Mobile ad hoc networks: from theory to reality
Mobile ad hoc networks are a collection of mobile nodes connected together over a wireless medium. These nodes can self-organise into arbitrary and temporary network topologies, allowing people and devices to interwork in areas with no pre-existing communication infrastructure. While wi-fi technology is ubiquitous, users seldom operate 802.11 wireless cards in ad hoc mode. This seems to be due to a gap between what end users might find useful and what technical issues research is addressing. Indeed, research largely concentrates on the optimisation of lower-layer protocols in unrealistic scenarios. Little attention is devoted to the Quality of service (QoS) that ad hoc networks provide to end users in realistic scenarios. The MOBILEMAN project aimed to reduce this gap by a research plan that combined theoretical analysis with experience gained with real ad hoc networks. A key facet of the research entailed an extensive measurement study, as popular simulation and analytical approximations may lead to inaccurate results. To understand the potentialities of this technology, the dependence of the communication range on several parameters was analysed. Emphasis was placed on characterising key networking features, such as the maximum communication distance between a small number of nodes and the interactions between simultaneously transmitting nodes. Besides the effect of environmental parameters, the effect of technology-dependent parameters was also analysed. Measurements under varying environment humidity and data transmission rate were used to revise formulations reported in computer networking handbooks and to define a more accurate channel model for 802.11 wireless cards. With the use of a more realistic channel model, well-known hidden and exposed node problems were addressed. A variety of extensions to the 802.11 Medium access control (MAC) protocol were investigated to cope with the lack of coordination among nodes. These modifications also addressed the problem of unequal allocation of channel resources to each node. The hardware/software platform, designed for experiments with the implementation of an enhanced 802.11 MAC card in realistic scenarios has also opened the way for a variety of new research directions. These include the design of shared memory components allowing the exchange of network information for all network layers, including the routing and transport layer.