<s>
In	O
digital	O
radio	O
,	O
packet	B-Application
radio	I-Application
is	O
the	O
application	O
of	O
packet	B-Protocol
switching	I-Protocol
techniques	O
to	O
digital	O
radio	O
communications	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
uses	O
a	O
packet	B-Protocol
switching	I-Protocol
protocol	O
as	O
opposed	O
to	O
circuit	B-Protocol
switching	I-Protocol
or	O
message	B-Protocol
switching	I-Protocol
protocols	O
to	O
transmit	O
digital	O
data	O
via	O
a	O
radio	O
communication	O
link	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
can	O
be	O
differentiated	O
from	O
other	O
digital	O
radio	O
switching	B-Protocol
schemes	O
by	O
the	O
following	O
attributes	O
:	O
</s>
<s>
This	O
is	O
very	O
similar	O
to	O
how	O
packets	O
of	O
data	O
are	O
transferred	O
between	O
nodes	B-Protocol
on	O
the	O
Internet	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
is	O
frequently	O
used	O
by	O
amateur	O
radio	O
operators	O
.	O
</s>
<s>
The	O
AX.25	B-Protocol
(	O
Amateur	O
X.25	B-Protocol
)	O
protocol	O
was	O
derived	O
from	O
the	O
X.25	B-Protocol
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
and	O
adapted	O
for	O
amateur	O
radio	O
use	O
.	O
</s>
<s>
Every	O
AX.25	B-Protocol
packet	O
includes	O
the	O
sender	O
's	O
amateur	O
radio	O
callsign	O
,	O
which	O
satisfies	O
the	O
US	O
FCC	O
requirements	O
for	O
amateur	O
radio	O
station	O
identification	O
.	O
</s>
<s>
AX.25	B-Protocol
allows	O
other	O
stations	O
to	O
automatically	O
repeat	O
packets	O
to	O
extend	O
the	O
range	O
of	O
transmissions	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
any	O
packet	O
station	O
to	O
act	O
as	O
a	O
digipeater	O
,	O
linking	O
distant	O
stations	O
with	O
each	O
other	O
through	O
ad	B-Protocol
hoc	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
This	O
makes	O
packet	B-Application
radio	I-Application
especially	O
useful	O
for	O
emergency	O
communications	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
can	O
be	O
used	O
in	O
mobile	O
communications	O
.	O
</s>
<s>
Some	O
mobile	O
packet	B-Application
radio	I-Application
stations	O
transmit	O
their	O
location	O
periodically	O
using	O
the	O
Automatic	B-Application
Packet	I-Application
Reporting	I-Application
System	I-Application
(	O
APRS	O
)	O
.	O
</s>
<s>
If	O
the	O
APRS	O
packet	O
is	O
received	O
by	O
an	O
"	O
igate	O
"	O
station	O
,	O
position	O
reports	O
and	O
other	O
messages	O
can	O
be	O
routed	O
to	O
an	B-Architecture
internet	I-Architecture
server	O
,	O
and	O
made	O
accessible	O
on	O
a	O
public	O
web	O
page	O
.	O
</s>
<s>
Some	O
packet	B-Application
radio	I-Application
implementations	O
also	O
use	O
dedicated	O
point-to-point	O
links	O
such	O
as	O
TARPN	O
.	O
</s>
<s>
In	O
cases	O
such	O
as	O
this	O
,	O
new	O
protocols	O
have	O
emerged	O
such	O
as	O
Improved	B-Protocol
Layer	I-Protocol
2	I-Protocol
Protocol	I-Protocol
(	O
IL2P	O
)	O
supporting	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
for	O
noisy	O
and	O
weak	O
signal	O
links	O
.	O
</s>
<s>
Since	O
radio	O
circuits	O
inherently	O
possess	O
a	O
broadcast	B-Operating_System
network	B-Architecture
topology	I-Architecture
(	O
i.e.	O
,	O
many	O
or	O
all	O
nodes	B-Protocol
are	O
connected	O
to	O
the	O
network	O
simultaneously	O
)	O
,	O
one	O
