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The IoT Domain Model :
The relation between Virtual and Physical Entity is usually achieved by embedding into, by attaching
to, or by simply placing in close vicinity of the Physical Entity one or more ICT Devices that provide
the technological interface for interacting with or gaining information about the Physical Entity. By
so doing the Device actually enhances the Physical Entity and allows the latter to be part of the
digital world. This can be achieved by using Devices of the same class, as in the case of body‐area
network nodes, or by using Devices of different classes, as in the case of an RFID tag and reader. A
Device thus mediates the interactions between Physical Entities (that have no projections in the
digital world) and Virtual Entities (which have no projections in the physical world), generating a
paired couple that can be seen as an extension of either one. Devices are thus technical artifacts for
bridging the real world of Physical Entities with the digital world of the Internet. This is done by
providing monitoring, sensing, actuation, computation, storage and processing capabilities. It is
noteworthy that a Device is also a Physical Entity and can be regarded as such, especially in the
context of certain applications.
An example for such an application is device management, whose main concern is the devices
themselves and not the objects or environments that these devices monitor.

From an IoT point of view, the following three basic types of Devices are of interest:
Sensors: These provide information about the Physical Entity they monitor. Information in this
context ranges from the identity of the Physical Entity to measures of the physical state of the
Physical Entity. Like other Devices, they can be attached or otherwise embedded in the physical
structure of the Physical Entity, or be placed in the environment and indirectly monitor entities. An
example for the latter is a face‐recognition enabled camera. Information from sensors can be
recorded for later retrieval.

Tags: These are used to identify Physical Entities to which they are usually attached to. The
identification process is called "reading" and it is carried out by specific sensor Devices, which are
usually called readers. The sole purpose of tags is to facilitate and increase the accuracy of the
identification process. This process can be optical, as in the case of barcodes and QR code, or it can
be RF‐based, as in the case of microwave car‐plate recognition systems and RFID. The actual physics
of the process as well as the many types of tags are however irrelevant for the domain model as
these technologies vary and change over time. These are important however when selecting the
right technology when implementing a concrete system.
Actuators: They can modify the physical state of a Physical Entity, like changing the state (translate,
rotate, stir, inflate, switch on/off.) of simple Physical Entities or activating/deactivating
functionalities of more complex ones.

The following figure shows the relationship between Augmented, Physical and Virtual Entities,
together with other terms and concepts.

Hardware concepts are shown in blue, software in green, animated objects in yellow, and concepts
that fit into either multiple or neither categories in brown.

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22 http://www.iot‐a.eu/arm/120613

268 ITU‐T's Technical Reports and Specifications