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MFJ-1278B MULTI-MODE ADVANCED OPERATION
While these settings seem extremely long, they will assure that the stations NOT using the
new protocol always get a shot at slot #1. With these settings, the users of the NEW protocol
will occasionally capture the channel.
Settings of 43 for 1200 baud and 64 for 300 baud could be used to assure that stations NOT
using the new protocol always have a shot at slot #0. This will result in users of the new
protocol almost never being able to capture the channel unless all traffic from users of the old
protocol has been cleared. This should NOT be done unless ALL stations are using some
type of slotted channel access procedure with compatible numbers of slots and compatible
slot times.
Remember that the objective of the new channel access procedures is to increase aggregate
throughput for the CHANNEL, not necessarily for an individual user (not even you!). It may
seem to you at first when you get on a very busy channel where you are the only station that
is using the new system, that you aren't getting to transmit very frequently. This is because
you aren't getting to transmit very frequently. However, the OTHER users on the channel
will see an incremental increase in the overall amount of data that the channel can pass per
unit time because you are being such a nice guy. Remember that transmitting isn't necessarily
communicating.
There is ONE benefit that you should be able to see even under the above circumstances. If
you are communicating with another station who is also running the new system, the channel
should appear more reliable to you even if quite slow. You should not be getting retried out
waiting for his ACK to come back & have to keep reconnecting.
As a higher and higher percentage of the users on the channel begin using the new system, the
performance of the channel should increase quite a bit. However, we all must remember that
if a channel is mathematically capable of transferring (for example) 1000 characters per unit
time, the very best it can possibly do (if CSMA is working right) is reliably pass about 620
characters per unit time. If it is offered more than this to handle, performance falls off
sharply with increased offered load. Currently the typical AX.25 simplex channels observed
on the HAM bands tend to hover around 10 to as high as 18 percent throughput. This is far
below the 62 percent that is possible when the protocol is working right.