of	O
the	O
first	O
technical	O
challenges	O
faced	O
in	O
the	O
implementation	O
of	O
packet	B-Application
radio	I-Application
networks	O
was	O
a	O
means	O
to	O
control	O
access	O
to	O
a	O
shared	O
communication	O
channel	O
to	O
avoid	O
collisions	O
of	O
signals	O
.	O
</s>
<s>
Professor	O
Norman	O
Abramson	O
of	O
the	O
University	O
of	O
Hawaii	O
led	O
development	O
of	O
a	O
packet	B-Application
radio	I-Application
network	O
known	O
as	O
ALOHAnet	O
and	O
performed	O
a	O
number	O
of	O
experiments	O
beginning	O
in	O
the	O
1970s	O
to	O
develop	O
methods	O
to	O
arbitrate	O
access	O
to	O
a	O
shared	O
radio	O
channel	O
by	O
network	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
in	O
1975	O
would	O
lead	O
Robert	O
Metcalfe	O
to	O
use	O
carrier-sense	O
multiple	O
access	O
(	O
CSMA	O
)	O
protocols	O
in	O
the	O
design	O
of	O
the	O
now	O
commonplace	O
Ethernet	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
technology	O
.	O
</s>
<s>
Over	O
1973	O
–	O
76	O
,	O
DARPA	O
created	O
a	O
packet	B-Application
radio	I-Application
network	O
called	O
PRNET	B-Protocol
in	O
the	O
San	O
Francisco	O
Bay	O
area	O
and	O
conducted	O
a	O
series	O
of	O
experiments	O
with	O
SRI	O
to	O
verify	O
the	O
use	O
of	O
ARPANET	O
(	O
a	O
precursor	O
to	O
the	O
Internet	O
)	O
communications	B-Protocol
protocols	I-Protocol
(	O
later	O
known	O
as	O
IP	B-Protocol
)	O
over	O
packet	B-Application
radio	I-Application
links	O
between	O
mobile	O
and	O
fixed	O
network	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
This	O
system	O
was	O
quite	O
advanced	O
,	O
as	O
it	O
made	O
use	O
of	O
direct	O
sequence	O
spread	B-Architecture
spectrum	I-Architecture
(	O
DSSS	O
)	O
modulation	O
and	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	B-Error_Name
)	O
techniques	O
to	O
provide	O
100	O
kbit/s	O
and	O
400	O
kbit/s	O
data	O
channels	O
.	O
</s>
<s>
These	O
experiments	O
were	O
generally	O
considered	O
to	O
be	O
successful	O
,	O
and	O
also	O
marked	O
the	O
first	O
demonstration	O
of	O
Internetworking	B-Architecture
,	O
as	O
in	O
these	O
experiments	O
data	O
was	O
routed	O
between	O
the	O
ARPANET	O
,	O
PRNET	B-Protocol
,	O
and	O
SATNET	O
(	O
a	O
satellite	O
packet	B-Application
radio	I-Application
network	O
)	O
networks	O
.	O
</s>
<s>
Throughout	O
the	O
1970s	O
and	O
1980s	O
,	O
DARPA	O
operated	O
a	O
number	O
of	O
terrestrial	O
and	O
satellite	O
packet	B-Application
radio	I-Application
networks	O
connected	O
to	O
the	O
ARPANET	O
at	O
various	O
military	O
and	O
government	O
installations	O
.	O
</s>
<s>
Amateur	O
radio	O
operators	O
began	O
experimenting	O
with	O
packet	B-Application
radio	I-Application
in	O
1978	O
,	O
when	O
—	O
after	O
obtaining	O
authorization	O
from	O
the	O
Canadian	O
government	O
—	O
Robert	O
Rouleau	O
,	O
VE2PY	O
;	O
Bram	O
Frank	O
,	O
VE2BFH	O
;	O
Norm	O
Pearl	O
,	O
VE2BQS	O
;	O
and	O
Jacques	O
Orsali	O
,	O
VE2EHP	O
of	O
the	O
Montreal	O
Amateur	O
Radio	O
Club	O
Montreal	O
,	O
Quebec	O
began	O
experimenting	O
with	O
transmitting	O
ASCII	B-Protocol
encoded	O
data	O
over	O
VHF	B-Algorithm
amateur	O
radio	O
frequencies	O
using	O
homebuilt	O
equipment	O
.	O
</s>
<s>
In	O
1980	O
,	O
Doug	O
Lockhart	O
VE7APU	O
,	O
and	O
the	O
Vancouver	O
Area	O
Digital	O
Communications	O
