MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 89
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 to cause even ACKs which are not from hidden terminals (&
therefore less susceptible to collision) to be delayed beyond even generous
FRACK timer settings when the channel gets busy.
Once the FRACK timer times out, even if the ACK finally makes it through
before the retry is sent, the original packet is retried anyway. This obviously
wastes a lot of time which could be better used clearing the channel of some of
the legitimate offered load.
It is this feature of the current AX.25 protocol that accounts for most of the
abysmally poor performance of the currently popular NETROM and THENET
nodes when they are used as omnidirectional single channel (or even
multichannel if there is more than a single other node on each channel) systems.
It should be noted that these node chips CAN handle point to point links to a
single other node perfectly adequately.
The prioritized ACK protocol avoids the above problems by giving ACKs
priority access to the channel. It does this in such a way that even ACKs coming
from hidden terminals are protected from collision.
The current protocol gives a limited version of this priority access only to
digipeated frames. Although it will be possible to support digipeating in a
compatible (with the new protocol) fashion, compatible digipeating support was
not an objective that was addressed in this release.
Ack prioritization works with slotted channel access in the following way:
1
Response frames (ACKs) are always sent immediately with no time delays
unrelated to hardware limitations applied. Ultimately, not even DCD will be