<s>
The	O
STM-4	B-Protocol
(	O
Synchronous	O
Transport	O
Module	O
)	O
is	O
a	O
SDH	B-Protocol
ITU-T	O
fiber	B-Architecture
optic	I-Architecture
network	O
transmission	O
standard	O
.	O
</s>
<s>
The	O
STM-4	B-Protocol
specification	O
is	O
designed	O
to	O
carry	O
7,680	O
8-bit	O
"	O
voice	O
"	O
frames	O
every	O
125	O
micro-seconds	O
for	O
a	O
total	O
payload	O
bit	O
rate	O
of	O
491.520	O
Mbit/s	O
.	O
</s>
<s>
The	O
other	O
levels	O
defined	O
by	O
the	O
SDH	B-Protocol
standard	O
are	O
STM-1	B-Protocol
,	O
STM-16	B-Protocol
,	O
STM-64	B-Protocol
and	O
STM-256	B-Protocol
.	O
</s>
<s>
Beyond	O
this	O
we	O
have	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	O
)	O
commonly	O
used	O
in	O
submarine	O
cabling	O
.	O
</s>
<s>
Although	O
STM-4	B-Protocol
is	O
comparable	O
to	O
OC-12	O
the	O
SDH	B-Protocol
frame	O
structure	O
allocates	O
more	O
space	O
to	O
overhead	O
than	O
that	O
of	O
SONET	B-Protocol
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
STM-4	B-Protocol
'	O
s	O
payload	O
bandwidth	O
differs	O
from	O
that	O
of	O
OC-12	O
.	O
</s>
<s>
SDH	B-Protocol
is	O
a	O
transport	O
hierarchy	O
based	O
on	O
multiples	O
of	O
155.52	O
Mbit/s	O
.	O
</s>
<s>
The	O
basic	O
unit	O
of	O
SDH	B-Protocol
is	O
STM-1	B-Protocol
.	O
</s>
<s>
Different	O
SDH	B-Protocol
rates	O
are	O
given	O
below	O
:	O
</s>
<s>
The	O
basic	O
STM-1	B-Protocol
carrier	O
frame	O
consists	O
of	O
eight	O
overheads	O
,	O
four	O
pointers	O
and	O
a	O
space	O
for	O
the	O
payload	O
.	O
</s>
<s>
The	O
overheads	O
are	O
the	O
regeneration	O
section	B-Protocol
overhead	I-Protocol
(	O
RSOH	O
)	O
,	O
associated	O
with	O
the	O
regenerators	O
,	O
and	O
the	O
multiplex	O
section	B-Protocol
overhead	I-Protocol
(	O
MSOH	O
)	O
,	O
associated	O
with	O
the	O
multiplexers	O
.	O
</s>
<s>
In	O
previous	O
sections	O
we	O
have	O
looked	O
at	O
an	O
example	O
in	O
which	O
a	O
140	O
Mbit/s	O
signal	O
was	O
mapped	O
into	O
a	O
VC-4	O
,	O
but	O
the	O
multiplexing	O
map	O
lets	O
the	O
STM-1	B-Protocol
signal	O
transport	O
other	O
types	O
of	O
Plesiochronous	O
signals	O
and	O
even	O
combinations	O
of	O
signals	O
.	O
</s>
<s>
The	O
basic	O
STM-1	B-Protocol
transport	O
rate	O
is	O
155.520	O
Mbit/s	O
and	O
is	O
defined	O
in	O
the	O
ITU-T	O
recommendation	O
G.707	B-Protocol
.	O
</s>
<s>
As	O
mentioned	O
above	O
,	O
the	O
overhead	O
of	O
an	O
STM-4	B-Protocol
signal	O
(	O
SOH	O
)	O
is	O
divided	O
into	O
two	O
parts	O
:	O
the	O
MSOH	O
and	O
the	O
RSOH	O
.	O
</s>
<s>
An	O
STM-4	B-Protocol
frame	O
consists	O
of	O
36	O
rows	O
each	O
containing	O
270	O
bytes	O
.	O
</s>
<s>
This	O
is	O
a	O
direct	O
multiple	O
of	O
STM-1	B-Protocol
,	O
which	O
consists	O
of	O
9	O
rows	O
each	O
containing	O
270	O
bytes	O
.	O
</s>
<s>
The	O
frame	O
frequency	O
of	O
32kHz	O
has	O
also	O
been	O
chosen	O
as	O
a	O
4x	O
multiple	O
of	O
that	O
of	O
STM-1	B-Protocol
,	O
so	O
that	O
one	O
byte	O
of	O
frame	O
corresponds	O
to	O
the	O
transmission	O
capacity	O
of	O
a	O
64	O
kbit/s	O
channel	O
.	O
</s>
<s>
The	O
first	O
36	O
bytes	O
of	O
each	O
row	O
of	O
a	O
frame	O
(	O
with	O
the	O
exception	O
of	O
row	O
16	O
)	O
contain	O
a	O
Section	B-Protocol
OverHead	I-Protocol
.	O
</s>
<s>
The	O
field	O
consisting	O
of	O
261X36	O
bytes	O
is	O
for	O
payload	O
transmission	O
(	O
The	O
STM	O
payload	O
is	O
known	O
as	O
the	O
Virtual	B-Protocol
Container	I-Protocol
(	O
VC	O
)	O
and	O
consists	O
of	O
the	O
actual	O
payload	O
,	O
known	O
as	O
the	O
Container	O
(	O
C	O
)	O
and	O
additional	O
overhead	O
known	O
as	O
Path	O
Over	O
Head	O
(	O
POH	O
)	O
,	O
for	O
path	O
related	O
information	O
)	O
.	O
</s>
<s>
The	O
Virtual	B-Protocol
Container	I-Protocol
is	O
not	O
located	O
rigidly	O
in	O
the	O
frame	O
but	O
can	O
move	O
around	O
freely	O
in	O
the	O
payload	O
section	O
.	O
</s>
<s>
This	O
allows	O
a	O
VC	O
received	O
in	O
the	O
multiplexer	O
to	O
be	O
matched	O
to	O
a	O
pre-specified	O
frame	O
phase	O
of	O
the	O
STM-4	B-Protocol
.	O
</s>
