MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 88
Slotting and Acknowledgment Priority
The
SLOTTING AND ACKNOWLEDGMENT PRIORITY
feature is
installed in the MFJ TNC in an attempt to prevent or minimize packet collisions
in a busy channel.
Use of the optional SLOTTING and ACKNOWLEDGMENT PRIORITY
procedures now in the MFJ TNC firmware significantly improve 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.
