a data transmission unit for use in a system wherein data to be transmitted is organized into packets which are assigned one of three states : idle , occupied , or released , and transmitted among interconnected units . each unit is assigned a window limit on the number of packets it may transmit and has one or more terminals associated therewith , which may or may not have data to transmit . the unit responds to the state of a received packet , the state of the window limit , and the presence or absence of transmit data to control whether the packet transmitted out of the unit is a packet from an internal switching buffer , a released packet from a released cell producing circuit , an occupied packet containing data from the unit &# 39 ; s own terminals , or a dummy cell . a released packet contains the address of the unit which generated it and is generated when there is no data to be transmitted and an idle cell is received by the unit . any unit other than the generating unit can occupy a released packet with data from its terminal , even though its allocated window is exhausted , thereby increasing data throughput .