<s>
The	O
VHF	B-Algorithm
Data	I-Algorithm
Link	I-Algorithm
or	O
VHF	B-Algorithm
Digital	O
Link	O
(	O
VDL	O
)	O
is	O
a	O
means	O
of	O
sending	O
information	O
between	O
aircraft	O
and	O
ground	O
stations	O
(	O
and	O
in	O
the	O
case	O
of	O
VDL	O
Mode	O
4	O
,	O
other	O
aircraft	O
)	O
.	O
</s>
<s>
Aeronautical	O
VHF	B-Algorithm
data	I-Algorithm
links	I-Algorithm
use	O
the	O
band	O
117.975	O
–	O
137MHz	O
assigned	O
by	O
the	O
International	O
Telecommunication	O
Union	O
to	O
Aeronautical	O
mobile	O
(	O
R	O
)	O
service	O
.	O
</s>
<s>
There	O
are	O
ARINC	O
standards	O
for	O
ACARS	B-Algorithm
on	O
VHF	B-Algorithm
and	O
other	O
data	O
links	O
installed	O
on	O
approximately	O
14,000	O
aircraft	O
and	O
a	O
range	O
of	O
ICAO	O
standards	O
defined	O
by	O
the	O
Aeronautical	O
Mobile	O
Communications	O
Panel	O
(	O
AMCP	O
)	O
in	O
the	O
1990s	O
.	O
</s>
<s>
Mode	O
2	O
is	O
the	O
only	O
VDL	O
mode	O
being	O
implemented	O
operationally	O
to	O
support	O
Controller	B-Device
Pilot	I-Device
Data	I-Device
Link	I-Device
Communications	I-Device
(	O
CPDLC	B-Device
)	O
.	O
</s>
<s>
It	O
was	O
the	O
same	O
as	O
VDL	O
Mode	O
2	O
except	O
that	O
it	O
used	O
the	O
same	O
VHF	B-Algorithm
link	O
as	O
VHF	B-Algorithm
ACARS	B-Algorithm
so	O
it	O
could	O
be	O
implemented	O
using	O
analog	O
radios	O
before	O
VHF	B-Algorithm
Digital	O
Radio	O
implementation	O
was	O
completed	O
.	O
</s>
<s>
It	O
has	O
been	O
implemented	O
in	O
a	O
Eurocontrol	O
Link	O
2000+	O
program	O
and	O
is	O
specified	O
as	O
the	O
primary	O
link	O
in	O
the	O
EU	O
Single	O
European	O
Sky	O
rule	O
adopted	O
in	O
January	O
2009	O
requiring	O
all	O
new	O
aircraft	O
flying	O
in	O
Europe	O
after	O
January	O
1	O
,	O
2014	O
to	O
be	O
equipped	O
with	O
CPDLC	B-Device
.	O
</s>
<s>
In	O
advance	O
of	O
CPDLC	B-Device
implementation	O
,	O
VDL	O
Mode	O
2	O
has	O
already	O
been	O
implemented	O
in	O
approximately	O
2,000	O
aircraft	O
to	O
transport	O
ACARS	B-Algorithm
messages	O
simplifying	O
the	O
addition	O
of	O
CPDLC	B-Device
.	O
</s>
<s>
The	O
Subnetwork	O
Layer	O
complies	O
with	O
the	O
requirements	O
of	O
the	O
ICAO	O
Aeronautical	B-Device
Telecommunication	I-Device
Network	I-Device
(	O
ATN	O
)	O
standard	O
which	O
specifies	O
an	O
end-to-end	O
data	O
protocol	O
to	O
be	O
used	O
over	O
multiple	O
air-ground	O
and	O
ground	O
subnetworks	O
including	O
VDL	O
.	O
</s>
<s>
The	O
VDL	O
Mode	O
2	O
Link	O
Layer	O
is	O
made	O
up	O
of	O
two	O
sublayers	O
:	O
a	O
Data	O
Link	O
service	O
and	O
a	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
sublayer	O
.	O
</s>
<s>
The	O
Data	O
Link	O
protocol	O
is	O
based	O
on	O
the	O
ISO	O
standards	O
used	O
for	O
dial-up	O
HDLC	O
access	O
to	O
X.25	B-Protocol
networks	O
.	O
</s>
<s>
The	O
MAC	B-Protocol
protocol	I-Protocol
is	O
a	O
version	O
of	O
Carrier	O
Sense	O
Multiple	O
Access	O
(	O
CSMA	O
)	O
.	O
</s>
<s>
The	O
VDL	O
Mode	O
2	O
Physical	O
Layer	O
specifies	O
the	O
use	O
in	O
a	O
25kHz	O
wide	O
VHF	B-Algorithm
channel	O
of	O
a	O
modulation	O
scheme	O
called	O
Differential	O
8-Phase-shift	O
keying	O
with	O
a	O
symbol	O
rate	O
of	O
10,500	O
symbols	O
per	O
second	O
.	O
</s>
<s>
This	O
required	O
the	O
implementation	O
of	O
VHF	B-Algorithm
digital	O
radios	O
.	O
</s>
<s>
Its	O
primary	O
function	O
was	O
to	O
provide	O
a	O
VHF	B-Algorithm
frequency	O
physical	O
layer	O
for	O
ADS-B	B-Application
transmissions	O
.	O
</s>
<s>
However	O
it	O
was	O
overtaken	O
as	O
the	O
link	O
for	O
ADS-B	B-Application
by	O
the	O
Mode	O
S	O
radar	O
link	O
operating	O
in	O
the	O
1,090	O
MHz	O
band	O
which	O
was	O
selected	O
as	O
the	O
primary	O
link	O
by	O
the	O
ICAO	O
Air	O
Navigation	O
Conference	O
in	O
2003	O
.	O
</s>
<s>
European	O
Air	O
Traffic	O
Management	O
modernization	O
trials	O
have	O
implemented	O
ADS-B	B-Application
and	O
air-ground	O
exchanges	O
using	O
VDL	O
Mode	O
4	O
systems	O
.	O
</s>
<s>
However	O
,	O
on	O
air	O
transport	O
aircraft	O
the	O
operational	O
implementations	O
of	O
ADS-B	B-Application
will	O
use	O
the	O
Mode	O
S	O
link	O
and	O
of	O
CPDLC	B-Device
will	O
use	O
VDL	O
Mode	O
2	O
.	O
</s>
