## 8 Cross Layer Optimization  
In the previous sections, we discussed the existing proposed solutions designed for WMSNs that follow the classical layered structure of the communication protocol stack. Some of these proposals may achieve a good performance in terms of some metrics related to each of their intended individual layers, but these performance metrics are not jointly optimized to maximize the overall network performance with minimum energy consumption. Moreover, most of these existing solutions do not provide enough support for multimedia applications since the work is done at the lower layers of the communication stack for optimizing functionalities such as optimal routing, reliable delivery, efficient resource management, and other tasks without taking into consideration the especial requirements of handling multimedia content over WMSNs. For example, multimedia compression and source coding algorithms at the application layer should be considered when designing the functionalities at the lower layers and vice versa:  
* Cross layer design between multimedia source coding techniques at the application layer and the routing protocol in the routing layer can be exploited for better multipath selection or in-network processing. * Cross layer design between the routing layer and the MAC layer can allow for packet-level service differentiation or priority-based scheduling and for more power efficient routing mechanisms. * Cross layer design between MAC layer and the physical layer, especially in the case of using UWB technology. The adoption of the UWB technology as the underlying transmission technique in WMSNs and the potential challenges in this area, appear as an interesting research topic. * Cross layer design between the routing layer and transport layer especially in the case of multiple paths routing for optimizing the selection of better or most adequate paths that guarantee the required QoS and reliable delivery for each type of multimedia content. Therefore, in WSNs in general and in WMSNs in particular as they are considered resource-constraint environments, we believe that the correlation characteristics and functionality interdependencies among the layers of the communication stack cannot be neglected and should be exploited for better performance and efficient communication, consequently, cross layer design stands as the most promising alternative to inefficient traditional layered protocol architectures. Recent works in WMSNs show that cross-layer integration and design techniques result in significant improvement in terms of quality of service, energy conservation, and exchanging information between different layers of the communication stack:  
A cross layer communication architecture is presented in [24] to provide QoS for WMSN applications based on the Time-Hopping Impulse Radio Ultra-Wide-Band (TH-IR-UWB) transmission technique. This architecture tries to solve the short-comes of using CSMA/CA for the MAC layer in WSN such as the variable and uncontrollable access delays from using random timer, idle listening from using carrier sense, and increased energy consumption occurs due to for example hidden node problem and to provide QoS for WMSN. The proposed system guarantees the end-to-end QoS requirements to handle multimedia content and packet level service differentiation at the network layer in terms of throughput, end-to-end packet error rate and delay by the joint hop-by-hop local decisions of the participating nodes. This is done by an admission control protocol where the sender node that wants to establish connection sends request packet describes its requirements to its neighbors, and among the replies the sender node selects the one who has the most positive advance toward the sink and able to satisfy the needed requirements and this continue iteratively until the end-to-end path is established to the sink.