<s>
Synchronous	B-Protocol
Optical	I-Protocol
Networking	I-Protocol
(	O
SONET	B-Protocol
)	O
and	O
Synchronous	B-Protocol
Digital	I-Protocol
Hierarchy	I-Protocol
(	O
SDH	O
)	O
are	O
standardized	O
protocols	O
that	O
transfer	O
multiple	O
digital	B-General_Concept
bit	O
streams	O
synchronously	O
over	O
optical	B-Architecture
fiber	I-Architecture
using	O
lasers	O
or	O
highly	O
coherent	O
light	O
from	O
light-emitting	O
diodes	O
(	O
LEDs	O
)	O
.	O
</s>
<s>
The	O
method	O
was	O
developed	O
to	O
replace	O
the	O
plesiochronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
(	O
PDH	O
)	O
system	O
for	O
transporting	O
large	O
amounts	O
of	O
telephone	O
calls	O
and	O
data	O
traffic	O
over	O
the	O
same	O
fiber	O
without	O
the	O
problems	O
of	O
synchronization	O
.	O
</s>
<s>
SONET	B-Protocol
and	O
SDH	O
,	O
which	O
are	O
essentially	O
the	O
same	O
,	O
were	O
originally	O
designed	O
to	O
transport	O
circuit	B-Protocol
mode	I-Protocol
communications	O
(	O
e.g.	O
,	O
DS1	B-Protocol
,	O
DS3	O
)	O
from	O
a	O
variety	O
of	O
different	O
sources	O
,	O
but	O
they	O
were	O
primarily	O
designed	O
to	O
support	O
real-time	O
,	O
uncompressed	O
,	O
circuit-switched	B-Protocol
voice	O
encoded	O
in	O
PCM	B-Algorithm
format	O
.	O
</s>
<s>
The	O
primary	O
difficulty	O
in	O
doing	O
this	O
prior	O
to	O
SONET/SDH	B-Protocol
was	O
that	O
the	O
synchronization	O
sources	O
of	O
these	O
various	O
circuits	O
were	O
different	O
.	O
</s>
<s>
SONET/SDH	B-Protocol
allowed	O
for	O
the	O
simultaneous	O
transport	O
of	O
many	O
different	O
circuits	O
of	O
differing	O
origin	O
within	O
a	O
single	O
framing	O
protocol	O
.	O
</s>
<s>
SONET/SDH	B-Protocol
is	O
not	O
a	O
complete	O
communications	O
protocol	O
in	O
itself	O
,	O
but	O
a	O
transport	O
protocol	O
(	O
not	O
a	O
'	O
transport	O
 '	O
in	O
the	O
OSI	O
Model	O
sense	O
)	O
.	O
</s>
<s>
Due	O
to	O
SONET/SDH	B-Protocol
'	O
s	O
essential	O
protocol	O
neutrality	O
and	O
transport-oriented	O
features	O
,	O
SONET/SDH	B-Protocol
was	O
the	O
obvious	O
choice	O
for	O
transporting	O
the	O
fixed	O
length	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
frames	O
also	O
known	O
as	O
cells	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
for	O
ATM	O
(	O
and	O
eventually	O
other	O
protocols	O
such	O
as	O
Ethernet	O
)	O
,	O
the	O
internal	O
complex	O
structure	O
previously	O
used	O
to	O
transport	O
circuit-oriented	O
connections	O
was	O
removed	O
and	O
replaced	O
with	O
a	O
large	O
and	O
concatenated	O
frame	O
(	O
such	O
as	O
STS-3c	O
)	O
into	O
which	O
ATM	O
cells	O
,	O
IP	B-Protocol
packets	O
,	O
or	O
Ethernet	O
frames	O
are	O
placed	O
.	O
</s>
<s>
Both	O
SDH	O
and	O
SONET	B-Protocol
are	O
widely	O
used	O
today	O
:	O
SONET	B-Protocol
in	O
the	O
United	O
States	O
and	O
Canada	O
,	O
and	O
SDH	O
in	O
the	O
rest	O
of	O
the	O
world	O
.	O
</s>
<s>
Although	O
the	O
SONET	B-Protocol
standards	O
were	O
developed	O
before	O
SDH	O
,	O
it	O
is	O
considered	O
a	O
variation	O
of	O
SDH	O
because	O
of	O
SDH	O
's	O
greater	O
worldwide	O
market	O
penetration	O
.	O
</s>
<s>
SONET	B-Protocol
is	O
subdivided	O
into	O
four	O
sublayers	O
with	O
some	O
factor	O
such	O
as	O
