<s>
An	O
optical	B-Protocol
mesh	I-Protocol
network	I-Protocol
is	O
a	O
type	O
of	O
optical	O
telecommunications	O
network	O
employing	O
wired	O
fiber-optic	B-Application
communication	I-Application
or	O
wireless	O
free-space	O
optical	O
communication	O
in	O
a	O
mesh	B-Architecture
network	I-Architecture
architecture	O
.	O
</s>
<s>
Most	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
use	O
fiber-optic	B-Application
communication	I-Application
and	O
are	O
operated	O
by	O
internet	O
service	O
providers	O
in	O
metropolitan	O
and	O
regional	O
but	O
also	O
national	O
and	O
international	O
scenarios	O
.	O
</s>
<s>
They	O
are	O
faster	O
and	O
less	O
error	O
prone	O
than	O
other	O
network	B-Architecture
architectures	I-Architecture
and	O
support	O
backup	O
and	O
recovery	O
plans	O
for	O
established	O
networks	O
in	O
case	O
of	O
any	O
disaster	O
,	O
damage	O
or	O
failure	O
.	O
</s>
<s>
Currently	O
planned	O
satellite	O
constellations	O
aim	O
to	O
establish	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
in	O
space	O
by	O
using	O
wireless	O
laser	O
communication	O
.	O
</s>
<s>
Transport	O
networks	O
,	O
the	O
underlying	O
optical	O
fiber-based	O
layer	O
of	O
telecommunications	O
networks	O
,	O
have	O
evolved	O
from	O
Digital	B-Architecture
cross	I-Architecture
connect	I-Architecture
system	I-Architecture
(	O
DCS	O
)	O
-based	O
mesh	B-Architecture
architectures	O
in	O
the	O
1980s	O
,	O
to	O
SONET/SDH	B-Protocol
(	O
Synchronous	O
Optical	O
Networking/Synchronous	O
Digital	O
Hierarchy	O
)	O
ring	O
architectures	O
in	O
the	O
1990s	O
.	O
</s>
<s>
In	O
DCS-based	O
mesh	B-Architecture
architectures	O
,	O
telecommunications	O
carriers	O
deployed	O
restoration	O
systems	O
for	O
DS3	O
circuits	O
such	O
as	O
AT&T	O
FASTAR	O
(	O
FAST	B-Protocol
Automatic	I-Protocol
Restoration	I-Protocol
)	O
and	O
MCI	O
Real	O
Time	O
Restoration	O
(	O
RTR	O
)	O
,	O
restoring	O
circuits	O
in	O
minutes	O
after	O
a	O
network	O
failure	O
.	O
</s>
<s>
In	O
SONET/SDH	B-Protocol
rings	O
,	O
carriers	O
implemented	O
ring	B-Architecture
protection	I-Architecture
such	O
as	O
SONET	B-Protocol
Unidirectional	O
Path	B-Protocol
Switched	O
Ring	O
(	O
UPSR	O
)	O
(	O
also	O
called	O
Sub-Network	B-Protocol
Connection	I-Protocol
Protection	I-Protocol
(	O
SCNP	O
)	O
in	O
SDH	B-Protocol
networks	O
)	O
or	O
SONET	B-Protocol
Bidirectional	O
Line	O
Switched	O
Ring	O
(	O
BLSR	O
)	O
(	O
also	O
called	O
Multiplex	B-Architecture
Section	O
-	O
Shared	O
Protection	O
Ring	O
(	O
MS-SPRing	O
)	O
in	O
SDH	B-Protocol
networks	O
)	O
,	O
protecting	O
against	O
and	O
recovering	O
from	O
a	O
network	O
failure	O
in	O
50ms	O
or	O
less	O
,	O
a	O
significant	O
improvement	O
over	O
the	O
recovery	O
time	O
supported	O
in	O
DCS-based	O
mesh	B-Architecture
restoration	O
,	O
and	O
a	O
key	O
driver	O
for	O
the	O
deployment	O
of	O
SONET/SDH	B-Protocol
ring-based	O
protection	O
.	O
</s>
<s>
G.841	O
)	O
,	O
"	O
P-cycles	B-Protocol
"	O
protection	O
,	O
next-generation	O
SONET/SDH	B-Protocol
equipment	O
that	O
can	O
handle	O
multiple	O
rings	O
,	O
or	O
have	O
the	O
ability	O
to	O
not	O
close	O
the	O
working	O
or	O
protection	O
ring	O
side	O
,	O
or	O
to	O
share	O
protection	O
capacity	O
among	O
rings	O
(	O
e.g.	O
,	O
with	O
Virtual	O
Line	O
Switched	O
Ring	O
(	O
VLSR	O
)	O
)	O
.	O
</s>
<s>
Technological	O
advancements	O
in	O
optical	O
transport	O
switches	O
in	O
the	O
