A network device for a communications network includes a port configured to transmit data to the network at a maximum transmit data rate. The device also includes a transmit buffer configured to buffer data units that are ready for transmission to the network, and a packet buffer configured to buffer data units before the data units are ready for transmission. The packet buffer is configured to output data units at a maximum packet buffer transmission rate faster than the maximum transmit data rate. The device includes a rate controller configured to control a transmission rate of data from the packet buffer to the transmit buffer so that averaged over a period, the transmission rate from the packet buffer to the transmit buffer is at most equal to the maximum transmit data rate, while allowing the transmission rate, at one or more time intervals, to exceed the maximum transmit data rate.