<s>
Class	B-Device
of	I-Device
service	I-Device
(	O
COS	O
or	O
CoS	O
)	O
is	O
a	O
parameter	O
used	O
in	O
data	O
and	O
voice	O
protocols	O
to	O
differentiate	O
the	O
types	O
of	O
payloads	O
contained	O
in	O
the	O
packet	B-Protocol
being	O
transmitted	O
.	O
</s>
<s>
COS	O
operates	O
only	O
on	O
802.1Q	O
VLAN	O
Ethernet	O
at	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
layer	B-Protocol
2	I-Protocol
)	O
,	O
while	O
other	O
QoS	O
mechanisms	O
(	O
such	O
as	O
DiffServ	O
,	O
also	O
known	O
as	O
DSCP	O
)	O
operate	O
at	O
the	O
IP	O
network	B-Application
layer	I-Application
(	O
layer	B-Application
3	I-Application
)	O
or	O
use	O
a	O
local	O
QoS	O
tagging	O
system	O
that	O
does	O
not	O
modify	O
the	O
actual	O
packet	B-Protocol
,	O
such	O
as	O
Cisco	O
's	O
"	O
QoS-Group	O
"	O
.	O
</s>
<s>
routers	O
,	O
switches	B-Protocol
,	O
etc	O
.	O
)	O
</s>
<s>
can	O
be	O
configured	O
to	O
use	O
existing	O
COS	O
values	O
on	O
incoming	O
packets	B-Protocol
from	O
other	O
devices	O
(	O
trust	O
mode	O
)	O
,	O
or	O
can	O
rewrite	O
the	O
COS	O
value	O
to	O
something	O
completely	O
different	O
.	O
</s>
<s>
As	O
related	O
to	O
legacy	O
telephone	O
systems	O
,	O
COS	O
is	O
often	O
used	O
to	O
define	O
the	O
permissions	O
an	O
extension	O
will	O
have	O
on	O
a	O
PBX	B-Device
or	O
Centrex	B-Device
.	O
</s>
<s>
The	O
term	O
can	O
be	O
used	O
generically	O
to	O
refer	O
to	O
the	O
classification	O
of	O
network	O
traffic	O
within	O
network	O
equipment	O
based	O
on	O
packet	B-Protocol
inspection	O
.	O
</s>
<s>
Cisco	O
implements	O
such	O
classification	O
through	O
either	O
access-lists	O
or	O
Network-Based	B-Protocol
Application	I-Protocol
Recognition	I-Protocol
(	O
NBAR	O
)	O
.	O
</s>
