<s>
AX.25	B-Protocol
(	O
Amateur	O
X.25	B-Protocol
)	O
is	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
originally	O
derived	O
from	O
layer	B-Protocol
2	I-Protocol
of	O
the	O
X.25	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
designed	O
for	O
use	O
by	O
amateur	O
radio	O
operators	O
.	O
</s>
<s>
It	O
is	O
used	O
extensively	O
on	O
amateur	O
packet	B-Application
radio	I-Application
networks	B-Architecture
.	O
</s>
<s>
AX.25	B-Protocol
v2.0	O
and	O
later	O
occupies	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
the	O
second	O
layer	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
It	O
is	O
responsible	O
for	O
establishing	O
link-layer	O
connections	O
,	O
transferring	O
data	O
encapsulated	O
in	O
frames	B-Protocol
between	O
nodes	B-Protocol
,	O
and	O
detecting	O
errors	O
introduced	O
by	O
the	O
communications	O
channel	O
.	O
</s>
<s>
As	O
AX.25	B-Protocol
is	O
a	O
pre-OSI-model	O
protocol	O
,	O
the	O
original	O
specification	O
was	O
not	O
written	O
to	O
cleanly	O
separate	O
into	O
OSI	O
layers	O
.	O
</s>
<s>
AX.25	B-Protocol
v2.2	O
(	O
1998	O
)	O
added	O
improvements	O
to	O
improve	O
efficiency	O
,	O
especially	O
at	O
higher	O
data	O
rates	O
.	O
</s>
<s>
Extended	O
sequence	O
numbers	O
(	O
7	O
vs.	O
3	O
bits	O
)	O
allow	O
a	O
larger	O
window	O
size	O
,	O
the	O
number	O
of	O
frames	B-Protocol
that	O
can	O
be	O
sent	O
before	O
waiting	O
for	O
acknowledgement	O
.	O
</s>
<s>
"	O
Selective	O
Reject	O
"	O
allows	O
only	O
the	O
missing	O
frames	B-Protocol
to	O
be	O
resent	O
,	O
rather	O
than	O
having	O
to	O
wastefully	O
resend	O
frames	B-Protocol
that	O
have	O
already	O
been	O
received	O
successfully	O
.	O
</s>
<s>
The	O
only	O
known	O
complete	O
implementation	O
of	O
v2.2	O
,	O
at	O
this	O
time	O
(	O
2020	O
)	O
,	O
is	O
the	O
software	O
TNC	B-Protocol
.	O
</s>
<s>
AX.25	B-Protocol
is	O
commonly	O
used	O
as	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
for	O
network	B-Application
layer	I-Application
such	O
as	O
IPv4	O
,	O
with	O
TCP	O
used	O
on	O
top	O
of	O
that	O
.	O
</s>
<s>
AX.25	B-Protocol
supports	O
a	O
limited	O
form	O
of	O
source	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
Although	O
it	O
is	O
possible	O
to	O
build	O
AX.25	B-Protocol
switches	O
similar	O
to	O
the	O
way	O
Ethernet	O
switches	O
work	O
,	O
this	O
has	O
not	O
yet	O
been	O
accomplished	O
.	O
</s>
<s>
AX.25	B-Protocol
does	O
not	O
define	O
a	O
physical	O
layer	O
implementation	O
.	O
</s>
<s>
In	O
practice	O
1200	O
baud	O
Bell	O
202	O
tones	O
and	O
9600baud	O
G3RUHDFSK	O
are	O
almost	O
exclusively	O
used	O
on	O
VHF	B-Algorithm
and	O
UHF	O
.	O
</s>
<s>
On	O
HF	B-Algorithm
the	O
standard	O
transmission	O
mode	O
is	O
300baud	O
Bell	O
103	O
tones	O
.	O
</s>
<s>
At	O
the	O
