## 3 Quality of Service
### Evaluation of the Selected Specifications
#### Priority Management  
In addition to medium access control, flow control with assignment of priority to the packets can be introduced to support real-time at higher level. A priority flag indicates to each device on the path if the arriving packet has to be transmitted without delay or can be buffered. The WirelessHART standard defines four main priority levels according to classes of data [36]. Control and configuration information as well as network diagnostic messages have the highest priority, followed by a second category composed of process data measurements and network statistic messages. The lowest priority is assigned to the fourth category, which includes packets reporting information about events and alarms. Additional classes of data are classified into the third category. In comparison, the ISA100.11a standard allows to prioritize the QoS contracts established between the devices and the system manager in addition to message priorities. The device wanting to transmit data first sends the required priorities in a contract proposal, as well as other additional parameters including reliability, periodicity and negotiability. The system manager replies to this request in a contract response and indicates the provided QoS for the requested communication. Depending on the traffic conditions, the system manager might not be able to provide the requested QoS. If the device indicates that the contract is negotiable, the system manager can propose another QoS level or postpone the contract to provide the requested QoS level. The contract priority is set during the contract establishment and concerns all messages exchanged during the contract duration. Depending on these priorities, the system manager manages the routing and the load balancing to provide the guarantees defined in the contract. Four levels of contract priority are available at network layer: network control, real time buffer, real time sequential and best effort queued. The first priority level can be used by the system manager to communicate critical information about the network management; whereas the second category is used for periodic data exchanges with buffer overwrite operations in case of fresher messages. The third class, real time sequential, is appropriate for applications requiring sequential and real-time data delivery like video or voice-based applications. The last level is adapted for client-server communications. Within the contract, message priority can also be assigned by setting one bit to either low or high. However, the contract priority takes precedence over the message priority. Each standard supports real-time communication in a different way. While the WISA specification and the 802.15.4e FA MAC layer target applications with strong real-time requirements, the ZigBee \begin{table}
\begin{tabular}{c|c|c|c|c|c}  & WISA & WirelessHART & ISA100.11a & ZigBee & 802.15.4e MAC \\ \hline \hline TDMA & x & x & x & x3  & x & & x \\ \hline CSMA/CA & & & & x & \\ \hline FDD & x & & & & \\ \hline Superframe & x & x & x & x4  & x \\ \hline Fixed timeslot duration & x & x & & & \\ Duration & 64 and 128 \(\mu s\) & 10 ms & & & \\ \hline Shared timeslots & & x & x & x4  & x \\ \hline Dedicated timeslots & x & x & x & x4  & x \\ \hline Superframe optimization & & x & x & & \\ \hline Message-based priority & & x & x & & \\ Priority levels & & 4 & 2 & & \\ \hline Contract-based priority & & & x & & \\ Priority levels & & & 4 & & \\ \hline \end{tabular}
\end{table} 
Table 3: Features relevant for real-time operation. and the WirelessHART standard are more adapted to applications with softer requirements. The recent ISA100.11a standard supports currently soft real-time requirements. More precisely, the evaluation of the selected technologies has shown that:  
* Dedicated timeslots are supported by all standards. Their utilization is a key feature to fulfill strong real-time requirements and is particularly appropriate, if the complete set of the sensors have a continuous stream of information send.