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(Value) to which meta-information (MetaData) can be associated. Important
meta-information is, e.g., at what time a value was measured (i.e. time stamp),
the location where a measurement took place, or the quality of the
measurement. Metadata can itself contain additional metadata, e.g. the unit in
which the metadata is measured. The association (Association) between a
Virtual Entity and a Service is detailed in the sense that is pertains to a certain
Attribute of the Virtual Entity. The serviceType can be set either to
INFORMATION, if the Service provides the attribute value to be read or to
ACTUATION, if the Service allows the Attribute value to be set, as resulting
of a corresponding change in the physical world.
3.4.1 Definition of the IoT Information Model
The diagram in Figure 11 shows the structure of the information that is handled
and processed in an IoT System. The main aspects are represented by the
elements VirtualEntity, ServiceDescription and Association. A
Virtual Entity models a Physical Entity and ServiceDescription describes a
Service that serves information about the Physical Entity itself or the
environment. Through an Association, the connection between an
Attribute of a Virtual Entity and the ServiceDescription is modelled,
e.g. the Service acts as a ―get‖ function for an Attribute value.
Every Virtual Entity needs to have a unique identifier (identifier) or entity
type (entityType), defining the type of the Virtual Entity representation, e.g. a
human, a car or a temperature sensor. Furthermore, a Virtual Entity can have
zero to many different attributes (Attribute class in Figure 11 ). The
entityType of the VirtualEntity class may refer to concepts in an
ontology that defines what attributes a Virtual Entity of this type has (see, for
instance, [Group, W3C OWL]). Each Attribute has a name
(attributeName), a type (attributeType), and one to many values
(ValueContainer). The attributeType specifies the semantic type of an
attribute, for example, that the value represents temperature. It can
reference an ontology-concepts, e.g., qu:temperature taken from ―Quantity
Kinds and Units‖-ontology [Lefort 2005]. This way, one can for instance,
model an attribute, e.g. a list of values, which itself has several values. Each
ValueContainer groups one Value and zero to many metadata information
units belonging to the given Value. The metadata can, for instance, be used
to save the timestamp of the Value, or other quality parameters, such as
accuracy or the unit of measurement. The Virtual Entity (VirtualEntity) is
also connected to the ServiceDescription via the <Service
Description / Virtual Entity> Association.
A ServiceDescription describes the relevant aspects of a Service,
including its interface. Additionally, it may contain one (or more)
ResourceDescription(s) describing a Resource whose functionality is