physical	O
layer	O
,	O
AX.25	B-Protocol
defines	O
only	O
a	O
"	O
physical	O
layer	O
state	O
machine	O
"	O
and	O
some	O
timers	O
related	O
to	O
transmitter	O
and	O
receiver	O
switching	O
delays	O
.	O
</s>
<s>
At	O
the	O
link	O
layer	O
,	O
AX.25	B-Protocol
uses	O
HDLC	O
frame	O
syntax	O
and	O
procedures	O
.	O
</s>
<s>
(	O
ISO3309	O
)	O
frames	B-Protocol
are	O
transmitted	O
with	O
NRZI	O
encoding	O
.	O
</s>
<s>
AX.25	B-Protocol
specifies	O
that	O
this	O
field	O
is	O
subdivided	O
into	O
multiple	O
addresses	O
:	O
a	O
source	O
address	O
,	O
zero	O
or	O
more	O
repeater	O
addresses	O
,	O
and	O
a	O
destination	O
address	O
,	O
with	O
embedded	O
control	O
fields	O
for	O
use	O
by	O
the	O
repeaters	O
.	O
</s>
<s>
AX.25	B-Protocol
supports	O
both	O
virtual-circuit	O
connected	O
and	O
datagram-style	O
connectionless	O
modes	O
of	O
operation	O
.	O
</s>
<s>
The	O
latter	O
is	O
used	O
to	O
great	O
effect	O
by	O
the	O
Automatic	B-Application
Packet	I-Application
Reporting	I-Application
System	I-Application
(	O
APRS	B-Application
)	O
.	O
</s>
<s>
A	O
simple	O
source	B-Protocol
routing	I-Protocol
mechanism	O
using	O
digipeaters	O
is	O
available	O
at	O
the	O
datalink	O
level	O
.	O
</s>
<s>
Digipeaters	O
act	O
as	O
simplex	O
repeaters	O
,	O
receiving	O
,	O
decoding	O
and	O
retransmitting	O
packets	B-Protocol
from	O
local	O
stations	O
.	O
</s>
<s>
The	O
AX.25	B-Protocol
specification	O
defines	O
a	O
complete	O
,	O
albeit	O
point	O
to	O
point	O
only	O
network	B-Application
layer	I-Application
protocol	O
,	O
but	O
this	O
has	O
seen	O
little	O
use	O
outside	O
of	O
keyboard-to-keyboard	O
or	O
keyboard-to-BBS	O
connections	O
.	O
</s>
<s>
NET/ROM	O
,	O
ROSE	O
,	O
and	O
TexNet	O
exist	O
to	O
provide	O
routing	O
between	O
nodes	B-Protocol
.	O
</s>
<s>
In	O
principle	O
,	O
a	O
variety	O
of	O
layer	O
3	O
protocols	O
can	O
be	O
used	O
with	O
AX.25	B-Protocol
,	O
including	O
the	O
ubiquitous	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
This	O
approach	O
is	O
used	O
by	O
AMPRNet	O
,	O
which	O
is	O
an	O
amateur	O
radio	O
TCP/IP	B-Protocol
network	I-Protocol
using	O
AX.25	B-Protocol
UI-frames	O
at	O
the	O
datalink	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
Traditionally	O
,	O
amateur	O
radio	O
operators	O
have	O
connected	O
to	O
AX.25	B-Protocol
networks	B-Architecture
through	O
the	O
use	O
of	O
a	O
terminal	B-Protocol
node	I-Protocol
controller	I-Protocol
,	O
which	O
contains	O
a	O
microprocessor	B-Architecture
and	O
an	O
implementation	O
of	O
the	O
protocol	O
in	O
firmware	B-Application
.	O
</s>
<s>
AX.25	B-Protocol
has	O
also	O
been	O
implemented	O
on	O
personal	B-Device
computers	I-Device
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
includes	O
native	O
support	O
for	O
