 independence-As the name implies, access independence means that IMS is intended to enable work with any network-be it fixed, wire- less, or mobile-and includes all the access options, such as UMTS, GPRS, The Next-Generation Network Infrastructure CDMA2000, WLAN, WiMax, DSL, and cable. Gateways are used to accom- modate older systems such as circuit-switched telephone networks and GSM cellular systems. The fact that open interfaces are used between the control and service layers (discussed later in this chapter) allows the ele- ments and sessions from various access networks to be combined. Different network architectures-IMS allows service providers and carri- ers to employ a variety of underlying network architectures. Terminal and user mobility-Terminal mobility is supported by roaming via the mobile network, and user mobility is enabled via IMS and SIP. Extensive IP-based services-Supporting a wide range of IP services is the ultimate goal, and IMS can enable support of applications ranging from VoIP to Push-to-Talk over Cellular (PoC), messaging, presence manage- ment, multiparty gaming, and content sharing. IMS Protocols IMS creates a telephony-oriented signaling network that overlays an underlying IP network. IMS uses Session Initiation Protocol (SIP), with specific extensions for IMS. As discussed in Chapter 9, SIP is a peer-to-peer protocol in which end devices, known as user agents, initiate sessions. It is an end-to-end signaling protocol, which means that all the logic is stored in end devices, except the routing of SIP messages. State is also stored in end devices only; there is no single point of fail- ure, and networks designed this way scale well. An IMS network comprises many SIP proxy servers that mediate all customer/user connections and access to net- work resources. The tradeoff for the distributiveness and scalability is higher mes- sage overhead because the messages are sent end to end. The aim of SIP is to provide the same functionality as the traditional PSTN, but because of their end- t