Cross-layer designs

Abstract

There exist exhaustive amount of research to enable efficient communication in wireless sensor networks (WSNs) (Akyildiz 2002). Most of the proposed communication protocols improve the energy efficiency to a certain extent by exploiting the collaborative nature of WSNs and its correlation characteristics. However, the main commonality of these protocols is that they follow the traditional layered protocol architecture. While these protocols may achieve very high performance in terms of the metrics related to each of these individual layers, they are not jointly optimised to maximise the overall network performance while minimising the energy expenditure. Considering the scarce energy and processing resources of WSNs, joint optimisation and design of networking layers, i.e., cross-layer design, stands as the most promising alternative to inefficient traditional layered protocol architectures. Accordingly, an increasing number of recent works have focused on cross-layer development of wireless sensor network protocols. In fact, recent papers on WSNs (Fang 2004; Hoesel 2004; Vuran 2005) reveal that cross-layer integration and design techniques result in significant improvement in terms of energy conservation. Generally, there are three main reasons behind this improvement. First, the stringent energy, storage, and processing capabilities of wireless sensor nodes necessitate such an approach. The significant overhead of layered protocols results in high inefficiency. Moreover, recent empirical studies necessitate that the properties of low power radio transceivers and the wireless channel conditions be considered in protocol design (Ganesan 2002; Zuniga 2004). Finally, the event-centric paradigm of WSNs requires application-aware communication protocols. Although a considerable amount of recent papers have focused on cross-layer design and improvement of protocols for WSNs, a systematic methodology to accurately model and leverage cross-layer interactions is still missing. With this respect, the design of networking protocols for multi-hop wireless ad hoc and sensor networks can be interpreted as the distributed solution of resource allocation problems at different layers. However, while most of the existing studies decompose the resource allocation problem at different layers, and consider allocation of resources at each layer separately, we review recent literature that has tried to estab lish sound cross-layer design methodologies based on the joint solution of resource allocation optimisation problems at different layers. Several open research problems arise in the development of systematic techniques for cross-layer design of WSN protocols. In this chapter, we describe the performance improvement and the consequent risks of a cross-layer approach. We review literature proposing precautionary guidelines and principles for cross-layer design, and suggest some possible research directions. We also present some concerns and precautionary considerations regarding cross-layer design architectures. A cross-layer solution, in fact, generally decreases the level of modularity, which may loosen the decoupling between design and development process, making it more difficult to further design improvements and innovations. Moreover, it increases the risk of instability caused by unintended functional dependencies, which are not easily foreseen in a non-layered architecture. This chapter is organised as follows. In next section, we overview the communication protocols devised for WSNs that focus on cross-layer design techniques. We classify these techniques based on the network layers they aim at replacing in the classical OSI (Open System Interconnection) network stack. In Section 3, a new communication paradigm, i.e., cross-layer module, is introduced. In Section 4, we discuss the resource allocation problems that relate to the cross-layer design and the proposed solutions in WSNs. Based on the experience in cross-layering in WSNs, in Section 5 we present the potential open problems that we foresee for WSNs. Then, we stress some reservations about cross-layer design by discussing its pros and cons in Section 6, and conclude the chapter in Section 7. © Springer-Verlag Berlin Heidelberg 2007.