PacComm TINY-2 - Page 33

OPERATING
INSTRUCTIONS
Packet
Formatting
Acknowledgments
of
digipeatéd
packets
are
made
from
end
to
end,
and
digipeaters
do
not
acknowledge
the
packets
they
relay.
If
there
are
several
intermediate
relays,
the
chance
of
either
the
original
packet
or
the
acknowledgment
to
be
lost
increases
drastically.
To
reduce
this
problem,
an
automatic
wait
time
can
be
imposed
on
any
station
not
transmitting
a
digipeated
packet.
This
usually
gives
the
digipeater
a
clear
chance
at
the
channel.
The
wait
time
is
set
by
the
command
DWait,
which
specifies
40
ms
intervals.
If
no
digipeating
is
being
done
in
the
local
area,
this
parameter
can
be
set
to
0,
but
in
any
event
it
should
be
set
to
the
same
value
by
all
members
of
a
local
packet
group.
In
order
to
avoid
unnecessary
packet
retries,
the
packet
control-
ler
implements
a
collision-avoidance
strategy
which
applies
to
all
packets
except
those
being
digipeated.
On
the
second
and
subsequent
transmissions
of
a
particular
packet,
the
packet
controller
waits
an
additional
random
time
after
detecting
a
clear
channel
before
beginning
transmission.
This
prevents
repeated
collisions
of
transmissions
by
the
same
two
stations.
The
random
time
is
a
multiple
(0-15)
of
the
TXDelay
time.
The
interval,
in
milliseconds,
between
the
packet
controller
detect-
ing
carrier-drop
and
beginning
to
transmit
is
eWait
time
=
DWait
*
10
for
the
first
transmission
of
a
packet.
For
subsequent
transmis-
sions
of
the
same
packet
the
interval
is
eWait
time
=
DWait
*
10
+
(r
*
TXDelay
)
*
10
where
r
is
2
random
number
from
0
to
15.
Thus,
if
your
packet
controller
is
forced
to
retransmit
packets,
you
will
occasionally
hear
a
fairly
long
delay
before
transmission
begins.
Multiple
packets
may
be
transmitted
before
waiting
for
an
acknowledgment.
This
permits
more
efficient
channel
use
when
large
amounts
of
data
are
being
transferred.
The
maximum
number
of
packets
which
the
packet
controller
will
send
before
waiting
for
acknowledgment
is
specified
by
the
command
MAXframe.
MAXframe
in
combination
with
Paclen,
which
sets
the
maximum
number
of
characters
ina
packet,
determines
how
much
information
can
be
sent
in
a
single
transmission.
Alternate
Transmit
Timing
An
alternate
method
of
transmit
timing
called
P-Persistance
may
be
implemented
by
the
command
PPersist
ON.
(This
dis-
ables
the
retry
timing
using
FRACK)
This
method
is
most
effective
if
all
users
on
a
channel
make
use
of
it.
Persist
uses
the
parameters
PErsist
and
SLottime.
When
a
frame
is
queued
for
transmission
and
the
DCD
is
not
active,
a
Operating
Manual,
4th
Ed.
©
PacComm,
1990