Image Processing Reference
In-Depth Information
29.5.6 Web Services
For purposes of management-level system integration, a common, preferably open format for access
to BAS data points (and possibly management functions as well) that is aligned with IT standards is
a key asset. HTTP and HTML certainly are popular in BAS as well, but are limited to user interfaces
since HTML is ill-suited for machine-to-machine communication. herefore, by tradition, OPC []
has fulfilled this role. However, it is in its classic design incarnation restricted to Windows plat-
forms due to its use of OLE/DCOM (Object Linking and Embedding/Distributed Component Object
Model).
Today, solutions emerge that follow the current mainstream trend of using WS for machine-to-
machine interaction over a network. WS are entirely platform independent. hey can be implemented
using any programming language and run on any hardware platform or operating system. his max-
imizes interoperability and greatly improves the reusability of the services they provide. Central to
WSaretheuseofXMLfordatarepresentationandastandardizedresourceaccessprotocol(typically
SOAP, sometimes also plain HTTP). WS follow a modular concept. This allows the use of off-the-
shelf standards for transmission, eventing, discovery, security, and many more. However, a drawback
is the additional overhead introduced by the XML encoding (especially if small amounts of data are
transmitted frequently) and the fact that servers cannot push information to clients (unless the latter
implement a server themselves). Given the hardware resources available at the management level,
this is typically not an issue, though.
The commitment to use WS technology determines low-level aspects of data representation and
transmission. Still, the application domain remains to be modeled. Today, three standards hold par-
ticular promises for bringing the world of WS to BA. First, an OPC XML variant for access to data
points has been defined, and implementations are available today. Moreover, the forthcoming OPC
UA (Unified Architecture) will, besides other significant architectural improvements, also be cen-
tered on XML and WS (but has not yet been disclosed to the public at large). Second, BACnet was
extended with a WS interface (BACnet/WS). hird, the oBIX (Open Building Information Exchange)
initiative added another challenger. All three provide historical data access and event and alarm man-
agement besides a data point abstraction. Both the BACnet/WS and oBIX standards, which have been
developed with special regard to building automation, are freely available to the general public.
29.5.6.1 BACnet/WS
Although published as BACnet Addendum -c in , BACnet/WS (Building Automation
and Control Network-WS) [] is not tied to BACnet as an underlying BAS. It can equally be used to
expose data from non-BACnet systems—although a mapping between the BACnet and BACnet/WS
data models is, of course, defined.
The fundamental primitive data element in the BACnet/WS data model is the
node
. Nodes are
arranged in a tree structure, having one mandatory parent node (except the root node) and an arbi-
trary number of children. From the network point of view, a node is mainly a static collection of
attributes
. An attribute represents a single aspect or quality of a node, such as its value or writabil-
ity. Each attribute has a value of a specific type. A static list of primitive value types is defined that
includes well-known types such as Boolean, Integer, Real, String and more, but also an enumeration
type (called Multistate). Besides, an attribute may also be an array of a primitive value type. For some
attributes, BACnet/WS supports localization (also for multiple locales simultaneously), for example,
a temperature value may be available in Fahrenheit, Celsius, and Kelvin at the same time, or a name
in various languages. Attributes may be mandatory or present under specific conditions only. hey
may also have subattributes.
Nodes and attributes are identified by their position in the tree structure. he hierarchical struc-
ture of nodes is reflected as a hierarchy of identifiers, resulting in a path used for naming. The
