subscriber data for the roaming user to VLR by sending MAP Insert Subscriber Data message via R-SGW. The message contains IMSI and other necessary parameters as defined in the UMTS/GSM specification. The message is passed through the R-SGW transparently while the SS7 to/from IP conversion is performed in R-SGW. 5. The serving VLR then acknowledges the receipt of the subscriber data to the HSS via R-SGW. 6. The HSS acknowledges the completion of location updating procedure to the MSC/VLR via R-SGW. 7. The MSC/VLR acknowledges the completion of location updating procedure to the 8. The HSS sends the MAP cancel location message to the old MSC/VLR (optional procedure). 9. Location cancellation is acknowledged to the HSS by the old MSC/VLR [7]. The steps 8 and 9 above assume that the UE was previously registered to a CN domain. The MAP messages between the MSC/VLR and HSS get passed transparently via the R-SGW. The R-SGW does not interpret the MAP messages in anyway, but performs only the lower level conversion between SS7 and IP. MSC/VLR R-SGW Old MSC/VLR 1. LU 2. UMTS/GSM Autentication 3. LU 4. Insert Subscriber Data Req 5. Insert Subscriber Data Ack. 6. Update Location Ack. 7. LU Ack. 8. Cancel Location Req. 9. Cancel Location Ack. Figure 9.13 Message sequence for roaming into a CN domain [7]. The UMTS Network and Radio Access Technology 9.5.5 Handover (HO) For HO of CS services involving the change of CN equipment (only CS-MGW or CS- MGW and MSC-server) the anchor principle applies, i.e. The first MSC server involved in a call will become the anchor MSC server for this call during and after HO, and will remain in the call until the call gets released. Every subsequent HO (intra and inter) will then be controlled by this MSC server The first CS-MGW involved in a call will become the anchor CS-MGW for this call during and after HO, and will remain in the call until the call is released. The Nc interface gets anchored in the CS-MGW, the correlation between MGW to PSTN and the MGW to UTRAN remain fixed until the call 