## 4 WirelessHART Scheduling Schemes
### Comparison of the Scheduling Protocols  
In order to compare the previously presented algorithm proposals for the WirelessHART technology, we present in this section a set of items to be compared between the algorithms. Those items are:  
* Objective: Developers of an algorithm may have different applications, situations and needs, which require different solutions. This leads to the development of a variety of different algorithms. Thus, the objective states what is the main thing that the proposal tries to improve when used in a network. * Metric: The algorithm needs to use parameters in order to make choices that will build the schedule and prioritize the developer's objectives for the algorithms. The metrics are those parameters (e.g., degree, deadlines) that are used to determine the links and channels that will be scheduled. * Multiple superframe: The WirelessHART specification defines that multiple publish rates must be supported in order to deal with different types of devices and processes. Devices with the same publish rate must be grouped in the same superframe. The multiple superframe item identifies whether the proposal explicitly deals with multiple superframes. * Redundancy: The proposals may view the scheduling problem in two different ways: one is to view the problem as a pool of transmissions that must be allocated in a schedule; the other is to see a group of end-to-end transmissions that should have each of their forming hops scheduled and provided with redundancy. The redundancy item describes whether the proposal uses or suggests a redundancy method. * Implementation: Different methods can be used in order to validate a proposal. This can range from simpler approaches, like an analysis of the solution to a full testbed implementation in real devices. Furthermore, there are cases where the proposal presents an actual code for the solution, but there are cases when just the policies that should be used are presented. In order to clarify those questions, the item implementation shows whether the paper presents actual pseudocode for the algorithm and which validation methods were used (e.g., testbed or simulation)
* Flow differentiation: The WirelessHART specification classifies the information flow to and from devices to the gateway (sink) as being an uplink and downlink, respectively. As these flows have different characteristics, the flow differentiation item identifies whether the paper specifically treats the uplink and downlink flows of the WirelessHART network. Table 6 presents the characteristics of each proposal for each item. The intention is to identify which trends prevail for the scheduling in WirelessHART, thus giving direction for further improvements in the field. Objective: We can see that there are two main trends for the objective. The first includes those which are mainly concerned with filling up the scheduling, like the Dang algorithm [78] and C-LLF, as this is a complex problem to be solved by itself. The second trend is formed by those algorithms that are proposed in order to specifically improve specific characteristics, like latency, reliability, time and channel usage. Metric: A balance between the three metrics, degree (this is the same as a hop count), deadline (laxity involves deadlines upon calculation) and publish rate, since it can be seen that all proposals use one or more of these. This reveals a clear trend for those metrics, but leaves space for new ones, like robustness. Multiple superframe: A concern for multiple superframes is only explicit in the proposals of the Dang algorithm and Han algorithm, which use publish rate as a metric for decisions. The others proposals did not specify how to deal with the issue and leave that for the implementation of the manager to solve. Redundancy: Three proposals presented a solution with a concern for redundancy, revealing themselves as more complete solutions.