an atm routing switch for bidirectional transmission of at least two types of cell , one type accepting variable bit rate of transmission and a second type accepting some loss of cells in transmission , includes first reserve buffer capacity for cells of the first type , a second reserve buffer capacity for cells of said second type and control circuitry for generating a flow control signal if a predetermined threshold for the first buffer capacity is reached by input of cells of said first type , and discarding input cells of said second type if a predetermined threshold for said second buffer capacity has been reached by input of cells of said second type .