From a system overview, initial 3G UMTS network architecture was more or less the same as the 2G, as it included both circuit and packet Core Networks. Eventually, a new domain was added on top of the PS domain: the IMS (IP Multimedia Subsystem).
The main objective of IMS was to allow the creation of standard and interoperable IP services (like Push-To-Talk, Presence or Instant Messaging) in a consistent way across 3GPP wireless networks. The interoperability of IMS-based services comes from the fact that IMS is based on flexible protocols like SIP (Session Initiation Protocol) developed by the IETF.
In addition, the IMS standard offers VoIP (Voice over IP) support and provides interworking with classical PSTN through signalling and media gateways.
As presented in Figure, the CS domain was still part of the 3G Core Network architecture, along with the PS/IMS structure. The main reason for keeping the CS domain was motivated by the need to support the – still dominant – circuit-switched voice services and H324M-based video-telephony support.
Although IMS was presented as an interesting step towards service integration, legacy network operators refrained from widely deploying and using it as a common platform for all services (including voice, real-time and nonreal-time services) because of the lack of support for voice services‘ seamless mobility between existing CS-based networks and IMS.