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A key word describing next-generation wireless communications, so called 4G, is "seamless". As part of the e-Japan plan promoted by the Japanese government, the MIRAI (multimedia integrated network by radio access innovation) project is aiming at the development of new technologies that will enable seamless integration of various wireless access systems for practical use by 2005. First, this paper describes heterogeneous wireless networks and introduces the concept of MIRAI. The most significant feature of MIRAI is the provision of a set of signaling functions: radio-access-network discovery and selection, heterogeneous paging, and vertical handoff. These functions make it possible to offer seamless services between different wireless systems. A medium access control (MAC) protocol for basic access networks, which are dedicated wireless systems providing the signaling, is then introduced. A proof-of-concept demonstration system is also briefly introduced. The delay performances of the signaling procedures are presented.