Group	O
(	O
VADCG	O
)	O
in	O
Vancouver	O
,	O
British	O
Columbia	O
began	O
producing	O
standardized	O
equipment	O
(	O
Terminal	B-Protocol
Node	I-Protocol
Controllers	I-Protocol
)	O
in	O
quantity	O
for	O
use	O
in	O
amateur	O
packet	B-Application
radio	I-Application
networks	O
.	O
</s>
<s>
Not	O
long	O
after	O
this	O
activity	O
began	O
in	O
Canada	O
,	O
amateurs	O
in	O
the	O
US	O
became	O
interested	O
in	O
packet	B-Application
radio	I-Application
.	O
</s>
<s>
In	O
1980	O
,	O
the	O
United	O
States	O
Federal	O
Communications	O
Commission	O
(	O
FCC	O
)	O
granted	O
authorization	O
for	O
United	O
States	O
amateurs	O
to	O
transmit	O
ASCII	B-Protocol
codes	I-Protocol
via	O
amateur	O
radio	O
.	O
</s>
<s>
Repeaters	O
may	O
be	O
designed	O
for	O
amateur	O
packet	B-Application
radio	I-Application
,	O
these	O
are	O
dubbed	O
"	O
digipeaters	O
"	O
.	O
</s>
<s>
The	O
first	O
known	O
amateur	O
packet	B-Application
radio	I-Application
activity	O
in	O
the	O
US	O
occurred	O
in	O
San	O
Francisco	O
during	O
December	O
1980	O
,	O
when	O
a	O
packet	O
repeater	O
was	O
put	O
into	O
operation	O
on	O
2	B-Algorithm
meters	I-Algorithm
by	O
Hank	O
Magnuski	O
KA6M	O
,	O
and	O
the	O
Pacific	O
Packet	B-Application
Radio	I-Application
Society	O
(	O
PPRS	O
)	O
.	O
</s>
<s>
In	O
keeping	O
with	O
the	O
dominance	O
of	O
DARPA	O
and	O
ARPANET	O
at	O
the	O
time	O
,	O
the	O
nascent	O
amateur	O
packet	B-Application
radio	I-Application
network	O
was	O
dubbed	O
the	O
AMPRNet	O
in	O
DARPA	O
style	O
.	O
</s>
<s>
Magnuski	O
obtained	O
IP	B-Protocol
address	I-Protocol
allocations	O
in	O
the	O
network	O
for	O
amateur	O
radio	O
use	O
worldwide	O
.	O
</s>
<s>
Many	O
groups	O
of	O
amateur	O
radio	O
operators	O
interested	O
in	O
packet	B-Application
radio	I-Application
soon	O
formed	O
throughout	O
the	O
country	O
including	O
the	O
Pacific	O
Packet	B-Application
Radio	I-Application
Society	O
(	O
PPRS	O
)	O
in	O
California	O
,	O
the	O
Tucson	O
Amateur	O
Packet	B-Application
Radio	I-Application
Corporation	O
(	O
TAPR	O
)	O
in	O
Arizona	O
and	O
the	O
Amateur	O
Radio	O
Research	O
and	O
Development	O
Corporation	O
(	O
AMRAD	O
)	O
in	O
Washington	O
,	O
D.C.	O
</s>
<s>
Packet	B-Application
radio	I-Application
started	O
becoming	O
more	O
and	O
more	O
popular	O
across	O
North	O
America	O
and	O
by	O
1984	O
the	O
first	O
packet-based	O
bulletin	B-Application
board	I-Application
systems	I-Application
began	O
to	O
appear	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
proved	O
its	O
value	O
for	O
emergency	O
operations	O
following	O
the	O
crash	O
of	O
an	O
Aeromexico	O
airliner	O
in	O
a	O
neighborhood	O
in	O
Cerritos	O
,	O
California	O
in	O
August	O
,	O
1986	O
.	O
</s>
<s>
Volunteers	O
linked	O
several	O
key	O
sites	O
to	O
pass	O
text	O
traffic	O
via	O
packet	B-Application
radio	I-Application
which	O
kept	O
voice	O
frequencies	O
clear	O
.	O
</s>
<s>
For	O
an	O
objective	O
description	O
of	O
early	O
developments	O
in	O
amateur	O
packet	B-Application
radio	I-Application
,	O
refer	O
to	O
the	O
article	O
"	O
Packet	B-Application
Radio	I-Application
in	O
the	O
Amateur	O
Service	O
"	O
.	O
</s>
<s>
taxis	O
,	O
tow	O
trucks	O
,	O
police	O
)	O
were	O
quick	O
to	O
note	O
the	O
value	O
of	O
packet	B-Application
radio	I-Application
