The present dissertation deals with the problem of under-utilization of collision channels and other related problems in wireless networks. It deals with the design of random access protocols for wireless systems and provides a mathematical framework for performance evaluation of multihop based heterogeneous wireless networks. This thesis is divided into three parts. In the first part, we propose new versions of slotted aloha incorporating power control, priority and hierarchy. Our simulations were important to understand the behaviour of such a system and the real impact of involved parameters (transmit power, transmit rate, arrival rate, hierarchy order). Both team problem (common objective function is maximized) and game problem (each user maximizes its own objective) were discussed. Introducing hierarchy seems to provide many promising improvement without/or with a low amount of external information. We also proposed two distributed algorithms to learn the desired throughput. Next, we developed in the second part an analytical Framework to evaluate performances of multihop based heterogeneous Wireless networks. We built a cross-layer model and derived expression of stability, end-to-end throughput and end-to-end delay. Furthermore, we provided an accurate approximation for the distribution of end-to-end delay in multihop ad hoc networks (operating with slotted aloha protocol). As a direct application, we highlighted how streaming and conversational flows could be supported in this class of ubiquitous networks. The third part of this thesis is devoted to understanding and modelling of IEEE 802.11e DCF/EDCF-operated multihop ad hoc networks. We indeed built a complete and simple APPLICATION/NETWORK/MAC/PHY cross-layered model with finite retries per packet per flow. We analyzed the stability of forwarding queues and derived expression of end-to-end throughput. We finally proposed a Fountain code-based MAC layer to improve the throughput/fairness over the network