Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 152)

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