 Since we know that these gateways are essentially software modules and components which can be implemented on one hardware or the other. So this results into some flexibility that can be utilized. Consequently, some interesting scenarios of deployment emerge and because of those interesting scenarios, we have corresponding interfaces which are activated. The serving gateway and the packet gateway are actually seen as two distinct gateways in the evolved packet core, we've seen those. And we also know that the serving gateway actually terminates the traffic which is coming from the user equipment and opens up an interface with the packet gateway on the GTP, GTPU tunnel. However, sometimes for the sake of cost cutting, optimization and ease of deployment, some vendors provide the serving gateway and packet gateway which are physically located into a single module. So the implementation of the corresponding interfaces is necessarily obviated. The need is finished because now the gateway which is known as the system architecture evolution gateway is now one piece of hardware that is implementing both these gateways. So it's an interesting scenario. Now the serving gateway and the packet gateway have their own responsibilities. If you remember with regards to the QS implementation and with regards to the charging and the policy rule functions, one was at the packet level, the other one was at the service level. So these two gateways are also connected through the policy and charging rule function implementation. The control interfaces now which allow this QS implementation and the charging and rule function implementation through the policy enforcement points which are the serving gateway and the packet gateway use the diameter protocol. The policy charging and enforcement functions are now located within the serving gateway and the packet gateway. For non 3GPP IP cans, both these gateways have to implement the diameter interface through the S6B interface and X6C interface. It actually means now the AAA functionality is implemented externally. Let's look at these two interfaces now. So we have S6B interface that is the packet gateway and 3GPP AAA interface. It is implemented in the control plane and you see it is implementing the diameter protocol and SCTP over IP and it's not shown in here the X6C interface but instead you see that we have the GXC interface which is again a control plane implementation. This is the policy and charging rule function server to the serving gateway implementation. It is a control functionality and it is implementing diameter over SCTP over IP.