White Paper - Session Control in IMS based Next Generation Networks
Roles of Session Border Control within a Converged Network
It is clear from the preferences of operators and the standards themselves that wireless networks require a signalling proxy function and wireline networks require both signalling and media proxies. Session border controllers are a good match for this functionality as we will see in this section but must be architected to allow physical separation of media and signalling, thus complying with standards requirements and allowing maximum flexibility of deployment with independent scaling of signalling and media.
It will also have become clear to the reader of the previous section that functions within converged networks and IMS definitions currently have considerable overlap. It is therefore important to consider the functional requirements at any particular point in the network rather than generate products to meet a functional block.
The Newport Networks 1460 has been designed to be carrier scale and capable of delivering signalling and media functions in physically separate devices. As such, the Newport Networks 1460 is well placed to perform the following functions.
SignallingProxy: Visited Network
 In this position, the SignallingProxy provides the first point of contact for signalling from the User Element and can locate the Home Network via DNS. Services destined for the visited network can also be directed at this point. Where required, Lawful Intercept of signalling and Instant Messaging can be provided, and Emergency Call Handling can be done in the local network, nearest to the emergency. At the interconnection towards the home network, the 1460 SignallingProxy can resolve issues with overlapping IP address spaces between the networks and also provide topology hiding. The signalling can be policed to prevent DoS attacks at the IP, UDP/TCP and SIP signalling level. A media device can be controlled to ensure that QoS negotiated at the signalling layer is not exceeded at the media layer. In contrast to the GGSN, this feature is provided on a per-media flow level. Billing records can be provided to record user SIP session activity in the visited network.
In summary, the Newport Networks 1460 SignallingProxy provides the P-CSCF, PDF, IBCF and THIG functions. These functions may reside in the same system or may be distributed as scaling demands.
SignallingProxy: Home Network
 Within the home network, the SignallingProxy again provides the network boundary function. It provides the first point of contact from visited networks and all functionality described in the visited networks section above. For example, traffic management, topology hiding and signalling policing are still likely to be key requirements. In addition, the I-CSCF provides a Diameter interface to retrieve the S-CSCF that should serve the user. Conversion between IPv4 and IPv6 could occur in either visited or home network, or tunnelling could be used through the visited network.
In standards terms, the SignallingProxy provides the P-CSCF, IBCF, and THIG functions and is well placed to provide the I-CSCF functions in the future. These functions may reside in the same system or may be distributed as scaling demands.
MediaProxy
 Mobile standards for IMS currently address signalling only, which leaves the packet network to deal with media. Consequently, the GGSN must police traffic within these networks. The disadvantages of this have already been covered. The GGSN is not available in access networks, other than GPRS, and therefore it makes sense to control the policing of media directly from the SignallingProxy. This means that sessions can be policed on a per-flow basis. This prevents the user from performing different actions in the media layer to that negotiated in the signalling layer. The MediaProxy can also provide NAPT and firewall functions for media crossing IP boundaries. This architecture is extremely robust in preventing unauthorized access at the media layer, as all ports on the MediaProxy are opened and closed under the control of the SignallingProxy.
This function equates to the Core Border Gateway Function (C-BGF) when deployed in the visited network toward the user and to the Interconnect Border Gateway Function (I-BGF) when deployed between networks. Again, these functions can be co-located or distributed as required.
Conclusions
Carrier-class session border controllers, such as the Newport Networks 1460, that can be deployed as a signalling proxy and media proxy can meet and exceed the requirements of next generation IMS-based networks. This gives the carrier the ability to deploy signalling proxies and media proxies today to meet existing network requirements, such as NAT traversal, Lawful Intercept and policing, in the full knowledge that investment in hardware and software will be reusable as their networks migrate towards a converged architecture. 
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