Epson MFJ-1278B manual Slotting and Acknowledgment Priority, Description

MFJ-1278B MULTI-MODE ADVANCED OPERATION

Slotting and Acknowledgment Priority

The addition of the SLOTTING AND ACKNOWLEDGMENT PRIORITY feature makes the MFJ-1278B more efficient on packet frequency channels. The MFJ-1278B achieves this greater efficiency by preventing or minimizing packet collisions on busy packet channels.

Use of the optional SLOTTING and ACKNOWLEDGMENT PRIORITY routine now in the MFJ-1278B firmware significantly improves the reliability of packet radio connections on shared channels. The greatest improvement is yielded when all stations in a network are so equipped and configured with the appropriate, matching parameters.

SLOTTING improves channel efficiency by requiring each TNC to "flip" a many sided die whenever the channel goes from busy to not-busy. The result of the "flip" is the number of 10ms DEADTIME intervals the TNC waits before transmitting.

SLOTTING solves most of the problems that occur when there are two or more stations waiting to transmit when a third is already transmitting. Before slotting, the first two stations are definitely going to transmit at the same time, guaranteeing a collision. With slotting, the first station will probably choose a "slot" different from the second. If everyone is hearing everyone else, collisions are very unlikely.

In conjunction with slotting, the ACKNOWLEDGMENT PRIORITY feature helps improve channel utilization by assigning priority to acknowledgments (AX.25 frame types "S"), i.e. acknowledgments are never delayed by slotting.

The reason this works is because each TNC recognizes when another is transmitting. It also recognizes the destination of each packet it hears. If a TNC hears a packet not addressed to it, it can assume that somewhere another TNC will send an acknowledgment. Therefore, on hearing packets for others, a TNC will always wait at least one slot time in case the addressed TNC acknowledges. And it will never delay it's own acknowledgments.

Description

The idea behind the prioritized acknowledgment (ACK) protocol is quite simple. The idea is to give ACKs priority access to the channel so that time is not wasted retrying packets that have already been correctly copied but for whatever reason, the ACK is not received within the time limit defined by the FRACK timer.

The present protocol does not handle a simplex LAN with hidden terminals as well as it possibly could. This is primarily because, the present protocol is more likely to synchronize collisions with acknowledgment packets than with any other type of packet.

To this collision synchronization mechanism the current version of AX.25 adds a propensity