the	O
path	O
,	O
line	O
,	O
section	O
and	O
physical	O
layer	O
.	O
</s>
<s>
The	O
SDH	O
standard	O
was	O
originally	O
defined	O
by	O
the	O
European	O
Telecommunications	O
Standards	O
Institute	O
(	O
ETSI	O
)	O
,	O
and	O
is	O
formalised	O
as	O
International	O
Telecommunication	O
Union	O
(	O
ITU	O
)	O
standards	O
G.707	B-Protocol
,	O
G.783	O
,	O
G.784	O
,	O
and	O
G.803	O
.	O
</s>
<s>
The	O
SONET	B-Protocol
standard	O
was	O
defined	O
by	O
Telcordia	O
and	O
American	O
National	O
Standards	O
Institute	O
(	O
ANSI	O
)	O
standard	O
T1.105	O
.	O
</s>
<s>
SDH	O
differs	O
from	O
Plesiochronous	B-Protocol
Digital	I-Protocol
Hierarchy	I-Protocol
(	O
PDH	O
)	O
in	O
that	O
the	O
exact	O
rates	O
that	O
are	O
used	O
to	O
transport	O
the	O
data	O
on	O
SONET/SDH	B-Protocol
are	O
tightly	O
synchronized	O
across	O
the	O
entire	O
network	O
,	O
using	O
atomic	O
clocks	O
.	O
</s>
<s>
Both	O
SONET	B-Protocol
and	O
SDH	O
can	O
be	O
used	O
to	O
encapsulate	B-Application
earlier	O
digital	B-General_Concept
transmission	O
standards	O
,	O
such	O
as	O
the	O
PDH	O
standard	O
,	O
or	O
they	O
can	O
be	O
used	O
to	O
directly	O
support	O
either	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
or	O
so-called	O
packet	B-Protocol
over	I-Protocol
SONET/SDH	I-Protocol
(	O
POS	O
)	O
networking	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
is	O
inaccurate	O
to	O
think	O
of	O
SDH	O
or	O
SONET	B-Protocol
as	O
communications	O
protocols	O
in	O
and	O
of	O
themselves	O
;	O
they	O
are	O
generic	O
,	O
all-purpose	O
transport	O
containers	O
for	O
moving	O
both	O
voice	O
and	O
data	O
.	O
</s>
<s>
The	O
basic	O
format	O
of	O
a	O
SONET/SDH	B-Protocol
signal	O
allows	O
it	O
to	O
carry	O
many	O
different	O
services	O
in	O
its	O
virtual	B-Protocol
container	I-Protocol
(	O
VC	O
)	O
,	O
because	O
it	O
is	O
bandwidth-flexible	O
.	O
</s>
<s>
SONET	B-Protocol
and	O
SDH	O
often	O
use	O
different	O
terms	O
to	O
describe	O
identical	O
features	O
or	O
functions	O
.	O
</s>
<s>
With	O
a	O
few	O
exceptions	O
,	O
SDH	O
can	O
be	O
thought	O
of	O
as	O
a	O
superset	O
of	O
SONET	B-Protocol
.	O
</s>
<s>
SONET	B-Protocol
is	O
a	O
set	O
of	O
transport	O
containers	O
that	O
allow	O
for	O
delivery	O
of	O
a	O
variety	O
of	O
protocols	O
,	O
including	O
traditional	O
telephony	O
,	O
ATM	O
,	O
Ethernet	O
,	O
and	O
TCP/IP	O
traffic	O
.	O
</s>
<s>
SONET	B-Protocol
therefore	O
is	O
not	O
in	O
itself	O
a	O
native	O
communications	O
protocol	O
and	O
should	O
not	O
be	O
confused	O
as	O
being	O
necessarily	O
connection-oriented	B-Protocol
in	O
the	O
way	O
that	O
term	O
is	O
usually	O
used	O
.	O
</s>
<s>
The	O
protocol	O
is	O
a	O
heavily	O
multiplexed	B-Architecture
structure	O
,	O
with	O
the	O
header	B-General_Concept
interleaved	O
between	O
the	O
data	O
in	O
a	O
complex	O
way	O
.	O
</s>
<s>
This	O
permits	O
the	O
encapsulated	O
data	O
to	O
have	O
its	O
own	O
frame	O
rate	O
and	O
be	O
able	O
to	O
"	O
float	O
around	O
"	O
relative	O
to	O
the	O
SDH/SONET	B-Protocol
frame	O
structure	O
and	O
rate	O
.	O
</s>
<s>
Extra	O
padding	O
is	O
allowed	O
for	O
the	O
multiplexed	B-Architecture
data	O
to	O
move	O
within	O
the	O
overall	O
framing	O
