## 7 Open Research Areas
### A Distributed Approach to Achieving Real-Time Operation  
Large-scale, distributed, real-time control applications require data to be transmitted over long distances through a multi-hop network in a timely manner. A distributed resource reservation algorithm is needed which would allow source nodes, based on the requirements of the application and traffic characteristic, to reserve network resources for its peer communications along their paths for addressing different QoS needs. The distributed nature allows the system to adapt quickly to disturbances or changes within the network to meet timing guarantees to support real-time control operation. While such mechanisms do not exist for present day sensor nodes in a distributed manner, relevant techniques from other networking-related domains could potentially be adapted to develop solutions that are suitable for wireless sensor and actuator networks. We briefly describe some of these relevant techniques.  
QoS in multi-hop networks can be supported by different mechanisms, such as _circuit switching, Asynchronous Transfer Mode (ATM) networks_, and _internet protocols_ (such as Integrated Services/RSVP, Differentiated Services, MPLS and constraint-based routing). It is also supported by _IEEE 802.11e_[65], ISA100.11a and WirelessHART in a centralized manner.  
ATM signaling protocols address certain performance issues in terms of reliability and timeliness of packet delivery that are of importance in industrial applications that require closed-loop, real-time control. The ATM protocol uses a switching technique that combines the concepts of circuit switching and packet switching. For example, similar to circuit switching, before initiating data transfer, a virtual circuit is first established between the source and destination. The protocol also includes admission control mechanisms that help determine whether the required QoS guarantees can be provided. ATMuses statistical multiplexing techniques, similar to those used in packet switching, in order to cope with variable bit rates (_i.e._, 'bursty' traffic).  
Internet protocols are mainly designed for multimedia applications. In those protocols, some mechanisms exist that allow a data receiver to request a special end-to-end quality of service for its data flows or classes of data. RSVP signaling is used by several internet protocols, such as _Integrated Services Architecture_, _differentiated service_, and MPLS, through which the application can reserve the resource and set up the path between the source and destinations.  
The IEEE 802.11e, based on _traffic classification_ mechanisms, provides different degrees of satisfaction for the users of the service. They define different priorities through which traffic can be delivered in several access categories. This differentiation is achieved by considering different amounts of time for sensing the channel to be idle and by considering different lengths of the contention window during backoff. This implies that high-priority traffic can access the channel by shorter back-offs than low-priority traffic. In addition, the packets are labeled with priority value and introduced into the corresponding queue in the path. Admission control in this standard as an important component limits the amount of traffic admitted into a particular service class so that the network resources can be efficiently utilized.