IMS: How does S-CSCF locate PGW for a terminating VoLTE 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

Hi Harshita,

SCSCF does not need to locate the address of the PGW. The way it works is that when user performs IMS registeration, the register request is routed to first to P CSCF function whih performs dns query to detemine the address of the icscf. Icscf queries HSS to retrieve user’s capabilities to route the registration to appropriate SCSCF. When request is receieved by scscf it registers subscriber and records the contact header which contains address of pcscf. And so when a call needs to terminate to a subscriber. Scscf have the address of the pcscf which routes the request towards UE’s ipv4 and ipv6 address. As this ipv4 and ipv6 subnets are advertised by specific pgw which is serving the customer. And this is how request reaches corrct pgw