 Network which involves legacy systems, for instance, PSTNs, PLMNs and POTS, need to eventually move to the next generation network. Next generation network with its all power features coming from the service stratum and the transport stratum is amazing in its own right. But when the legacy systems have to evolve or to migrate to NGN, certain challenges arise. So in this module we are going to appreciate the process of migration with its own complexities, challenges and various approaches which are applicable. The migration or the evolution is the process in which parts or segments of the existing plain old telephone systems, the examples which I have already mentioned are upgraded or fully replaced. This process of upgradation or complete replacement could be done across multiple phases. The eventual goal is to enable the NGN services by incorporating certain elements, finally reaching to a point where the legacy hardware is done away. We can achieve this by considering the functionalities as in different network elements, which could be replaced either by adopting the emulation approach or the simulation approach. So this emulation or simulation could be considered as a process that is going to affect a number of entities. These entities could be at the transport network, for instance, the access and core networks, the user side, the network side, the access network itself, the classical form of switching that we know as exchange in both core and exercise along with all their interfaces. Then we have the signaling infrastructure that is the dialing process which involves the overlay such as signaling standard number seven. The functionalities which are embedded with this signaling are usually located at the exchange or the switch itself. So how are these going to be emulated or simulated? Then we have the management services, for instance, the classical F caps model, including the fault management, configuration management, the performance management, security management, and the access control management. And then the services itself in public land mobile networks roaming, such as in GSM networks, is a given. How could we think about migrating this to the NGN architecture? And then there are some value-added services. We'd look at these in due course of time. So for all these, the issues are going to emerge, how to transport it from the classical network to the NGN. If it gets any more complicated by assuming that there are lease lines versus the NGN, which is purely IP based. And then the services with their own coupling, in fact, strong coupling with their underlying classical network architecture. And then the supplementary services, including call forwarding, call barring, etc. Last but not the least, the numbering system, which is used in, for instance, ports like E.164, which starts off from the leftmost side, including the country code, the city code, the neighborhood, and the client or the customer number as such. Now, these are the issues that have to be handled with very carefully. There are a lot of approaches, which sometimes overlap, or which could be seen distinctly. So we'll say that the migration process or the evolutionary process could be implemented by choosing a call server, kind of a soft switch within the core network. So what will happen is, we'll immediately transfer some of the switching to packet based networking, including the routers and switches. So let's assume that we are going to start off at the local exchange. So this actually is the first scenario. Here we are going to replace the local exchange, which is a switched network with IP enabled access gateway. If we start off from the transit exchange, we can think about incorporating IP enabled trunking media gateways and the signaling side is incorporated through signaling gateways. This could be the second possibility or we could go more aggressive and we could replace the local exchange and the transit exchange in a one step fashion. So it means we are going to consider top down replacement of TEs and the bottom up replacement of LEs. Instead of using a call server, we could replace the core network functionality by a complete IP multimedia subsystem, which is used in NGN. In this approach, the IP multimedia subsystem that integrates very seamlessly with NGN, the end users access services using IMS with NGN enabled user equipment or if some legacy user equipment or terminal equipment is to be used, then it could be used via the access gateway and the services which could readily become available are the voice over IP, IP television with a certain QoS grade and best effort internet service. The trunking media gateways, which not only carry voice but multimedia traffic and the signaling gateways, which are implementing part of the SS7 kind of overlay signaling network functionality are deployed for interconnecting the NGN with the classical PSTN or integrated services digital network based networks. So this allows the coexistence or backward compatibility of a migrated part of the entire network with the legacy part of the network. So we can finally conclude by looking at the migration process. Here the call server based migration is not shown because there's a lot of complexity there but the eventual goal is to move the existing legacy systems to NGN using IP multimedia subsystem. So here you can look at the upper part which involves the PSTN, ISTN temporary arrangement, the legacy arrangement with the local exchanges and the transit exchanges but once the migration is to be done then IP multimedia subsystem replaces the four switching elements and the access gateways and the trunking media gateway and the signaling gateway all communicate via the IMS. If you notice something very interesting we have the left hand side which involves legacy terminal equipment accessing IMS for NGN services through the access gateways but on the right hand side we have equipment which is fully IP enabled such as IP phones or IP TV which can directly communicate and exchange that is send or receive signaling information with IMS thereby this IP multimedia subsystem migration takes care of the responsibility of supporting the legacy networks so that sooner or later the components are completely replaced by NGN compatible or NGN based hardware.