<s>
Usage	B-Protocol
Parameter	I-Protocol
Control	I-Protocol
(	O
UPC	O
)	O
and	O
Network	B-Protocol
Parameter	I-Protocol
Control	I-Protocol
(	O
NPC	O
)	O
are	O
functions	O
that	O
may	O
be	O
performed	O
in	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
UPC	B-Protocol
and	I-Protocol
NPC	I-Protocol
may	O
involve	O
traffic	O
shaping	O
,	O
where	O
traffic	O
is	O
delayed	O
until	O
it	O
conforms	O
to	O
the	O
expected	O
levels	O
and	O
timing	O
,	O
or	O
traffic	O
policing	O
,	O
where	O
non-conforming	O
traffic	O
is	O
either	O
discarded	O
immediately	O
,	O
or	O
reduced	O
in	O
priority	O
so	O
that	O
it	O
may	O
be	O
discarded	O
downstream	O
in	O
the	O
network	O
if	O
it	O
would	O
cause	O
or	O
add	O
to	O
congestion	B-Protocol
.	O
</s>
<s>
The	O
actions	O
for	O
UPC	B-Protocol
and	I-Protocol
NPC	I-Protocol
in	O
the	O
ATM	O
protocol	O
are	O
defined	O
in	O
ITU-T	O
Recommendation	O
I.371	O
Traffic	O
control	O
and	O
congestion	B-Protocol
control	O
in	O
B	O
ISDN	O
and	O
the	O
ATM	O
Forum	O
's	O
User-Network	O
Interface	O
(	O
UNI	O
)	O
Specification	O
.	O
</s>
<s>
These	O
provide	O
a	O
conformance	O
definition	O
,	O
using	O
a	O
form	O
of	O
the	O
leaky	B-Algorithm
bucket	I-Algorithm
algorithm	I-Algorithm
called	O
the	O
Generic	B-Algorithm
Cell	I-Algorithm
Rate	I-Algorithm
Algorithm	I-Algorithm
(	O
GCRA	B-Algorithm
)	O
,	O
which	O
specifies	O
how	O
cells	O
are	O
checked	O
for	O
conformance	O
with	O
a	O
cell	O
rate	O
,	O
or	O
its	O
reciprocal	O
emission	O
interval	O
,	O
and	O
jitter	O
tolerance	O
:	O
either	O
a	O
Cell	O
Delay	O
Variation	O
tolerance	O
(	O
CDVt	O
)	O
for	O
testing	O
conformance	O
to	O
the	O
Peak	O
Cell	O
Rate	O
(	O
PCR	O
)	O
or	O
a	O
Burst	O
Tolerance	O
or	O
Maximum	O
Burst	O
Size	O
(	O
MBS	O
)	O
for	O
testing	O
conformance	O
to	O
the	O
Sustainable	O
Cell	O
Rate	O
(	O
SCR	O
)	O
.	O
</s>
<s>
UPC	B-Protocol
and	I-Protocol
NPC	I-Protocol
define	O
a	O
Maximum	O
Burst	O
Size	O
(	O
MBS	O
)	O
parameter	O
on	O
the	O
average	O
or	O
Sustained	O
Cell	O
Rate	O
(	O
SCR	O
)	O
,	O
and	O
a	O
Cell	O
Delay	O
Variation	O
tolerance	O
(	O
CDVt	O
)	O
on	O
the	O
Peak	O
Cell	O
Rate	O
(	O
PCR	O
)	O
at	O
which	O
the	O
bursts	O
are	O
transmitted	O
.	O
</s>
<s>
UPC	B-Protocol
and	I-Protocol
NPC	I-Protocol
are	O
normally	O
performed	O
on	O
a	O
per	O
Virtual	O
Channel	O
(	O
VC	O
)	O
or	O
per	O
Virtual	O
Path	O
(	O
VP	O
)	O
basis	O
,	O
i.e.	O
</s>
<s>
This	O
is	O
often	O
done	O
using	O
Content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
,	O
where	O
the	O
VCI	O
and	O
or	O
VPI	O
form	O
the	O
addressable	O
content	O
.	O
</s>
<s>
discarded	O
,	O
or	O
reduced	O
in	O
priority	O
so	O
that	O
they	O
may	O
be	O
discarded	O
downstream	O
if	O
there	O
is	O
congestion	B-Protocol
.	O
</s>
<s>
The	O
GCRA	B-Algorithm
,	O
while	O
,	O
possibly	O
,	O
complicated	O
to	O
describe	O
and	O
understand	O
,	O
can	O
be	O
implemented	O
very	O
simply	O
.	O
</s>
<s>
Transmissions	O
onto	O
an	O
AFDX	B-Protocol
network	O
are	O
required	O
to	O
be	O
limited	O
to	O
a	O
Bandwidth	O
Allocation	O
Gap	O
(	O
BAG	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
token	B-Algorithm
bucket	I-Algorithm
algorithm	O
is	O
recommended	O
for	O
AFDX	B-Protocol
,	O
and	O
a	O
version	O
that	O
allows	O
for	O
variable	O
length	O
frames	O
(	O
one	O
that	O
counts	O
bytes	O
)	O
is	O
preferred	O
over	O
one	O
that	O
only	O
counts	O
frames	O
and	O
assumes	O
that	O
all	O
frames	O
are	O
of	O
the	O
maximum	O
permitted	O
length	O
.	O
</s>
