Wireless Sensor and Actuator Networks: Algorithms and Protocols for Scalable Coordination and Data Communication

5.3 RATE-BASED MULTICASTING

Rate-based multicast, or multiratecasting, is a generalization of multicasting in which the data sent from a source to the destinations is possibly sent at a different rate for each destination. Let us consider the example of backup base stations in which the sensed data is stored for further analyzes. To ensure fault tolerance, several base stations can be deployed hierarchically, each one collecting the data at a different rate (the highest rate for the primary, a slightly inferior rate for the secondary, etc.). If the primary base station fails, then the secondary base station takes over, and a specific protocol is run to shift the rate requirements among base stations (from the n-ary to the n + 1-ary) and to inform the nodes about this change. Finally, the normal report mechanism can be resumed. Other examples may include overlapping sensing area, where the actuators collect data from sensors at a rate inversely proportional to their relative distance. Obviously, these protocols do not generate optimal multiratecast routing paths (which is an NP-complete problem, as generalizing the optimal multicast tree problem in wireless networks has been proven NP-complete in Ruiz and Gomez-Skarmeta (2005)).

The problem of rate-based multicast is very recent. A rate-adaptive multicast protocol has been proposed for mobile ad hoc networks in Nguyen et al. (2006). This protocol adapts the rate of communication to the quality of the links in order to reduce the ...