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via Web Services and the non-BAS systems can be easily integrated as well.
For example, a weather bureau could offer a Web Service that allows a BA
system to automatically retrieve temperature forecast data for use by various
control algorithms. Similarly, the BA system itself could offer a Web Service
that allows a tenant's accounting system to obtain up-to-the-minute figures
on energy consumption.
However, since the SOAP request/response is enveloped in XML format, it
is too complex for control communications in some situations and increases
the need for processor power and additional response time.
5.8.3 Application issues
While traditional distributed communication technology platforms, including
DCOM, CORBA and JAVA/RMI, provide great implementation vehicles for
integration, none of them is a clear winner. The strengths of Web Services
as an information integrator and distributor, namely simplicity of access
and ubiquity, are important in resolving the fragmented middleware world
where interoperability is hard to come by. Web Services complements these
platforms by providing a uniform and widely accessible interface and access
glue over services that are more efficiently implemented in a traditional mid-
dleware platform. The combination of OPC DCOM and Web Services is a
good example.
On an intranet, OPC DCOM configuration and security can be addressed
relatively easily in comparison with the situation on the Internet. OPC
DCOM can achieve better communication performance on intranet com-
pared with Web Services. Therefore, OPC DCOM is a better choice on
intranet. However, when it is extended to the Internet, Web Services can
provide better security and integration performance compared with OPC
DCOM. In conclusion, the best way is to combine OPC DCOM and Web
Services in application when the TCP/IP-based control network is connected
to the Internet.
Intranet
OPC Server
Internet
Client (web browser)
Monitoring, Supervising
Web Services
Provider
SOAP
OPC Server
Web Server
Control Net
Figure 5.10
BAS integration combining the use of OPC DCOM and Web Services.
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