PacComm TINY-2 - HDLC Frames and AX.25 Level Two

PACKET RADIO PROTOCOL HDLC Frames
Level two accomplishes this task by partitioning data to be
transferred by level one into individual frames, each with its own
error detection field and frame identification fields. The packet
controller
supports
two
versions
of
a
level-two
layer,
AX.25
Version
1.0
and
AX.25
Version
2.0.
Each
of
these
protocols
is
based
on
HDLC,
the
High-Level
Data
Link
Control
protocol
defined
by
the
ISO.Version
1
is
supported
for
compatibility
with
older
packet
controllers
only.
|
FLAG
|
ADDRESS
|
CONTROL
|
PID
&DATA
|
FCS
|
FLAG
|
FLAG.
A
unique
bit
sequence
(01111110)
used
to
detect
frame
boundaries.
A
technique
called
“bit
stuffing”
is
used
to
keep
all
other
parts
of
the
frame
from
looking
like
a
flag.
ADDRESS.
A
field
normalh
specifying
the
destination
address.
AX.25
uses
a
minimum
of
14
bytes
and
a
maximum
of
70
bytes
containing
the
actual
call
signs
of
the
source,
destination,
and
optionally
up
to
eight
digipeaters.
CONTROL.
A
byte
which
identifies
the
frame
type.
In
the
AX.25
protocol,
the
control
field
may
include
frame
numbers
in
one
or
two
3-bit
fields.
PID,
A
Protocol
Identification
byte
appears
as
the
first
byte
of
the
HDLC
DATA
field
in
AX.25
Level
‘Two
information
frames,
and
identifies
which
Level
3
protocol
is
implemented,
if
any.
In
the
case
where
no
Level
3
protocol
is
implemented,
PID
=
$F0.
DATA.
This
field
contains
the
actual
information
to
be
trans-
ferred.
This
field
need
not
be
present.
Most
frames
used
only
for
link
control
do
not
have
data
fields.
FCS.
Frame
Check
Sequence,
a
16-bit
error
detection
field.
The
communications
chip
recognizes
the
opening
and
closi
fags
and
passes
the
address,
control,
and
data
(including
PID)
fields
to
the
software.
The
FCS
field
is
a
Frame
CheckSequence
computed
by
the
transmitting
chip
and
sent
with
the
frame.
The
receiving
chip
recomputes
the
FCS
based
on
the
data
received
and
rejects
any
frames
in
which
the
received
FCS
does
not
match
the
computed
FCS.
There
is
virtu:
ally
no
chance
of
an
un-
detected
bad
frame
using
this
method.
This
satisfies
the
level
two
task
of
bad
data
detection.
The
circuitry
used
in
the
packet
controller
encodes
the
trans-
mitted
data
in
NRZI
form,
which
encodes
a
“0"
data
bit
as
a
transition
in
the
encoded
bit
stream
and
a
”1"
data
bit
as
no
transition.
This,
in
combination
with
the
“bit-stuffing”
which
ensures
that
no
more
than
five
“1"s
occur
in
a
row
except
when
ees
et
eee
ee
40
Operating
Manual,
4th
Ed.
©
PacComm,
1990