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The UK will probably have the first regulated Inter-Service Provider VoIP interconnect anywhere in the world. The standards for this interconnect are being created under the auspices of the UK Network Interoperability Consultative Committee (NICC) and is the result of the combined efforts of interested parties in the main Service Providers, both mobile and fixed, and vendors that operate within the UK.
The boundary conditions that have been set for the service offered at the interconnect are intended to mirror those available for PSTN service within the UK. This is quite natural since the service being provided by the IP system is a PSTN replacement service.
Consequently, one of the key objectives is to have service restored within half a second of a failure in the interconnect. This means that not only does a resilient multiple node architecture have to be used, but also the IP protocols and networking technologies have to be specially configured to meet this stringent requirement.
Router components in the interconnect network are avoided, with interconnection being performed at layer 2, so that questions of ownership of IP layer products and systems are easier to resolve. In the longer run, the evolution of the interconnect specification will take in VLAN technology which is beneficial in economic and management terms.
In the tradition of standards bodies, the definition of the interconnect takes on the form of distributed logical functions, so that no particular vendor is favoured. However, most of the border functions correspond closely to the functionality provided by session border controllers. The interconnect is divided into signalling interconnect and media interconnect and thus requires SBC to be able to do the same.
The protocol used is ISUP over SIP (SIP-I), as defined in the ITU document Q.1912.5 (Mode C), with some detailed profiling to meet the needs of the existing UK SS7 based service model.
Since the service is a PSTN simulation service, many of the issues are resolved through reference to the UK implementation of ISUP, and indeed many of the contributors to the interconnect definition documents are from this background.
Networking Models
The basic network model that can be used to realise the interconnect is shown below. At the boundary of the network are signalling and media firewall functions, designed to protect the Service Provider's infrastructure, while offering the required quality of service levels.
In the notional schematic above, the User Agent may be an IP client, a media gateway or indeed a normal PSTN phone connected through the media gateway. Their signalling (MGCP/Megaco) will be translated by a Call Agent to SIP-I, but the media will flow directly from the IP client / media gateway device to the media border function.
However, this diagram, while showing the border functions, has a missing functional link, that is the link between either the Call Agent or Signalling Boundary Function to the Media Boundary Function. This link is necessary to open media pinholes for the calls to flow through the Media Boundary Function.
Traditional firewall functions for media are impractical because of the huge and variable number of sources of media and the fact that there are incoming calls. Any traditional firewall would have to be opened to virtually any incoming traffic. Thus, the Service Providers network would be open to their peers. This would be unacceptable to Service Providers.
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