0 (IMT-2000) process. Network Domains The 3G network, as defined by 3GPP, is divided into three different domains: the circuit-switched domain, the packet-switched domain, and the IP multimedia domain (commonly known as the IP multimedia subsystem). The first of these domains employs circuit-switching technology to pro- vide voice and multimedia over circuits. This domain is not dissimilar to 2G systems, where all services are provided through circuit switching. The packet-switched domain provides IP connectivity to the terminal. A terminal obtains access to the Internet through this domain. Users can surf the Web, send and receive e-mails, and do most of what can be done in the Internet. This domain does not define any particular architecture on top of IP; it can be seen primarily as an access technology. In earlier mobile gen- erations, people used a dial-up connection or an Internet Services Digital Network (ISDN) line for this purpose; 3G systems will provide Internet access through the packet-switched domain. What the user does with this access is strictly up to the user. The third domain is the most important one from our point of view. The IP multimedia domain provides IP-based multimedia services to users Chapter 7 employing SIP as the principal signalling protocol. In 3G architectures, multimedia services take on new importance and are separated out for closer examination. The IP Multimedia Domain The first question that a reader might ask is how the IP multimedia domain differs from the packet-switched domain. A 3G terminal in the packet-switched domain could theoretically use its cel- lular Internet access to contact the SIP server of the user's choice and get the service he or she wants. However, although the logic behind this sce- nario is real, the characteristics of cellular access make it unlikely to hap- pen. In order to obtain efficient data transport, a radio interface must be configured with full awareness of the characteristics of the data that will be transported. Otherwise, data transfers s