## 3 Quality of Service
### Open Issues  
The previous evaluation has highlighted that the selected specifications differ with respect to the supported QoS requirements for industrial applications, particularly concerning real-time support and reliability. Open Standard for Factory Automation  
The first conclusion that can be drawn is that no standard provides currently an open solution to _factory automation_ with strict real-time requirements. The WISA specification is dedicated to this kind of deployments, but it is based on the IEEE 802.15.1 standard that consumes more energy than the IEEE 802.15.4-based standards. Moreover, the WISA specification is proprietary, thus locking the user into a single vendor as well as the design and maintenance of a proprietary set of interfaces. Even if WISA successfully fulfills the requirements of factory automation, it does not support openness and interoperability. The 802.15.4 Factory Automation MAC provides a promising perspective, but is still under development. Furthermore, the recently released ISA100.11a standard is expected to be enhanced by future addenda to make it fit to applications with stronger real-time requirements. As a consequence, additional research and development is necessary in order to obtain open and IEEE 802.15.4-based standards, which suit the needs of factory automation. QoS Support in Heterogeneous Networks  
In the _process automation_ domain, the standards WirelessHART and ISA100.11a occupy the first positions and a potential convergence is analyzed by the ISA100.12 Working Group. WirelessHART devices are foreseen to be deployed within ISA100.11a-based WSNs, as illustrated in Fig. 12. In parallel to the ISA100.12 Working Group, the ISA100 Wireless Backhaul Backbone Network Working Group addresses potential interoperability between the ISA100.11a and the ZigBee-based standards. Additionally, the ISA100.11a standard is compatible with existing wired standards including HART, FOUNDATION Fieldbus, Modbus, and Profi bus. The ISA100.11a standard offers therefore a wide panel of compatible standards, which allows manufacturers to reuse existing devices. However, questions regarding the provided QoS capabilities among heterogeneous WSNs have to be raised. Indeed, such compatibility requires translation between the standards at the gateways, which may increase the end-to-end transmission delay of end systems belonging to different kinds of networks. Moreover, options implemented in one standard may not be supported or may have to be disabled to support interoperability. In such case, segments of the network that do not support the same priority scheme or the QoS contracts established in the ISA100.11a standard may ignore the indicated priority leading to a best-effort QoS. Thus, the efforts to support the QoS requirements would be wasted. A central network manager for both or more network segments based on different standards would be a possible solution because options, configuration settings and capabilities could be centrally managed. The network manager would be able to obtain a global view of the network and thus is able to take the appropriate decisions. However, a single central network manager remains a single point of failure that threatens the resilience of the network, if it is not deployed redundantly. In case of malfunctions, the whole network would be affected and could catastrophically fail. Therefore, the support of interoperability between the investigated standards remains an open issue. Although addressing the heterogeneity has a number of interesting research challenges, its practical feasibility may be limited by the technical complexity and financial costs incurred to adapt different technologies. The surveyed standards dedicated to _process automation_ can also be deployed in mesh topology, thus raising the question of QoS support over multi-hop routes. The WirelessHART and ISA100.11a standards propose solutions based on priority mechanisms, which are centrally managed by the network manager.