| 摘要 |
There is disclosed an improved modem (78) for a single frequency, modulated RF carrier local data network for a distributed data processing system and a method for line (64) acquisition and contention resolution. The protocol established for line acquisition and contention resolution is implemented by a modem controller (104). The controller (104) causes the receiver section (82) to listen for foreign carriers to determine when the line is busy. When the client device (66,70,72,74,68) of the modem (78) desires to send a data packet to another unit, the modem controller (104) causes the receiver (82) 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 (82) 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 (82) to listen to the line for other carriers. If none are found the controller (104) causes the transmitter (78) to transmit a data packet preceded by a preamble (115) of 100% modulation level RF carrier. During the preamble period, all receivers (82) in the system set their respective automatic gain control levels to a level for comfortable reception. To gain level so established depends upon the strength of the received signal. This amplification level is maintained throughout the entire data packet. There is also disclosed herein a data link controller (530) and an interface specification for a generic class of network interface modems (78), one of which is disclosed herein for a client data processing unit for use in a data communications network. The specific modem (78) described herein is for use in a broadband local area data network with collision avoidance protocol. The data link controller (530) features hardware acknowledge circuitry that acknowledges the receipt of a perfect data packet immediately upon receipt thereof without request to or assistance by the client data processing unit. The DLC (530) prepares the acknowledgment packet from the incoming data packet and places it on the data link without going through a line acquisition protocol. Each received acknowledgment packet is placed in a transmit buffer (571) at a known location relative to the data packet which was just transmitted, for which it is an acknowledgment so that each acknowledgment packet may be located in less time by the client data processing unit. The acknowledgment packets are logically part of the transmit process and are therefore placed in the transmit buffer. |
