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 Server network information 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 order to provide compatibility between ISUP and SIP signalling systems, there is a need for a user part gateway connected to PLMN/PSTN/ISDN (sometimes denoted GSTN, general switched telephone network), and the Internet. This gateway has to handle calls, which originate in either of the two sides. The mapping between ISUP and SIP is based on the type of the message received when a connection is being established (e.g. ACM, IAM, INVITE, BY