<s>
PL-4	B-Protocol
or	O
POS-PHY	O
Level	O
4	O
was	O
the	O
name	O
of	O
the	O
interface	O
that	O
the	O
interface	O
SPI-4.2	B-Protocol
is	O
based	O
on	O
.	O
</s>
<s>
The	O
name	O
means	O
Packet	B-Protocol
Over	I-Protocol
SONET	I-Protocol
Physical	O
layer	O
level	O
4	O
.	O
</s>
<s>
PL-4	B-Protocol
was	O
developed	O
by	O
PMC-Sierra	O
in	O
conjunction	O
with	O
the	O
Saturn	O
Development	O
Group	O
.	O
</s>
<s>
The	O
first	O
,	O
exemplified	O
by	O
PCI-Express	O
and	O
HyperTransport	B-Device
,	O
supports	O
reads	O
and	O
writes	O
of	O
memory	O
addresses	O
.	O
</s>
<s>
The	O
second	O
broad	O
category	O
carries	O
user	O
packets	O
over	O
1	O
or	O
more	O
channels	O
and	O
is	O
exemplified	O
by	O
the	O
IEEE	O
802.3	O
family	O
of	O
Media	B-Protocol
Independent	I-Protocol
Interfaces	I-Protocol
and	O
the	O
Optical	O
Internetworking	O
Forum	O
family	O
of	O
System	B-Protocol
Packet	I-Protocol
Interfaces	I-Protocol
.	O
</s>
<s>
Of	O
these	O
last	O
two	O
,	O
the	O
family	O
of	O
System	B-Protocol
Packet	I-Protocol
Interfaces	I-Protocol
is	O
optimized	O
to	O
carry	O
user	O
packets	O
from	O
many	O
channels	O
.	O
</s>
<s>
The	O
family	O
of	O
System	B-Protocol
Packet	I-Protocol
Interfaces	I-Protocol
is	O
the	O
most	O
important	O
packet-oriented	O
,	O
chip-to-chip	O
interface	O
family	O
used	O
between	O
devices	O
in	O
the	O
Packet	B-Protocol
over	I-Protocol
SONET	I-Protocol
and	O
Optical	B-Protocol
Transport	I-Protocol
Network	I-Protocol
,	O
which	O
are	O
the	O
principal	O
protocols	O
used	O
to	O
carry	O
the	O
internet	O
between	O
cities	O
.	O
</s>
<s>
PL-4	B-Protocol
was	O
designed	O
to	O
be	O
used	O
in	O
systems	O
that	O
support	O
OC-192	O
SONET	B-Protocol
interfaces	O
and	O
is	O
sometimes	O
used	O
in	O
10	O
Gigabit	O
Ethernet	O
based	O
systems	O
.	O
</s>
<s>
A	O
typical	O
application	O
of	O
PL-4	B-Protocol
(	O
SPI-4.2	B-Protocol
)	O
is	O
to	O
connect	O
a	O
framer	O
device	O
to	O
a	O
network	O
processor	O
.	O
</s>
<s>
Implementations	O
of	O
SPI-4.2	B-Protocol
(	O
PL-4	B-Protocol
)	O
have	O
been	O
produced	O
which	O
allow	O
somewhat	O
higher	O
clock	O
rates	O
.	O
</s>
<s>
The	O
name	O
is	O
an	O
acronym	O
of	O
an	O
acronym	O
of	O
an	O
acronym	O
as	O
the	O
P	O
in	O
PL	O
stands	O
for	O
POS-PHY	O
and	O
the	O
S	O
in	O
POS-PHY	O
stands	O
for	O
SONET	B-Protocol
(	O
Synchronous	B-Protocol
Optical	I-Protocol
Network	I-Protocol
)	O
.	O
</s>
<s>
PL-4	B-Protocol
is	O
a	O
descendant	O
of	O
PL-3	B-Protocol
which	O
itself	O
is	O
a	O
descendant	O
of	O
the	O
ATM	O
Forum	O
UTOPIA	O
family	O
of	O
standards	O
.	O
</s>
