, multipath-based and coherent-based protocols. The key idea in the design of negotiation-based routing protocols is to provide energy-efficient communication by reducing data redundancy during data transmission. In these protocols (e.g., SPIN family of protocols [15]), each sensor node adds a high-level data description to its collected data and performs some negotiations with its neighboring nodes to eliminate duplicated data packets. In the query-based routing protocols, a sink node propagates a query message throughout the network regarding the desired sensing task. If a node senses any related information, it sends back its collected data towards the sink node through the reverse path. Directed Diffusion [14] and Rumor Routing [16] are two examples of the primary query-based routing protocols. The third group of routing protocols (_i.e._, QoS-based routing protocols) is mainly designed to satisfy QoS demands of different applications (e.g., delay, reliability, and bandwidth). The main aim of these approaches is to establish a trade-off between energy consumption and data quality. Sequence Assignment Routing (SAR) protocol [27], SPEED [28], Multipath Multi-SPEED (MMSPEED) [29], Energy-aware QoS Routing Protocol [30] and Delay-minimum Energy-aware Routing Protocol (DERP) [31] can be considered as the QoS-aware routing protocols. In contrast with single-path routing techniques, multipath routing protocols allow each source node to find multiple paths towards the sink node to improve network performance. There are several protocols in this category such as Braided Multipath Routing [32] and N-to-1 Multipath Routing Protocol [33]. Since, in wireless sensor networks, all the network nodes cooperatively process the flooded data in the network, the last group of routing algorithms is dedicated to the coherent data processing-based routing protocols. In this group, data packets are sent to the aggregators in order to reduce data redundancy. Therefore, energy efficiency is the main purpose of these routing protocols. Routing protocols such as Directed Diffusion [14], SPIN [15] and SAR [27] utilize data aggregation and can be categorized as the coherent data processing-based routing protocols. From this point onward, we focus on the multipath routing approach and the related issues that should be considered in the design of these protocols for wireless sensor networks.