## 4 Taxonomy of the Existing Multipath Routing Protocols
### Concurrent Multipath Routing
#### 4.2.1 Multipath Routing Protocols for Reliable Data Transmission
Nevertheless, using such a simple flooding strategy cannot result in constructing high-quality paths with minimum interference. According to the operation of this protocol, all the constructed paths are located in physical proximity of each other and concurrent data transmission over these paths may reduce the network performance. _H-SPREAD_[61] combines the introduced path construction process in N-to-1 Multipath Routing Protocol with a hybrid data transmission technique to improve reliability and security of data transmission in wireless sensor networks. H-SPEAD takes advantages of a _threshold secret sharing_ scheme and path diversity of multipath data forwarding to increase path resilient against node failure or compromised paths. According to the security property of the _threshold secret sharing_ scheme, data packets can be safely forwarded towards the sink node even when a small number of nodes or paths have failed or are compromised during the data transmission process. In this algorithm, the source node divides each data packet to the multiple shares, _M1_, _M2_, _M3_,..., _Mn_, through using the secret sharing strategy and then transmits them towards the sink node through different paths. Based on the special characteristics of the _threshold secret sharing_ mechanism, even when a certain number of paths have failed due to link or node failures, the original message can still be retrieved via other received shares at the destination node. However, since this approach utilizes the N-to-1 multipath routing algorithm to construct multiple paths, this protocol may suffer from the effects of wireless interference. Therefore, high packet loss ratio caused by interference can reduce the probability of successful packet retrieval at the sink node. Moreover, H-SPREAD only improves reliability and security of data delivery in the network, but it cannot enhance security of individual nodes. _Multipath Multispeed Protocol (MMSPEED)_[29] is designed based on the cross-layer design approach between network and MAC layer to provide QoS differentiation in terms of reliability and timeliness. From a timeliness perspective, MMSPEED extends the SPEED protocol [28] through introducing multiple speed levels to guarantee timeliness packet delivery. The utilized speed notion in  
Figure 6: **(a)** Spanning tree constructed by initial flooding in N-to-1 Multipath Routing Protocol. **(b)** Multipath discovery using multipath extension flooding mechanism. this protocol can be realized through Figure 7. Suppose node \(A\) forwards a data packet to its immediate neighboring node \(B\), which can reduce the remaining geographic distance to the destination (_i.e._, node \(C\)) about \(d\) meters. According to the estimated delay of data transmission over link \(A\)-\(B\) (_i.e._, _delay\({}_{A\text{-}B}\)_), the achievable progress speed towards the destination through forwarding this data packet to the node \(B\) can be calculated as \(\textit{Speed}_{A\text{-}B}=(\textit{distance}_{A\text{-}C}-\textit{distance}_{B\text{-}C})/ \textit{delay}_{A\text{-}B}\). In the reliability domain, reliability demands of different applications are satisfied through using a probabilistic multipath forwarding strategy. In order to satisfy the delay requirements of various applications, MMSPEED extends the SPEED protocol to provide different speed layers over a single network. Accordingly, for \(M\) virtual speed layers there exists \(M\) different _SetSpeeds_. In this protocol, data packets are assigned to the appropriate speed layer to be placed in the suitable queue according to their speed category. After that, data packets are serviced in the FCFS policy. This mechanism ensures that high-priority packets are serviced before low-priority packets. However, as contention-based MAC protocols utilize CSMA/CA mechanism to perform channel access [62, 63], employing a local priority data transmission scheme at the network layer does not necessarily prioritize data transmission at the link layer.