Considering new market realities characterized by factors such as: open competition among operators due to deregulation of markets, explosion of digital traffic, e. g., due to the increasing use of the Internet, increasing demand for new multimedia services, increasing demand for a general mobility, convergence of networks and services, etc., the NGN (Next Generation Network) is conceived as a concrete implementation of the GII (Global Information Infrastructure). Recommendations in the Y series provide the foundation of the Next Generation Networks (NGN). However, implementation issues were not adequately addressed in GII. As a consequence, the NGN should be understood as the further step in the realization of GII concept. The target of NGN is to ensure that all elements required for interoperability and network capabilities support applications globally across the NGN while maintaining the concept of separation between transport, services and applications. This Recommendation is intended to be used as background information to assist the development of Recommendations, standards and of implementation guidelines for the realization of Next Generation Networks.