The invention provides a variable length frame exchanging method adapted to a fixed length cell handling exchange into which a terminal for use with a data communication method by a variable length frame is accommodated, a variable length frame exchanging interface apparatus and a call acceptance determination system, which are improved in that a variable length frame can be exchanged at a high speed in a fixed length cell handling exchange network and a terminal can be connected to and accommodated into the exchange in the same conditions as an existing service. The terminal is connected to the exchange by way of a subscriber line including a plurality of channels. A variable length frame from the terminal is disassembled into fixed length cells, and a use channel number or numbers and a DLCI of the subscriber line are converted into a VPI and a VCI. The VPI and the VCI are added to each of the fixed length cells, respectively, and the resulted fixed length cells are inputted to the exchange so that the frame is exchanged by the exchange.
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