vice core networks' may result in a core network role as mediating middleware between various accesses. Many access operators with different technologies may connect to few core/backbone network operators with fairly homo- geneous backbone technologies. The core network develops towards a hierarchical IP transport with hierarchical servers according to Figure 15.4. When many interworking actors cooperate to produce a service, achieve end-to-end QoS and end-to-end security, the real challenges will come. There is no dictator that sees to it that everything works, rather there is a kind of democracy that reigns. The situation must therefore be solved by means of agreements. Service level agreement (SLA) is thus a common expression in this chapter. Whereas IP is the tool to interconnect users globally, web services using XML might become the glue at the service layer level. In future rather than software components being developed and bound together to form a single rigid solution, systems could be developed as a 'federation' of services at the point of execution. This will enable alternatively located software components to be substituted. A type of software interconnection appears. See also Appendix 1. XML and IP might then be accompanied by SIP signalling end to end. A target network? This chapter can only cover a small amount of the important aspects. Since 'a picture says more than a thousand words' the chapter is rich in figures, and some contexts are probably left to the reader to finalize. Basically, interconnection or interworking is a matter of relevant knowledge of the environment, including agreements. One could compare a large network, operated by actor A, and two networks, each half the size of the first one, operated by actors B and C. How can a B + C connection be as good as an A connection, provided the conditions are equal in all other aspects? Understanding Changing Telecommunications - Building a Successful Telecom Business Local Transit Transit Local H.248 H.248 Site router Aggregation Regional 