,	O
as	O
the	O
data	O
is	O
clocked	O
at	O
a	O
different	O
rate	O
than	O
the	O
frame	O
rate	O
.	O
</s>
<s>
The	O
protocol	O
is	O
made	O
more	O
complex	O
by	O
the	O
decision	O
to	O
permit	O
this	O
padding	O
at	O
most	O
levels	O
of	O
the	O
multiplexing	B-Architecture
structure	O
,	O
but	O
it	O
improves	O
all-around	O
performance	O
.	O
</s>
<s>
The	O
basic	O
unit	O
of	O
framing	O
in	O
SDH	O
is	O
a	O
STM-1	B-Protocol
(	O
Synchronous	O
Transport	O
Module	O
,	O
level	O
1	O
)	O
,	O
which	O
operates	O
at	O
155.520	O
megabits	O
per	O
second	O
(	O
Mbit/s	O
)	O
.	O
</s>
<s>
SONET	B-Protocol
refers	O
to	O
this	O
basic	O
unit	O
as	O
an	O
STS-3c	O
(	O
Synchronous	O
Transport	O
Signal	O
3	O
,	O
concatenated	O
)	O
.	O
</s>
<s>
When	O
the	O
STS-3c	O
is	O
carried	O
over	O
OC-3	O
,	O
it	O
is	O
often	O
colloquially	O
referred	O
to	O
as	O
OC-3c	O
,	O
but	O
this	O
is	O
not	O
an	O
official	O
designation	O
within	O
the	O
SONET	B-Protocol
standard	O
as	O
there	O
is	O
no	O
physical	O
layer	O
(	O
i.e.	O
</s>
<s>
SONET	B-Protocol
offers	O
an	O
additional	O
basic	O
unit	O
of	O
transmission	O
,	O
the	O
STS-1	O
(	O
Synchronous	O
Transport	O
Signal	O
1	O
)	O
or	O
OC-1	O
,	O
operating	O
at	O
51.84	O
Mbit/s	O
—	O
exactly	O
one	O
third	O
of	O
an	O
STM-1/STS-3c/OC	O
-3c	O
carrier	O
.	O
</s>
<s>
This	O
speed	O
is	O
dictated	O
by	O
the	O
bandwidth	O
requirements	O
for	O
PCM-encoded	O
telephonic	O
voice	O
signals	O
:	O
at	O
this	O
rate	O
,	O
an	O
STS-1/OC	O
-1	O
circuit	O
can	O
carry	O
the	O
bandwidth	O
equivalent	O
of	O
a	O
standard	O
DS-3	O
channel	O
,	O
which	O
can	O
carry	O
672	O
64-kbit/s	O
voice	O
channels	O
.	O
</s>
<s>
In	O
SONET	B-Protocol
,	O
the	O
STS-3c	O
signal	O
is	O
composed	O
of	O
three	O
multiplexed	B-Architecture
STS-1	O
signals	O
;	O
the	O
STS-3c	O
may	O
be	O
carried	O
on	O
an	O
OC-3	O
signal	O
.	O
</s>
<s>
In	O
packet-oriented	O
data	O
transmission	O
,	O
such	O
as	O
Ethernet	O
,	O
a	O
packet	O
frame	O
usually	O
consists	O
of	O
a	O
header	B-General_Concept
and	O
a	O
payload	O
.	O
</s>
<s>
The	O
header	B-General_Concept
is	O
transmitted	O
first	O
,	O
followed	O
by	O
the	O
payload	O
(	O
and	O
possibly	O
a	O
trailer	B-Device
,	O
such	O
as	O
a	O
CRC	O
)	O
.	O
</s>
<s>
In	O
synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
,	O
this	O
is	O
modified	O
slightly	O
.	O
</s>
<s>
The	O
header	B-General_Concept
is	O
termed	O
the	O
overhead	O
,	O
and	O
instead	O
of	O
being	O
transmitted	O
before	O
the	O
payload	O
,	O
is	O
interleaved	O
with	O
it	O
during	O
transmission	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
an	O
STS-1	O
,	O
the	O
frame	O
is	O
810	O
octets	O
in	O
size	O
,	O
while	O
the	O
STM-1/STS	O
-3c	O
frame	O
is	O
2,430	O
octets	O
in	O
size	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
an	O
STS-3c/STM	O
-1	O
,	O
which	O
operates	O
three	O
times	O
faster	O
than	O
an	O
STS-1	O
,	O
nine	O
octets	O
of	O
overhead	O
are	O
transmitted	O
,	O
followed	O
by	O
261	O
octets	O
of	O
payload	O
.	O
</s>
<s>
For	O
both	O
SONET	B-Protocol
and	O
SDH	O
,	O
this	O
is	O
often	O
represented	O
by	O
displaying	O
the	O
frame	O
graphically	O
:	O
as	O
a	O
block	O
of	O
90	O
columns	O
