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

Abstract

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.

Keywords:

• Wireless Sensor Networks
• Data Transport
• Multi-hop Networks
• Event Detection
• Distortion
• Stability
• ns-2 Simulations
• Path stability
• Shadowing

Notes

¹By using the theorem in [4], we can say that a simple 2-regular network is almost surely strongly 2-connected.

²Other MAC factors affect the reception process, for example the Carrier Sensing Threshold (CST) and Capture Threshold (CP) of IEEE.802.11 used in ns-2.

³These figures reproduce the results obtained in [2].

Figure 1. Analysis of stability.

⁴Since the number of Scheduler events within a simulated WSN can be very high, we applied a patch against the scheduler module of NS-2 in order to speed up the simulation time [14].