AX.25	B-Protocol
.	O
</s>
<s>
The	O
computers	O
can	O
also	O
interconnect	O
to	O
other	O
computers	O
or	O
be	O
bridged	O
or	O
routed	O
to	O
TNCs	O
and	O
transceivers	O
located	O
elsewhere	O
using	O
BPQ	O
over	O
Ethernet	O
framing	O
,	O
which	O
is	O
also	O
natively	O
supported	O
by	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
to	O
facilitate	O
more	O
modern	O
setups	O
with	O
the	O
actual	O
transceivers	O
directly	O
placed	O
under	O
or	O
in	O
the	O
antenna	O
mast	O
,	O
creating	O
a	O
'	O
low	O
loss	O
 '	O
,	O
shorter	O
RF	O
wiring	O
need	O
,	O
and	O
replacing	O
expensive	O
and	O
long	O
and	O
thick	O
coax	O
cables	O
and	O
amplifiers	O
with	O
cheap	O
fiber	O
(	O
RFI	O
(	O
both	O
ways	O
)	O
/EMP/lightning	O
resistant	O
)	O
or	O
copper	O
Ethernet	O
wiring	O
.	O
</s>
<s>
BPQ	O
Ethernet	O
framing	O
allows	O
connecting	O
entire	O
stacks	O
of	O
TNC+transceiver	O
pairs	O
to	O
any	O
existing	O
network	O
of	O
computers	O
which	O
then	O
can	O
all	O
access	O
all	O
radio	O
links	O
offered	O
simultaneously	O
(	O
transparently	O
bridged	O
)	O
,	O
communicate	O
with	O
each	O
other	O
internally	O
over	O
AX.25	B-Protocol
,	O
or	O
with	O
filtered	O
routing	O
select	O
specific	O
TNCs/radio	O
frequencies	O
.	O
</s>
<s>
Note	O
that	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
AX.25	B-Protocol
implementation	O
does	O
contain	O
some	O
bugs	O
,	O
mainly	O
in	O
disconnecting	O
sessions	O
(	O
this	O
bug	O
appears	O
to	O
have	O
slipped	O
in	O
over	O
time	O
due	O
to	O
maintenance	O
by	O
some	O
people	O
using	O
it	O
exclusively	O
for	O
APRS	B-Application
and	O
not	O
in	O
virtual	O
channel	O
connected	O
mode	O
)	O
,	O
and	O
for	O
example	O
TAP	O
interfaces	O
not	O
supporting	O
the	O
full	O
7	O
bytes	O
of	O
call	O
sign	O
string	O
length	O
as	O
some	O
interface	O
drivers	O
expect	O
a	O
6	O
byte	O
HW	O
address	O
(	O
MAC	O
)	O
,	O
not	O
an	O
up-to-7	O
byte	O
one	O
like	O
AX.25	B-Protocol
use	O
requires	O
.	O
</s>
<s>
is	O
a	O
free	O
open-source	O
replacement	O
for	O
the	O
1980s-style	O
TNC	B-Protocol
.	O
</s>
<s>
It	O
contains	O
DSP	O
software	O
modems	O
and	O
a	O
complete	O
implementation	O
of	O
AX25	B-Protocol
v2.2	O
plus	O
FX.25	B-Protocol
forward	I-Protocol
error	I-Protocol
correction	I-Protocol
.	O
</s>
<s>
It	O
can	O
function	O
as	O
a	O
digital	O
repeater	O
,	O
GPS	O
tracker	O
,	O
and	O
APRS	B-Application
Internet	O
Gateway	O
(	O
IGate	O
)	O
without	O
any	O
additional	O
software	O
.	O
</s>
<s>
AX.25	B-Protocol
is	O
often	O
used	O
with	O
a	O
TNC	B-Protocol
that	O
implements	O
the	O
KISS	O
framing	O
as	O
a	O
low-cost	O
alternative	O
to	O
using	O
expensive	O
and	O
uncommon	O
HDLC	O
controller	O
cards	O
.	O
</s>
<s>
The	O
KISS	O
framing	O
is	O
not	O
part	O
of	O
the	O
AX.25	B-Protocol
protocol	O