systems	I-Application
to	O
provide	O
simple	O
mobile	O
data	O
systems	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
rapid	O
development	O
of	O
a	O
number	O
of	O
commercial	O
packet	B-Application
radio	I-Application
systems	I-Application
:	O
</s>
<s>
One	O
of	O
the	O
first	O
challenges	O
faced	O
by	O
amateurs	O
implementing	O
packet	B-Application
radio	I-Application
is	O
that	O
almost	O
all	O
amateur	O
radio	O
equipment	O
(	O
and	O
most	O
surplus	O
commercial/military	O
equipment	O
)	O
has	O
historically	O
been	O
designed	O
to	O
transmit	O
voice	O
,	O
not	O
data	O
.	O
</s>
<s>
Like	O
any	O
other	O
digital	O
communications	O
system	O
that	O
uses	O
analog	O
media	O
,	O
packet	B-Application
radio	I-Application
systems	I-Application
require	O
a	O
modem	B-Application
.	O
</s>
<s>
Since	O
the	O
radio	O
equipment	O
to	O
be	O
used	O
with	O
the	O
modem	B-Application
was	O
intended	O
for	O
voice	O
,	O
early	O
amateur	O
packet	O
systems	O
used	O
AFSK	O
modems	B-Application
that	O
followed	O
telephone	O
standards	O
(	O
notably	O
the	O
Bell	B-Device
202	I-Device
standard	O
)	O
.	O
</s>
<s>
When	O
using	O
a	O
direct	O
FSK	O
modulation	O
like	O
G3RUH	O
's	O
packet	B-Application
radio	I-Application
modem	B-Application
,	O
a	O
9,600	O
baud	O
transmission	O
is	O
easily	O
made	O
in	O
the	O
same	O
channel	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
baseband	B-Algorithm
characteristics	O
of	O
the	O
audio	O
channel	O
provided	O
by	O
voice	O
radios	O
are	O
often	O
quite	O
different	O
from	O
those	O
of	O
telephone	O
audio	O
channels	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
need	O
in	O
some	O
cases	O
to	O
enable	O
or	O
disable	O
pre-emphasis	O
or	O
de-emphasis	O
circuits	O
in	O
the	O
radios	O
and/or	O
modems	B-Application
.	O
</s>
<s>
Another	O
problem	O
faced	O
by	O
early	O
"	O
packeteers	O
"	O
was	O
the	O
issue	O
of	O
asynchronous	O
versus	O
synchronous	B-Operating_System
data	O
transfer	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
most	O
personal	B-Device
computers	I-Device
had	O
asynchronous	O
RS-232	O
serial	B-Protocol
ports	I-Protocol
for	O
data	O
communications	O
between	O
the	O
computer	O
and	O
devices	O
such	O
as	O
modems	B-Application
.	O
</s>
<s>
Unfortunately	O
,	O
the	O
simple	O
AFSK	O
modems	B-Application
typically	O
used	O
provide	O
no	O
timing	O
signal	O
to	O
indicate	O
the	O
start	O
of	O
a	O
packet	O
frame	B-Protocol
.	O
</s>
<s>
That	O
led	O
to	O
the	O
need	O
for	O
a	O
mechanism	O
to	O
enable	O
the	O
receiver	O
to	O
know	O
when	O
to	O
start	O
assembling	O
each	O
packet	O
frame	B-Protocol
.	O
</s>
<s>
The	O
receiver	O
looks	O
for	O
the	O
"	O
frame	B-Protocol
boundary	O
octet	O
,	O
"	O
then	O
begins	O
decoding	O
the	O
packet	O
data	O
that	O
follows	O
it	O
.	O
</s>
<s>
Another	O
frame	B-Protocol
boundary	O
octet	O
marks	O
the	O
end	O
of	O
the	O
packet	O
frame	B-Protocol
.	O
</s>
<s>
A	O
basic	O
packet	B-Application
radio	I-Application
station	O
consists	O
of	O
a	O
computer	O
or	O
dumb	O
terminal	O
,	O
a	O
modem	B-Application
,	O
and	O
a	O
transceiver	O
with	O
an	O
antenna	O
.	O
</s>
<s>
Traditionally	O
,	O
the	O
computer	O
and	O
modem	B-Application
are	O
combined	O
in	O
one	O
unit	O
,	O
the	O
terminal	B-Protocol
