<s>
A	O
unidirectional	B-Device
network	I-Device
(	O
also	O
referred	O
to	O
as	O
a	O
unidirectional	O
gateway	O
or	O
data	B-Device
diode	I-Device
)	O
is	O
a	O
network	O
appliance	O
or	O
device	O
that	O
allows	O
data	O
to	O
travel	O
in	O
only	O
one	O
direction	O
.	O
</s>
<s>
Data	B-Device
diodes	I-Device
can	O
be	O
found	O
most	O
commonly	O
in	O
high	O
security	O
environments	O
,	O
such	O
as	O
defense	O
,	O
where	O
they	O
serve	O
as	O
connections	O
between	O
two	O
or	O
more	O
networks	O
of	O
differing	O
security	O
classifications	O
.	O
</s>
<s>
Given	O
the	O
rise	O
of	O
industrial	B-Device
IoT	I-Device
and	O
digitization	O
,	O
this	O
technology	O
can	O
now	O
be	O
found	O
at	O
the	O
industrial	O
control	O
level	O
for	O
such	O
facilities	O
as	O
nuclear	O
power	O
plants	O
,	O
power	O
generation	O
and	O
safety	B-Application
critical	I-Application
systems	I-Application
like	O
railway	O
networks	O
.	O
</s>
<s>
After	O
years	O
of	O
development	O
,	O
data	B-Device
diodes	I-Device
have	O
evolved	O
from	O
being	O
only	O
a	O
network	O
appliance	O
or	O
device	O
allowing	O
raw	O
data	O
to	O
travel	O
only	O
in	O
one	O
direction	O
,	O
used	O
in	O
guaranteeing	O
information	O
security	O
or	O
protection	O
of	O
critical	O
digital	O
systems	O
,	O
such	O
as	O
industrial	O
control	O
systems	O
,	O
from	O
inbound	O
cyber	O
attacks	O
,	O
to	O
combinations	O
of	O
hardware	O
and	O
software	O
running	O
in	O
proxy	O
computers	O
in	O
the	O
source	O
and	O
destination	O
networks	O
.	O
</s>
<s>
Data	B-Device
Diodes	I-Device
are	O
now	O
capable	O
of	O
transferring	O
multiple	O
protocols	O
and	O
data	O
types	O
simultaneously	O
.	O
</s>
<s>
It	O
contains	O
a	O
broader	O
range	O
of	O
cybersecurity	O
features	O
like	O
secure	O
boot	O
,	O
certificate	O
management	O
,	O
data	O
integrity	O
,	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
,	O
secure	O
communication	O
via	O
TLS	B-Protocol
,	O
among	O
others	O
.	O
</s>
<s>
A	O
unique	O
characteristic	O
is	O
that	O
data	O
is	O
transferred	O
deterministically	O
(	O
to	O
predetermined	O
locations	O
)	O
with	O
a	O
protocol	O
"	O
break	O
"	O
that	O
allows	O
the	O
data	O
to	O
be	O
transferred	O
through	O
the	O
data	B-Device
diode	I-Device
.	O
</s>
<s>
Data	B-Device
diodes	I-Device
are	O
commonly	O
found	O
in	O
high	O
security	O
military	O
and	O
government	O
environments	O
,	O
and	O
are	O
now	O
becoming	O
widely	O
spread	O
in	O
sectors	O
like	O
oil	O
&	O
gas	O
,	O
water/wastewater	O
,	O
airplanes	O
(	O
between	O
flight	O
control	O
units	O
and	O
in-flight	O
entertainment	O
systems	O
)	O
,	O
manufacturing	O
and	O
cloud	B-Architecture
connectivity	I-Architecture
for	O
industrial	B-Device
IoT	I-Device
.	O
</s>
<s>
The	O
first	O
data	B-Device
diodes	I-Device
were	O
developed	O
by	O
governmental	O
organizations	O
in	O
the	O
eighties	O
and	O
nineties	O
.	O
</s>
<s>
Primary	O
solutions	O
used	O
by	O
these	O
organizations	O
were	O
air	B-Architecture
gaps	I-Architecture
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
utilities	O
and	O
oil	O
and	O
gas	O
companies	O
have	O
used	O
data	B-Device
diodes	I-Device
for	O
several	O
years	O
,	O
and	O
regulators	O
have	O
encouraged	O
their	O
use	O
to	O
protect	O
equipment	O
and	O
processes	O
in	O
safety	O
instrumented	O
systems	O
(	O
SISs	O
)	O
.	O
</s>
<s>
The	O
Nuclear	O
Regulatory	O
Commission	O
(	O
NRC	O
)	O
now	O
mandates	O
the	O
use	O
of	O
data	B-Device
diodes	I-Device
and	O
many	O
other	O
sectors	O
,	O
in	O
addition	O
to	O
