High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 156

Figure 10. Example deployments that may require node 2 to serve as a bridge for the nodes 1 and 3. In
(a) the nodes 1 and 3 do not share an event region due to their distance. In (b) these nodes share their
regions indeed, but cannot communicate directly due to an obstacle between them.

requirements. In particular, the following ques-
tions are of primary concern:

the ACK-based data exchange requires at least two
transmissions per detection interval and node, i.e.,
one request and one acknowledgement message.
In usual application, such a request is spread as
broadcast and hence, several nodes may answer
to particular request. To simplify matters, this
possibility is ignored but would of course further
increase the required traffic for ACK-based data
exchange. Hence, here we only consider the
idealized-ACK data exchange requiring two mes-
sages per interval. The publish/subscribe approach
maintains the exchange of EDT node values and
reduces the traffic by submitting only changes
of node values to achieve longer time intervals
without any transmission. The focus is on reducing
the package payload for application data as well
as the number of transmissions. The simulation
results presented in a later section show that the
proposed publish/subscribe scheme outperforms
idealized ACK-based variants.
Since every bit to be transmitted is expensive
with regard to energy consumption, the amount
of exchanged data has to be minimized. EDTs ef-

Is there a need to transmit some event information?

If yes, which node has to transmit what informa-
tion?

Is there really a sensor node that receives the data?

Additionally, the amount of exchanged data
ought to be kept as small as possible. This section
presents an adaptive and easy-to-scale mechanism
to efficiently share EDT evaluation results based
on a publish/subscribe approach. A comprehensive
comparison of other approaches using publish/
subscribe is given in (Heinzelman, W. B., Murphy,
A. L., Carvalho, H. S. & Perillo, M. A., 2004).
It is quite obvious that request-acknowledge-
ment-based (ACK) communication schemes can
be used for reliable collaboration. But they produce
a huge amount of traffic and are therefore inef-
ficient for sensor networks. In idealized scenarios,