 Now, let's expand our understanding of NGN functional architecture and we start looking at all the entities which are part of the NGN to be tethered into a fabric through some kind of connectivity and media. In this module, we should go into different kinds of connectivity models by looking at their requirement and their implementation uniqueness. First of all, we need to be sure about the requirement to have connectivity. We all know that NGN has standardized the functionalities. These functionalities are distributed across the whole network. Some of these are on the user side, some of these are applications, some are services and some are related to the network implementation. But when we talk about NGN, a next generation network that could be one of the next generation networks, we need to talk about certain connectivity mechanisms that allows these entities or the functional entities to be connected to each other. For that, we can talk about a concept called interfaces. In NGN, the open interfaces are promoted. The first one is naturally the user-to-network interface, UNI. UNI actually is responsible for providing connectivity between the N terminals and the NGN. The N terminals could be user, user application, user handheld device, a desktop or any other kind of input-output device. The user network could also be the part of the user community. It could be a home network or it could be a corporate network from where a certain user is connecting to the NGN in the form of a group. Then we have the network-to-network interface, the NNI. NNI provides a mechanism to exchange the call signaling and control information over the network from one particular kind of network-related protocols to another kind of network-related protocols. So it means some kinds of protocol translation or also called media translation could be required. For that, the notion of media codec, that is encoder-decoder, is relevant. The network-to-network interface essentially can help us to connect entities within one NGN to each other that use different protocols. To connect one NGN to another NGN. Connecting NGN to certain legacy networks or proprietary networks. The service-to-network interface is what would allow a service to reach NGN and a service to be accessible from NGN. Once a service is provided by a service provider, a service provider would also have the user plane, the control plane. So it means essentially the service-to-network interface handles both the user plane side and the control plane side. An example of the service-to-network interface is the requirement of defining interfaces when IPTV content from a third party is to be integrated into NGN. How would that happen? Let's look at the IPTV content first. Let's assume that a certain third-party company thinks about offering its services to the NGN users on the content that it has created. Now there's a requirement not only to establish the user-level interfaces but since the content is coming from a third party, so the control plane is also required to be activated. So these two planes are important. Now if we recall, the service-to-network interface is related to the service that we talked about in NGN. But once we have the application, the application is relatively simpler because it doesn't involve the underlying control infrastructure to be activated. So the application-to-network interface provides all the necessary interactions, the syntax, the semantics, the message flow which are related to the content delivery over the control plane. It means since the application is more closed, so how exactly the application has encoded the data is not much of a concern because the application is being integrated into NGN. So compared to the service, the application in NGN does not address the user part. An NGN itself can be an application provider. If it is itself an application provider, it means the user part is already covered because the IP stream is developed in-house. So the content type, the codec, is compatible to the NGN because the content has been developed within the NGN community. So it means only the control information is required to be executed. This is the final illustration here we see that we have the NGN shown as a cloud and appropriate interfaces such as the UNI, NNI, ANI and SNI are shown in here. You can see that certain types of service providers or equipment or users are shown to be connected to the NGN through their respective interfaces.