Kantronics KPC-4 - HF Packet Operation

Getting
Out
of
Transparent
Getting
into
the
Transparent
Mode
is
easy,
you
just
type
a
"t"
in
Command
Mode.
But
since
Transparent
Mode
allows
the
sending
of
all
characters
you
can
not
get
out
of
Transparent
Mode
by
just
typing
a
Ctrl-C
(COMMAND
character)
as
in
Convers
Mode.
In
order
to
get
out
of
Transparent
Mode
you
must
follow
a
special
sequence,
or
No
use
a
modem
break
if
your
program
supports
one.
The
special
sequence
must
be
followed
precisely.
This
example
assumes
the
COMMAND
character
i
Ctrl-C
and
CMDTIME
is
1
second:
e
Wait
at
least
1
second
since
the
last
character
was
sent
from
the
computer
to
the
TNC
Type
a
Ctrl-C
Within
1
second
type
a
second
Ctrl-C
Within 1 second type a third Ctrl-C
Wait
1
second
and
the
cmd:
prompt
should
appear
If
the
guard
time
of
one
second
before
and
after
the
three
Ctrl-Cs
is
not
there
the
TNC
assumes
that
they
are
data
and
sends
them
to
the
radio.
Don't
get
impatient,
one
second
can
seem
longer
than
you
think
it
should.
HF Packet Operation
All
of
the
information
previously
presented
applies
to
HF
packet
as
well
as
VHF
packet,
but
the
major
differences
all
center
around
the
timing
parameters.
The
reason
for
this
is
the
band
conditions
on
HF
generally
cause
more
of
what
is
called
the
hidden
station
concept.
A
simple
example
will
demonstrate:
If
I'm
located
in
Lawrence,
Kansas,
and
I'm
talking
to
a
station
in
Boston,
Massachusetts,
all
goes
very
well
as
long
as
we
are
the
only
two
stations
on
frequency.
In
the
real
world,
there
are
many
other
stations
on
at
the
same
time.
Now,
I
senda
packet
of
data
to
my
friend
in
Boston,
and
a
station
in
San
Diego
has
data
to
send
ho»
also.
He
doesn't
transmit
because
he
hears
my
signal.
The
Boston
station
receives
my packet, and I quit transmitting. Now the Boston station keys up to send his
acknowledgment back to me, but the San Diego station also keys up to send his data
because he can't hear the Boston station. Here we have a collision. I hear both of them,
so the acknowledge doesn't arrive.
On the other side of that coin, if the Boston station sends me a packet, I may receive
it fine, then the San Diego station keys up to send a packet of data. Perhaps this is
several long packets which cause him to transmit for 5 seconds. I didn't transmit
because I heard the San Diego station. Now, the San Diego station stops sending, I
start my DWAIT or SLOTTIME wait, and a station in Seattle keys up to answer the
San Diego station. Again I don't transmit because I hear the Seattle station. By this
time, perhaps 8 seconds or so have gone by. The Boston station didn't receive my
ack — of course not! I haven't even sent it yet because the channel isn't clear! But, his
FRACK time is expired and he thinks I missed it. Even if I acknowledge right now, he
will still have to go through the POLL/RESPONSE sequence since his FRACK time
expired.
What we see here is the need on HF, with more hidden stations, and slower baud rates,
to increase our FRACK time to give the receiving station a reasonable opportunity to
ack our packets before we retry. Otherwise, we have lots of retries, and very little data.
20 PACKET -
E O Copyright 1989, Kantronics, Inc. All Rights Reserved.
Version 2.85 Duplication of this manual or the firmware without
Operations Manual permission of Kantronics, Inc. is prohibited.