Hello ThanhND,

I would recommend (if you haven't done so already) that you install the Fokus Open IMS Core on a machine and run multiple instances of the S-CSCF. The HSS provides a means to configure the core for multiple S-CSCF's, and assign a particular S-CSCF to the profile of any user.

Application Servers are assigned to a service in the user's service profile - meaning an Application Server can be used by more than one S-CSCF.

Unfortunately I haven't ever tried to access an AS from another IMS domain, although I have a feeling this may not be possible. Perhaps other readers could answer this one in more detail..

Hope this helps. Good luck with your research

Regards,
Mike