first	O
decade	O
of	O
the	O
21st	O
century	O
,	O
along	O
with	O
continuous	O
deployment	O
of	O
dense	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
DWDM	O
)	O
systems	O
,	O
have	O
led	O
telecommunications	O
service	O
providers	O
to	O
replace	O
their	O
SONET	B-Protocol
ring	I-Protocol
architectures	O
by	O
mesh-based	O
architectures	O
for	O
new	O
traffic	O
.	O
</s>
<s>
The	O
new	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
support	O
the	O
same	O
fast	O
recovery	O
previously	O
available	O
in	O
ring	O
networks	O
while	O
achieving	O
better	O
capacity	O
efficiency	O
and	O
resulting	O
in	O
lower	O
capital	O
cost	O
.	O
</s>
<s>
Such	O
fast	O
recovery	O
(	O
in	O
the	O
tens	O
to	O
hundreds	O
of	O
milliseconds	O
)	O
in	O
case	O
of	O
failures	O
(	O
e.g.	O
,	O
network	O
link	O
or	O
node	O
failure	O
)	O
is	O
achieved	O
through	O
the	O
intelligence	O
embedded	O
in	O
these	O
new	O
optical	O
transport	O
equipment	O
,	O
which	O
allows	O
recovery	O
to	O
be	O
automatic	O
and	O
handled	O
within	O
the	O
network	O
itself	O
as	O
part	O
of	O
the	O
network	O
control	B-Protocol
plane	I-Protocol
,	O
without	O
relying	O
on	O
an	O
external	O
network	O
management	O
system	O
.	O
</s>
<s>
Optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
refer	O
to	O
transport	O
networks	O
that	O
are	O
built	O
directly	O
off	O
the	O
mesh-like	O
fiber	O
infrastructure	O
deployed	O
in	O
metropolitan	O
,	O
regional	O
,	O
national	O
,	O
or	O
international	O
(	O
e.g.	O
,	O
trans-oceanic	O
)	O
areas	O
by	O
deploying	O
optical	O
transport	O
equipment	O
that	O
are	O
capable	O
of	O
switching	B-Protocol
traffic	O
(	O
at	O
the	O
wavelength	O
or	O
sub-wavelength	O
level	O
)	O
from	O
an	O
incoming	O
fiber	O
to	O
an	O
outgoing	O
fiber	O
.	O
</s>
<s>
In	O
addition	O
to	O
switching	B-Protocol
wavelengths	O
,	O
the	O
equipment	O
is	O
typically	O
also	O
able	O
to	O
multiplex	B-Architecture
lower	O
speed	O
traffic	O
into	O
wavelengths	O
for	O
transport	O
,	O
and	O
to	O
groom	O
traffic	O
(	O
as	O
long	O
as	O
the	O
equipment	O
is	O
so-called	O
opaque	O
-	O
see	O
subsection	O
on	O
transparency	O
)	O
.	O
</s>
<s>
As	O
most	O
of	O
the	O
transport	O
networks	O
evolve	O
toward	O
mesh	B-Architecture
topologies	I-Architecture
utilizing	O
intelligent	O
network	O
elements	O
(	O
optical	B-Protocol
cross-connects	I-Protocol
or	O
optical	O
switches	O
)	O
for	O
provisioning	O
and	O
recovery	O
of	O
services	O
,	O
new	O
approaches	O
have	O
been	O
developed	O
for	O
the	O
design	O
,	O
deployment	O
,	O
operations	O
and	O
management	O
of	O
mesh	B-Architecture
optical	O
networks	O
.	O
</s>
<s>
Optical	O
switches	O
build	O
by	O
companies	O
such	O
as	O
Sycamore	O
and	O
Ciena	O
(	O
with	O
STS-1	B-Protocol
granularity	O
of	O
switching	B-Protocol
)	O
and	O
Tellium	O
(	O
with	O
STS-48	B-Protocol
granularity	O
of	O
switching	B-Protocol
)	O
have	O
been	O
deployed	O
in	O
operational	O
mesh	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Calient	O
has	O
built	O
all-optical	O
switches	O
based	O
on	O
3D	O
MEMS	B-General_Concept
technology	O
.	O
</s>
<s>
Optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
today	O
not	O
only	O
provide	O
trunking	O
capacity	O
to	O
higher-layer	O
networks	O
,	O
such	O
as	O
inter-router	O
or	O
inter-switch	O
connectivity	O
in	O
an	O
IP	B-Protocol
,	O
MPLS	B-Protocol
,	O
or	O
Ethernet-centric	O
packet	O
infrastructure	O
,	O
