You have to download the latest specs from ONVIF and use their WSDLs to generate code for calling the ONVIF compliant webservices
http://www.onvif.org/Documents/Specifications.aspx[
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I am not going to lie to you, there is quite a lot of work to do if you plan on supporting most of the features.
There is a company that have developed an application called ONVIF Device Manager and they have made the application and source code available on SourceForge:
http://sourceforge.net/projects/onvifdm/[
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You should take a look at the ONVIF Device Manager, but there is a lot of code and stuff to go through in order to figure out how it works.
Soren Madsen