Communication switch including input throttling to reduce output congestion

A switch for a data communication system includes a multiplicity of ports 1-4 operative to receive (11-14) data packets; a multiplicity of ports operative to transmit (41-44) data packets, means for storing (51-54) data packets prior to transmission from the ports that are operative to transmit data packets, said means establishing respective transmit queues of data packets for the ports that are operative to transmit data packets; and means 37,38 for establishing measures of the contribution of each of a plurality of ports that are operative to receive data to the queue of data for each of the ports that are operative to transmit data. The switch is characterised by a control process which responds to said measures and to an indication that a queue of data packets for a port operative to transmit data is excessive. The process identifies at least one port that provides data packets to said excessively long transmit queue and enables means 21-24 for providing flow control to a device which is coupled to the identified port 1-4. This is done using MAC control frames and a leaky bucket counter, Fig.5, (not shown). The control frame signals a reduction in the supply of data packets to the device coupled to the identified port.