Stability Problems of Transport of Event Data in Realistic Wireless Sensor Networks

In general, wireless sensor networks have a certain degree of spatial redundancy, which means that a high number of nodes can sense the same event at the same time. This redundancy permits to simplify both MAC protocol and the transport of sensed data. In particular, in this paper, we review a widely accepted transport technique which makes use of the event-reliability concept. A system implementing this technique is essentially a control system, where the controlled variables are the reporting rates of sensor nodes and the control variable is a function of the perceived event reliability at the sink node. As control system, we are interested in stable operating points. If we respect the capacity limit of the network, such stable points exist. However, if we take into account also the irregularities of radio links, well documented in recent measurements on real sensors testbeds, possible unstable states may arise. Here, we study this problem by means of simulation of a wireless sensor network under the assumption of two simplified versions of the radio model. We found that indeed the routing protocols should be carefully engineered in order to support event reliability based techniques.