 When the transport stratum does its job well and Connection is established for the user equipment to a network attachment point The service has to be provisioned Preferably in a seamless manner for that the service stratum is invoked for managing the mobility Let's look at some of the highlights of mobility management at the service stratum You may argue that once the transport stratum has established The requirements has has fulfilled the requirements for mobility management. Why do we need it at the service stratum? Let's recall the Kinds of handovers that could take place we we discussed them before we started with the transport stratum If the handover is taking place at the link layer, it means the IP addressing is not changing It is within the same Radio access network. So it's a horizontal handover. We know it can be handled at the transport stratum when we have Different mobile environments and the technology is changing We do handover at the network layer and we call it as the vertical handoff or handover These two are relatively easier to be implemented at the transport stratum But when we are required to do handover at the application layer it means that We are now talking about switching the service parameters from the Leaving network to the joining network It has to be implemented only at the service stratum And when it comes to the service stratum at the In the ngn architecture, it is the primary responsibility of the ip multimedia subsystem Let's take an example since we are talking about the application layer and the network layer is Below even not adjacent to the application layer if the ip address changes then It's quite likely that The service is disrupted So it means the service stratum Um assumes That the transport stratum provides it certain information about connectivity um It means that there is some kind of media independent handover provisioning In fact, there's a standard by IEEE known as IEEE 8 or 2.21 MIH or media independent handover What happens here is that the link layer information is used At the upper layer To make sure that the um handover at the service stratum is smooth um and is not affected by any significant link layer change at the transport stratum so the mobility management at the service stratum Can be seen in conjunction with the mobility management at the transport stratum Here in this diagram you see that we have the transport stratum doing its job The service stratum has now certain mobility management related entities. For instance, we have the service proxy Serving in proxy call control session functions the call session control function element Then we have the handover control functional element or entity These are interacting with the transport stratum and the user equipment or both depending upon the requirement You see here that the user equipment has shown to you here through the mobile phone diagram This mobile phone icon or symbol is now interacting not only With the transport stratum. It is also interacting live with the call session control functional element or entity At the same time it is communicating or interacting with the transport stratum which in turn is now Also talking to the service stratum. So you see that service stratum is Talking simultaneously to the transport stratum as well as the user equipment So we see that the service stratum mobility management basically is the interaction of the function elements or functional entities of the service control function and the transport functions the handover That we actually understand very well now is going to be implemented through exchange of control messages between The functional elements of the transport stratum as well as the service stratum because now the service stratum has to get some information like media independent handover in terms of the device identification like the Gateway access gateway or the edge gateway the controllers and the controller addresses to establish the Tunnels and data paths through the transport stratum because remember service stratum in itself cannot realize The service provisioning without the help of the transport stratum The service control function that is shown to you in the diagram through different the call session control function element There are certain additional elements as well. For instance, we have the Authorization and authentication function element We have the user profile which has to be consulted when a user equipment moves from one network to to other its profile has to be Tallied or consulted if the service can be provisioned to the user or not And then the subscription to certain set or suite of services is also an important concern So this is what the essential Mobility management at the service stratum is you can have a look at it once more. We have the Interactions shown to you either through the continuous lines or through dotted lines and That is pretty much all