itself	O
nor	O
is	O
it	O
sent	O
over	O
the	O
air	O
.	O
</s>
<s>
It	O
merely	O
serves	O
to	O
encapsulate	B-Protocol
the	O
protocol	O
frames	B-Protocol
in	O
a	O
way	O
that	O
can	O
successfully	O
pass	O
over	O
a	O
serial	O
link	O
to	O
the	O
TNC	B-Protocol
.	O
</s>
<s>
With	O
SLIP	O
,	O
these	O
were	O
the	O
two	O
SLIP-connected	O
hosts	O
;	O
with	O
KISS	O
,	O
it	O
is	O
assumed	O
that	O
the	O
KISS	O
framing	O
link	O
is	O
over	O
serial	O
with	O
only	O
the	O
host	O
computer	O
and	O
the	O
TNC	B-Protocol
involved	O
.	O
</s>
<s>
AX.25	B-Protocol
has	O
most	O
frequently	O
been	O
used	O
to	O
establish	O
direct	O
,	O
point-to-point	B-Architecture
links	O
between	O
packet	B-Application
radio	I-Application
stations	O
,	O
without	O
any	O
additional	O
network	B-Application
layers	I-Application
.	O
</s>
<s>
This	O
is	O
sufficient	O
for	O
keyboard-to-keyboard	O
contacts	O
between	O
stations	O
and	O
for	O
accessing	O
local	O
bulletin	B-Application
board	I-Application
systems	I-Application
and	O
DX	O
clusters	O
.	O
</s>
<s>
In	O
recent	O
years	O
,	O
APRS	B-Application
has	O
become	O
a	O
popular	O
application	O
.	O
</s>
<s>
For	O
tunneling	B-Protocol
of	O
AX.25	B-Protocol
packets	B-Protocol
over	O
IP	O
,	O
AXIP	O
and	O
AXUDP	O
are	O
used	O
to	O
encapsulate	B-Protocol
AX.25	B-Protocol
into	O
IP	O
or	O
UDP	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
At	O
the	O
speeds	O
commonly	O
used	O
to	O
transmit	O
packet	B-Application
radio	I-Application
data	O
(	O
rarely	O
higher	O
than	O
9,600	O
bit/s	O
,	O
and	O
typically	O
1,200	O
bit/s	O
)	O
,	O
the	O
use	O
of	O
additional	O
network	B-Application
layers	I-Application
with	O
AX.25	B-Protocol
is	O
impractical	O
due	O
to	O
the	O
data	O
overhead	O
involved	O
.	O
</s>
<s>
This	O
is	O
not	O
a	O
limitation	O
of	O
AX.25	B-Protocol
per	O
se	O
,	O
but	O
places	O
constraints	O
on	O
the	O
sophistication	O
of	O
applications	O
designed	O
to	O
use	O
it	O
.	O
</s>
<s>
The	O
AX.25	B-Protocol
implementation	O
of	O
HDLC	O
includes	O
sender	O
and	O
destination	O
station	O
call-sign	O
plus	O
four-bit	O
Secondary	O
Station	O
Identifier	O
(	O
SSID	O
)	O
value	O
in	O
range	O
0	O
through	O
15	O
in	O
the	O
frame	O
address	O
.	O
</s>
<s>
However	O
,	O
AX.25	B-Protocol
has	O
a	O
built-in	O
hard	O
limit	O
of	O
six	O
characters	O
,	O
which	O
means	O
a	O
seven-character	O
callsign	O
cannot	O
be	O
used	O
in	O
an	O
AX.25	B-Protocol
network	O
.	O
</s>
<s>
AX.25	B-Protocol
lacks	O
an	O
explicit	O
port	B-Protocol
(	O
or	O
SAP	B-Protocol
)	O
;	O
the	O
SSID	O
often	O
assumes	O
this	O
role	O
.	O
</s>
<s>
Thus	O
there	O
can	O
be	O
only	O
one	O
service	O
per	O
AX.25	B-Protocol
station	O
SSID	O
address	O
,	O
which	O
is	O
often	O
kludged	O
around	O
with	O
varying	O
degrees	O
of	O
