Abstract: A port concentrator communication system which allows a large number of end user stations to share a limited number of trunk lines to a packet switch network is described. Specifically, the system comprises a number of end user stations each coupled to a port concentration intelligent unit, with these units being coupled together in cascade fashion and the end unit in the cascade being coupled to a set of trunk lines leading to a packet switch network. Each of the trunk line ports corresponds to a channel in the DDS DSOB data streams network and in the reverse direction to the set of end user stations. Special code bytes, indicating the status of trunk line ports, appear in the DSOB data streams and play an integral role in the method whereby the port concentration intelligent units associated with the end user stations can request, obtain access to and communicate over, and relinquish trunk lines.