<s>
In	O
telecommunications	O
and	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
network	B-Protocol
packet	I-Protocol
is	O
a	O
formatted	O
unit	O
of	O
data	B-General_Concept
carried	O
by	O
a	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
A	O
packet	B-Protocol
consists	O
of	O
control	O
information	O
and	O
user	O
data	B-General_Concept
;	O
the	O
latter	O
is	O
also	O
known	O
as	O
the	O
payload	O
.	O
</s>
<s>
Control	O
information	O
provides	O
data	B-General_Concept
for	O
delivering	O
the	O
payload	O
(	O
e.g.	O
,	O
source	O
and	O
destination	O
network	O
addresses	O
,	O
error	B-Error_Name
detection	I-Error_Name
codes	I-Error_Name
,	O
or	O
sequencing	O
information	O
)	O
.	O
</s>
<s>
Typically	O
,	O
control	O
information	O
is	O
found	O
in	O
packet	B-General_Concept
headers	I-General_Concept
and	O
trailers	B-Device
.	O
</s>
<s>
In	O
packet	B-Protocol
switching	I-Protocol
,	O
the	O
bandwidth	O
of	O
the	O
transmission	O
medium	O
is	O
shared	O
between	O
multiple	O
communication	O
sessions	O
,	O
in	O
contrast	O
to	O
circuit	B-Protocol
switching	I-Protocol
,	O
in	O
which	O
circuits	O
are	O
preallocated	O
for	O
the	O
duration	O
of	O
one	O
session	O
and	O
data	B-General_Concept
is	O
typically	O
transmitted	O
as	O
a	O
continuous	O
bit	O
stream	O
.	O
</s>
<s>
In	O
the	O
seven-layer	O
OSI	O
model	O
of	O
computer	B-Architecture
networking	I-Architecture
,	O
packet	B-Protocol
strictly	O
refers	O
to	O
a	O
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
at	O
layer	B-Application
3	I-Application
,	O
the	O
network	B-Application
layer	I-Application
.	O
</s>
<s>
A	O
data	B-General_Concept
unit	O
at	O
layer	B-Protocol
2	I-Protocol
,	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
is	O
a	O
frame	B-Protocol
.	O
</s>
<s>
In	O
layer	B-Architecture
4	I-Architecture
,	O
the	O
transport	B-Architecture
layer	I-Architecture
,	O
the	O
data	B-General_Concept
units	O
are	O
segments	O
and	O
datagrams	O
.	O
</s>
<s>
Thus	O
,	O
in	O
the	O
example	O
of	O
TCP/IP	O
communication	O
over	O
Ethernet	O
,	O
a	O
TCP	O
segment	O
is	O
carried	O
in	O
one	O
or	O
more	O
IP	O
packets	B-Protocol
,	O
which	O
are	O
each	O
carried	O
in	O
one	O
or	O
more	O
Ethernet	B-Protocol
frames	I-Protocol
.	O
</s>
<s>
The	O
basis	O
of	O
the	O
packet	B-Protocol
concept	O
is	O
the	O
postal	O
letter	O
:	O
the	O
header	B-General_Concept
is	O
like	O
the	O
envelope	O
,	O
the	O
payload	O
is	O
the	O
entire	O
content	O
inside	O
the	O
envelope	O
,	O
and	O
the	O
footer	O
would	O
be	O
your	O
signature	O
at	O
the	O
bottom	O
.	O
</s>
<s>
Network	B-Architecture
design	I-Architecture
can	O
achieve	O
two	O
major	O
results	O
by	O
using	O
packets	B-Protocol
:	O
error	B-Error_Name
detection	I-Error_Name
and	O
multiple	O
host	O
addressing	O
.	O
</s>
<s>
Communications	O
protocols	O
use	O
various	O
conventions	O
for	O
distinguishing	O
the	O
elements	O
of	O
a	O
packet	B-Protocol
and	O
for	O
formatting	O
the	O
user	O
data	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
in	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
,	O
the	O
packet	B-Protocol
is	O
formatted	O
in	O
8-bit	O
bytes	B-Application
,	O
and	O
special	O
characters	O
are	O
used	O
to	O
delimit	O
elements	O
.	O
</s>
<s>
Other	O
protocols	O
,	O
like	O
Ethernet	O
,	O
establish	O
the	O
start	O
of	O
the	O
header	B-General_Concept
and	O
data	B-General_Concept
elements	O
by	O
their	O
location	O
relative	O
