<s>
In	O
telecommunications	O
,	O
a	O
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
(	O
PDU	O
)	O
is	O
a	O
single	O
unit	O
of	O
information	O
transmitted	O
among	O
peer	O
entities	O
of	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
implements	O
a	O
connection-oriented	O
transfer	O
mode	O
,	O
and	O
the	O
PDU	O
of	O
this	O
protocol	O
is	O
called	O
a	O
segment	B-Protocol
,	O
while	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
uses	O
datagrams	B-Protocol
as	O
protocol	B-Protocol
data	I-Protocol
units	I-Protocol
for	O
connectionless	B-Protocol
communication	I-Protocol
.	O
</s>
<s>
A	O
layer	O
lower	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
at	O
the	O
Internet	B-Protocol
layer	I-Protocol
,	O
the	O
PDU	O
is	O
called	O
a	O
packet	B-Protocol
,	O
irrespective	O
of	O
its	O
payload	O
type	O
.	O
</s>
<s>
In	O
the	O
context	O
of	O
packet	B-Protocol
switching	I-Protocol
data	B-Architecture
networks	I-Architecture
,	O
a	O
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
(	O
PDU	O
)	O
is	O
best	O
understood	O
in	O
relation	O
to	O
a	O
service	O
data	O
unit	O
(	O
SDU	O
)	O
.	O
</s>
<s>
The	O
physical	B-Application
layer	I-Application
sends	O
ones	O
and	O
zeros	O
across	O
a	O
wire	O
or	O
fiber	O
.	O
</s>
<s>
The	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
then	O
organizes	O
these	O
ones	O
and	O
zeros	O
into	O
chunks	O
of	O
data	O
and	O
gets	O
them	O
safely	O
to	O
the	O
right	O
place	O
on	O
the	O
wire	O
.	O
</s>
<s>
The	O
network	B-Application
layer	I-Application
transmits	O
the	O
organized	O
data	O
over	O
multiple	O
connected	O
networks	O
,	O
and	O
the	O
transport	B-Architecture
layer	I-Architecture
delivers	O
the	O
data	O
to	O
the	O
right	O
software	O
application	O
at	O
the	O
destination	O
.	O
</s>
<s>
In	O
order	O
to	O
do	O
this	O
,	O
the	O
protocol	O
(	O
lower	O
)	O
layer	O
will	O
add	O
to	O
the	O
SDU	O
certain	O
data	O
it	O
needs	O
to	O
perform	O
its	O
function	O
;	O
which	O
is	O
called	O
encapsulation	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
it	O
might	O
add	O
a	O
port	O
number	O
to	O
identify	O
the	O
application	O
,	O
a	O
network	O
address	O
to	O
help	O
with	O
routing	O
,	O
a	O
code	O
to	O
identify	O
the	O
type	O
of	O
data	O
in	O
the	O
packet	B-Protocol
and	O
error-checking	O
information	O
.	O
</s>
<s>
All	O
this	O
additional	O
information	O
,	O
plus	O
the	O
original	O
service	O
data	O
unit	O
from	O
the	O
higher	O
layer	O
,	O
constitutes	O
the	O
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
at	O
this	O
layer	O
.	O
</s>
<s>
The	O
SDU	O
and	O
metadata	O
added	O
by	O
the	O
lower	O
layer	O
can	O
be	O
larger	O
than	O
the	O
maximum	O
size	O
of	O
that	O
layer	O
's	O
PDU	O
(	O
known	O
as	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
;	O
MTU	B-Protocol
)	O
.	O
</s>
<s>
When	O
this	O
is	O
the	O
case	O
,	O
the	O
PDU	O
must	O
be	O
split	O
into	O
multiple	O
payloads	O
of	O
a	O
size	O
suitable	O
for	O
transmission	O
or	O
processing	O
by	O
the	O
lower	O
layer	O
;	O
a	O
process	O
known	O
as	O
IP	B-Protocol