electrical	O
and	O
nuclear	O
,	O
also	O
use	O
data	B-Device
diodes	I-Device
effectively	O
.	O
</s>
<s>
In	O
Europe	O
,	O
regulators	O
and	O
operators	O
of	O
several	O
safety-critical	B-Application
systems	I-Application
started	O
recommending	O
and	O
implementing	O
regulations	O
on	O
the	O
use	O
of	O
unidirectional	O
gateways	O
.	O
</s>
<s>
In	O
2013	O
the	O
working	O
,	O
Industrial	O
Control	O
System	O
Cybersecurity	O
,	O
directed	O
by	O
the	O
French	O
Network	O
and	O
Information	O
Security	O
Agency	O
(	O
ANSSI	O
)	O
stated	O
that	O
is	O
forbidden	O
to	O
use	O
firewalls	B-Application
to	O
connect	O
any	O
class	O
3	O
network	O
,	O
such	O
as	O
railway	O
switching	O
systems	O
,	O
to	O
a	O
lower	O
class	O
network	O
or	O
corporate	O
network	O
,	O
only	O
unidirectional	O
technology	O
is	O
permitted	O
.	O
</s>
<s>
Unidirectional	B-Device
network	I-Device
devices	O
are	O
typically	O
used	O
to	O
guarantee	O
information	O
security	O
or	O
protection	O
of	O
critical	O
digital	O
systems	O
,	O
such	O
as	O
Industrial	O
control	O
systems	O
,	O
from	O
cyber	O
attacks	O
.	O
</s>
<s>
The	O
physical	O
nature	O
of	O
unidirectional	B-Device
networks	I-Device
only	O
allows	O
data	O
to	O
pass	O
from	O
one	O
side	O
of	O
a	O
network	O
connection	O
to	O
another	O
,	O
and	O
not	O
the	O
other	O
way	O
around	O
.	O
</s>
<s>
In	O
the	O
second	O
case	O
,	O
a	O
safety-critical	B-Application
physical	O
system	O
can	O
be	O
made	O
accessible	O
for	O
online	O
monitoring	O
,	O
yet	O
be	O
insulated	O
from	O
all	O
Internet-based	O
attacks	O
that	O
might	O
seek	O
to	O
cause	O
physical	O
damage	O
.	O
</s>
<s>
There	O
are	O
two	O
general	O
models	O
for	O
using	O
unidirectional	B-Device
network	I-Device
connections	O
.	O
</s>
<s>
In	O
the	O
classical	O
model	O
,	O
the	O
purpose	O
of	O
the	O
data	B-Device
diode	I-Device
is	O
to	O
prevent	O
export	O
of	O
classified	O
data	O
from	O
a	O
secure	O
machine	O
while	O
allowing	O
import	O
of	O
data	O
from	O
an	O
insecure	O
machine	O
.	O
</s>
<s>
The	O
majority	O
of	O
unidirectional	B-Device
network	I-Device
applications	O
in	O
this	O
category	O
are	O
in	O
defense	O
,	O
and	O
defense	O
contractors	O
.	O
</s>
<s>
These	O
organizations	O
traditionally	O
have	O
applied	O
air	B-Architecture
gaps	I-Architecture
to	O
keep	O
classified	O
data	O
physically	O
separate	O
from	O
any	O
Internet	O
connection	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
unidirectional	B-Device
networks	I-Device
in	O
some	O
of	O
these	O
environments	O
,	O
a	O
degree	O
of	O
connectivity	O
can	O
safely	O
exist	O
between	O
a	O
network	O
with	O
classified	O
data	O
,	O
and	O
a	O
network	O
with	O
an	O
Internet	O
connection	O
.	O
</s>
<s>
If	O
users	O
at	O
each	O
security	O
level	O
share	O
a	O
machine	O
dedicated	O
to	O
that	O
level	O
,	O
and	O
if	O
the	O
machines	O
are	O
connected	O
by	O
data	B-Device
diodes	I-Device
,	O
the	O
Bell-Lapadula	O
constraints	O
can	O
be	O
rigidly	O
enforced	O
.	O
</s>
<s>
Traditionally	O
,	O
when	O
the	O
IT	B-General_Concept
network	I-General_Concept
provides	O
DMZ	B-General_Concept
server	O
access	O
for	O
an	O
authorized	O
user	O
,	O
the	O
data	O
is	O
vulnerable	O
to	O
intrusions	O
from	O
the	O
IT	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
However	O
,	O
with	O
a	O
unidirectional	O
gateways	O
separating	O
a	O
critical	O
side	O
or	O
OT	O
network	O
with	O
sensitive	O
data	O
from	O
an	O
open	O
side	O
with	O
business	O
and	O
Internet	O
connectivity	O
,	O
normally	O
IT	B-General_Concept
