<s>
The	O
System	B-Protocol
Packet	I-Protocol
Interface	I-Protocol
(	O
SPI	O
)	O
family	O
of	O
Interoperability	O
Agreements	O
from	O
the	O
Optical	O
Internetworking	O
Forum	O
specify	O
chip-to-chip	O
,	O
channelized	O
,	O
packet	O
interfaces	O
commonly	O
used	O
in	O
synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
and	O
Ethernet	O
applications	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>
SPI-4.2	B-Protocol
System	B-Protocol
Packet	I-Protocol
Interface	I-Protocol
Level	O
4	O
(	O
SPI-4	O
)	O
Phase	O
2	O
:	O
OC-192	O
System	O
Interface	O
for	O
Physical	O
and	O
Link	O
Layer	O
Devices	O
.	O
</s>
<s>
These	O
agreements	O
grew	O
out	O
of	O
the	O
donation	O
to	O
the	O
OIF	O
by	O
PMC-Sierra	O
of	O
the	O
POS-PHY	O
interface	O
definitions	O
PL-3	B-Protocol
and	O
PL-4	B-Protocol
,	O
which	O
themselves	O
came	O
from	O
the	O
ATM	O
Forum	O
's	O
Utopia	O
definitions	O
.	O
</s>
<s>
System	B-Protocol
Packet	I-Protocol
Interface	I-Protocol
or	O
SPI	O
as	O
it	O
is	O
widely	O
known	O
is	O
a	O
protocol	O
for	O
packet	O
and	O
cell	O
transfers	O
between	O
PHY	O
and	O
LINK	O
layer	O
devices	O
in	O
multi-gigabit	O
applications	O
.	O
</s>
<s>
The	O
Interlaken	B-Protocol
protocol	O
,	O
a	O
close	O
variant	O
of	O
SPI-5	O
replaced	O
the	O
System	B-Protocol
Packet	I-Protocol
Interface	I-Protocol
in	O
the	O
marketplace	O
.	O
</s>
<s>
To	O
ensure	O
optimal	O
use	O
of	O
the	O
rx/tx	O
buffers	O
in	O
devices	O
connected	O
with	O
SPI	B-Protocol
interface	I-Protocol
,	O
the	O
RBUF/TBUF	O
element	O
size	O
in	O
those	O
devices	O
should	O
match	O
the	O
SPI-4.2	B-Protocol
data	O
burst	O
size	O
.	O
</s>
