 Service overlay networks is a very interesting concept and a very relevant one for the next-generation networks because it provides the abstraction and the decoupling for applications provisioning by using multiple services. When we say that it's a service overlay network, it means it's an overlay of a logical network on top of a physical NGN that uses logical services from different entities spread over the entire NGN. So let's start with the understanding of what SONs are and then we are going to look at a generalized model that deals with the explanation of SONs. Now the NGN once deployed is a completely utilizable and available infrastructure to deploy different services for various applications since some kind of flexibility and abstraction is required for NGNs so that different kinds of services which are diverse in their design, in their availability, in their customer base and choices has to be provided. So a more logically designed services offering has to be ensured. For that, SON actually emerged as a logically interconnected network. Physical interconnection is always done through wires or through physical medium. Wireless is also one option. So once we say logical, it means for a given physical infrastructure, different combinations can be established to link different nodes depending upon different requirements. So SONs offer different services in the form of functionality which are used by the applications which users are subscribing to. So SON is essentially an ITUT mandated proposal that has been there for quite some time. In fact, we'll study it shortly that SONs actually have always been there as signaling signaling signaling networks which have been there for telecommunication networks. But lately these have been termed as service overlays. So once a SON is deployed as an overlay onto a physical NGN then emerges the requirement to use these services. These services can be used to establish application provisioning by using these services, modifying the behavior of these services, subscribing to unregistering or even extending the services. SON since is an overlay, it means NGN has to interact with it. NGN interacts with SON either via ANI interface or SNI interface. So ANI basically is the application to network interface. We'll quickly look at the figure in the next slide or through the service to network interface. Here, once we are talking about the service to network interface, it means we are actually considering the service overlay network coming from a third party. It means that we have a service provider which is offering its service to the NGN. So it opens an interface known as SNI but for a complete service overlay network, the application to network interface is going to be required. Now whatever we are discussing, we are looking at service overlay as software implementation. So it essentially turns out to be an application layer phenomenon. So looking at the TCP IP stack, we can assume that the service overlay network is an application layer phenomenon. Let us look at a very generalized simple view of service overlay networks over NGN. Here you can see we have the entire NGN represented through the main cloud at the core with routers connected. And we have different users coming from residential networks, enterprises, customer premises, networks, etc. So all these are connecting from the exercise to the NGN. Now this NGN interacts with the service overlay network through the ANI-SNI interface. The service overlay network, if you notice very interestingly, is offering its services to the application layer. It means the user applications are using the services which are being offered through the service overlay network. So this is a quick rundown of what service overlay networks are. We are going to look at it in more detail.