The IP Multimedia Subsystem (IMS) delivers an architecture for supporting multimedia services using Session Initiation Protocol (SIP) over Internet Protocol (IP) networks with managed quality of service (QoS). However, the IMS network was not designed to be "open" and therefore does not allow interworking with "non-IMS" user elements or networks. The IMS standards basically assume that media-rich end-to-end sessions over IP are only possible between IMS subscribers. We believe that this assumption is unnecessarily restrictive, since it does not allow IMS subscribers to interwork with the fast growing base of non-IMS IP broadband subscribers, who nevertheless are equipped to perform media-rich voice and video SIP sessions. This paper will show that allowing IMS subscribers to interwork with non-IMS (but still SIP-enabled) broadband subscribers is easy to achieve from a technical and architectural point of view. The proposed solution is safe, provides authentication, and allows IMS subscribers to take advantage of their terminating QoS even though they are accessed from the public IP network by a non-IMS subscriber. We will also show that this capability can generate new revenues in several ways and may also help to discourage wireless and wireline broadband subscribers from embracing nontraditional service providers such as Google, Skype, and Vonage.
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