ATM switches

The invention relates to data switching and particularly to a method and apparatus for transferring data arranged in cells across a data switch having at least one input port and a plurality of output ports, the cells each having a data section and address information, the method comprising the steps of sending a control cell from each output port to each input port to indicate whether the respective output port is able to receive data, receiving a data cell at said at least one input port, reading said address information from said received data cell, sending said data cell to each output port indicated by said address information only if said output cell is able to receive data, and otherwise retaining said data cell. The invention is particularly relevant to ATM switching and the B-ISDN protocol.