<s>
In	O
fiber-optic	B-Application
communications	I-Application
,	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	O
)	O
is	O
a	O
technology	O
which	O
multiplexes	B-Architecture
a	O
number	O
of	O
optical	B-Protocol
carrier	I-Protocol
signals	O
onto	O
a	O
single	O
optical	B-Architecture
fiber	I-Architecture
by	O
using	O
different	O
wavelengths	O
(	O
i.e.	O
,	O
colors	O
)	O
of	O
laser	O
light	O
.	O
</s>
<s>
This	O
technique	O
enables	O
bidirectional	O
communications	O
over	O
a	O
single	O
strand	O
of	O
fiber	O
,	O
also	O
called	O
wavelength-division	B-Architecture
duplexing	I-Architecture
,	O
as	O
well	O
as	O
multiplication	O
of	O
capacity	O
.	O
</s>
<s>
The	O
term	O
WDM	O
is	O
commonly	O
applied	O
to	O
an	O
optical	B-Protocol
carrier	I-Protocol
,	O
which	O
is	O
typically	O
described	O
by	O
its	O
wavelength	O
,	O
whereas	O
frequency-division	B-Protocol
multiplexing	I-Protocol
typically	O
applies	O
to	O
a	O
radio	O
carrier	O
which	O
is	O
more	O
often	O
described	O
by	O
frequency	O
.	O
</s>
<s>
A	O
WDM	O
system	O
uses	O
a	O
multiplexer	B-Protocol
at	O
the	O
transmitter	O
to	O
join	O
the	O
several	O
signals	O
together	O
and	O
a	O
demultiplexer	O
at	O
the	O
receiver	O
to	O
split	O
them	O
apart	O
.	O
</s>
<s>
With	O
the	O
right	O
type	O
of	O
fiber	O
,	O
it	O
is	O
possible	O
to	O
have	O
a	O
device	O
that	O
does	O
both	O
simultaneously	O
and	O
can	O
function	O
as	O
an	O
optical	B-Protocol
add-drop	I-Protocol
multiplexer	I-Protocol
.	O
</s>
<s>
By	O
using	O
WDM	O
and	O
optical	O
amplifiers	O
,	O
they	O
can	O
accommodate	O
several	O
generations	O
of	O
technology	O
development	O
in	O
their	O
optical	O
infrastructure	O
without	O
having	O
to	O
overhaul	O
the	O
backbone	B-General_Concept
network	O
.	O
</s>
<s>
The	O
capacity	O
of	O
a	O
given	O
link	O
can	O
be	O
expanded	O
simply	O
by	O
upgrading	O
the	O
multiplexers	B-Protocol
and	O
demultiplexers	O
at	O
each	O
end	O
.	O
</s>
<s>
Most	O
WDM	O
systems	O
operate	O
on	O
single-mode	B-Device
fiber	I-Device
optical	I-Device
cables	I-Device
which	O
have	O
a	O
core	O
diameter	O
of	O
9µm	O
.	O
</s>
<s>
Certain	O
forms	O
of	O
WDM	O
can	O
also	O
be	O
used	O
in	O
multi-mode	B-Device
fiber	I-Device
cables	I-Device
(	O
also	O
known	O
as	O
premises	O
cables	O
)	O
which	O
have	O
core	O
diameters	O
of	O
50	O
or	O
62.5µm	O
.	O
</s>
<s>
Coarse	B-Architecture
WDM	I-Architecture
provides	O
up	O
to	O
16	O
channels	O
across	O
multiple	O
transmission	O
windows	O
of	O
silica	O
fibers	O
.	O
</s>
<s>
New	O
amplification	O
options	O
(	O
Raman	B-Algorithm
amplification	I-Algorithm
)	O
enable	O
the	O
extension	O
of	O
the	O
usable	O
wavelengths	O
to	O
the	O
L-band	O
(	O
1565	O
–	O
1625nm	O
)	O
,	O
more	O
or	O
less	O
doubling	O
these	O
numbers	O
.	O
</s>
<s>
Coarse	B-Architecture
wavelength	I-Architecture
division	I-Architecture
multiplexing	I-Architecture
(	O
CWDM	O
)	O
,	O
in	O
contrast	O
to	O
DWDM	O
,	O
uses	O
increased	O
channel	O
spacing	O
to	O
allow	O
less-sophisticated	O
and	O
thus	O
cheaper	O
transceiver	O
designs	O
.	O
</s>
<s>
WDM	O
,	O
CWDM	O
and	O
DWDM	O
are	O
based	O
on	O
the	O
same	O
concept	O
of	O
using	O
multiple	O
wavelengths	O
of	O
