## 3 State-of-the-Art for WirelessHART Routing Schemes
### Non-WirelessHART Routing Algorithms  
In order to provide some suggestions and ideas for developing new WirelessHART routing algorithms, this subsection will outline some non-WirelessHART routing protocols. The ideas presented may serve as an inspiration for new WirelessHART algorithms in the future. For the ISA100.11a technology, [67, 68] applied the packet reception rate (PPR) mathematical model in order to define connectivity regions; such regions are classified by the success rate of their routes. Two graphs are built from the topology: the first one is based on the energy consumption of the nodes; the other is latency-based. Then, a scheduling algorithm is applied to the two graphs in order to make a trade-off between energy consumption and latency. The same authors also produced in [69] an industrial cognitive radio sensor network-focused routing algorithm based in ISA 100.11a. The algorithm estimates the maximum throughput for each path and sends the data for most optimal of them. The algorithm presented in [69] uses a hierarchical cluster-based structure in which stations (called clusters) are responsible for sensing and allocating channels to their neighboring nodes. A cluster head is a special node that uses two separate radio transceivers at the same time, allowing the use of the idle time of the ISM licensed channels. Each cluster can estimate the state of the transmission mean using a method called window mean with exponentially moving average. The hydro protocol, presented in [70], has the objective of reducing the overhead of the P2P transmissions. Hydro uses the ETXmetric [71] to measure the link quality and adopts three primitives to reach the proposed objective. The first primitive determines the formation of a distributed DAG (directed acyclic graph); thus, each node must have a list of default route devices that will lead to the border routers. The second primitive is the global topology construction that defines a topology report, which is piggybacked onto the data traffic. This report contains information about the best neighbors in the list of default routes, so that a graph can be built. Finally, there is a centralized route installation, which determines whether routes should be installed/updated in the devices by the border routers by a route install message. Although the work presented in [72] is very interesting due to the focus on the challenges of the IWSN, the present work does not conform to the WirelessHART current specification, becoming more of an option for future updates for the technology. Thus, we will emphasize it in this subsection due to the innovative approach presented. The work in [72] proposes the REALFLOWprotocol, which focuses on industrial wireless sensor networks. The protocol includes a routing algorithm, which provides multipath diversity (in order to increase reliability), real-time performance, tolerance to sudden changes in the topology and a workload division among the devices in order to allow the network manager to save processing time in routing information calculation. In order to achieve the aforementioned objectives, the authors divide the solution into two parts: a routing establishing and management part and a packet forwarding part. The relationship among those parts and the network structure are illustrated in Figure 10.  
Figure 10: REALFLOW proceeding summary (adapted from [72]). Steps 1-3 in Figure 10 represent the three control messages used for the routing establishment and management. The networking discovery is performed by the list-update message, which is broadcast from the gateway to the devices carrying information about signal levels, hop count and the previous node address. When a device receives a list-update message, it updates the parameters contained in the message and rebroadcasts the updated message as needed in order to reach the whole network.