PACKET
15
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 rate (HBAUD) the longer 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 re-
peated 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 lets 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 acknowledge 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 acknowledge 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.
Let's now assume that station a sends frame number 3 to station B. Station B does not receive the
frame and therefore no acknowledge is received by station A. With version 1, the entire packet is
retransmitted (with the same frame number) again to station B and this continues until station A
receives an acknowledge from station B. This acknowledge can two basic forms. The first time sta-
tion B receives frame 3, he will send an acknowledge of the form "ready to receive frame 4"
<rr4>
. If this acknowledge is sent and station A did not receive it, station A will again send frame
3. Since station B already received frame 3, he would acknowledge it with the from "I've already
got frame number 3"
<rej4>
. This is also known as Reject Frame sent. This process would contin-
ue until the retry count is exceeded when, under version 1, the sending TNC will initiate a discon-
nect and dump the packet into the air UNPROTO. (The monitoring of the commands in
< >
de-
pends on the settings of MRESP and MCON.)
Now let's look at the same conditions under version 2 (AX25L2C2 ON). Station B does not receive
the frame and therefore no acknowledge is received by station A. This time, station A sends a
POLL or question to station B saying, in effect, "did you receive my frame number 3?"
<<RR3>>
.
Since station B did not receive the frame, he would respond with "no I did not"
<<rr3>>
. This
really says "I am ready to receive frame 3". At this point, station A, upon receiving the rr3 would
immediately resend the entire frame. If station B had already received frame 3 once but the ac-
knowledge never got to station A, the question from station A for the retry would be the same.
Station B's response however, would be different. He would respond with "ready to receive frame
4"
<<rr4>>
. If station A does not receive station B's reply this POL/REPLY sequence would con-
tinue for the number of retries set in the sending TNC and if no response was received, the TNC at
station A would then begin to issue connect requests to station B since there is still an outstanding