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two constants. The result of a relation between
two constants is of constant Boolean type, too.
In that case, the event specification is redesigned
and the Boolean result of such relation is directly
inserted instead.
To support definition of complex phenomena,
configured thresholds can be composed by logic
operations. Logic operations are also specified by
XML tags, namely: <AND>, <OR>, <NOT>. Sim-
ilar to relational elements, the elements <AND>
and <OR> define a respective logic operation
between two relational or two logic elements or a
mix of both. Since both operations are commuta-
tive, the order of the linked elements is irrelevant.
The <NOT> element specifies an unary operation
and can only be used on top of one relational or
logic element. As known from Boolean algebra, it
inverses the Boolean result of the underlying ele-
ment. Of course, it is also possible to link several
logic operations together. Logic elements further
allow to define 2-bounded intervals for certain
sensing capability by combining several primitive
events. For example, measuring a temperature
between 20 and 25 results in a combination of
two primitive events, exemplified in Listing 1.
To simplify matters only three logic elements
are available, but these are sufficient to define
every possible logic combination. Supporting
more language elements may slightly increase the
usability for end-users but even implies to imple-
ment more complex interpretation means on the
sensor nodes. This would require using more
processing and memory resources. For this reason
and to keep the language quite simple, the integra-
tion of further logic elements like NAND or NOR,
is omitted. This is vitally important for implement-
ing a language interpreter on sensor nodes, which
provide scarce resources only. Since the event
specification is created by support of a user as-
sistant, a respective mapping of the required
logical operation into the available logic elements
can be achieved automatically. Further, the com-
mutative algebraic and logic operations might
even have more than two operands. The expres-
siveness of XML even allows for specification of
such multi-nary operations. But with regard to
aspects of simplicity and minimization of the
binary event specification, we limit to binary
operations.
Definition of Hardware-
Independent Parameters
A complete event specification consists of three
mandatory and one optional element represented
by respective tags. As already introduced, the
<SENSORDATA> element defines the required
sensing capabilities for primitive or complex
Listing 1. Example specification of a phenomenon as complex event, which detects temperature between
20 and 25 centigrade.
<AND>
<GREATER>
<VARIABLE> temperature </VARIABLE>
<CONSTANT unit=”centigrade”>20</CONSTANT>
</GREATER>
<LESS>
<VARIABLE> temperature </VARIABLE>
<CONSTANT unit=”centigrade”>25</CONSTANT>
</LESS>
</AND>
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