and	O
nine	O
rows	O
for	O
STS-1	O
,	O
and	O
270	O
columns	O
and	O
nine	O
rows	O
for	O
STM1/STS	O
-3c	O
.	O
</s>
<s>
The	O
internal	O
structure	O
of	O
the	O
overhead	O
and	O
payload	O
within	O
the	O
frame	O
differs	O
slightly	O
between	O
SONET	B-Protocol
and	O
SDH	O
,	O
and	O
different	O
terms	O
are	O
used	O
in	O
the	O
standards	O
to	O
describe	O
these	O
structures	O
.	O
</s>
<s>
Their	O
standards	O
are	O
extremely	O
similar	O
in	O
implementation	O
,	O
making	O
it	O
easy	O
to	O
interoperate	O
between	O
SDH	O
and	O
SONET	B-Protocol
at	O
any	O
given	O
bandwidth	O
.	O
</s>
<s>
The	O
Synchronous	O
Transport	O
Module	O
,	O
level	O
1	O
(	O
STM-1	B-Protocol
)	O
frame	O
is	O
the	O
basic	O
transmission	O
format	O
for	O
SDH	O
—	O
the	O
first	O
level	O
of	O
the	O
synchronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
.	O
</s>
<s>
The	O
STM-1	B-Protocol
frame	O
is	O
transmitted	O
in	O
exactly	O
125μs	O
,	O
therefore	O
,	O
there	O
are	O
8,000	O
frames	O
per	O
second	O
on	O
a	O
155.52Mbit/s	O
OC-3	O
fiber-optic	B-Architecture
circuit	O
.	O
</s>
<s>
The	O
STM-1	B-Protocol
frame	O
consists	O
of	O
overhead	O
and	O
pointers	O
plus	O
information	O
payload	O
.	O
</s>
<s>
The	O
first	O
nine	O
columns	O
of	O
each	O
frame	O
make	O
up	O
the	O
section	B-Protocol
overhead	I-Protocol
and	O
administrative	O
unit	O
pointers	O
,	O
and	O
the	O
last	O
261	O
columns	O
make	O
up	O
the	O
information	O
payload	O
.	O
</s>
<s>
Also	O
within	O
the	O
administrative	O
unit	O
are	O
one	O
or	O
more	O
virtual	B-Protocol
containers	I-Protocol
(	O
VCs	O
)	O
.	O
</s>
<s>
The	O
section	B-Protocol
overhead	I-Protocol
(	O
SOH	O
)	O
of	O
a	O
STM-1	B-Protocol
signal	O
is	O
divided	O
into	O
two	O
parts	O
:	O
the	O
regenerator	O
section	B-Protocol
overhead	I-Protocol
(	O
RSOH	O
)	O
and	O
the	O
multiplex	O
section	B-Protocol
overhead	I-Protocol
(	O
MSOH	O
)	O
.	O
</s>
<s>
Section	B-Protocol
overhead	I-Protocol
Called	O
regenerator	O
section	B-Protocol
overhead	I-Protocol
(	O
RSOH	O
)	O
in	O
SDH	O
terminology	O
:	O
27	O
octets	O
containing	O
information	O
about	O
the	O
frame	O
structure	O
required	O
by	O
the	O
terminal	O
equipment	O
.	O
</s>
<s>
Line	O
overhead	O
Called	O
multiplex	O
section	B-Protocol
overhead	I-Protocol
(	O
MSOH	O
)	O
in	O
SDH	O
:	O
45	O
octets	O
containing	O
information	O
about	O
error	O
correction	O
and	O
Automatic	O
Protection	O
Switching	O
messages	O
(	O
e.g.	O
,	O
alarms	O
and	O
maintenance	O
messages	O
)	O
as	O
may	O
be	O
required	O
within	O
the	O
network	O
.	O
</s>
<s>
The	O
error	O
correction	O
is	O
included	O
for	O
STM-16	B-Protocol
and	O
above	O
.	O
</s>
<s>
Administrative	O
unit	O
(	O
AU	O
)	O
pointer	O
Points	O
to	O
the	O
location	O
of	O
the	O
J1	O
byte	O
in	O
the	O
payload	O
(	O
the	O
first	O
byte	O
in	O
the	O
virtual	B-Protocol
container	I-Protocol
)	O
.	O
</s>
<s>
When	O
the	O
DS3	O
enters	O
a	O
SONET	B-Protocol
network	O
,	O
path	O
overhead	O
is	O
added	O
,	O
and	O
that	O
SONET	B-Protocol
network	O
element	O
(	O
NE	O
)	O
is	O
said	O
to	O
be	O
a	O
path	O
generator	O
and	O
terminator	O
.	O
</s>
<s>
The	O
SONET	B-Protocol
NE	O
is	O
line	O
terminating	O
if	O
it	O
