## 6 Future Challenges and Open Research Issues  
The energy efficiency in CPSN is one of the prime issues researchers have been working on for years. In order for sensing planes to perform their monitoring tasks for longer periods of time and due to the nature of their sensing in harsh environments, battery replacement is not an easy task and sometimes even impossible [75]. For extending network lifetime, much research work has been done on MAC protocols and routing in addition to energy harvesting. MAC protocols provide efficient resource sharing and contribute in saving power by utilizing the node's hardware only when it is needed. Major causes for energy consumption in wireless nodes are due to collision of transmitted packets, overhearing of transmitted packets, packet overheads and idle listening of the radios [8,9]. Numerous MAC protocols have been proposed but only a few manage to bear the harsh physical environment for sensing applications. Some of the widely used MAC protocols for WSN are SMAC, TMAC and BMAC [2]; however, the need for efficient MACs for pipeline monitoring applications with linear and hierarchical layouts is still an open issue. An optimum and well-designed antenna use can not only improve the energy efficiency of the system and transmission range but also provide reliable communication making the sensing node small enough to fit inside the sensing environment [11]. Wireless sensor nodes consume power in data sharing, processing, transmission/reception and in data routing. Routing protocol needs to be simple by reducing computational complexity and power efficiency in order to help increase the network lifetime [1]. There is a need for such cross layer efficient routing algorithms, particularly in different pipeline monitoring applications. Some initial work has been done in determining an optimum routing protocol [76,77] but still extensive efforts are required in this field. Reliability and robustness complements each other, reliability being the most desired aspect of the sensor system in order to perform and maintain its functions in normal as well as hostile environments [77], while robustness enables the system to handle errors during execution. Middleware offers the ability to assimilate and reprocess software components on demand and help abstract the dissemination and heterogeneity of the underlying computing environment and services. It also supports the addition of non-functional values such as interoperability, load balancing, scalability, reliability, availability, usability, extensibility, manageability, reusability, services discovery, Quality of Service (QoS), stability, efficiency and security [21,22,51]. To address the design and implementation issues related to CPSN applications, a new approach to integrate the middleware layer has been proposed in [78]. The middleware is present between an operating system and the application layer in a sensor node. It can be divided into many sub-middleware functions some of which include time synchronization, location detection, battery-power control and networking [25]. In traditional computing devices, operating systems are well established, but for sensor nodes the applications are executed on bare hardware without a separate operating system [79]. Hence, the identification and implementation of appropriate operating systems and middleware in CPSN is still a research focus [51,80]. Research could be initiated on developing different reliable and robust sensor nodes for diverse types of pipelines with oil and gas monitoring as summarized in Table 7.