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
Person
Service
Device
Service
Device
Service
Device
Presence and Instant Messaging 235