node	I-Protocol
controller	I-Protocol
(	O
TNC	O
)	O
,	O
with	O
a	O
dumb	O
terminal	O
(	O
or	O
terminal	B-Application
emulator	I-Application
)	O
used	O
to	O
input	O
and	O
display	O
data	O
.	O
</s>
<s>
Increasingly	O
,	O
personal	B-Device
computers	I-Device
are	O
taking	O
over	O
the	O
functions	O
of	O
the	O
TNC	O
,	O
with	O
the	O
modem	B-Application
either	O
a	O
standalone	O
unit	O
or	O
implemented	O
entirely	O
in	O
software	O
.	O
</s>
<s>
Alternatively	O
,	O
multiple	O
manufacturers	O
(	O
including	O
Kenwood	O
and	O
Alinco	O
)	O
now	O
market	O
handheld	O
or	O
mobile	O
radios	O
with	O
built-in	O
TNCs	O
,	O
allowing	O
connection	O
directly	O
to	O
the	O
serial	B-Protocol
port	I-Protocol
of	O
a	O
computer	O
or	O
terminal	O
with	O
no	O
other	O
equipment	O
required	O
.	O
</s>
<s>
The	O
computer	O
is	O
responsible	O
for	O
managing	O
network	O
connections	O
,	O
formatting	O
data	O
as	O
AX.25	B-Protocol
packets	O
,	O
and	O
controlling	O
the	O
radio	O
channel	O
.	O
</s>
<s>
Frequently	O
it	O
provides	O
other	O
functionality	O
as	O
well	O
,	O
such	O
as	O
a	O
simple	O
bulletin	B-Application
board	I-Application
system	I-Application
to	O
accept	O
messages	O
while	O
the	O
operator	O
is	O
away	O
.	O
</s>
<s>
Following	O
the	O
OSI	O
model	O
,	O
packet	B-Application
radio	I-Application
networks	O
can	O
be	O
described	O
in	O
terms	O
of	O
the	O
physical	O
,	O
data	O
link	O
,	O
and	O
network	O
layer	O
protocols	O
on	O
which	O
they	O
rely	O
.	O
</s>
<s>
Modems	B-Application
used	O
for	O
packet	B-Application
radio	I-Application
vary	O
in	O
throughput	O
and	O
modulation	O
technique	O
,	O
and	O
are	O
normally	O
selected	O
to	O
match	O
the	O
capabilities	O
of	O
the	O
radio	O
equipment	O
in	O
use	O
.	O
</s>
<s>
The	O
first	O
amateur	O
packet	B-Application
radio	I-Application
stations	O
were	O
constructed	O
using	O
surplus	O
Bell	B-Device
202	I-Device
1,200	O
bit/s	O
modems	B-Application
,	O
and	O
despite	O
its	O
low	O
data	O
rate	O
,	O
Bell	B-Device
202	I-Device
modulation	O
has	O
remained	O
the	O
standard	O
for	O
VHF	B-Algorithm
operation	O
in	O
most	O
areas	O
.	O
</s>
<s>
At	O
HF	B-Algorithm
frequencies	O
,	O
Bell	O
103	O
modulation	O
is	O
used	O
,	O
at	O
a	O
rate	O
of	O
300	O
bit/s	O
.	O
</s>
<s>
Upon	O
adding	O
a	O
turn	O
the	O
transmitter	O
on	O
output	O
signal	O
(	O
"	O
PTT	O
"	O
)	O
for	O
transmitter	O
control	O
,	O
one	O
has	O
made	O
a	O
radio	O
modem	B-Application
.	O
</s>
<s>
Due	O
to	O
this	O
simplicity	O
,	O
and	O
just	O
having	O
suitable	O
microchips	O
at	O
hand	O
,	O
the	O
Bell	B-Device
202	I-Device
modulation	O
became	O
standard	O
way	O
to	O
send	O
the	O
packet	B-Application
radio	I-Application
data	O
over	O
the	O
radio	O
as	O
two	O
distinct	O
tones	O
.	O
</s>
<s>
Ways	O
to	O
achieve	O
higher	O
speeds	O
than	O
1,200	O
bits/s	O
,	O
include	O
using	O
telephone	B-Application
modem	I-Application
chips	O
via	O
the	O
microphone	O
and	O
audio	O
out	O
connectors	O
.	O
</s>
<s>
This	O
has	O
been	O
proven	O
to	O
work	O
at	O
speeds	O
up	O
to	O
4,800	O
bit/s	O
using	O
fax	O
V.27	O
modems	B-Application
in	O
half-duplex	O
mode	O
.	O
</s>
<s>
These	O
modems	B-Application
use	O
phase-shift	B-General_Concept
keying	I-General_Concept
which	O
works	O
fine	O
when	O
there	O
is	O
no	O
