Collision avoiding system, apparatus and protocol for a multiple access digital communications system including variable length packets

摘要

A communications path having at least two stations coupled thereto. During time intervals when no station on the path has a packet to transmit, start-of-cycle packets for timing and synchronizing the stations are routinely transmitted between respective ends of the path. When a station has a packet of information to transmit, the station monitors the path for detecting whether or not the path is busy. On the one hand, if the path is not busy, a predetermined delay is introduced before the station is permitted to transmit a packet. The delay is related to the electrical distance between the station, which has the packet to transmit, and a station, which priorly transmitted a packet including a start-of-cycle packet. Also, the delay begins when the priorly transmitted packet is received by the station. On the other hand, if the path is busy, the station monitors the path until the path becomes not busy at which point the predetermined delay is introduced, the delay being related to the electrical distance between the station and that station which caused the path to be busy. On either hand, at the expiration of the delay and if the path has remained not busy during all of the predetermined delay, the station then transmits its packet; otherwise the station does not transmit its packet until a subsequent cycle. Thereby collisions are avoided and variable length packets are transmitted and received.