network	I-General_Concept
,	O
organizations	O
can	O
achieve	O
the	O
best	O
of	O
both	O
worlds	O
,	O
enabling	O
the	O
connectivity	O
required	O
and	O
assuring	O
security	O
.	O
</s>
<s>
This	O
holds	O
true	O
even	O
if	O
the	O
IT	B-General_Concept
network	I-General_Concept
is	O
compromised	O
,	O
because	O
the	O
traffic	O
flow	O
control	O
is	O
physical	O
in	O
nature	O
.	O
</s>
<s>
No	O
reported	O
cases	O
of	O
data	B-Device
diodes	I-Device
being	O
bypassed	O
or	O
exploited	O
to	O
enable	O
two-way	O
traffic	O
.	O
</s>
<s>
Cost	O
;	O
data	B-Device
diodes	I-Device
were	O
originally	O
expensive	O
,	O
although	O
lower	O
cost	O
solutions	O
are	O
now	O
available	O
.	O
</s>
<s>
The	O
simplest	O
form	O
of	O
a	O
unidirectional	B-Device
network	I-Device
is	O
a	O
modified	O
,	O
fiber-optic	B-Architecture
network	I-Architecture
link	I-Architecture
,	O
with	O
send	O
and	O
receive	O
transceivers	O
removed	O
or	O
disconnected	O
for	O
one	O
direction	O
,	O
and	O
any	O
link	B-Protocol
failure	I-Protocol
protection	I-Protocol
mechanisms	I-Protocol
disabled	O
.	O
</s>
<s>
All-optical	O
data	B-Device
diodes	I-Device
can	O
support	O
very	O
high	O
channel	O
capacities	O
and	O
are	O
among	O
the	O
simplest	O
.	O
</s>
<s>
In	O
2019	O
,	O
Controlled	O
Interfaces	O
demonstrated	O
its	O
(	O
now	O
patented	O
)	O
one-way	O
optical	B-Architecture
fiber	I-Architecture
link	O
using	O
100G	O
Commercial	O
Off	O
The	O
Shelf	O
transceivers	O
in	O
a	O
pair	O
of	O
Arista	O
network	O
switch	O
platforms	O
.	O
</s>
<s>
The	O
German	O
companies	O
INFODAS	O
and	O
GENUA	O
have	O
developed	O
software	O
based	O
(	O
"	O
logical	O
"	O
)	O
data	B-Device
diodes	I-Device
that	O
use	O
a	O
Microkernel	O
Operating	O
system	O
to	O
ensure	O
unidirectional	O
data	O
transfer	O
.	O
</s>
<s>
Due	O
to	O
the	O
software	O
architecture	O
these	O
solutions	O
offer	O
higher	O
speed	O
than	O
conventional	O
hardware	O
based	O
data	B-Device
diodes	I-Device
.	O
</s>
<s>
ST	O
Engineering	O
,	O
have	O
developed	O
its	O
own	O
Secure	O
e-Application	O
Gateway	O
,	O
consisting	O
of	O
multiple	O
data	B-Device
diodes	I-Device
and	O
other	O
software	O
components	O
,	O
to	O
enable	O
real-time	O
bi-directional	O
HTTP(S )	O
web	O
services	O
transactions	O
over	O
the	O
internet	O
while	O
protecting	O
the	O
secured	O
networks	O
from	O
both	O
malicious	O
injects	O
and	O
data	O
leakage	O
.	O
</s>
<s>
In	O
2018	O
,	O
Siemens	O
Mobility	O
released	O
an	O
industrial	O
grade	O
unidirectional	O
gateway	O
solution	O
in	O
which	O
the	O
data	B-Device
diode	I-Device
,	O
Data	O
Capture	O
Unit	O
,	O
uses	O
electromagnetic	O
induction	O
and	O
new	O
chip	O
design	O
to	O
achieve	O
an	O
EBA	O
safety	O
assessment	O
,	O
guaranteeing	O
secure	O
connectivity	O
of	O
new	O
and	O
existing	O
safety	B-Application
critical	I-Application
systems	I-Application
up	O
to	O
Safety	O
integrity	O
level	O
(	O
SIL	O
)	O
4	O
to	O
enable	O
secure	O
IoT	B-Operating_System
and	O
provide	O
data	O
analytics	O
and	O
other	O
cloud	B-Architecture
hosted	I-Architecture
digital	O
services	O
.	O
</s>
<s>
The	O
US	O
Naval	O
Research	O
Laboratory	O
(	O
NRL	O
)	O
has	O
developed	O
its	O
own	O
unidirectional	B-Device
network	I-Device
called	O
the	O
Network	O
Pump	O
.	O
</s>
<s>
A	O
data	B-Device
diode	I-Device
intended	O
for	O
industrial	O
use	O
,	O
however	O
,	O
may	O
not	O
have	O
or	O
require	O
third	O
party	O
certification	O
and	O
accreditation	O
at	O
all	O
,	O
depending	O
on	O
the	O
application	O
.	O
</s>
