A private user network interface (UNI) for end-user access to an ATM network from equipment such as a bridge, router, or DSU. At the Data Link Layer, DXI encapsulates the user payload in a frame, employing a variation of the HDLC protocol.The data communications equipment (DCE) converts the frame to the appropriate, class-specific ATM protocol through segmentation and reassembly (SAR) and presents the data to the ATM network switch in ATM cell format. DXI modes correspond to AAL 3/4 and AAL 5. See also AAL 3/4, AAL 5, ATM, bridge, cell, Data Link Layer, DCE, DSU, encapsulate, HDLC, payload, private UNI, router, and SAR.