High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 157

ficiently share local evaluation results using a few
bytes only. In contrast to existing approaches that
need to share raw sensor data, which is usually
between two and four byte per value plus identi-
fier, here only the Boolean value of a certain EDT
node is of interest. Thus, a data transmission has
to contain only the event identifier, the number of
the respective EDT node and the current Boolean
value assigned to that node. Remember, pre-order
numbering the complete EDT before pruning as-
sures that each EDT node at each device in the
network possess the same numbering. This label-
ing scheme allows to efficiently describe the node
of interest and the assigned value with one byte
only. That byte consists of one bit representing
the Boolean value and seven bits representing the
number of the EDT node. Hence, 128 different
nodes in one EDT can be addressed. If an EDT
contains more than 128 nodes, an extra byte for
addressing is used. In addition, the event identi-
fier must be submitted given that a sensor node is
enabled to configure several EDTs concurrently. If
the event identifier is chosen to be a unique number
less than 256, e.g., this number may be generated
while preparing the binary event description,
all necessary information can be transmitted by
two bytes only. Hence, this scheme reduces the
required data payload for collaborative exchange
by at least 50 percent.
As mentioned, missing values of undecidable
EDT nodes must be obtained by suitable other
sources, e.g., neighboring nodes. Thus, the sensor
node broadcasts a data interest (subscription) into
the network to find suitable information suppli-
ers. If the binary event specification defined a
collaboration region, this subscription must also
contain the location of the subscribing sensor
node. On receiving a subscription, the sensor node
compares the location data to determine whether
both nodes share the collaboration region. Only if
that holds true or if the request contains no loca-
tion data, i.e., assuming the one-hop neighborhood
collaboration region, the received subscription is
of interest. The receiving sensor node searches its

own respective EDT to determine whether it can
provide the requested information. The requested
EDT node is marked with a toPublish flag and the
sensor node answers the request by providing the
current value of the requested EDT node. In all
other cases, the node discards the received sub-
scription without further processing. Subscriptions
can also consist of many concurrent data interests
in case of requiring information about several un-
decidable EDT nodes of one or more configured
EDTs. That significantly reduces processing and
communication effort required for packaging,
addressing, transmission etc.
On EDT evaluation, the current state of each
EDT node is determined. Results at nodes marked
with the toPublish flag are also important for
other devices in the network and hence, ought to
be published. To save resources these evaluation
results are not transmitted periodically. Only
first-time subscriptions and state changes require
transmission of the current node state. If a device
accepts a received subscription for the first time,
it answers with the current node state to provide
an initial value. Since a node state is of Boolean
type, only state changes must be submitted to up-
date the node state at the subscriber. If node states
change rarely, the number of required publications
is significantly reduced. Even in the worst case,
i.e., the node state changes at each evaluation,
this scheme requires the same overhead as usual
methods where values are transmitted repeatedly
at every evaluation period.
Using a publish/subscribe scheme is rather
simple if reliable communication architectures and
fixed network structures are provided, but WSNs
are subject to unpredictable behavior caused by
sudden changes in context, connectivity, working
mode etc. That especially holds true if mobility
of nodes is provided. To ensure a certain level of
robustness and efficiency, some fundamentals
have to be considered from respective points of
view of subscribers and publishers. How does
the subscribing node know, whether some other
node received the subscription, accepted it or is