 Another important functional requirement for QS provisioning in NGN is signalling. In fact, when we were discussing Admission Control, signalling actually came under discussion. However, we need to look at it in isolation as well. Just like Admission Control, that makes sure that certain types of traffic are entertained by the network and certain services are denied, certain user requests are rejected. Signalling is basically the mechanics or the implementation that helps great deal in Admission Control. So, we say that Admission Control and Signalling go hand in hand. So, let's look at the need for signalling and then we'd look at the adoption of protocol and standard by the NGN. So, signalling is basically an inquiry or a query response mechanism which is needed to implement a certain call session. Signalling is basically the discovery of the resources and sharing of such information with the participating entities. Through signalling, we can determine if there are enough resources available between the calling and call party. For instance, if bandwidth is a concern to admit a certain call, then using signalling, it can be determined if enough bandwidth is available between the two hosts. Signalling is basically a requirement prior to call establishment, however, it is also required to ensure that the call continues at a certain quality of service level. So, it means that signalling can go on while a call is established to make sure that certain QS levels are maintained. The signalling standard which has been adopted by NGN is the Session Initiation Protocol. SIP is a very comprehensive protocol stack that involves multiple protocols which work together to provide signalling support for multimedia traffic. So, NGN as in ITUT has adopted SIP because it provides media independence that is regardless of the stream type that SIP shall work. And then SIP allows signalling to take any form that is between two parties that is unicast or if multicasting or even broadcasting is required, then signalling provides very efficient and highly effective signalling support for such conference call kind of environments. The platform that has been adopted for implementing SIP signalling is the IP Multimedia subsystem. The obvious reason for adopting this is it is a highly distributed and decentralized architecture. So, using this decentralization, the functional elements and the implementation complexity can be distributed across the network.