The increasing demand of multimedia contents such as video streaming, voice over IP services, gaming, and the massive peer-to-peer phenomena, arise the need for very high performance communications networks. These applications do not only require high data rates but also `quality of service` (reliability, delay, security...) need be provided. Moreover, the convergence among different technologies is required for providing more integrated and flawless services. In order to fulfill all these requirements, next generation networks has to be more efficient, flexible and reconfigurable.
The layered protocol architecture based on the ISO/OSI and the TCP/IP stacks, though of fundamental importance in allowing for an easy design and for understanding the basic network functionalities, has been shown to impose great limitations to the design of optimized solutions. The investigation of the relations among different layers and the study of mechanisms which break the classic layering is usually referred to as `cross-layer design`.
In this presentation, an introduction to the concept of cross-layer design is provided according to the literature, both from an architectural and analytical point of view. Two specific cases are then presented. The first one uses an analytical framework to define an algorithm for the joint congestion control, channel allocation and scheduling in multi-channel ad-hoc networks where each node is provided with multiple interfaces. In the second one, a simple mechanism spanning PHY, MAC and LL layers is investigated in the context of cellular networks, addressing the problem of trading fairness for physical layer efficiency.