processes	O
the	O
line	O
overhead	O
.	O
</s>
<s>
SONET	B-Protocol
regenerators	O
terminate	O
the	O
section	O
,	O
but	O
not	O
the	O
paths	O
or	O
line	O
.	O
</s>
<s>
Each	O
VTG	O
can	O
then	O
be	O
subdivided	O
into	O
four	O
VT1.5	B-Protocol
signals	O
,	O
each	O
of	O
which	O
can	O
carry	O
a	O
PDH	O
DS1	B-Protocol
signal	O
.	O
</s>
<s>
The	O
SDH	O
equivalent	O
of	O
a	O
VTG	O
is	O
a	O
TUG-2	O
;	O
VT1.5	B-Protocol
is	O
equivalent	O
to	O
VC-11	O
,	O
and	O
VT2	O
is	O
equivalent	O
to	O
VC-12	O
.	O
</s>
<s>
Three	O
STS-1	O
signals	O
may	O
be	O
multiplexed	B-Architecture
by	O
time-division	B-Protocol
multiplexing	I-Protocol
to	O
form	O
the	O
next	O
level	O
of	O
the	O
SONET	B-Protocol
hierarchy	O
,	O
the	O
OC-3	O
(	O
STS-3	O
)	O
,	O
running	O
at	O
155.52Mbit/s	O
.	O
</s>
<s>
The	O
signal	O
is	O
multiplexed	B-Architecture
by	O
interleaving	O
the	O
bytes	O
of	O
the	O
three	O
STS-1	O
frames	O
to	O
form	O
the	O
STS-3	O
frame	O
,	O
containing	O
2,430	O
bytes	O
and	O
transmitted	O
in	O
125μs	O
.	O
</s>
<s>
For	O
example	O
,	O
four	O
STS-3	O
or	O
AU4	O
signals	O
can	O
be	O
aggregated	O
to	O
form	O
a	O
622.08Mbit/s	O
signal	O
designated	O
OC-12	O
or	O
STM-4	B-Protocol
.	O
</s>
<s>
The	O
highest	O
rate	O
commonly	O
deployed	O
is	O
the	O
OC-768	O
or	O
STM-256	B-Protocol
circuit	O
,	O
which	O
operates	O
at	O
rate	O
of	O
just	O
under	O
38.5	O
Gbit/s	O
.	O
</s>
<s>
Where	O
fiber	O
exhaustion	O
is	O
a	O
concern	O
,	O
multiple	O
SONET	B-Protocol
signals	O
can	O
be	O
transported	O
over	O
multiple	O
wavelengths	O
on	O
a	O
single	O
fiber	O
pair	O
by	O
means	O
of	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
,	O
including	O
dense	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
DWDM	O
)	O
and	O
coarse	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
CWDM	O
)	O
.	O
</s>
<s>
The	O
Gigabit	O
Ethernet	B-Protocol
Alliance	I-Protocol
created	O
two	O
10	O
Gigabit	O
Ethernet	O
variants	O
:	O
a	O
local	O
area	O
variant	O
(	O
LAN	O
PHY	O
)	O
with	O
a	O
line	O
rate	O
of	O
10.3125Gbit/s	O
,	O
and	O
a	O
wide	O
area	O
variant	O
(	O
WAN	O
PHY	O
)	O
with	O
the	O
same	O
line	O
rate	O
as	O
OC-192/STM	O
-64	O
(	O
9,953,280	O
kbit/s	O
)	O
.	O
</s>
<s>
The	O
WAN	O
PHY	O
variant	O
encapsulates	O
Ethernet	O
data	O
using	O
a	O
lightweight	O
SDH/SONET	B-Protocol
frame	O
,	O
so	O
as	O
to	O
be	O
compatible	O
at	O
a	O
low	O
level	O
with	O
equipment	O
designed	O
to	O
carry	O
SDH/SONET	B-Protocol
signals	O
,	O
whereas	O
the	O
LAN	O
PHY	O
variant	O
encapsulates	O
Ethernet	O
data	O
using	O
64B/66B	O
line	O
coding	O
.	O
</s>
<s>
However	O
,	O
10	O
Gigabit	O
Ethernet	O
does	O
not	O
explicitly	O
provide	O
any	O
interoperability	O
at	O
the	O
bitstream	O
level	O
with	O
other	O
SDH/SONET	B-Protocol
systems	O
.	O
</s>
<s>
This	O
differs	O
from	O
WDM	O
system	O
transponders	O
,	O
including	O
both	O
coarse	O
and	O
dense	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
systems	O
(	O
CWDM	O
and	O
DWDM	O
)	O
that	O
currently	O
support	O
OC-192	O
SONET	B-Protocol
signals	O
,	O
which	O
can	O
normally	O
support	O
thin-SONET	O
–	O
framed	O
10	O
Gigabit	O
Ethernet	O
.	O
</s>
<s>
User	O
throughput	O
must	O
not	O
