In asynchronous transfer mode (ATM), a flow control feedback mechanism that communicates to the originating end-user device to change the transfer characteristics of the connection during periods of congestion.The network buffers cells and advises the sender to throttle back on the rate of transmission. The available bit rate (ABR) class of service makes use of RMCells. ABR is a best-effort category in which the network attempts to pass the maximum number of cells, but with no absolute guarantees. See also ABR, best effort, buffer, cell, congestion, and flow control.