PacComm TINY-2 - Packet Radio Protocol Overview; Physical and Data Link Layers

PACKET RADIO PROTOCOL
The following material is intended to supply an overview of the
protocol used to transmit data by the packet controller. It is
somewhat tutorial in nature for those who have not had previous
exposure to layered network protocols, but it presumes some
knowledge of general communications hardware and software.
Persons already well versed in networking may want to skip this
chapter and refer to the primary defining document, Amateur
Packet Radio Link-Layer Protocol, AX.25 Version 2.0, avail-
able from the ARRL, 225 Main Street, Newington, CT 06111.
The PacComm packet controller hardware and software ar-
chitecture is organized in accordance with the International
Standards
Organization
(ISO)
layered
network
model.
The
model describes seven levels and is officially known as the ISO
Reference
Model
of
Open
Systems
Interconnection,
or
simply
the
ISO
Model.
The
model
and
many
other
interesting
topics
are
discussed
in
Computer
Networks
by
Andrew
S.
Tanenbaum.
The ISO
model
provides
for
layered
processes,
each
supplying
a
set
of
services
to
a
higher
leve!
process.
The
PacComm
packet
controller
currently
implements
the
first
two
layers,
the
Physical
layer
and
the
Data
Link
layer.
Physical
Layer
enn
The
duty
of
the
Physical
Layer,
layer
one,
is
to
provide
for
the
transmission
and
reception
of
data
at
the
bit
level.
It
is
con-
cerned
only
with
how
each
bit
is
physically
transmitted,
i.e.,
voltages
on
a
cable
or
modem
tones
on
phone
or
RF
links
The
physical
layer
of
the
packet
controller
is
described
in
the
TECHNICAL
REFERENCE
MANUAL.
It
is
compatible
with
the
various
packet
controllers
currently
available.
The
actual
modem
interface
is
compatible
with
the
Bell
202
standard
which
is
similar
to
the
CC
V.23
standard.
Any
other
hardware
device
which
is
compatible
with
the
Bell
202
standard
should
be
compatible
with
the
PacComm
packet
controller,
at
least
at
level
one
of
the
ISO
reference
model.
Data Link Layer
The
duty
of
the
Data
Link
layer
is
to
supply
an
error-free
stream
of
data
to
higher
levels.
Since
level
one
simply
passes
any
bits
received
to
level
two
and
is
unaware
of
the
content
or
overlyi
structure
of
the
data,
transmission
errors
are
not
detectable
at
level
one.
Level
two
carries
the
responsibility
of
detecting
and
rejecting
bad
data,
retransmitting
rejected
data,
and
detecting
the
reception
of
duplicate
data.
Operating
Manual,
4th
Ed.
©
PacComm,
1990
39