light	O
on	O
a	O
single	O
fiber	O
but	O
differ	O
in	O
the	O
spacing	O
of	O
the	O
wavelengths	O
,	O
number	O
of	O
channels	O
,	O
and	O
the	O
ability	O
to	O
amplify	O
the	O
multiplexed	B-Architecture
signals	O
in	O
the	O
optical	O
space	O
.	O
</s>
<s>
EDFA	O
provide	O
an	O
efficient	O
wideband	O
amplification	O
for	O
the	O
C-band	O
,	O
Raman	B-Algorithm
amplification	I-Algorithm
adds	O
a	O
mechanism	O
for	O
amplification	O
in	O
the	O
L-band	O
.	O
</s>
<s>
Originally	O
,	O
the	O
term	O
coarse	B-Architecture
wavelength	I-Architecture
division	I-Architecture
multiplexing	I-Architecture
(	O
CWDM	O
)	O
was	O
fairly	O
generic	O
and	O
described	O
a	O
number	O
of	O
different	O
channel	O
configurations	O
.	O
</s>
<s>
Prior	O
to	O
the	O
relatively	O
recent	O
ITU	O
standardization	O
of	O
the	O
term	O
,	O
one	O
common	O
definition	O
for	O
CWDM	O
was	O
two	O
or	O
more	O
signals	O
multiplexed	B-Architecture
onto	O
a	O
single	O
fiber	O
,	O
with	O
one	O
signal	O
in	O
the	O
1550nm	O
band	O
and	O
the	O
other	O
in	O
the	O
1310nm	O
band	O
.	O
</s>
<s>
Some	O
GBIC	B-Protocol
and	O
small	O
form	O
factor	O
pluggable	O
(	O
SFP	B-Protocol
)	O
transceivers	O
utilize	O
standardized	O
CWDM	O
wavelengths	O
.	O
</s>
<s>
GBIC	B-Protocol
and	O
SFP	B-Protocol
CWDM	O
optics	O
allow	O
a	O
legacy	O
switch	O
system	O
to	O
be	O
"	O
converted	O
"	O
to	O
enable	O
wavelength	O
multiplexed	B-Architecture
transport	O
over	O
a	O
fiber	O
by	O
selecting	O
compatible	O
transceiver	O
wavelengths	O
for	O
use	O
with	O
an	O
inexpensive	O
passive	O
optical	O
multiplexing	B-Architecture
device	O
.	O
</s>
<s>
The	O
10GBASE-LX4	O
10	O
Gbit/s	O
physical	B-Application
layer	I-Application
standard	O
is	O
an	O
example	O
of	O
a	O
CWDM	O
system	O
in	O
which	O
four	O
wavelengths	O
near	O
1310nm	O
,	O
each	O
carrying	O
a	O
3.125gigabit-per-second	O
(	O
Gbit/s	O
)	O
data	O
stream	O
,	O
are	O
used	O
to	O
carry	O
10Gbit/s	O
of	O
aggregate	O
data	O
.	O
</s>
<s>
Dense	B-Architecture
wavelength	I-Architecture
division	I-Architecture
multiplexing	I-Architecture
(	O
DWDM	O
)	O
refers	O
originally	O
to	O
optical	O
signals	O
multiplexed	B-Architecture
within	O
the	O
1550nm	O
band	O
so	O
as	O
to	O
leverage	O
the	O
capabilities	O
(	O
and	O
cost	O
)	O
of	O
erbium	O
doped	O
fiber	O
amplifiers	O
(	O
EDFAs	O
)	O
,	O
which	O
are	O
effective	O
for	O
wavelengths	O
between	O
approximately	O
1525	O
–	O
1565nm	O
(	O
C	O
band	O
)	O
,	O
or	O
1570	O
–	O
1610nm	O
(	O
L	O
band	O
)	O
.	O
</s>
<s>
EDFAs	O
were	O
originally	O
developed	O
to	O
replace	O
SONET/SDH	B-Protocol
optical-electrical-optical	O
(	O
OEO	O
)	O
regenerators	O
,	O
which	O
they	O
have	O
made	O
practically	O
obsolete	O
.	O
</s>
<s>
In	O
terms	O
of	O
multi-wavelength	O
signals	O
,	O
so	O
long	O
as	O
the	O
EDFA	O
has	O
enough	O
pump	O
energy	O
available	O
to	O
it	O
,	O
it	O
can	O
amplify	O
as	O
many	O
optical	O
signals	O
as	O
can	O
be	O
multiplexed	B-Architecture
into	O
its	O
amplification	O
band	O
(	O
though	O
signal	O
densities	O
are	O
limited	O
by	O
choice	O
of	O
modulation	O
format	O
)	O
.	O
</s>
<s>
The	O
EDFA	O
's	O
cost	O
is	O
thus	O
leveraged	O
across	O
as	O
many	O
channels	O
