Image Processing Reference
In-Depth Information
node identifiers are delimited by a slash, with the attribute identifier following after a colon (e.g.,
/building_1/west_wing/boiler/temp:value ).
Each node has a mandatory attribute specifying the node's type. The node type may be chosen
from a predefined list of strings. It describes the semantics of a node or subtree and determines the
mandatory attributes. For example, a point node resembles a data point from the underlying sys-
tem and has a mandatory value attribute. While a point is (reasonably) a leaf node, area nodes or
system nodes are used as directory nodes and provide semantic information for their subtree (e.g.,
“west wing” or “HVAC”). he special node type reference allows multiple use of a node's data in the
hierarchy by providing the path to its referent and reflecting most of its attributes.
BACnet/WS operate upon node attributes. Services are provided for retrieving and modifying sin-
gle primitive values as well as “plural” versions of these services operating on a collection of arbitrary
multiple nodes in order to reduce network traffic by batching the operations. Additional services
allow read-only access to entire arrays, historic data, and locale information.
29.5.6.2 oBIX
The oBIX initiative is hosted by OASIS []. The oBIX specification defines a flexible object model
for describing the data and operations available on a server. In oBIX, everything is an object: objects
are also used to describe data types (classes) and operations (method signatures). Together with the
possibilitytocustomdeineanykindofobjectbysubtypingandcomposition,thismakestheoBIX
data model highly extensible.
Conceptually, every object is derived from the root object . It specifies a number of mandatory
attributes, such as an object's name, which are inherited by all other objects. Like in every data model,
various object types holding primitive values are defined. These are Booleans, Integer and Floating
point numbers, enumerations, Strings, points in time and time spans. oBIX base object types also
cover a number of universally applicable concepts: lists and feeds (containers with either static con-
tent or event queue semantics), errors, references to other objects, and operations. Custom classes
can be derived from any object type. Multiple inheritance is supported. Every custom-built class can
be handled by any oBIX client.
For flagging primitive values as coming from the automation system, oBIX offers Point classes.
Read-only points are effectively an empty class (acting merely as a semantic marker); writable points
additionally specify an operation for altering the corresponding value. oBIX also provides a very
flexible SI-based system for describing engineering units, which is again based on objects.
Furthermore,theoBIXstandardlibrarycontainsacoupleofpredeinedspecialpurposeclasses
which a client can use to access functions of an oBIX server. This includes a service which relieves
the client from monitoring an object's state (Watches), batch operations to reduce communication
overhead, and historical data access.
Historical trends can be represented via the oBIX History Record, which groups a point value and
a time stamp. The History object consists of a list of history records and methods to query them.
Query filters can be specified and extended to a rollup calculation (e.g., for average values).
Eventually, oBIX defines a normalized model to query, watch, and acknowledge alarms. An oBIX
server supplies feeds of alarm objects. Every time the server detects that the value of an object meets
a predefined alarm condition, an alarm object is added to the feed. his object contains a timestamp
andpointstoitssource.Alarmscanbestateful(i.e.,thepointintimewhenthesourcereturnedto
normal is recorded) and they can record if (and by whom) the alarm was acknowledged.
oBIX follows what is called a RESTful approach (Representational State Transfer), a resource cen-
tric architectural style for WS. Central concepts of the RESTful approach are resources that share a
uniform interface and a highly restricted set of operations on these resources. his approach mimics
the World Wide Web where only the commands GET, PUT, and POST are used to access countless
resources.
 
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