fragmentation	I-Protocol
.	O
</s>
<s>
The	O
addition	O
of	O
addressing	O
and	O
control	O
information	O
(	O
encapsulation	B-Protocol
)	O
to	O
an	O
SDU	O
to	O
form	O
a	O
PDU	O
and	O
the	O
passing	O
of	O
that	O
PDU	O
to	O
the	O
next	O
lower	O
layer	O
as	O
an	O
SDU	O
repeats	O
until	O
the	O
lowest	O
layer	O
is	O
reached	O
and	O
the	O
data	O
passes	O
over	O
some	O
medium	O
as	O
a	O
physical	O
signal	O
.	O
</s>
<s>
Protocol	B-Protocol
data	I-Protocol
units	I-Protocol
of	O
the	O
OSI	O
model	O
are	O
:	O
</s>
<s>
The	O
Layer	B-Architecture
4	I-Architecture
:	O
transport	B-Architecture
layer	I-Architecture
PDU	O
is	O
the	O
segment	B-Protocol
or	O
the	O
datagram	B-Protocol
.	O
</s>
<s>
The	O
Layer	B-Application
3	I-Application
:	O
network	B-Application
layer	I-Application
PDU	O
is	O
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
Layer	B-Protocol
2	I-Protocol
:	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
PDU	O
is	O
the	O
frame	B-Protocol
.	O
</s>
<s>
The	O
Layer	B-Application
1	I-Application
:	O
physical	B-Application
layer	I-Application
PDU	O
is	O
the	O
bit	O
or	O
,	O
more	O
generally	O
,	O
symbol	O
.	O
</s>
<s>
Protocol	B-Protocol
data	I-Protocol
units	I-Protocol
for	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
are	O
:	O
</s>
<s>
The	O
Internet	B-Protocol
layer	I-Protocol
PDU	O
is	O
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
link	B-Protocol
layer	I-Protocol
PDU	O
is	O
the	O
frame	B-Protocol
.	O
</s>
<s>
On	O
TCP/IP	B-Protocol
over	O
Ethernet	O
,	O
the	O
data	O
on	O
the	O
physical	B-Application
layer	I-Application
is	O
carried	O
in	O
Ethernet	B-Protocol
frames	I-Protocol
.	O
</s>
<s>
For	O
IPX	O
,	O
the	O
IPX	O
packet	B-Protocol
is	O
the	O
network	B-Application
layer	I-Application
PDU	O
is	O
the	O
for	O
IPX	O
,	O
and	O
can	O
simultaneously	O
be	O
used	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
as	O
well	O
.	O
</s>
<s>
The	O
SPX	O
packet	B-Protocol
on	O
top	O
of	O
IPX	O
,	O
is	O
a	O
transport	B-Architecture
layer	I-Architecture
PDU	O
.	O
</s>
<s>
The	O
link-layer	B-Protocol
PDU	O
is	O
the	O
frame	B-Protocol
.	O
</s>
<s>
The	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
PDU	O
in	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
networks	O
is	O
called	O
a	O
cell	O
.	O
</s>
<s>
A	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
(	O
MAC	O
PDU	O
or	O
MPDU	O
)	O
is	O
a	O
message	O
that	O
is	O
exchanged	O
between	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
entities	O
in	O
a	O
communication	O
system	O
based	O
on	O
the	O
layered	O
OSI	O
model	O
.	O
</s>
<s>
In	O
systems	O
where	O
the	O
MPDU	O
may	O
be	O
larger	O
than	O
the	O
MAC	O
service	O
data	O
unit	O
(	O
MSDU	O
)	O
,	O
the	O
MPDU	O
may	O
include	O
multiple	O
MSDUs	O
as	O
a	O
result	O
of	O
packet	B-Protocol
aggregation	I-Protocol
.	O
</s>
<s>
In	O
systems	O
where	O
the	O
MPDU	O
is	O
smaller	O
than	O
the	O
MSDU	O
,	O
then	O
one	O
MSDU	O
may	O
generate	O
multiple	O
MPDUs	O
as	O
a	O
result	O
of	O
packet	B-Protocol
segmentation	I-Protocol
.	O
</s>