success	O
.	O
</s>
<s>
Some	O
amateurs	O
,	O
notably	O
Phil	O
Karn	O
KA9Q	O
,	O
have	O
argued	O
that	O
AX.25	B-Protocol
is	O
not	O
well-suited	O
to	O
operation	O
over	O
noisy	O
,	O
limited-bandwidth	O
radio	O
links	O
,	O
citing	O
its	O
lack	O
of	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
and	O
automatic	O
data	B-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
However	O
,	O
a	O
viable	O
widely	O
adopted	O
successor	O
to	O
AX.25	B-Protocol
has	O
yet	O
to	O
emerge	O
.	O
</s>
<s>
a	O
large	O
existing	O
deployment	O
of	O
recycled	O
narrowband	O
FM	O
radios	O
and	O
especially	O
existing	O
APRS	B-Application
applications	O
,	O
</s>
<s>
adoption	O
of	O
newer	O
line	B-Protocol
codings	I-Protocol
potentially	O
including	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
takes	O
more	O
effort	O
than	O
the	O
1,200	O
bit/s	O
AFSK	O
of	O
Bell	O
202	O
.	O
</s>
<s>
Previously	O
sufficient	O
small	O
8-bit	O
microprocessors	B-Architecture
with	O
128	O
bytes	O
of	O
RAM	O
would	O
not	O
be	O
enough	O
,	O
and	O
new	O
ones	O
might	O
cost	O
US$30	O
instead	O
of	O
US$3	O
.	O
</s>
<s>
Phil	O
Karn	O
did	O
demo	O
decoding	O
of	O
this	O
new	O
modulation	O
of	O
his	O
by	O
running	O
it	O
on	O
a	O
Pentium	B-General_Concept
II	I-General_Concept
machine	O
some	O
10	O
years	O
later	O
,	O
mid-level	O
embedded	B-Architecture
microprocessors	I-Architecture
are	O
capable	O
enough	O
to	O
do	O
the	O
same	O
with	O
under	O
US$50	O
system	O
cost	O
.	O
</s>
<s>
Despite	O
these	O
limitations	O
,	O
an	O
extension	O
to	O
the	O
AX.25	B-Protocol
protocol	O
,	O
supporting	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
,	O
has	O
been	O
created	O
by	O
the	O
TAPR	O
.	O
</s>
<s>
This	O
extension	O
is	O
called	O
FX.25	B-Protocol
.	O
</s>
<s>
Transmitting	O
interleaved	O
Reed-Solomon	B-Error_Name
FEC	I-Error_Name
signal	O
in	O
some	O
smart	O
modulation	O
needs	O
a	O
lot	O
fewer	O
resources	O
than	O
reception	O
of	O
the	O
same	O
signal	O
,	O
thus	O
a	O
sufficient	O
microprocessor	B-Architecture
might	O
cost	O
just	O
US$5	O
instead	O
of	O
US$30	O
and	O
a	O
system	O
cost	O
might	O
stay	O
below	O
US$50	O
,	O
transmitter	O
included	O
.	O
</s>
<s>
However	O
,	O
in	O
recent	O
years	O
,	O
the	O
ability	O
to	O
receive	O
as	O
well	O
as	O
send	O
using	O
cheap	O
microcontrollers	O
(	O
such	O
as	O
the	O
Atmel	B-Architecture
AVR	I-Architecture
or	O
the	O
Motorola	B-Device
68HC08	I-Device
families	O
)	O
has	O
been	O
demonstrated	O
.	O
</s>
<s>
Most	O
recently	O
,	O
a	O
wholly	O
new	O
protocol	O
with	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
has	O
been	O
created	O
by	O
Nino	O
Carillo	O
,	O
KK4HEJ	O
,	O
called	O
Improved	B-Protocol
Layer	I-Protocol
2	I-Protocol
Protocol	I-Protocol
(	O
IL2P	O
)	O
.	O
</s>
