 Now that we have discussed the core components of the IP Multimedia sub system, it is appropriate now to look at the additional functional elements which are equally important in IMS. These include BCGF, MGCF and MRF. As you can see in this diagram, the breakout gateway control function sets on the right side of the core, it actually interacts with the serving call session control function and through SCSCF it interacts with other entities. Similarly, the media gateway control function is also important that is not part of the core but performs important functions. And then we have the multimedia resource function MRF that has two jobs, one is the function controller and the other one is functional processor. Let's look at their functionality one by one. Breakout control gateway function actually is responsible for an exception that is it is a node that processes the user requests which come for routing from the serving SCSCF when the serving SCSCF realizes that it cannot provide translation or it cannot find a valid session to be provided through the DNS and enum numbering system. It means the call is being made to a call party which does not have valid DNS registry or enum record. This can be an example where a PSTN telephone is a call party. In such case, the call has to terminate so this breakout control gateway function determines which next hop shall be used for forwarding of this tip message. If the breakout occurs in the same IMS network where the calling party is also present, the breakout control gateway function selects a media gateway control function for interacting with PSTN because it has to activate a node or a functionality that can now talk to PSTN. If the breakout occurs into another network, it means it is now impossible to entertain this particular call within the same IMS. So the SIP request is forwarded through the interrogator SCSCF to another IMS which can handle it. Since there is interaction between multiple IMSs in the later case and even for the same IMS in the former case, appropriate charging and billing records have to be activated. The next important functional element in IMS is the media gateway controller function. We have briefly touched upon its functionality just now but we are going to look at it in a little more detail. It actually allows translation of the signaling from the NGN standard of SIP and any other standard which is not understandable by the NGN architecture functional element. An example can be the integrated services digital network user part that is signaling standard 7 which is used by PSTN. So if the signaling has to be translated from the calling party which is sitting in the IMS to a called party which is there in PSTN to the media gateway controller function now has to translate this from one form to the other. This media gateway controller function is the same for the system architecture evolution as in PGPP, 4G and so forth. There it interfaces the serving gateway through the stream control transport protocol over IP. So it means that media gateway controller function has actually been borrowed by the NGN architecture from the PGPP alliance. Then we have another important functionality performed by servers known as the multimedia resource functions. The multimedia resource functions actually deals with the resources required to handle the multimedia traffic. The multimedia resource functions are split into two parts multimedia resource function controller and function processor. As far as the MRFC is concerned it is a signaling node that understands the information which is coming from the application server and the serving CSCF and it uses it to control the media stream whichever is coming in the session. It actually means in simple words that multimedia resource function controller takes the control information coming in the SIP messages from the application server and applies it on to the traffic which is being sent in the control user plane. Then we have the multimedia resource function processor. It is a user plane node. It means it carries user traffic. It has a functionality that it can mix different media streams. For instance it can be used for multicasting. It can be used for adding audio, video, textual layers for receiving parties each having its own particular format to it which it has subscribed in the service level agreement and the contract. At times this particular node or this server can itself become the source of media stream for instance it can provide announcements for the multimedia traffic to its users and it can also provide media stream processing. Media stream processing actually means an example is flow control which user is going to be allowed admission into the network to attend for instance a conference call. So if there is a conference call between multiple users MRFP is responsible to make sure to make sure which participants can take part in the conference session.