 Let us look at the service overlay network from the viewpoint of how the functionalities can be offered. So it means we are going to look at the framework of SON from the functional perspective and then we are going to look at the functions themselves by grouping them into different functional groups. So service overlay networks actually offer the functions in the same way like NGN functional architecture. So it means that the overlaying of SON on NGN is not going to be at odds or in any kind of inconsistency with the well-known NGN architecture of the service stratum and the transport stratum. SONs essentially, as I had earlier said, are not much different from the traditional telecommunication network signaling. There we had SS7 for signaling which was overlaid onto physical GSM architecture. Here also we can think about SON as a kind of next generation service overlay network. Like any other overlay, different elements of SON like different servers which provide different services are connected to each other in a logical manner and communicate with each other in a peer-to-peer manner. What we conclude from here is that there could be a master SON element and there could be a slave SON element but generally speaking each SON element is fairly independent and is deployed as an independent entity with complete control and freedom to talk to any other element in the SON overlay. This is the functional model of SON. If you look at the figure in a top-down manner, you will see that we have the user applications. These user applications are separated or partitioned in a separate rectangle and the big box below is the service overlay network. So the application layer talks to the service overlay network through the application to network interface. Then we have the SON or service overlay network functional modules. We are going to look at each one of these but all you have to understand for now is that the application is connected to the service overlay network through dedicated interface. We are going to look at that interface and this interface is also then used to provide the connectivity of SON with the NGN. On the right hand side, we see a third party. A third party is offering a unique service. For instance, this service is available to the NGN via the service to network interface. Let's look at each one of these in parts. So the first one is the application to network interface and service to network interface networking function or internet working function. What it simply means is a discovery mechanism that is required between SON overlay as a network and NGN. It is a very essential function because it allows the discovery of SON elements by the NGN and the NGN functional elements by SON. So the information which is exchanged between the application to discover the availability of an overlay and the NGN is useful in establishing a link or a connection between SON and NGN. So services can then be controlled and can then be offered managed through service control functions and it is already understood that the ANI is used for connecting the SON to the NGN and SNI is used to connect the third party SON with the application and the NGN. The next one that we have is the interface. What you saw in the figure at in the SON box at the top and at the bottom where the interface is. It means that an application actually is able to talk to SON through a specific well-defined interface and it again it is defined by the ITUT. This interface allows the exchange of relevant information for the application to create a service and to even compose a service so that an application can use one or more services. The next important functionality in the service overlay network is the control function itself. This is at the heart of service overlay because without having the control functionality including the registration, authorization, authentication etc. it is not possible to manage the execution of the overlay. The control is provided partially to the users through their application interfaces for the service providers to manage the service release. The application can be modified or personalized depending upon how a user wants an application to behave by binding certain services at the SON. Then we have the service composition function although it is quite obvious that the composition function is going to compose the inner details of a service as such and could also relate or bind multiple services together. That is why the service composition is one of the most challenging tasks because depending upon the context or depending upon the conditions and the requirements for instance emergency service provisioning which means that there are going to be definite bounds in delay and on performance. User preferences is an important concern. Security, quality of service and the adaptability are other considerations. And lastly we have the management functions because in order to maintain the sustainability and the continuity of the overlay functionality some kind of measurements have to be made and some configuration changes have to be incorporated because once an application binds certain services for a session that session has to continue, certain QoS requirements have to be managed. For instance, is it going to be QoS stringent requirement for flow that is the source and destination ports are the consideration or it is going to be a certain session ID based QoS consideration or if it is going to be a class of service which is a group based QoS service provisioning requirement. So in all QoS is a very important management concern for the son to successfully run then some kind of AAA mechanism including accounting in this case because the authorization and authentication have already been done but accounting is an important consideration and surely security management is what sons have to be taking care of as well.