## 1 Introduction  
Industrial control applications can be categorized into two main classes: (i) factory automation, and (ii) process control. Factory automation applications involve machines (e.g., robots) that perform discrete actions and are highly sensitive to message delays. Thus, such applications may require latency in the region of 2-50 ms. Process control, however, is typically used for monitoring and controlling the continuous production stream of fluid materials (e.g., oil and gas refinery) [1, 2]. Due to the non-critical nature of the process control applications, latency requirements are not usually stringent (\(>\)100 ms) [1]. Based on the criticality and importance of the applications, the International Society of Automation (ISA) considers six classes of wireless communication, from critical control to monitoring applications, in which the importance of the message response time and Quality of Service (QoS) requirements vary [3]. In the more critical applications, sensor/process data need to be transmitted to the destination in a reliable, timely and accurate manner. Process control applications cover class 1 to 5 [1]. The details of the classes are shown in Table 1.  
ISA100.11a [3] and WirelessHART [4] standards are designed for process control and monitoring applications. ISA100.11a supports industrial applications from class 1 to 5, and WirelessHART supports industrial applications ranging from class 2 to 5 [1]. Traditional wireless sensor networks (WSNs) are deployed in class 4-5 applications, in which low-power consumption is given priority over providing a bounded response time delay. ZigBee Pro [5], as one of the first standard for WSNs, is designed for applications which have softer real-time and reliability requirements, so it can not address the requirements of industrial control applications [6]. This paper will address the (i) real-time and (ii) reliable communication requirements of periodic monitoring and process control applications from class 1 to 5 in industrial harsh and dynamic environments. Those applications generally involve mostly static field devices (_i.e._, sensors and actuators). Most of the traffic over the network consists of real-time sensor data that is published periodically (with a rate of 1 per second to 1 per hour [2]) toward the other sensors, actuators (in this paper, we assume that the actuators support a set of function blocks for controlling purposes) or the gateway for closed-loop process control and monitoring applications. In closed-loop process control applications (either critical or non-critical), the sensor data will travel in the field toward the actuator without involving the central control station, also called _control in the field_. Certain Quality of Service (QoS) mechanisms are used by communication networks to meet the real-time requirements. These mechanisms can generally be categorized into: (i) _traffic classification \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Category** & **Class** & **Application** & **Description** \\ \hline Safety & 0 & Emergency action & Always critical \\ \hline \multirow{3}{*}{Control} & 1 & Closed-loop regulatory control & Often critical \\  & 2 & Closed-loop supervisory control & Usually noncritical \\  & 3 & Open-loop control & Human in loop \\ \hline \multirow{2}{*}{Monitoring} & 4 & Alerting & Short-term operational consequence \\  & 5 & Logging and downloading/uploading & No immediate operational consequence \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Different classes of applications as defined by ISA. and (ii) _resource reservation_. The _traffic classification_ mechanism can be used for channel access and packet delivery along the path between the endpoints, by labeling the packets with a priority value and placing them on the corresponding queue in the path. The _resource reservation_ technique allocates the communication resources along the path between two end-points for a specific traffic or class of traffic to achieve the desired QoS requirement [7].