as	O
can	O
be	O
multiplexed	B-Architecture
into	O
the	O
1550nm	O
band	O
.	O
</s>
<s>
A	O
DWDM	O
terminal	O
multiplexer	B-Protocol
.	O
</s>
<s>
The	O
terminal	O
multiplexer	B-Protocol
contains	O
a	O
wavelength-converting	O
transponder	O
for	O
each	O
data	O
signal	O
,	O
an	O
optical	O
multiplexer	B-Protocol
and	O
where	O
necessary	O
an	O
optical	O
amplifier	O
(	O
EDFA	O
)	O
.	O
</s>
<s>
Each	O
wavelength-converting	O
transponder	O
receives	O
an	O
optical	O
data	O
signal	O
from	O
the	O
client-layer	O
,	O
such	O
as	O
Synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
[	O
SONET	B-Protocol
/SDH	O
]	O
or	O
another	O
type	O
of	O
data	O
signal	O
,	O
converts	O
this	O
signal	O
into	O
the	O
electrical	O
domain	O
and	O
re-transmits	O
the	O
signal	O
at	O
a	O
specific	O
wavelength	O
using	O
a	O
1,550	O
nm	O
band	O
laser	O
.	O
</s>
<s>
These	O
data	O
signals	O
are	O
then	O
combined	O
into	O
a	O
multi-wavelength	O
optical	O
signal	O
using	O
an	O
optical	O
multiplexer	B-Protocol
,	O
for	O
transmission	O
over	O
a	O
single	O
fiber	O
(	O
e.g.	O
,	O
SMF-28	O
fiber	O
)	O
.	O
</s>
<s>
The	O
terminal	O
multiplexer	B-Protocol
may	O
or	O
may	O
not	O
also	O
include	O
a	O
local	O
transmit	O
EDFA	O
for	O
power	O
amplification	O
of	O
the	O
multi-wavelength	O
optical	O
signal	O
.	O
</s>
<s>
An	O
intermediate	O
optical	O
terminal	O
,	O
or	O
optical	B-Protocol
add-drop	I-Protocol
multiplexer	I-Protocol
.	O
</s>
<s>
At	O
the	O
remote	O
site	O
,	O
the	O
terminal	O
de-multiplexer	O
consisting	O
of	O
an	O
optical	O
de-multiplexer	O
and	O
one	O
or	O
more	O
wavelength-converting	O
transponders	O
separates	O
the	O
multi-wavelength	O
optical	O
signal	O
back	O
into	O
individual	O
data	O
signals	O
and	O
outputs	O
them	O
on	O
separate	O
fibers	O
for	O
client-layer	O
systems	O
(	O
such	O
as	O
SONET/SDH	B-Protocol
)	O
.	O
</s>
<s>
Originally	O
,	O
this	O
de-multiplexing	O
was	O
performed	O
entirely	O
passively	O
,	O
except	O
for	O
some	O
telemetry	O
,	O
as	O
most	O
SONET	B-Protocol
systems	O
can	O
receive	O
1,550	O
nm	O
signals	O
.	O
</s>
<s>
However	O
,	O
in	O
order	O
to	O
allow	O
for	O
transmission	O
to	O
remote	O
client-layer	O
systems	O
(	O
and	O
to	O
allow	O
for	O
digital	O
domain	O
signal	O
integrity	O
determination	O
)	O
such	O
de-multiplexed	O
signals	O
are	O
usually	O
sent	O
to	O
O/E/O	O
output	O
transponders	O
prior	O
to	O
being	O
relayed	O
to	O
their	O
client-layer	O
systems	O
.	O
</s>
<s>
Transponders	O
in	O
some	O
systems	O
supporting	O
40GHz	O
nominal	O
operation	O
may	O
also	O
perform	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
via	O
digital	B-Protocol
wrapper	I-Protocol
technology	I-Protocol
,	O
as	O
described	O
in	O
the	O
ITU-T	O
G.709	O
standard	O
.	O
</s>
<s>
It	O
is	O
the	O
multi-wavelength	O
analogue	O
to	O
SONET	B-Protocol
's	O
DCC	O
(	O
or	O
supervisory	O
channel	O
)	O
.	O
</s>
<s>
The	O
introduction	O
of	O
the	O
ITU-T	O
G.694.1	O
frequency	O
grid	O
in	O
2002	O
has	O
made	O
it	O
easier	O
to	O
integrate	O
WDM	O
with	O
older	O
but	O
more	O
standard	O
SONET/SDH	B-Protocol
systems	O
.	O
</s>
<s>
The	O
main	O
grid	O
is	O
placed	O
inside	O
the	O
optical	B-Architecture