amplitude-shift	O
keying	O
,	O
but	O
at	O
faster	O
speeds	O
such	O
as	O
9,600	O
bit/s	O
,	O
signal	O
levels	O
become	O
critical	O
and	O
they	O
are	O
extremely	O
sensitive	O
to	O
group	O
delay	O
in	O
the	O
radio	O
.	O
</s>
<s>
1,200	O
bit/s	O
AFSK	O
node	B-Protocol
controllers	O
on	O
2	B-Algorithm
meters	I-Algorithm
(	O
144	O
–	O
148MHz	O
)	O
are	O
the	O
most	O
commonly	O
found	O
packet	B-Application
radio	I-Application
.	O
</s>
<s>
For	O
1,200	O
/2	O
,	O
400	O
bit/s	O
UHF/VHF	O
packet	B-Application
radio	I-Application
,	O
amateurs	O
use	O
commonly	O
available	O
narrow	O
band	O
FM	O
voice	O
radios	O
.	O
</s>
<s>
For	O
HF	B-Algorithm
packet	O
,	O
300	O
bit/s	O
data	O
is	O
used	O
over	O
single	B-Device
sideband	I-Device
(	O
SSB	B-Device
)	O
modulation	O
.	O
</s>
<s>
Custom	O
modems	B-Application
have	O
been	O
developed	O
which	O
allow	O
throughput	O
rates	O
of	O
19.2	O
kbit/s	O
,	O
56	O
kbit/s	O
,	O
and	O
even	O
1.2Mbit/s	O
over	O
amateur	O
radio	O
links	O
on	O
FCC	O
permitted	O
frequencies	O
of	O
440MHz	O
and	O
above	O
.	O
</s>
<s>
The	O
interface	O
between	O
the	O
"	O
modem	B-Application
"	O
and	O
the	O
"	O
radio	O
"	O
is	O
at	O
the	O
intermediate	O
frequency	O
part	O
of	O
the	O
radio	O
as	O
opposed	O
to	O
the	O
audio	O
section	O
used	O
for	O
1,200	O
bit/s	O
operation	O
.	O
</s>
<s>
In	O
many	O
commercial	O
data	O
radio	O
applications	O
,	O
audio	O
baseband	B-Algorithm
modulation	O
is	O
not	O
used	O
.	O
</s>
<s>
The	O
2.4GHz	O
"	O
Wi-Fi	O
"	O
band	O
partially	O
overlaps	O
an	O
amateur	O
radio	O
band	O
,	O
so	O
commercial	O
Wi-Fi	O
hardware	O
can	O
be	O
adapted	O
and	O
used	O
by	O
licensed	O
amateur	O
radio	O
operators	O
at	O
higher	O
power	O
levels	O
,	O
although	O
restrictions	O
on	O
amateur	O
radio	O
limit	O
the	O
appeal	O
of	O
using	O
packet	B-Application
radio	I-Application
to	O
connect	O
to	O
the	O
internet	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
networks	O
rely	O
on	O
the	O
AX.25	B-Protocol
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
,	O
derived	O
from	O
the	O
X.25	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
intended	O
specifically	O
for	O
amateur	O
radio	O
use	O
.	O
</s>
<s>
Despite	O
its	O
name	O
,	O
AX.25	B-Protocol
defines	O
both	O
the	O
physical	O
and	O
data	B-Protocol
link	I-Protocol
layers	I-Protocol
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Packet	B-Application
radio	I-Application
has	O
most	O
often	O
been	O
used	O
for	O
direct	O
,	O
keyboard-to-keyboard	O
connections	O
between	O
stations	O
,	O
either	O
between	O
two	O
live	O
operators	O
or	O
between	O
an	O
operator	O
and	O
a	O
bulletin	B-Application
board	I-Application
system	I-Application
.	O
</s>
<s>
No	O
network	O
services	O
above	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
are	O
required	O
for	O
these	O
applications	O
.	O
</s>
<s>
To	O
provide	O
automated	O
routing	O
of	O
data	O
between	O
stations	O
(	O
important	O
for	O
the	O
delivery	O
of	O
electronic	O
mail	O
)	O
,	O
several	O
network	O
layer	O
protocols	O
have	O
been	O
developed	O
for	O
use	O
with	O
AX.25	B-Protocol
.	O
</s>
<s>
In	O
principle	O
,	O
any	O
network	O
layer	O
protocol	O
may	O
be	O
used	O
,	O
including	O
the	O
ubiquitous	O
Internet	B-Protocol
Protocol	I-Protocol
.	O
</s>