but	O
also	O
support	O
efficient	O
routing	B-Protocol
and	O
fast	O
failure	O
recovery	O
of	O
high-bandwidth	O
point-to-point	O
Ethernet	O
and	O
SONET/SDH	B-Protocol
services	O
.	O
</s>
<s>
Several	O
planned	O
satellite	O
constellations	O
such	O
as	O
SpaceX	O
Starlink	O
intended	O
for	O
global	O
internet	O
provisioning	O
aim	O
to	O
establish	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
in	O
space	O
.	O
</s>
<s>
The	O
interconnected	O
network	B-Architecture
architecture	I-Architecture
allows	O
for	O
direct	O
routing	B-Protocol
of	O
user	O
data	O
from	O
satellite	O
to	O
satellite	O
and	O
enables	O
seamless	O
network	O
management	O
and	O
continuity	O
of	O
service	O
.	O
</s>
<s>
Optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
support	O
the	O
establishment	O
of	O
circuit-mode	O
connection-oriented	B-Protocol
services	I-Protocol
.	O
</s>
<s>
Multiple	O
recovery	O
mechanisms	O
that	O
provide	O
different	O
levels	O
of	O
protection	O
or	O
restoration	O
against	O
different	O
failure	O
modes	O
are	O
available	O
in	O
mesh	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Channel-	O
,	O
link-	O
,	O
segment	B-Architecture
-	I-Architecture
and	O
path	B-Protocol
-	O
protection	O
are	O
the	O
most	O
common	O
protection	O
schemes	O
.	O
</s>
<s>
P-cycles	B-Protocol
is	O
another	O
type	O
of	O
protection	O
that	O
leverages	O
and	O
extends	O
ring-based	O
protection	O
.	O
</s>
<s>
In	O
path-protected	O
mesh	B-Architecture
networks	I-Architecture
,	O
some	O
connections	O
can	O
be	O
unprotected	O
;	O
others	O
can	O
be	O
protected	O
against	O
single	O
or	O
multiple	O
failures	O
in	O
various	O
ways	O
.	O
</s>
<s>
A	O
connection	O
can	O
be	O
protected	O
against	O
a	O
single	O
failure	O
by	O
defining	O
a	O
backup	O
path	B-Protocol
,	O
diverse	O
from	O
the	O
primary	O
path	B-Protocol
taken	O
by	O
the	O
connection	O
over	O
the	O
mesh	B-Architecture
network	I-Architecture
.	O
</s>
<s>
The	O
backup	O
path	B-Protocol
and	O
associated	O
resources	O
can	O
be	O
dedicated	O
to	O
the	O
connection	O
(	O
Dedicated	O
Backup	O
Path	B-Protocol
Protection	I-Protocol
,	O
aka	O
dedicated	O
(	O
1+1	O
)	O
path	B-Protocol
protection	I-Protocol
,	O
Subnetwork	B-Protocol
Connection	I-Protocol
Protection	I-Protocol
(	O
SNCP	O
)	O
in	O
SDH	B-Protocol
networks	O
,	O
or	O
UPSR	O
in	O
SONET	B-Protocol
ring	I-Protocol
networks	O
)	O
,	O
or	O
shared	O
among	O
multiple	O
connections	O
(	O
)	O
,	O
typically	O
ones	O
whose	O
primary	O
paths	O
are	O
not	O
likely	O
to	O
fail	O
at	O
the	O
same	O
time	O
,	O
thereby	O
avoiding	O
contention	O
for	O
the	O
shared	O
resources	O
in	O
case	O
of	O
a	O
single	O
link	O
or	O
node	O
failure	O
.	O
</s>
<s>
Finally	O
,	O
multiple	O
diverse	O
routes	O
can	O
be	O
designed	O
so	O
that	O
a	O
connection	O
has	O
multiple	O
recovery	O
routes	O
and	O
can	O
recover	O
even	O
after	O
multiple	O
failures	O
(	O
examples	O
of	O
mesh	B-Architecture
networks	I-Architecture
across	O
the	O
Atlantic	O
and	O
Pacific	O
oceans	O
)	O
.	O
</s>
<s>
Traditional	O
transport	O
networks	O
are	O
made	O
of	O
optical	O
fiber-based	O
links	O
between	O
telecommunications	O
offices	O
,	O
where	O
multiple	O
wavelengths	O
are	O
multiplexed	B-Architecture
to	O
increase	O
the	O
capacity	O
of	O
the	O
fiber	O
.	O
</s>
<s>
The	O
wavelengths	O
are	O
terminated	O
on	O
electronic	O
devices	O
called	O
transponders	B-Architecture
,	O
undergoing	O
