The PIDF standard has great flexibility because of the XML data elements, as
shown in the example, and this flexibility can lead to confusion. To avoid con-
fusion, a data model is helpful and, as a consequence, a data presence model
has been developed.
The Data Model for Presence
The data model for presence [19] introduces some useful definitions that we
will mention here briefly:
■■ Service—Such as IM or telephony.
■■ Device—PC/laptop, PDA, phone.
■■ Person—The end user.
■■ Occurrence—There can be multiple occurrences for a service or multiple
person occurrences in a presence document. Ambiguities are best
resolved by the watcher.
■■ Presentity—The complete picture that combines the person, services,
and devices for the user’s presence status on the network.
■■ PresentityURI—The unique identifier for the presentity on the network.
■■ Data component—Part of the presence document that describes the per-
son, service, or device.
■■ Status—Dynamic information about a person, service, or device.
■■ Characteristics—Static information about a person, service, or device.
■■ Attribute—A single piece of presence information (also called a presence
attribute).
■■ Composition—Combining presence data into a coherent picture of
presentity.The main presence data elements and their relationships can be represented
graphically as in Figure 13.8.
Figure 13.8 The presence data model
PersonServiceDeviceServiceDeviceServiceDevicePresence and Instant Messaging 235