 Although we are not looking at various aspects which are not included in the engine reference architecture in due detail, but sometimes it is interesting to digress a little. In this module, we should look at how the engine topology is managed and how is it configured. After all, it is the physical topology that is part of the engine reference architecture. So here we shall start with how the topology is there as such physically and how it is logically composed and then we look at various subnets or modular parts of it. So first we understand that when the engine topology is to be understood, it is a physical topology but it has to be understood in a logical manner. The logical decomposition of a very large complex network into subnetworks is the key. But remember that this particular logical decomposition cannot be as such seen as the physical decomposition and it has a reason. The reason is sometimes because of the technology availability, because how the vendors and the OEMs, the original equipment manufacturers actually launch and roll out their equipment into the industry. So it is possible at times that there is a network, let's say single network, which can be orchestrated, which can be tailored to serve both as a customer premises access network and the core network. So definitely you might have guessed already that we are going to talk about the network division or decomposition in terms of the core network part, the access network part and then we have the customer premises or the customer network. Appropriate interfaces which connect one network to the other, either at the inter-network level or the intra-network level are shown to you here. At the inter-network level we have the user-to-network interface that connects the customer premises equipment to the access network. Now the access network is not one network necessarily, depending on different administrative domains, different access networks and access technologies may exist. So the customer equipment can be connected to one or more of these access networks through the user-to-network interface. In the access network, each of these networks which belong to different vendors or different service providers form the domains. These administrative domains have more than one network equipment. The connectivity between these network equipments is done through the internal network-to-network interface. Why? Because this functionality is not exposed to the external networks. Whereas when we have different networks, for instance we have at the core level, again different domains which are offering core network services, we can see that there are two different domains which are connected through network-to-network interfaces. And you can see that since each access network is connected to a core network, so the connectivity is within these two networks, it is also implemented through the internal network-to-network interface. It is so because sometimes the administrative domains are related to the same organizations. So from the organizational perspective, if the domain is the same, let's say domain one or domain A, it needs internal interfaces. Whereas if the domains are different, for instance between domain two and domain three, the interfaces have to be network-to-network interfaces. The network-to-network interfaces are supposed to be more open and they need to be visible and they are implemented with the consent of both the participating agencies. Now let's look at the subnets which are formed in these customer network, the access network and the core network. So the customer network is essentially a home network, an enterprise or an organizational network and it is connected to the user equipment via the user-to-network interface. The access network is where the end user traffic is carried on behalf of the end user to the towards the core network. So it means the access network is a kind of a transit network that carries the user equipment traffic to the core side. So essentially if there are more than one users, so their traffic is aggregated and sent to the core network. The core network is where the actual data traffic porting is done. So we have the transmission of the user data. At the same time, to manage the transmission of user data, control and management information is also transmitted. Now the core networks are connected to different network elements on the access side. So as a consequence, it could have different domain connectivities. You've just seen in the figure, if you may recall, we have the domain one which is both in the access side as well as the core side and domain two and domain three which are there in the core and access side. So depending on the diversity of domains, the respective interfaces have to be utilized. Now these are the interfaces that we have already discussed. But for the sake of recalling, let's see that the network-to-network interface is between different core network domains or between a core network and an access network if these two belong to different administrative domains. The internal network-to-network interface is within a single administrative domain. Now the scoping of the network is also important to be understood. Now the core networks actually belong to both the services stratum as well as the transport stratum. And why is that? Because the user applications and user services have to be invoked and have to be implemented onto the network. So core network has to deal with the services stratum and of course it has to carry traffic so the transport stratum is also an important function. Now the core networks are connected to each other via the transit networks. These are sometimes known as the internet exchange points or the POPS, the points of presence. The access networks however only belong to the transport stratum because the responsibility of the services is normally delegated to the user equipment and to the application to network interface.