1. A method of flow control in a network communications device, said network communications device receiving packets, comprising: enqueuing said packets in one or more packet queue structures; scheduling said packets for dequeuing from said one or more packet queue structures according to a packet hierarchy; dequeuing said packets; segmenting each said dequeued packet into one or more ATM cells; enqueuing said ATM cells into one or more cell queue structures; scheduling said ATM cells for dequeuing from said one or more cell queue structures according to said packet hierarchy; and transmitting said ATM cells through an ATM interface; wherein said dequeuing said packets is controlled by the presence or absence of one or more feedback signals from said cell queue structures and wherein said scheduling said ATM cells for dequeuing from said one or more cell queue structures employs a first in, first-out buffer.