The paper presents and analyzes two alternatives of interworking modes for interconnection of IEEE 802.6 based Metropolitan Area Networks (MANs) and ATM based B-ISDN. The two alternatives are the `cell-to-slot interworking' and the `frame interworking.' The cell-to-slot mode modifies ATM-cells to DQDB-slots and vice versa, whereas the frame interworking mode reassembles ATM-cells and converts the resulting AAL frames to DQDB IMPDU frames. In the paper, simulation results of a DQDB network are presented, where one of the DQDB stations represents an IWU getting aggregated traffic from B-ISDN. The aggregated traffic is modelled by a mixture of various constant bitrate and variable bitrate traffic sources representing different real-time sources and background traffic. It turns out that the cell-to-slot interworking mode is much more useful for keeping bandwidth requirements of real-time traffic than the frame interworking mode which causes additional burstiness due to reassembly. Based on these results one can conclude that the cell-to-slot interworking mode is much more appropriate for real-time applications than the frame interworking mode.