 the last step, the signalling IP address gets allocated in association with PDP context activation and not on an incoming call basis, then the selected CSCF becomes the serv- ing CSCF2. From the point of view of the latter, the IP address provided by the UE cor- responds to the address where the UE is reachable for MT call control signalling and/or any other type of MT signalling. Whether a procedure gets activated individually by the UE or automatically depends on the implementation of the terminal and on the UE's configuration. For example, a UE multimedia application may start the application level registration and steps 2-4 would need to follow in response to support the operation initiated by the application. 9.5.4 Location Management Figure 9.12 illustrates the registration concept for a R00 subscriber roaming into a UMTS/GSM CN domain. Visited GSM Nwk R-SGW MSC/VLR Update Home R'00 PS-IM/CS Nwk Location (IMSI) Visited UMTS Nwk MSC/VLR Update Location (IMSI) UMTS (R99-CS, and R00-CS in case of non-IP transport) subscriber data download GSM (CS) subscriber data download Figure 9.12 A roaming model for registration in a CN domain. From Ref. [7] Figure 9.13 illustrates the detailed message sequence chart for a UMTS ROO subscriber roaming into a CN domain. The sequence can be summarized as follows: 1. The UE initiates the UMTS R99/GSM location update (LU) procedure with the MSC/VLR of the visited network, where the LU message contains the IMSI of the subscriber. 2 Note that the S-CSCF can be either in the home or a visited network. Towards IP Based Networks 2. The UMTS/GSM authentication gets performed as per the existing UMTS R99/ GSM specifications. 3. The MSC/VLR initiates the MAP location update procedure towards the HSS of the user via R-SGW. The HSS stores the VLR address etc. The message contains IMSI and other parameters as defined in UMTS R99/GSM specifications. The message is passed through the R-SGW transparently while the SS7 to/from IP conversion is performed in the R-SGW. 4. The HSS provides the 