cycle involves opening a VISA session to each of the instru- ments used in the laboratory. The syntax for identifying a remote instrument is as follows: visa://ip_address_ of_the_webserverlinstrument_ID After opening a VISA session, an instrument can be controlled using standard SCPI-Standard Commands for Programmable Instruments commands (e.g., the syntax for measuring a DC voltage is: MEAS:Voltage:DC:?). Such commands are sent to the instrument using the VISAWrite.VI LabVIEW function. The measured value can be viewed by calling the VISARead.VI function, whose output is con- verted to a double format and placed in an array. After being discovered-as described before-the VISA entities can be used for a LabVIEW program (e.g., the SCPI command to a remote DMM, for the mea- surement of a DC voltage-Figure 12.36). If LAbVIEW "full-development system" is not available, one can download and install the free NI LabVIEW Run-Time Engine, a kind of "LabVIEW player" (http://sine.ni.com/apps/utf8/niup.ni?ap = GB_NIDU&ip = 209&loc = en-US& du = =http://joule.ni.com/nidu/cds/view/p/id/679/lang/en) and open the VI appli- cation in .EXE format. visa://82.78.149.213/GPIB0::22::INSTR MEAS:VOLT:DC? Figure 12.36 VISA measurement on a remote DMM. Service Delivery Platforms 12.3.2.1.1 Software Technologies Overview In this section, we will make a short description of the used technologies and soft- ware libraries, and then we will focus on the implementation of the Web service that will allow remote access to VISA instrumentation. Web services are powered by XML and three other core technologies: WSDL, SOAP, and UDDI. XML (eXtensible Markup Language): XML forms the basis for all modern Web services, which use XML-based technologies to describe their interfaces and to encode their messages. WSDL, SOAP, and UDDI all use XML-based messaging that any machine can interpret WSDL (Web Services Description Language): The WSDL definition acts as the initial Web service interface, providing clients with all the information they need to interact