## 5 Challenges and Open Research Issues  
Most of the existing QoS techniques only support QoS-aware routing protocols. QoS-aware routing is a crucial part for maintaining the Quality of Service framework to improve the lifetime of wireless networks. The data delivery paths are analyzed using knowledge resource accessibility along with other requirements under QoS routing schemes. There are numerous issues that are to be focused on when designing the QoS routing protocols for WMSNs. We have determined some important factors that can improve the design:  
* Parameter selection (the delay, bandwidth and path computation). * Timeliness and reliability. * QoS state maintenance and propagation. * Scalability and mobility. * Maintaining the network adaptability and balancing the efficiency for low latency. There are many introduced routing protocols, but some of them focus on maintaining QoS. In general, the main job of WSN is to sense the environment and to send the sensed data to the base station. Therefore, QoS provisioning in WMSN faces significant challenges that are discussed as follows:  
* _Heterogeneity_: This is one of the big challenges in WMSN for maintaining the QoS because the sensors used for detecting the events are different from each other. There are some applications that require heterogeneous sensors to monitor the events and to capture images and videos of moving objects such as handling disaster situations, surveillance systems and the military battlefield environment. These applications generate data from sensors at varying rates based on different QoS limitations and delivery models. Hence, these diversified WMSNs may impose significant challenges for the provision of QoS. * _Limitation of resources_: Efficient energy utilization is one of the significant challenges for maintaining the QoS. When sensor nodes are communicating they may run out of battery power. This situation can be worse when sensor nodes are underground as then they are not replaceable or rechargeable. * _Bandwidth utilization_: This is generally a challenge in WSNs because WSNs involve real time and non-real time traffic. Thus, the bandwidth allocation should be maintained to balance the traffic flow between real time and non-real time communication. However, the introduction of the multimedia will pose more challenges because there will be high traffic and more bandwidth demand to process such data. * _Network adaptability_: Link failures and node failures can be caused by mobility. As a result, the network topology is changed which is the issue of concern. The network encompasses the densely deployed hundreds to thousands of nodes in a landscape of interest. In this situation, a number of sensor nodes may join or leave the network which affects the QoS. * _Data Redundancy_: Sensor nodes are deployed in the area of interest for sensing the situation, but most of the generated data is redundant, while this redundancy affects the reliability and fault tolerance process. As a result, a significant amount of energy is wasted. Data fusion and data aggregation are a solution to handle the redundancy. For instance, the data of sensors that generate the same image and point in the same direction can be aggregated. However, data fusion techniques and data aggregation could create problems for QoS design. * _Unreliable Medium_: Radio is the communication medium in WMSNs, which is less reliable bacause of the inherent features of Medium Access Control (MAC) protocols. Furthermore, wireless links are also highly affected by several environmental factors including signal interference and noise. * _Assorted Data Pattern_: This is an important issue for designing the QoS routing protocol because data can arrive in periodic and non-periodic forms. The sensing data can periodically be created in some applications at unpredictable times due to exposure to some serious events. Similarly, some sensory data can be generated at regular intervals, such as when monitoring real time environmental applications.