Is it necessary HSS and S-CSCF both the nodes belongs to home network for an IMS call ?

Question

Is it possible HSS belongs to home network and S-CSCF belongs to visited network for the Cx interface ? If not then why ?

Answer 1

No. This is not possible.
To initiate a call, roaming subscriber would send the Invite to visited PCSCF.
Based on service-route hdr information(stored by this PCSCF during registration), PCSCF would send the request to home SCSCF.
Home SCSCF now would contact HSS to get the caller profiles for this user.

So Hss and Scscf both belonged to home domain.

Answer 2

Just based on the understanding , two Diameter nodes can communicate in case of the inter-network . For example : V-PCRF talks to H-PCRF (S9 interface) . I don't have clear answer and expecting some IMS network expert would put some light in this area.