 The concept of mobility management is not new. We have seen how we can manage mobility in the context of mobile networks, but from the viewpoint of next generation networks, the concept of mobility management is rooted in certain varieties of device, the types of mobility and the way mobility is handled. In this module and couple of subsequent modules, we are going to appreciate how mobility is perceived in the context of the next generation network, particularly the role of the IP multimedia subsystem. So we'll start with certain basics of mobility management. First thing, mobility management is a core functionality that is not part of the transport function. It means it has to be carried out out of band. It could be carried out in user plane or data plane, but it has to be an independent functionality than the transport of data. It is performed through either explicit signaling or certain admission control functions that we can call as control operations. Mobility management control function is an important module or a service which is available at the next generation network, transport as well as service stratum. We have looked at the role of different modules and services in the NGN architecture, assuming that we have a good understanding, we will proceed further. Mobility can be categorized to be of different types, just on the basis of the connectivity of the user equipment with the network and any variation of the connectivity that happens during the device is mobile. So we say that the point of contact is going to change for a user equipment. If it is going to be changed significantly, then we say that the mobility management has to be invoked or activated. So depending upon the point of contact to the network by a user equipment, the change in the point of contact in terms of the scope or the magnitude determines the type of mobility. We call this point of contact to be changing either at the access network level or at the core network level. Or we may even assume that the point of contact does not change. So there are three types. Naturally, the first and the most obvious one is when the core network changes. So it means the access network before and after the handover is now connected to different core networks. It means now a mobile or a user equipment is moving from one network to the other and the core technology has changed. It's a very big change. So it means that theoretically speaking, we can imagine that a lot of control signaling is going to take place because the overall core functions of two networks are different. A lesser magnitude mobility is when the core network does not change, but when a mobile or user equipment is moving, there is a handoff requirement in which the access network changes. It means that the core network is the same, but the access technology has changed. And the simplest one is when we can imagine that the old and new network attachment points belong to the same access network technology let alone the core network. So you can imagine the complexity to be decreasing in terms of mobility management for all these three scenarios. Let's look at it in terms of this diagram. Here we see that on the left hand side, we have a core network operated by operator one. On the right hand side, we have a core network operated by operator two. Each of these core networks has its own access network. Now taking up the mobility management scenarios for three mobility instances in which the network attachment point is changing, if a mobile moves from one core network to the other core network, it is the inter core network mobility. So the inter core network mobility management is going to be the severest of them all. Then we have the inter access network mobility. It is shown to you here for both these instances for operator one core network access networks as well as those for operator two. So let's look at the left hand side in operator one when a mobile or a user equipment moves from one access network to the other. So the core network connectivity remains the same but the access network technology changes and the most simple scenario is in which the mobile moves within the access network. So it means access network network attachment point can change but the technology is not changing. So it is the intra access network mobility. So the mobility management requirements are going to be generated out of these scenarios.