 Now we shall look at a very important aspect of service provisioning, that is, if a certain user is using a service at a point in time, a user has to have a gratifying experience from the network which is providing a service. So it means a service has to start and a service has to continue and a service has to terminate as per the user requirements. In all this is known as the service continuity. We look at how NGN looks at service continuity, what are the options which it has in order to make sure that the service continues and what are different features which are available at the transport and service stratum. So service continuity actually means that a service should not be disrupted whether it is the movement of a certain user within a certain network that is the access network remains unchanged and the only wireless connectivity is being switched from one wireless access point to the other. Then inter access network that is moving from one access network to the other. Complete movement of a user such that the IP address also requires to be changed and a user can switch from one terminal to another and the service should not be disrupted. So it means that service continuity can be termed as the inability to maintain an ongoing service or a session of a certain service. It involves some deep insight into what is the current state of variables, parameters, memory utilization, etc. The network environment for the user should remain fairly consistent. That is a user should not feel a certain drop in service because from the user's perspective the network certainly has changed. Then the sessions which have already been established should not be revoked and there should be no requirement to reestablish a certain session so that the service continuity is realized in full spirit. NGN actually considers the service continuity from the network perspective. That is the networks could be all wired, could be wired and wireless. So under the umbrella of the fixed mobile convergence, using the IP multimedia subsystem, IMS, complete architecture, the service continuity is realized. Service continuity also implies that there could be interaction between circuit switched and packet switched networks. For instance, a call is being initiated from the IP domain and it is terminating into the legacy network, such as a PSTN or PLMN. Likewise, TV service, once it is being broadcast from traditional broadcasting station and if it is going to be streamed live on the PCs or the tablets of end users, then the circuit switching and packet switching or the broadcasting stations have to collaborate with each other in real time. This is not easy, but this is exactly what NGN realizes through the immaculate system of IMS. At the transport stratum, the basic support is provisioned through the mobile IP that is IP version 4, mobile IP as such, that uses the concept of registration and tunneling and the handover, the care of addresses, the home network, the foreign network, etc. In service stratum, this is a little more tricky because providing transport or the physical access is the first step, but the features which are associated with the service are so many that it needs separate discussion that we shall shortly do. One effort by TF has actually been the initiation and experimentation with the host identity protocol. Basically, it is a mechanism to decouple or disassociate the IP address which plays a pivotal role in identifying a certain service from the socket point of view because we know that a socket actually is identified through source IP, destination port, destination IP. So IP address becomes so important that once a certain network interface with a certain IP address is involved in a certain service at the initiation and if there's a requirement to change it, it becomes quite complicated and difficult to change the IP address. So this host identity protocol, aka hip, introduces something new. It is basically a 128 bit host ID tag which is above the network layer. So it means now the transport layer does not interact with the IP address to open the socket rather this 128 bit host ID tag is used. It's a very interesting concept because now the namespace of socket is going to be extended, but the overall impact on the global internet remains to be seen. At the network layer, if you look at the options we have above, we've already covered, but below IP there are certain initiatives. For instance, the 802.21, the media-independent handover. It is a mechanism through which the local identifiers of different networks are exchanged with each other to allow seamless handover. Now this particular handover is IP agnostic. It means the IP address is not changing, but the physical data link layer identifiers are changing. So certain protocols like Wi-Fi, WiMax, Ethernet, UMTS, etc. use their own identifiers against the same end-to-end socket to make sure that the service stays continuously available. As we had briefly earlier said, that service stratum needs more time. So here we are going to now look at how the service stratum considers service continuity as a very important aspect. So first of all, we need to understand that the services need to adapt dynamically to the changing environment, changing network conditions, the dynamism of the network. For instance, the traffic load could change in no time. The QS capabilities could vary from one network which was earlier adopted to the network to which the user is switching. Then the capacity of the egress network and coming down to the ingress network could be a time-wearing phenomenon with regards to the active number of users. The user preferences could change in the middle once the service is being provisioned. Then there could be a certain service level agreement between the operators and inter-operator handover is required so that the service should continue. For this complicated wish list, there are two options. The first one is again focusing on multimedia traffic as such. So the multimedia service continuity actually considers all these services which are required to be there uninterrupted and the overall coordination, synchronization and their availability for providing the multimedia traffic. Now all the particular multimedia traffic types such as voice, video, data, interactive games, etc. need to be provided through the always on connectivity and synchronization of services which would offer this multimedia traffic. Then there's a specific case for voice because the voice is something which is relatively straightforward as compared to the enormity of the multimedia traffic. So seamless voice services also need to be synchronized. These have to be available. Let's look at certain interesting service continuity scenarios. We have three scenarios here. On the left hand side in A we have service continuity on the same terminal once the access changes from for instance mobile access to Wi-Fi access or the other way around. So it means here the connectivity is the inter access network connectivity as an issue. In B we have the service continuity between different multimedia terminals for instance for the same service on the same network. Now the traffic needs to be provided from the mobile terminal which could be on UMTS, which could be on 3G, Wi-Fi, etc. or Wi-Max. And then it needs to be switched to for instance laptop. In that case the service is going to be continuing only if proper set of services, functionality and resource availability is ensured by the network. And then lastly we have the service continuity once the telephone terminals change. This is a typical example of how the circuit switched and packet switched networks interact with each other. We see here that we have a mobile access on the left hand side and we have a plain old telephone system that is a circuit switched network. So it means now a whole new set of services would be required to make sure that signaling which is different in both these networks takes place. The services which are required on both the ends are uniform and they are synchronized with each other.