deduct	O
path	O
overhead	O
from	O
the	O
payload	O
bandwidth	O
,	O
but	O
path-overhead	O
bandwidth	O
is	O
variable	O
based	O
on	O
the	O
types	O
of	O
cross-connects	B-Architecture
built	O
across	O
the	O
optical	O
system	O
.	O
</s>
<s>
The	O
only	O
exception	O
is	O
OC-24	O
,	O
which	O
is	O
standardized	O
in	O
ANSI	O
T1.105	O
,	O
but	O
not	O
a	O
SDH	O
standard	O
rate	O
in	O
ITU-T	O
G.707	B-Protocol
.	O
</s>
<s>
The	O
ATM	O
and	O
SDH	O
layers	O
are	O
the	O
regenerator	O
section	O
level	O
,	O
digital	B-General_Concept
line	O
level	O
,	O
transmission	O
path	O
level	O
,	O
virtual	O
path	O
level	O
,	O
and	O
virtual	O
channel	O
level	O
.	O
</s>
<s>
The	O
physical	O
layer	O
is	O
modeled	O
on	O
three	O
major	O
entities	O
:	O
transmission	O
path	O
,	O
digital	B-General_Concept
line	O
and	O
the	O
regenerator	O
section	O
.	O
</s>
<s>
The	O
photonic	O
layer	O
is	O
the	O
lowest	O
SONET	B-Protocol
layer	O
and	O
it	O
is	O
responsible	O
for	O
transmitting	O
the	O
bits	O
to	O
the	O
physical	O
medium	O
.	O
</s>
<s>
It	O
provides	O
synchronization	O
and	O
multiplexing	B-Architecture
for	O
multiple	O
paths	O
.	O
</s>
<s>
The	O
path	O
layer	O
is	O
SONET	B-Protocol
's	O
highest	O
level	O
layer	O
.	O
</s>
<s>
Network	O
management	O
systems	O
are	O
used	O
to	O
configure	O
and	O
monitor	O
SDH	O
and	O
SONET	B-Protocol
equipment	O
either	O
locally	O
or	O
remotely	O
.	O
</s>
<s>
Transport	O
of	O
network	O
management	O
data	O
between	O
the	O
'	O
network	O
management	O
system	O
terminal	O
 '	O
and	O
the	O
SONET/	O
SDH	O
equipment	O
e.g.	O
</s>
<s>
Transport	O
of	O
network	O
management	O
data	O
between	O
SDH/	O
SONET	B-Protocol
equipment	O
using	O
'	O
dedicated	O
embedded	O
data	O
communication	O
channels	O
 '	O
(	O
DCCs	O
)	O
within	O
the	O
section	O
and	O
line	O
overhead	O
.	O
</s>
<s>
Although	O
this	O
can	O
be	O
done	O
locally	O
,	O
through	O
a	O
craft	O
interface	O
,	O
it	O
is	O
normally	O
done	O
through	O
a	O
network	O
management	O
system	O
(	O
sitting	O
at	O
a	O
higher	O
layer	O
)	O
that	O
in	O
turn	O
operates	O
through	O
the	O
SONET/SDH	B-Protocol
network	O
management	O
network	O
.	O
</s>
<s>
Network-element	O
software	O
upgrades	O
are	O
done	O
mostly	O
through	O
the	O
SONET/SDH	B-Protocol
management	O
network	O
in	O
modern	O
equipment	O
.	O
</s>
<s>
Local	O
"	O
craftspersons	O
"	O
(	O
telephone	O
network	O
engineers	O
)	O
can	O
access	O
a	O
SDH/	O
SONET	B-Protocol
network	O
element	O
on	O
a	O
"	O
craft	O
port	O
"	O
and	O
issue	O
commands	O
through	O
a	O
dumb	O
terminal	O
or	O
terminal	O
emulation	O
program	O
running	O
on	O
a	O
laptop	O
.	O
</s>
<s>
This	O
interface	O
can	O
also	O
be	O
attached	O
to	O
a	O
console	B-Device
server	I-Device
,	O
allowing	O
for	O
remote	O
out-of-band	B-Application
management	I-Application
and	O
logging	O
.	O
</s>
<s>
SONET	B-Protocol
equipment	O
is	O
often	O
managed	O
with	O
the	O
TL1	O
protocol	O
.	O
</s>
<s>
TL1	O
is	O
a	O
telecom	O
language	O
for	O
managing	O
and	O
reconfiguring	O
SONET	B-Protocol
network	O
elements	O
.	O
</s>
<s>
The	O
command	O
language	O
used	O
by	O
a	O
SONET	B-Protocol
network	O
element	O
,	O
such	O
as	O
TL1	O
,	O
must	O
be	O
carried	O
