Abstract: An asynchronous transfer mode (ATM) layer function processing apparatus with an enlarged structure is disclosed including: an ATM layer receiving cell processor for storing a cell start signal and cell input data extracted by use of a cell read clock in a buffer when a cell transmissible signal is received from an input signal of a subscriber physical layer function processor, extracting a flag according to the number of stored cells, and performing a multiplexing operation according to a scheduling algorithm using a cell interrupt signal and the extracted flag; a processor interface for forming a cell from data received from a microprocessor, storing the cell in a buffer, generating the cell interrupt signal, and controlling each constituent of the ATM layer function processing apparatus; an ATM layer transmitting cell processor for performing an input cell routing function according to an output port identifier value when a corresponding routing value and a match signal are generated from a connection table