A method and apparatus is disclosed for interfacing an asynchronous network with a synchronous network and in particular for efficiently utilizing available bandwidth of a synchronous network transmit opportunity. In one embodiment asynchronous traffic arrives via an asynchronous network at a network device, such as a switch, for transmission over a synchronous network. The traffic is parsed into cells and after switching, a reassembly unit is provided for processing one or more cells buckets. Write operations occur based on an ingress pointer while read operations are controlled by an egress pointer. Upon occurrence of a transmit opportunity on the synchronous network, the entire bandwidth of the transmit opportunity is utilized by loading awaiting cells from bucket memory on to the synchronous network. Sufficient cells are stored in memory between the memory locations identified by the ingress pointer and the egress pointer to insure total utilization of transmit opportunity bandwidth.