by	O
other	O
management	O
protocols	O
,	O
such	O
as	O
SNMP	B-Protocol
,	O
CORBA	B-Architecture
,	O
or	O
XML	B-Protocol
.	O
</s>
<s>
With	O
the	O
convergence	O
of	O
SONET	B-Protocol
and	O
SDH	O
on	O
switching	O
matrix	O
and	O
network	O
elements	O
architecture	O
,	O
newer	O
implementations	O
have	O
also	O
offered	O
TL1	O
.	O
</s>
<s>
Most	O
SONET	B-Protocol
NEs	O
have	O
a	O
limited	O
number	O
of	O
management	O
interfaces	O
defined	O
:	O
</s>
<s>
The	O
electrical	O
interface	O
,	O
often	O
a	O
50-ohm	O
coaxial	O
cable	O
,	O
sends	O
SONET	B-Protocol
TL1	O
commands	O
from	O
a	O
local	O
management	O
network	O
physically	O
housed	O
in	O
the	O
central	O
office	O
where	O
the	O
SONET	B-Protocol
network	O
element	O
is	O
located	O
.	O
</s>
<s>
This	O
is	O
for	O
local	O
management	O
of	O
that	O
network	O
element	O
and	O
,	O
possibly	O
,	O
remote	O
management	O
of	O
other	O
SONET	B-Protocol
network	O
elements	O
.	O
</s>
<s>
SONET	B-Protocol
and	O
SDH	O
have	O
dedicated	O
data	O
communication	O
channels	O
(	O
DCCs	O
)	O
within	O
the	O
section	O
and	O
line	O
overhead	O
for	O
management	O
traffic	O
.	O
</s>
<s>
Generally	O
,	O
section	B-Protocol
overhead	I-Protocol
(	O
regenerator	O
section	O
in	O
SDH	O
)	O
is	O
used	O
.	O
</s>
<s>
Dual	O
(	O
IP+OSI	O
)	O
stack	O
using	O
PPP	B-Protocol
or	O
LAP-D	O
with	O
tunneling	O
functions	O
to	O
communicate	O
between	O
stacks	O
.	O
</s>
<s>
With	O
advances	O
in	O
SONET	B-Protocol
and	O
SDH	O
chipsets	O
,	O
the	O
traditional	O
categories	O
of	O
network	O
elements	O
are	O
no	O
longer	O
distinct	O
.	O
</s>
<s>
Traditional	O
regenerators	O
terminate	O
the	O
section	B-Protocol
overhead	I-Protocol
,	O
but	O
not	O
the	O
line	O
or	O
path	O
.	O
</s>
<s>
Also	O
,	O
some	O
of	O
the	O
functionality	O
of	O
regenerators	O
has	O
been	O
absorbed	O
by	O
the	O
transponders	O
of	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
systems	O
.	O
</s>
<s>
The	O
low-speed	O
side	O
takes	O
in	O
low-speed	O
signals	O
,	O
which	O
are	O
multiplexed	B-Architecture
by	O
the	O
network	O
element	O
and	O
sent	O
out	O
from	O
the	O
high-speed	O
side	O
,	O
or	O
vice	O
versa	O
.	O
</s>
<s>
Recent	O
digital	B-Architecture
cross	I-Architecture
connect	I-Architecture
systems	I-Architecture
(	O
DCSs	O
or	O
DXCs	O
)	O
support	O
numerous	O
high-speed	O
signals	O
,	O
and	O
allow	O
for	O
cross-connection	O
of	O
DS1s	O
,	O
DS3s	O
and	O
even	O
STS-3s/12c	O
and	O
so	O
on	O
,	O
from	O
any	O
input	O
to	O
any	O
output	O
.	O
</s>
<s>
SONET	B-Protocol
and	O
SDH	O
have	O
a	O
limited	O
number	O
of	O
architectures	O
defined	O
.	O
</s>
<s>
the	O
ability	O
to	O
transmit	O
traffic	O
even	O
when	O
part	O
of	O
the	O
network	O
has	O
failed	O
)	O
,	O
and	O
are	O
fundamental	O
to	O
the	O
worldwide	O
deployment	O
of	O
SONET	B-Protocol
and	O
SDH	O
for	O
moving	O
digital	B-General_Concept
traffic	O
.	O
</s>
<s>
Every	O
SDH/SONET	B-Protocol
connection	O
on	O
the	O
optical	O
physical	O
layer	O
uses	O
two	O
optical	B-Architecture
fibers	I-Architecture
,	O
regardless	O
of	O
the	O
transmission	O
speed	O
.	O
</s>
<s>
The	O
SDH	O
equivalent	O
of	O
UPSR	O
is	O
subnetwork	B-Protocol
connection	I-Protocol