fiber	I-Architecture
amplifier	O
bandwidth	O
,	O
but	O
can	O
be	O
extended	O
to	O
wider	O
bandwidths	O
.	O
</s>
<s>
In	O
addition	O
,	O
since	O
DWDM	O
provides	O
greater	O
maximum	O
capacity	O
it	O
tends	O
to	O
be	O
used	O
at	O
a	O
higher	O
level	O
in	O
the	O
communications	O
hierarchy	O
than	O
CWDM	O
,	O
for	O
example	O
on	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
and	O
is	O
therefore	O
associated	O
with	O
higher	O
modulation	O
rates	O
,	O
thus	O
creating	O
a	O
smaller	O
market	O
for	O
DWDM	O
devices	O
with	O
very	O
high	O
performance	O
.	O
</s>
<s>
3R	O
Transponders	O
were	O
fully	O
digital	O
and	O
normally	O
able	O
to	O
view	O
SONET/SDH	B-Protocol
section	O
layer	O
overhead	O
bytes	O
such	O
as	O
A1	O
and	O
A2	O
to	O
determine	O
signal	O
quality	O
health	O
.	O
</s>
<s>
Many	O
systems	O
will	O
offer	O
2.5Gbit/s	O
transponders	O
,	O
which	O
will	O
normally	O
mean	O
the	O
transponder	O
is	O
able	O
to	O
perform	O
3R	O
regeneration	O
on	O
OC-3/12/48	O
signals	O
,	O
and	O
possibly	O
gigabit	O
Ethernet	O
,	O
and	O
reporting	O
on	O
signal	O
health	O
by	O
monitoring	O
SONET/SDH	B-Protocol
section	O
layer	O
overhead	O
bytes	O
.	O
</s>
<s>
Muxponder	B-Device
The	O
muxponder	B-Device
(	O
from	O
multiplexed	B-Architecture
transponder	O
)	O
has	O
different	O
names	O
depending	O
on	O
vendor	O
.	O
</s>
<s>
It	O
essentially	O
performs	O
some	O
relatively	O
simple	O
time-division	O
multiplexing	B-Architecture
of	O
lower-rate	O
signals	O
into	O
a	O
higher-rate	O
carrier	O
within	O
the	O
system	O
(	O
a	O
common	O
example	O
is	O
the	O
ability	O
to	O
accept	O
4	O
OC-48s	O
and	O
then	O
output	O
a	O
single	O
OC-192	O
in	O
the	O
1,550	O
nm	O
band	O
)	O
.	O
</s>
<s>
More	O
recent	O
muxponder	B-Device
designs	O
have	O
absorbed	O
more	O
and	O
more	O
TDM	O
functionality	O
,	O
in	O
some	O
cases	O
obviating	O
the	O
need	O
for	O
traditional	O
SONET/SDH	B-Protocol
transport	O
equipment	O
.	O
</s>
<s>
For	O
DWDM	O
the	O
range	O
between	O
C21-C60	O
is	O
the	O
most	O
common	O
range	O
,	O
for	O
Mux/Demux	O
in	O
8	O
,	O
16	O
,	O
40	O
or	O
96	O
sizes	O
.	O
</s>
<s>
With	O
a	O
ROADM	O
,	O
network	O
operators	O
can	O
remotely	O
reconfigure	O
the	O
multiplexer	B-Protocol
by	O
sending	O
soft	O
commands	O
.	O
</s>
<s>
Cisco	O
's	O
Enhanced	O
WDM	O
system	O
combines	O
1	O
Gb	O
Coarse	O
Wave	B-Architecture
Division	I-Architecture
Multiplexing	I-Architecture
(	O
CWDM	O
)	O
connections	O
using	O
SFPs	O
and	O
GBICs	B-Protocol
with	O
10	O
Gb	O
Dense	B-Architecture
Wave	I-Architecture
Division	I-Architecture
Multiplexing	I-Architecture
(	O
DWDM	O
)	O
connections	O
using	O
XENPAK	B-Protocol
,	O
X2	O
or	O
XFP	B-Protocol
DWDM	O
modules	O
.	O
</s>
<s>
In	O
addition	O
to	O
this	O
,	O
CFP	O
modules	O
deliver	O
100	O
Gbit/s	O
Ethernet	O
suitable	O
for	O
high	O
speed	O
Internet	B-General_Concept
backbone	I-General_Concept
connections	O
.	O
</s>
<s>
Shortwave	O
WDM	O
uses	O
vertical-cavity	O
surface-emitting	O
laser	O
(	O
VCSEL	O
)	O
transceivers	O
with	O
four	O
wavelengths	O
in	O
the	O
846	O
to	O
953nm	O
range	O
over	O
single	O
OM5	O
fiber	O
,	O
or	O
2-fiber	O
connectivity	O
for	O
OM3/OM4	O
fiber	O
.	O
</s>
