## 4 WirelessHART Scheduling Schemes
### Non-WirelessHART Scheduling Algorithms
The second part is to provide the transmission order together with a conflict graph (see [89, 90]) to the scheduling algorithm (the one used was a Bellman-Ford algorithm), so that the transmissions can be accommodated in the schedule without conflicts. The work in [91] presents a new medium access protocol-based on the WirelessHART and ISA100.11a that focuses on addressing IWSN critical traffic and latency issues. As a main feature, it uses the initial and final parts of the traditional time slots, so that higher-priority transmission may signal to devices that a higher-priority transmission will occur; thus, devices should hold low-priority transmission scheduled for that time slot. This scheme allows higher-priority packets to hijack the bandwidth from lower-priority packets during the network operation. Although this technique is not applicable to the current WirelessHART specification, the signaling scheme may be used in future installments of the IWSN standard specifications. The work in [92] combines traditional TDMA with slotted aloha in order to produce an optimal schedule for a single hop delay-constrained sensor network. This algorithm aims to improve reliability, while avoiding the drawback of not providing efficient usage of resources, a common situation in traditional TDMA schemes. It improves the works presented by [93, 94] by using slotted aloha instead of CSMA, thus avoiding the variable delay drawback introduced by exponential CSMA backoff. Thus, two algorithms are presented: one that computes an optimal schedule and a heuristic algorithm that manages to compute solutions with a success probability close to the optimal solution, but with much more modest computational complexity. In summary, the strategy consists of the following: based on a Bernoulli loss model, each transmission has a success probability \(p_{i}\), and from that, the algorithms distribute the transmission in the schedule in order to maximize the success probability of the schedule. Each time slot can be a dedicated slot, where only one transmission is scheduled to occur, or a shared slot. In a shared slot, the devices scheduled to occur may attempt a transmission using the slotted alohascheme if they have failed their respective dedicated slot attempts to transmit. The optimal solutions consist of trying all of the possible combinations in order to maximize the success probability of the schedule. The heuristic solution consists of a greedy algorithm that chooses the next device(s) to be scheduled in order to maximize the probability of the controller receiving one more packet in the _k + 1_ step based on the already defined previous _k_-th step. The numerical simulation results of [92] show that their scheme outperforms the traditional exclusive slot TDMA approaches, thus making the use of shared slots a relevant topic for future works. The paper [95] focuses on integration of security and safety in IWSN and proposes a full framework for providing secure and safe communications for the network. As the focus of our present work is routing and scheduling, it is worth noting that the paper [95] expresses concern about the focus of the current WirelessHART implementations on the uplink transmissions over the downlink transmissions and how this would affect the safety of WirelessHART-based distributed control systems (DCS). For example, a delay in delivering a turnoff signal sent from the manager to an actuator in a critical state may provoke safety issues. The authors state that in order to achieve good results, all of the set points for the actuators must be distributed for the devices in the same cycle, and jitter and delays should be reduced to a minimum. Thus, they propose the implementation of a new WirelessHART command that the control application can use to set periodic transmissions up to the actuators (downlink transmissions). The work in [96] has to be objective to maximize reliability for end-to-end packet delivery in IWSN. The paper organizes the network topology into disjointed sets of hypernodes.