to	O
the	O
start	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
Some	O
protocols	O
format	O
the	O
information	O
at	O
a	O
bit	O
level	O
instead	O
of	O
a	O
byte	B-Application
level	O
.	O
</s>
<s>
A	O
packet	B-Protocol
may	O
contain	O
any	O
of	O
the	O
following	O
components	O
:	O
</s>
<s>
The	O
routing	B-Protocol
of	O
network	B-Protocol
packets	I-Protocol
requires	O
two	O
network	O
addresses	O
,	O
the	O
source	O
address	O
of	O
the	O
sending	O
host	O
,	O
and	O
the	O
destination	O
address	O
of	O
the	O
receiving	O
host	O
.	O
</s>
<s>
Error	B-Error_Name
detection	I-Error_Name
and	I-Error_Name
correction	I-Error_Name
is	O
performed	O
at	O
various	O
layers	O
in	O
the	O
protocol	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
Network	B-Protocol
packets	I-Protocol
may	O
contain	O
a	O
checksum	B-Algorithm
,	O
parity	B-Error_Name
bits	I-Error_Name
or	O
cyclic	O
redundancy	O
checks	O
to	O
detect	O
errors	O
that	O
occur	O
during	O
transmission	O
.	O
</s>
<s>
At	O
the	O
transmitter	O
,	O
the	O
calculation	O
is	O
performed	O
before	O
the	O
packet	B-Protocol
is	O
sent	O
.	O
</s>
<s>
When	O
received	O
at	O
the	O
destination	O
,	O
the	O
checksum	B-Algorithm
is	O
recalculated	O
,	O
and	O
compared	O
with	O
the	O
one	O
in	O
the	O
packet	B-Protocol
.	O
</s>
<s>
If	O
discrepancies	O
are	O
found	O
,	O
the	O
packet	B-Protocol
may	O
be	O
corrected	O
or	O
discarded	O
.	O
</s>
<s>
Any	O
packet	B-Protocol
loss	I-Protocol
due	O
to	O
these	O
discards	O
is	O
dealt	O
with	O
by	O
the	O
network	O
protocol	O
.	O
</s>
<s>
In	O
some	O
cases	O
modifications	O
of	O
the	O
network	B-Protocol
packet	I-Protocol
may	O
be	O
necessary	O
while	O
routing	B-Protocol
,	O
in	O
which	O
cases	O
checksums	B-Algorithm
are	O
recalculated	O
.	O
</s>
<s>
Under	O
fault	O
conditions	O
,	O
packets	B-Protocol
can	O
end	O
up	O
traversing	O
a	O
closed	O
circuit	O
.	O
</s>
<s>
If	O
nothing	O
was	O
done	O
,	O
eventually	O
the	O
number	O
of	O
packets	B-Protocol
circulating	O
would	O
build	O
up	O
until	O
the	O
network	O
was	O
congested	O
to	O
the	O
point	O
of	O
failure	O
.	O
</s>
<s>
Time	B-Protocol
to	I-Protocol
live	I-Protocol
is	O
a	O
field	O
that	O
is	O
decreased	O
by	O
one	O
each	O
time	O
a	O
packet	B-Protocol
goes	O
through	O
a	O
network	B-Protocol
hop	I-Protocol
.	O
</s>
<s>
If	O
the	O
field	O
reaches	O
zero	O
,	O
routing	B-Protocol
has	O
failed	O
,	O
and	O
the	O
packet	B-Protocol
is	O
discarded	O
.	O
</s>
<s>
Ethernet	O
packets	B-Protocol
have	O
no	O
time-to-live	B-Protocol
field	O
and	O
so	O
are	O
subject	O
to	O
broadcast	B-General_Concept
radiation	I-General_Concept
in	O
the	O
presence	O
of	O
a	O
switching	B-Architecture
loop	I-Architecture
.	O
</s>
<s>
There	O
may	O
be	O
a	O
field	O
to	O
identify	O
the	O
overall	O
packet	B-Protocol
length	O
.	O
</s>
<s>
A	O
protocol	O
identifier	O
field	O
specifies	O
a	O
packet	B-Protocol
's	O
protocol	O
and	O
allows	O
the	O
protocol	B-Protocol
stack	I-Protocol
to	O
process	O
many	O
types	O
of	O
packets	B-Protocol
.	O
</s>
<s>
Some	O
networks	O
implement	O
quality	O
of	O
service	O
which	O
can	O
prioritize	O
some	O
types	O
of	O
packets	B-Protocol
above	O
others	O
.	O
</s>
<s>
This	O
field	O
indicates	O
which	O
packet	B-Protocol
queue	O
should	O
be	O
used	O
;	O
a	O
high	O
priority	O
queue	O
is	O
emptied	O
more	O
quickly	O
than	O
lower	O
priority	O
queues	O
at	O
points	O
in	O
the	O
network	O
where	O
congestion	O
is	O
