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The main concepts and interfacing and multiplexing techniques for an NNI (network node interface) as set forth in CCITT (International Consultive Committee for Telephone and Telegraph) Recommendations are described. The objectives and history of the NNI standardization are reviewed together with concepts, basic requirements, and features of the NNI. The nine-row-based frame structure and the virtual container (VC) concept, which are the main features of the NNI, are introduced. The multiplexing principle and method, the detailed frame structure and overhead, and mapping methods related to the NNI are described. Applications of the NNI in the synchronous network and international interworking are presented. The future application of asynchronous transfer mode (ATM) is discussed.< >