## 4 Taxonomy of the Existing Multipath Routing Protocols
### Concurrent Multipath Routing
#### 4.2.1 Multipath Routing Protocols for Reliable Data Transmission
Therefore, at the end of the route discovery process, each source node has discovered a set of partially disjoint paths that can additively satisfy delay and reliability demands of the target application. Figure 8 demonstrates a set of discovered paths using MCMP protocol. According to the structure of the constructed paths, source and intermediate nodes, which have discovered multiple sub-paths towards the sink node, should send several copies of the original data packets to the sink node through different sub-paths to provide reliability. For instance, in Figure 8, node \(G\) should forward two copies of its received data packets towards the sink node through node \(H\) and node \(I\). The introduced data redundancy of MCMP is the main disadvantage of this protocol. Furthermore, since partially disjoint paths are usually located nearby, high data rate transmission causes significant interference. This highly affects the maximum achievable data transmission rate using this protocol. _Energy Constrained Multipath Routing (ECMP)_[64] is the extended version of MCMP to provide energy-efficient communication, while it also satisfies the delay and reliability requirements of each application. In the MCMP protocol described earlier, intermediate nodes select the set of their neighboring nodes that satisfies the delay and reliability requirements of the data source, regardless of the energy consumed for data transmission over individual links. In contrast, ECMP introduces an energy optimization problem. This problem is constrained by delay, reliability and geo-spatial energy consumption to provide multi-constrained QoS routing in sensor networks. Accordingly, the main motivation in designing ECMP is to support multi-constrained QoS routing with minimum energy consumption. To demonstrate this issue, consider Figure 9, where node \(A\) has two neighboring nodes that can equivalently satisfy the delay and reliability requirements of the intended application. As it can be seen from this figure, the distance between node A and node B is shorter than the distance between node \(A\) and node \(C\). Since the required energy for data transmission can be related to the distance between sender and receiver [65], energy consumption for data transmission over link _A_-_B_ is lower than the energy consumption for data transmission over link _A_-_C_. Therefore, selecting node \(B\) as the next-hop neighboring node \(A\) can result in lower energy consumption. However, in MCMP, nodes randomly select their next-hop neighboring nodes without considering the amount of energy consumption over the chosen link. Therefore, compared to MCMP, ECMP refines the set of next-hop nodes to a smaller set through considering the energy efficiency of the links towards the neighboring nodes. According to the experimental results [64], both MCMP and ECMP provide an equivalent delivery ratio and transmission delay. However, ECMP results in lower energy consumption, compared with MCMP. _Delay-Constrained High-Throughput Protocol for Multipath Transmission (DCHT)_[56] is the modified version of Directed Diffusion [14] that propounds the idea of using multipath routing approach to support high-quality video streaming in low-power wireless sensor networks. DCHT introduces a novel path reinforcement method and uses a new routing cost function, which considers  
Figure 8: Partially disjoint paths established by MCMP. Figure 9: Link selection according to the geo-spatial energy consumption constraint. the expected transmission count (ETX) [2] and delay metrics to discover high-quality paths with minimum end-to-end latency. Similar to the Directed Diffusion, routing operation in this protocol is initialized by flooding an _interest_ message throughout the network. Moreover, in order to calculate data transmission latency over each path, sink node adds a timestamp (\(t_{o}\)) to the _interest_ message.