Local modulated carrier data network with a collision avoidance protocol

Abstract

A modem for a single frequency, modulated RF carrier local data network for a distributed data processing system and a method for line acquisition and contention resolution. The protocol established for line acquisition and contention resolution is implemented by a modem controller. The controller causes the receiver section to listen for foreign carriers to determine when the line is busy. When the client device of the modem desires to send a data packet to another unit, the modem controller causes the receiver to listen for a certain pre-burst period and then causes the transmitter to send a burst of RF carrier out on the line and simultaneously causes the receiver to listen to the line for interference beating, i.e. changes in amplitude on the line which indicates another unit is requesting the line. To insure beat patterns, the frequency of the burst carrier is swept over a range during the burst. If a contention is found, a resolution thereof is made utilizing a random delay and retry protocol. After the burst, the controller causes the receiver to listen to the line for other carriers. If none are found, the controller causes the transmitter to transmit a data packet preceded by a preamble of 100% modulation level RF carrier. During the preamble period, all receivers in the system set their respective automatic gain control levels to a level for comfortable reception. The gain level so established depends upon the strength of the received signal. This amplification level is maintained throughout the entire data packet.