Interface apparatus

An interface apparatus in a communication network in which when a frame that has been received from a digital transmission line and that has overhead and a payload is converted to AAL Type-1 ATM cells, part of the overhead data, which indicates each of the starting positions of data blocks that have been multiplexed into the frame, and the payload are defined as being at least a cell conversion zone and a predetermined position of the cell conversion zone is adopted as a reference position, wherein a true pointer that specifies this reference position is included in a prescribed cell and, if a true pointer cannot be placed in any one of eight cells of one cycle (SC=0 to 7), then a cell for which SC=6 holds is regarded as a P-format cell in which a pointer will be placed, a false pointer is placed in this cell and the cell is sent to an ATM network, and a frame is assembled by AAL Type-1 cells that have been received from the ATM network and the frame is sent to a digital transmission line, said interface apparatus comprising:

• a receive buffer for storing, in sync with a buffer write clock, an AAL Type-1 cell payload that has been received from an ATM network, and for reading out the cell payload in sync with a buffer read-out clock;
• a detector for detecting a pointer from a payload that has been read out of said receive buffer;
• selector means for outputting data of the payload from a third byte onward if a pointer is detected and outputting data of the payload from a second byte onward if a pointer is not detected;
• a controller having a frame counter, which is preset to (R-1-pointer) when a true pointer is detected, for subsequently counting up the byte timing signal and outputting 0 to (R-1) cyclically (where R represents the total number of bytes of a frame), said controller outputting a starting position signal, which indicates the starting position of the cell conversion zone, when the value of the count is a value that corresponds to the reference position;
• and
• a frame assembler for assembling a frame using data output from said selector means on the basis of the starting position signal.