Logical diagram of the xPhoNet transit exchange is shown below. In principle, it is the central interconnecting point of telephone networks (i.e. nation-wide nodal point creating a star of telephone networks).
It is a common NET PhoNet 7000 internet telephone exchange, with a special algorithm for call routing implemented, its routing tables being generated from numbering plans of so called xPhoNet partner networks (i.e. from the data saved in the register of networks). This way, call routing is in compliance with the data about numbering ranges assigned by Telecommunication Office (TO), extended by telephone numbers borrowed and taken over from other networks and narrowed by telephone numbers handed over and lent to other networks. Telephone calls among all xPhoNet partner networks then need not be done by transit via one or more large classic TDM networks. Source telephone network need not utilize service of another network represented by completion-routing of telephone calls routed to transferred numbers. Destination network need not necessarily take over the calls routed to borrowed numbers via the operator which has these numbers assigned by TO. Each telephone network connected newly to xPhoNet automatically acquires connection to all other partner networks. From the point of view of VoIP networks, transit exchange is thus able to create an overlay telephone network interconnecting VoIP networks into a common national voice network working on packet switching principle.
Towards partner networks, the transit exchange may be connected via VoIP bundles with SIP signalling or via E1 bundles with SS7 signalling. Various xPhoNet centres may be mutually interconnected via VoIP bundles with SIP signalling. Each xPhoNet national centre then will be connected to supra-national centre of telephone networks (to several supra-national centres, possibly). This way, an overlay packet telephone network determined for transit of international calls will arise.
Star-type interconnection shows an advantage of least number of bundles necessary for complete interconnection of all TDM, mobile and VoIP networks. However, this topology shows a disadvantage of easy vulnerability of transit since a fault of one bundle will cause "cut-off" of the damaged network from all other networks and a fault of transit exchange then would damage all networks. Reliability of star-type interconnection may be partially enhanced by a tandem of two transit exchanges where each TDM, mobile and VoIP network may be connected to both centres. However, in practice, a combination of direct voice peer-to-peer connections with star-type connection created by xPhoNet is expected. Peer-to-peer connections are and will be advantageous for basic interconnection of networks with significant mutual telephone traffic.
Small and new VoIP networks will easily and quickly acquire an interconnection into large group of other networks. Large VoIP networks will acquire a path towards other networks without peer-to-peer interconnections and furthermore as back-up path for cases of overloading or fault of peer-to-peer connections. Via xPhoNet, small networks will acquire basic interconnection and large networks alternative interconnection to other networks in a given country.