The growing desire of network providers to introduce support for voice over IP (VoIP) has created interesting challenges in the area of interoperability with existing wireless circuit networks. The 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2) standards have defined the IP Multimedia Subsystem (IMS) as the platform for convergence. By definition, IMS is access agnostic; it provides services and features through a common core network, regardless of the means of transport. However, the IMS standards are just beginning to address the challenges associated with interworking with existing cellular circuit networks. Achieving seamless mobility involves supporting both roaming and handoff between networks. This paper discusses the issues involved in providing seamless mobility for subscribers across the packet and circuit domains and proposes network‐based solutions. © 2006 Lucent Technologies Inc.

The evolution from traditional circuit-based networks to packet-based networks is a salient feature of today's telecommunications industry. As IntroductionPacket networks based on Session Initiation Protocol (SIP) and Internet Protocol (IP) are showing great promise for the telecommunications industry. Industry analysts believe this new technology will enable a large number of new revenue-generating services, reduce the cost of operating a network, and revolutionize the nature of telecommunications. As SIP networks mature to support mixed endpoint types and replace existing portions of the public switched telephone network (PSTN), interesting issues arise relating to interoperability among the various endpoints.This paper begins with a discussion of the various endpoint types that may coexist in a SIP-based service network. It then explores some of the issues that surface when interoperating between mixed endpoint types. The issues discussed include interworking between ISDN User Part (ISUP) and SIP, call progress handling and early media, media transport, and signaling transport. Endpoint TypesSeveral different types of endpoints (e.g., native SIP endpoints, gateways to PSTN endpoints, transit nodes, mobile switching centers [MSCs], and application servers) may coexist in a SIP-based network. Native SIP endpoints use SIP to set up call sessions; gateways to PSTN endpoints provide access to the Bell Labs Technical Journal 9(3), 183-197 (2004) Native SIP EndpointsA native SIP endpoint is an endpoint that uses SIP signaling to establish a call session. SIP signaling is used to create, modify, and terminate multimedia sessions. Networks that support native SIP endpoints must make allowance for a number of characteristics common to such endpoints. For example, native SIP endpoints use a uniform resource identifier (URI) and a dynamic addressing scheme that requires domain name server (DNS) resolution. They also support a wide variety of codec types. Finally, they are not homogeneous; they may have a variety of capabilities and user interfaces that can be used to establish sessions. For example, when a callee is being alerted, a native SIP endpoint with a graphical user interface (GUI) may display a message or a picture of a phone ringing, while native SIP endpoints lacking such an interface may simply provide a local ringing tone, as most existing SIP endpoints do. Because of such differences, the progress of a call is communicated by having a terminating SIP endpoint generate a SIP signaling message and send it out-of-band to the originating SIP endpoint. This allows the originating endpoint to provide distinct behavior based on its capabilities; for example, an endpoint with a GUI may show a picture of a phone ringing instead of providing an audible ringing tone. SIP endpoints typically provide call progress indication using SIP provisional messages out-of-band.A preconditions feature can be used to minimize ghost rings to the originator of a session. (The term "ghost rings" refers to an immediate alert that ...