| 摘要 |
<p>An unslotted radio frequency multiple access bus communications system employing a contention listen-while-talk(LWT) protocol. The system includes a plurality of remotely located terminals coupled to a communications bus. The bus includes a pair of oppositely directed, unidirectional signal paths which are connected at one end so that one path is an inbound path to that end and the second is an outbound path from that end. Each of the remote terminals includes a bus interface unit (BIU) coupled to both the inbound and outbound paths at the remote location. Each BIU is adapted to transmit and receive modulated carrier signals on the inbound and outbound paths, respectively. When a terminal wishes to send a message to another terminal, the terminal's BIU initially performs an rf carrier sensing operation to make an initial determination as to whether or not any other subscriber is transmitting on the inbound path. In the event no carrier is detected, the BIU confirms this determination by first beginning to transmit a message packet modulated on a carrier signal and addressed to the desired receiving terminal on the inbound path, and then monitoring the outbound path during a collision window. If the transmitted message is received intact by the transmitting terminal within this collision window, the transmitting terminal's BIU determines that no collision has taken place between its message and a message sent by another terminal or noise, and that the transmitting terminal has gained access to the bus Thereafter, the BIU may transmit any remaining portion of its message packet on the inbound path However, in the event a collision is detected, the transmitting terminal's BIU aborts its transmission and then backs off for a random time period before again attempting to gain access to the bus and transmit its message.</p> |