an	O
optical-to-electrical	O
conversion	O
for	O
signal	O
Reamplification	O
,	O
Reshaping	O
,	O
and	O
Retiming	O
(	O
3R	O
)	O
.	O
</s>
<s>
Inside	O
a	O
telecommunications	O
office	O
,	O
the	O
signals	O
are	O
then	O
handled	O
to	O
and	O
switched	O
by	O
a	O
transport	O
switch	O
(	O
aka	O
optical	B-Protocol
cross-connect	I-Protocol
or	O
optical	O
switch	O
)	O
and	O
either	O
are	O
dropped	O
at	O
that	O
office	O
,	O
or	O
directed	O
to	O
an	O
outgoing	O
fiber	O
link	O
where	O
they	O
are	O
again	O
carried	O
as	O
wavelengths	O
multiplexed	B-Architecture
into	O
that	O
fiber	O
link	O
towards	O
the	O
next	O
telecommunications	O
office	O
.	O
</s>
<s>
When	O
the	O
incoming	O
wavelengths	O
do	O
not	O
undergo	O
an	O
optical-to-electrical	O
conversion	O
and	O
are	O
switched	O
through	O
a	O
telecommunications	O
office	O
in	O
the	O
optical	O
domain	O
using	O
all-optical	O
switches	O
(	O
also	O
called	O
photonic	B-Protocol
cross-connect	I-Protocol
,	O
optical	B-Protocol
add-drop	I-Protocol
multiplexer	I-Protocol
,	O
or	O
Reconfigurable	O
Optical	B-Protocol
Add-Drop	I-Protocol
Multiplexer	I-Protocol
(	O
ROADM	O
)	O
systems	O
)	O
,	O
the	O
network	O
is	O
considered	O
to	O
be	O
transparent	O
.	O
</s>
<s>
ROADM-based	O
transparent	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
have	O
been	O
deployed	O
in	O
metropolitan	O
and	O
regional	O
networks	O
since	O
the	O
mid-2000s	O
.	O
</s>
<s>
Routing	B-Protocol
is	O
a	O
key	O
control	O
and	O
operational	O
aspect	O
of	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
In	O
transparent	O
or	O
all-optical	O
networks	O
,	O
routing	B-Protocol
of	O
connections	O
is	O
tightly	O
linked	O
to	O
the	O
wavelength	O
selection	O
and	O
assignment	O
process	O
(	O
so-called	O
Routing	B-Protocol
and	I-Protocol
Wavelength	I-Protocol
Assignment	I-Protocol
,	O
or	O
"	O
RWA	O
"	O
)	O
.	O
</s>
<s>
In	O
an	O
opaque	O
network	O
,	O
the	O
routing	B-Protocol
problem	O
is	O
one	O
of	O
finding	O
a	O
primary	O
path	B-Protocol
for	O
a	O
connection	O
and	O
if	O
protection	O
is	O
needed	O
,	O
a	O
backup	O
path	B-Protocol
diverse	O
from	O
the	O
primary	O
path	B-Protocol
.	O
</s>
<s>
Several	O
algorithms	O
can	O
be	O
used	O
and	O
combined	O
to	O
determine	O
a	O
primary	O
path	B-Protocol
and	O
a	O
diverse	O
backup	O
path	B-Protocol
(	O
with	O
or	O
without	O
sharing	O
of	O
resource	O
along	O
the	O
backup	O
path	B-Protocol
)	O
for	O
a	O
connection	O
or	O
service	O
,	O
such	O
as	O
:	O
shortest	O
path	B-Protocol
,	O
including	O
Dijkstra	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
;	O
k-shortest	O
path	B-Protocol
,	O
such	O
as	O
Yen	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
;	O
edge	B-Protocol
and	I-Protocol
node-diverse	I-Protocol
or	I-Protocol
disjoint	I-Protocol
routing	I-Protocol
,	O
including	O
Suurballe	B-Protocol
's	I-Protocol
algorithm	I-Protocol
;	O
and	O
numerous	O
heuristics	B-Algorithm
.	O
</s>
<s>
In	O
general	O
,	O
however	O
,	O
the	O
problems	O
of	O
optimal	O
routing	B-Protocol
for	O
Dedicated	O
Backup	O
Path	B-Protocol
Protection	I-Protocol
with	O
arbitrary	O
Shared	O
Risk	O
Link	O
Groups	O
(	O
SRLGs	O
)	O
,	O
and	O
for	O
Shared	O
Backup	O
Path	B-Protocol
Protection	I-Protocol
are	O
NP-complete	O
.	O
</s>
<s>
Mesh	B-Architecture
networking	I-Architecture
in	O
general	O
and	O
wireless	O
mesh	B-Architecture
networking	I-Architecture
in	O
particular	O
.	O
</s>
