ormation including identification, security, location and service profiles. For reliability reasons there must be more than one HSS/HLR. The node can also support other systems such as WLAN. The 3G HLR is part of the HSS functionality. It also needs to communicate via new IP-based interfaces. Like the HLR, the HSS contains or has access to the authentication centres/servers (e.g. AUC, AAA) and SMS nodes, as well as the FNR register. The HSS may consist of the elements and interfaces shown in Figure 14.18. The DNS and the location server are sometimes referred to as the user mobility server (UMS). 14.9 THE DOMAIN OF (VOICE AND) SIGNALLING OVER IP A typical feature of this domain is that digitized voice encapsulated in RTP (real-time transport protocol) packets is sent over carrier-grade IP networks using the services for Control Network Application and services Subscription information Server network Location server GMSC server 2G GMSC MSC server R-SGW 2G MSC Figure 14.18 The central role of HSS as a spider component for short- and long-term subscriber data, such as location, security, numbering and mobility management transport, provided by the layers below RTP (UDP, IP). At the receiving end of the VoIP network the packets are de-capsulated, and the original information is recreated. Another associated feature is the use of the SIP protocol for signalling with H.323 as an alternative. SIP often interfaces ISUP signalling, where IP and PSTN/ISDN meet. So far, large-scale public voice over the Internet has not been used very much for QoS reasons. The VoIP architecture of the pertinent domain is called Softswitch. Softswitch is the con- cept of separating the network hardware/traffic resources from network software/hardware control. Softswitch is nowadays often interpreted as a specific control layer component, also called media gateway controller or call agent. See Figure 14.19. The separation of traffic resources and control is valid for other domains as well. 14.9.1 USER PART MAPPING BETWEEN ISUP AND SIP In orde