A technical prerequisite for a converged network is a common service delivery platform that is independent from the access networks below. The 3GPP SIP-based IP Multimedia Subsystem is defined for this purpose. By specification, IMS is the first implementation towards reaching converged communications which allows users to communicate with video, audio and multimedia content, via any fixed, mobile and wireless access network type, with controllable QoS. This article analyses the mechanisms of resource allocation and admission control by employing logical interfaces that carry SIP messages. Keywords: IMS, TISPAN, PSTN emulation, Resource Admission, Metro IntroductionThe concept of Next Generation Network (NGN) provides a new network infrastructure with features and capabilities that support the provision of value-added multimedia services over multiple and heterogeneous QoS enabled transport technologies. In this respect, the ETSI TISPAN group [1] is working on the specification of an NGN based on the IP Multimedia Subsystem (IMS). IMS was introduced in the release 5 of 3GPP standards in 2002, as an IP-based architecture to control of the new value-added services with QoS requirements that were envisioned for UMTS. But, although IMS has conceptually been designed to be independent from the technology used in the access network, the standards developed by the 3GPP are mainly focused on the UMTS IP connectivity access network. From the previous work done by the 3GPP, ETSI and 3GPP started to cooperate in 2004 in the ETSI TISPAN group, in order to define a Core IMS suitable for wireless and wire line networks. TISPAN has published a first release of ETSI IMS standards and is currently working on a second release. We could say that 3GPP describes the point of view of mobile operators (support of new applications), while TISPAN adds the wire line operators specifications (convergence). TISPAN makes specifications for several non IMS subsystems like Network Attachment Subsystem (NASS) and the Resource Admission Control Subsystem (RACS) [2]. Most of the IMS protocols are standardized by the Internet Engineering Task Force (IETF) (e.g. the Session Initiation Protocol (SIP)). Other standardization bodies are involved in the development of IMS. On the other hand, with the rapid development of broadband services, carriers have gradually shifted their focus to broadband AN after expanding and reconstructing the core and service access control layers of MAN. This leads to an analysis of Metro AN (Metro Access Network) technology development. This paper aims at depicting the overall IMS architecture, protocols and technologies of metro access, as well as the related motivation. This paper is divided NGN Basic principles in IP Multimedia SubsystemIMS is a converged driver around IP-centric networks for delivering multimedia services from any access technology. One of the basic principles of IMS is Access independence. IMS will eventually work with any network (fixed, mobile or wireless) with packet switching functions...

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