Kantronics KPC–3 Plus User Manual page 70

number was 83, then the TNC would not start the key-up of the transmitter since 83 is
greater than the 63 PERSIST value. Instead, it would wait an additional 100 ms, and if
the channel is still clear, generate a new random number. This time, let's say it comes
up with the number 27. Since this is less than the PERSIST value, we now start the key-
up of the transmitter to send the packet.
Txdelay
TXDELAY should be adjusted to allow your radio sufficient time to switch from receive
mode to transmit mode and develop full power output. If the TNC sends the packet
before the radio is at full power, the beginning of the packet will be lost and no one will
be able to decode it. TXDELAY also allows the station you are talking to sufficient time
to switch from transmit mode back to receive mode. The TNC sends flags during the
TXDELAY period, so if someone has this set extra long you will hear a repetitive sound
at the beginning of the packet.
Frack (Frame Acknowledgment Time)
If the TNC expects an acknowledgment of a packet it has sent, it will wait FRACK
seconds for the acknowledgment. If the acknowledgment is not received it will either
send a poll or retransmit the packet, depending on the setting of AX25L2V2. When digis
are used, extra time is allowed for each transmission using the following equation:
FRACK * ((2 * n) + 1) seconds
where n is the number of digipeaters. The lower the baud (HBAUD) the higher this
parameter should be set, because everything is slower. The length of the transmission
(determined by PACLEN and MAXFRAME) also needs to be taken into account when
deciding how to set FRACK. Longer packets (and more of them) require more time to
be transmitted, more time to be repeated by the digipeater, and so on down the line.
The FRACK timer begins when PTT is released (the packet has been sent) and is
suspended when data carrier from the radio is present, or when your station is
transmitting.
Retries AX.25 Level 2, Version 1 vs. Version 2
The way retries are accomplished depends on AX25L2V2 being OFF or ON. To explain
this we will follow a conversation through its path. First let's assume station ―A‖ is
connected to station ―B‖ with Version 1 protocol (AX25L2V2 OFF). When station A
sends a packet to station B, he expects to receive an acknowledgment back indicating
that station B has received the information. In order to verify that the proper packet (or
frame) has been acknowledged each frame has a number. This number is sent as a
part of the frame so the receiving station knows where this packet belongs in the
conversation. The frame numbers range from 0-7 and because of this, we are limited to
a MAXFRAME of 7 (we do not want the same frame number reused in the same
transmission). This is also true for Version 2. If the first acknowledgment is received,
there is really no difference between the two versions, practically speaking. The
difference shows up with retries, so let's assume that the packet did not get through on
the first attempt.
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