in its domain and routes them to the proper S-CSCF. For outgoing sessions, the I- CSCF receives requests from the user and routes them to the associated S- CSCF. The IP multimedia domain of a provider's network contains many CSCFs of each of these three types. Another vital node in the IP multimedia domain, albeit not a SIP node, is the Home Subscriber Server (HSS). The HSS contains the S-CSCF asso- ciated with an individual user and his or her profile. As a result, it knows where the user is reachable and which services he or she is subscribed to. CSCFs consult the HSS when they need such information. Call Flow Examples The following examples will help us understand the role of the different nodes in this architecture. User Registration Figure 7-1 shows terminal registration in a 3G net- work. The point of such a registration is to assign an S-CSCF to the user. The S-CSCF will be in charge of providing services to the user. This REG ISTER from the terminal is sent, as any request from the terminal would Laura's Figure 7-1 terminal P-CSCF I-CSCF S-CSCF Registration in a 3G network. (1) REGISTER (2) REGISTER (3) Query (4) Response (5) REGISTER (6) Query (7) Response (8) 200 OK (9) 200 OK (10) 200 OK Chapter 7 be, to the P-CSCF. The P-CSCF is the point of contact between the termi- nal and the network. The P-CSCF sends the REGISTER to the I-CSCF. The I-CSCF has to choose an S-CSCF for the user. To do that, the I-CSCF con- sults the HSS. Note that the protocol used between them is not SIP. The HSS can be thought of as a location server. It interacts with SIP entities, but does not speak SIP. The response from the HSS contains enough information SO that the I- CSCF can select a particular S-CSCF for the user. Then the REGISTER is forwarded to the chosen S-CSCF. Upon receipt of the REGISTER, the S- CSCF downloads a user profile from the HSS, telling it which services the user requires. Finally, it answers the REGISTER with a 200 OK response. Registration from a Visited Domain The previous scenario becomes more complex