The following is edited from the 3GPP documents:
IP is being introduced in both fixed and mobile networks as a more cost-effective alternative to circuit switched technology in the legacy PSTN/PLMN, as well as the underpinning transport for delivering IMS based multi-media services.
In order to ensure carrier grade end to end performance, appropriate interconnect solutions are required to support communications between users connected to different networks. There are currently a number of initiatives underway outside 3GPP addressing IP Interconnection of services scenarios and commercial models to achieve this; for example, the GSM Association has developed the IPX (IP Packet Exchange). Also, ETSI has recently defined requirements and use case scenarios for IP Interconnection of services. These initiatives require the use of appropriate technical solutions and corresponding technical standards, some of which are already available and others which will require development in 3GPP.
Moreover, new models of interconnection may emerge in the market where Network Operators expose network capabilities to 3rd party Application Providers including user plane connectivity for the media related to the service.
The main objective of IPXS is thus:
To specify the technical requirements for carrier grade inter-operator IP Interconnection of Services for the support of Multimedia services provided by IMS and for legacy voice PTSN/PLMN services transported over IP infrastructure (e.g. VoIP).
These technical requirements should cover the new interconnect models developed by GSMA (i.e. the IPX interconnect model) and take into account interconnect models between national operators (including transit functionality) and peering based business trunking.
Any new requirements identified should not overlap with requirements already defined by other bodies (e.g. GSMA, ETSI TISPAN). Specifically the work will cover:
• Service level aspects for direct IP inter-connection between Operators, service level aspects for national transit IP interconnect and service level aspects for next generation corporate network IP interconnect (peer-to-peer business trunking).
• Service layer aspects for interconnection of voice services (e.g. toll-free, premium rate and emergency calls).
• Service level aspects for IP Interconnection (service control and user plane aspects) between Operators and 3rd party Application Providers.
To ensure that requirements are identified for the Stage 2 & 3 work to identify relevant existing specifications, initiate enhancements and the development of the new specifications as necessary.
The following is a related presentation on Release-8 II-NNI with an introduction to Rel-9 and Rel-10 features.
The 3GPP references can be seen from the presentation above.
European Commission conducted a study on this topic back in 2008 and produced a lengthy report on this. Since the report is 187 pages long, you can also read the executive summary to learn about the direction in technical, economic and public policy.