occurring	O
.	O
</s>
<s>
In	O
general	O
,	O
the	O
payload	O
is	O
the	O
data	B-General_Concept
that	O
is	O
carried	O
on	O
behalf	O
of	O
an	O
application	O
.	O
</s>
<s>
When	O
necessary	O
,	O
some	O
networks	O
can	O
break	O
a	O
larger	O
packet	B-Protocol
into	O
smaller	O
packets	B-Protocol
.	O
</s>
<s>
IP	O
packets	B-Protocol
are	O
composed	O
of	O
a	O
header	B-General_Concept
and	O
payload	O
.	O
</s>
<s>
The	O
header	B-General_Concept
consists	O
of	O
fixed	O
and	O
optional	O
fields	O
.	O
</s>
<s>
The	O
payload	O
appears	O
immediately	O
after	O
the	O
header	B-General_Concept
.	O
</s>
<s>
An	O
IP	O
packet	B-Protocol
has	O
no	O
trailer	B-Device
.	O
</s>
<s>
However	O
,	O
an	O
IP	O
packet	B-Protocol
is	O
often	O
carried	O
as	O
the	O
payload	O
inside	O
an	O
Ethernet	B-Protocol
frame	I-Protocol
,	O
which	O
has	O
its	O
own	O
header	B-General_Concept
and	O
trailer	B-Device
.	O
</s>
<s>
Per	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
IP	O
networks	O
do	O
not	O
provide	O
guarantees	O
of	O
delivery	O
,	O
non-duplication	O
,	O
or	O
in-order	O
delivery	O
of	O
packets	B-Protocol
.	O
</s>
<s>
However	O
,	O
it	O
is	O
common	O
practice	O
to	O
layer	O
a	O
reliable	B-Protocol
transport	B-Architecture
protocol	I-Architecture
such	O
as	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
on	O
top	O
of	O
the	O
packet	B-Protocol
service	O
to	O
provide	O
such	O
protection	O
.	O
</s>
<s>
The	O
Consultative	O
Committee	O
for	O
Space	O
Data	B-General_Concept
Systems	O
(	O
CCSDS	O
)	O
packet	B-Protocol
telemetry	O
standard	O
defines	O
the	O
protocol	O
used	O
for	O
the	O
transmission	O
of	O
spacecraft	O
instrument	O
data	B-General_Concept
over	O
the	O
deep-space	O
channel	O
.	O
</s>
<s>
Under	O
this	O
standard	O
,	O
an	O
image	O
or	O
other	O
data	B-General_Concept
sent	O
from	O
a	O
spacecraft	O
instrument	O
is	O
transmitted	O
using	O
one	O
or	O
more	O
packets	B-Protocol
.	O
</s>
<s>
Packetized	O
elementary	O
stream	O
(	O
PES	O
)	O
is	O
a	O
specification	O
associated	O
with	O
the	O
MPEG-2	B-Algorithm
standard	O
that	O
allows	O
an	O
elementary	O
stream	O
to	O
be	O
divided	O
into	O
packets	B-Protocol
.	O
</s>
<s>
The	O
elementary	O
stream	O
is	O
packetized	O
by	O
encapsulating	O
sequential	O
data	B-General_Concept
bytes	B-Application
from	O
the	O
elementary	O
stream	O
between	O
PES	O
packet	B-General_Concept
headers	I-General_Concept
.	O
</s>
<s>
A	O
typical	O
method	O
of	O
transmitting	O
elementary	O
stream	O
data	B-General_Concept
from	O
a	O
video	O
or	O
audio	O
encoder	O
is	O
to	O
first	O
create	O
PES	O
packets	B-Protocol
from	O
the	O
elementary	O
stream	O
data	B-General_Concept
and	O
then	O
to	O
encapsulate	O
these	O
PES	O
packets	B-Protocol
inside	O
an	O
MPEG	O
transport	O
stream	O
(	O
TS	O
)	O
packets	B-Protocol
or	O
an	O
MPEG	O
program	O
stream	O
(	O
PS	O
)	O
.	O
</s>
<s>
The	O
TS	O
packets	B-Protocol
can	O
then	O
be	O
transmitted	O
using	O
broadcasting	O
techniques	O
,	O
such	O
as	O
those	O
used	O
in	O
an	O
ATSC	B-Device
and	O
DVB	B-Application
.	O
</s>
<s>
This	O
means	O
that	O
the	O
FM	B-Application
or	O
AM	B-Device
regular	O
mono	O
sound	O
carrier	O
is	O
left	O
alone	O
for	O
reception	O
by	O
monaural	O
receivers	O
.	O
</s>
<s>
The	O
NICAM	O
packet	B-Protocol
(	O
except	O
for	O
the	O
header	B-General_Concept
)	O
is	O
scrambled	O
with	O
a	O
nine-bit	O
pseudo-random	O
bit-generator	O
before	O
transmission	O
.	O
</s>
