ATM transmission equipment provided between existing exchange or terminals of carrier equipment that, for example, can emulate existing transmission paths, and also implement T point interfaces for an in-home unit. The ATM transmission equipment detects communication anomalies or extracts maintenance/operation information from data strings on STM lines, such as high-speed digital lines or ISDN lines, and replaces only the necessary information resulting from those extractions or detections with alarm signals. The alarm signals are then transmitted together with information channels to the ATM network via a cell assembler/dissembler. The information channels and alarm information that arrive from the ATM network are rearranged into STM line data strings and output to LT units, DSUs, or terminals.