Abstract: An interface apparatus has a transmitting section in which first transmission-timing information generating means generates transmission timing information XH, which constitutes timing information of a sending user clock CTU, using a network clock CN and the sending user clock CTU, second transmission-timing information generating means generates test transmission timing information XH′ having any value, a selector selects the transmission timing information XH, which is output by the first transmission-timing information generating means, during ordinary communication, and selects the test transmission timing information XH′ during a test, and cell assembling means assembles sending user data DTU and the transmission timing information XH or XH′ selected by the selector into a fixed-length cell and sends the cell to an opposite interface apparatus.

Abstract: An internal timing information producing portion produces internal timing information IRTS1 to IRTS4 which is the timing information on a data reception clock by using a network clock C.sub.N and the data reception clock C.sub.RU. A calculator calculates the difference between the internal timing information IRTS1 to IRTS4 and the transmission timing information RRTS1 to RRTS4 contained in a received cell, and a data reception clock producing portion adjusts the timing of the data reception clock C.sub.RU so that the difference is zero and outputs the adjusted timing. An ATM cell decomposing portion outputs the user data D.sub.RU in synchronism with the data reception clock C.sub.RU which is input from the data reception clock producing portion.

Abstract: An internal timing information producing portion produces internal timing information IRTS1 to IRTS4 which is the timing information on a data reception clock by using a network clock C.sub.N and the data reception clock C.sub.RU. A calculator calculates the difference between the internal timing information IRTS1 to IRTS4 and the transmission timing information RRTS1 to RRTS4 contained in a received cell, and a data reception clock producing portion adjusts the timing of the data reception clock C.sub.RU so that the difference is zero and outputs the adjusted timing. An ATM cell decomposing portion outputs the user data D.sub.RU in synchronism with the data reception clock C.sub.RU which is input from the data reception clock producing portion.