<s>
Packet	B-Protocol
over	I-Protocol
SONET/SDH	I-Protocol
,	O
abbreviated	O
POS	O
,	O
is	O
a	O
communications	O
protocol	O
for	O
transmitting	O
packets	B-Protocol
in	O
the	O
form	O
of	O
the	O
Point	B-Protocol
to	I-Protocol
Point	I-Protocol
Protocol	I-Protocol
(	O
PPP	O
)	O
over	O
SDH	B-Protocol
or	O
SONET	B-Protocol
,	O
which	O
are	O
both	O
standard	O
protocols	O
for	O
communicating	O
digital	O
information	O
using	O
lasers	O
or	O
light	O
emitting	O
diodes	O
(	O
LEDs	O
)	O
over	O
optical	O
fibre	O
at	O
high	O
line	O
rates	O
.	O
</s>
<s>
POS	O
is	O
defined	O
by	O
RFC	O
2615	O
as	O
PPP	O
over	O
SONET/SDH	B-Protocol
.	O
</s>
<s>
PPP	O
is	O
the	O
Point	B-Protocol
to	I-Protocol
Point	I-Protocol
Protocol	I-Protocol
that	O
was	O
designed	O
as	O
a	O
standard	O
method	O
of	O
communicating	O
over	O
point-to-point	O
links	O
.	O
</s>
<s>
Since	O
SONET/SDH	B-Protocol
uses	O
point-to-point	O
circuits	O
,	O
PPP	O
is	O
well	O
suited	O
for	O
use	O
over	O
these	O
links	O
.	O
</s>
<s>
Scrambling	O
is	O
performed	O
during	O
insertion	O
of	O
the	O
PPP	O
packets	B-Protocol
into	O
the	O
SONET/SDH	B-Protocol
frame	O
to	O
solve	O
various	O
security	O
attacks	O
including	O
denial-of-service	O
attacks	O
and	O
the	O
imitation	O
of	O
SONET/SDH	B-Protocol
alarms	O
.	O
</s>
<s>
This	O
modification	O
was	O
justified	O
as	O
cost-effective	O
because	O
the	O
scrambling	O
algorithm	O
was	O
already	O
used	O
by	O
the	O
standard	O
used	O
to	O
transport	O
ATM	O
cells	O
over	O
SONET/SDH	B-Protocol
.	O
</s>
<s>
However	O
,	O
scrambling	O
can	O
optionally	O
be	O
disabled	O
to	O
allow	O
a	O
node	O
to	O
be	O
compatible	O
with	O
another	O
node	O
that	O
uses	O
the	O
now	O
obsoleted	O
RFC	O
1619	O
version	O
of	O
Packet	B-Protocol
over	I-Protocol
SONET/SDH	I-Protocol
which	O
lacks	O
the	O
scrambler	O
.	O
</s>
<s>
The	O
most	O
important	O
application	O
of	O
POS	O
is	O
to	O
support	O
sending	O
of	O
IP	B-Protocol
packets	B-Protocol
across	O
wide	O
area	O
networks	O
.	O
</s>
<s>
POS	O
is	O
also	O
one	O
of	O
the	O
link	O
layers	O
used	O
by	O
the	O
Resilient	O
Packet	B-Protocol
Ring	O
standard	O
known	O
as	O
IEEE	O
802.17	O
.	O
</s>
<s>
POS	O
(	O
packet	B-Protocol
over	I-Protocol
SONET	I-Protocol
)	O
is	O
a	O
double-nested	O
abbreviation	O
.	O
</s>
<s>
The	O
S	O
represents	O
"	O
SONET/SDH	B-Protocol
"	O
,	O
which	O
itself	O
stands	O
for	O
"	O
Synchronous	O
Optical	O
Network/Synchronous	O
Digital	O
Hierarchy	O
"	O
.	O
</s>
<s>
Given	O
this	O
information	O
,	O
POS	O
technically	O
stands	O
for	O
"	O
Packet	B-Protocol
over	O
Synchronous	O
Optical	O
Network/Synchronous	O
Digital	O
Hierarchy	O
"	O
.	O
</s>
<s>
The	O
System	B-Protocol
Packet	I-Protocol
Interface	I-Protocol
series	O
of	O
standards	O
from	O
the	O
Optical	O
Internetworking	O
Forum	O
including	O
SPI-4.2	B-Protocol
and	O
SPI-3	B-Protocol
and	O
their	O
predecessors	O
PL-4	B-Protocol
and	O
PL-3	B-Protocol
are	O
commonly	O
used	O
as	O
the	O
interface	O
between	O
packet	B-Protocol
processing	O
devices	O
and	O
framer	O
devices	O
that	O
implement	O
POS	O
.	O
</s>
<s>
The	O
acronym	O
PL-4	B-Protocol
means	O
POS-PHY	O
Layer	O
4	O
.	O
</s>