protection	I-Protocol
(	O
SNCP	O
)	O
;	O
SNCP	O
does	O
not	O
impose	O
a	O
ring	O
topology	O
,	O
but	O
may	O
also	O
be	O
used	O
in	O
mesh	O
topologies	O
.	O
</s>
<s>
The	O
interface	O
is	O
often	O
a	O
DS1	B-Protocol
,	O
with	O
sync-status	O
messages	O
supplied	O
by	O
the	O
clock	O
and	O
placed	O
into	O
the	O
DS1	B-Protocol
overhead	O
.	O
</s>
<s>
SONET/SDH	B-Protocol
development	O
was	O
originally	O
driven	O
by	O
the	O
need	O
to	O
transport	O
multiple	O
PDH	O
signals	O
—	O
like	O
DS1	B-Protocol
,	O
E1	O
,	O
DS3	O
,	O
and	O
E3	O
—	O
along	O
with	O
other	O
groups	O
of	O
multiplexed	B-Architecture
64kbit/s	O
pulse-code	B-Algorithm
modulated	I-Algorithm
voice	O
traffic	O
.	O
</s>
<s>
In	O
order	O
to	O
support	O
large	O
ATM	O
bandwidths	O
,	O
concatenation	O
was	O
developed	O
,	O
whereby	O
smaller	O
multiplexing	B-Architecture
containers	O
(	O
e.g.	O
,	O
STS-1	O
)	O
are	O
inversely	O
multiplexed	B-Architecture
to	O
build	O
up	O
a	O
larger	O
container	O
(	O
e.g.	O
,	O
STS-3c	O
)	O
to	O
support	O
large	O
data-oriented	O
pipes	O
.	O
</s>
<s>
For	O
example	O
,	O
fitting	O
a	O
100Mbit/s	O
Fast	B-Protocol
Ethernet	I-Protocol
connection	O
inside	O
a	O
155Mbit/s	O
STS-3c	O
container	O
leads	O
to	O
considerable	O
waste	O
.	O
</s>
<s>
This	O
problem	O
was	O
overcome	O
with	O
the	O
introduction	O
of	O
Virtual	B-Protocol
Concatenation	I-Protocol
.	O
</s>
<s>
Virtual	B-Protocol
concatenation	I-Protocol
(	O
VCAT	O
)	O
allows	O
for	O
a	O
more	O
arbitrary	O
assembly	O
of	O
lower-order	O
multiplexing	B-Architecture
containers	O
,	O
building	O
larger	O
containers	O
of	O
fairly	O
arbitrary	O
size	O
(	O
e.g.	O
,	O
100Mbit/s	O
)	O
without	O
the	O
need	O
for	O
intermediate	O
network	O
elements	O
to	O
support	O
this	O
particular	O
form	O
of	O
concatenation	O
.	O
</s>
<s>
Virtual	B-Protocol
concatenation	I-Protocol
leverages	O
the	O
X.86	O
or	O
Generic	B-Protocol
Framing	I-Protocol
Procedure	I-Protocol
(	O
GFP	O
)	O
protocols	O
in	O
order	O
to	O
map	O
payloads	O
of	O
arbitrary	O
bandwidth	O
into	O
the	O
virtually	O
concatenated	O
container	O
.	O
</s>
<s>
The	O
Link	B-Protocol
Capacity	I-Protocol
Adjustment	I-Protocol
Scheme	I-Protocol
(	O
LCAS	B-Protocol
)	O
allows	O
for	O
dynamically	O
changing	O
the	O
bandwidth	O
via	O
dynamic	O
virtual	B-Protocol
concatenation	I-Protocol
,	O
multiplexing	B-Architecture
containers	O
based	O
on	O
the	O
short-term	O
bandwidth	O
needs	O
in	O
the	O
network	O
.	O
</s>
<s>
The	O
set	O
of	O
next-generation	O
SONET/SDH	B-Protocol
protocols	O
that	O
enable	O
Ethernet	O
transport	O
is	O
referred	O
to	O
as	O
Ethernet	B-Protocol
over	I-Protocol
SONET/SDH	I-Protocol
(	O
EoS	O
)	O
.	O
</s>
<s>
SONET/SDH	B-Protocol
was	O
used	O
by	O
internet	O
access	O
providers	O
for	O
large	O
customers	O
,	O
and	O
is	O
no	O
longer	O
competitive	O
in	O
the	O
supply	O
of	O
private	O
circuits	O
.	O
</s>
<s>
Development	O
has	O
stagnated	O
for	O
the	O
last	O
decade	O
(	O
2020	O
)	O
and	O
both	O
suppliers	O
of	O
equipment	O
and	O
operators	O
of	O
SONET/SDH	B-Protocol
networks	O
are	O
migrating	O
to	O
other	O
technologies	O
such	O
as	O
OTN	O
and	O
wide	O
area	O
Ethernet	O
.	O
</s>
