<s>
An	O
anomaly-based	B-Protocol
intrusion	I-Protocol
detection	I-Protocol
system	I-Protocol
,	O
is	O
an	O
intrusion	O
detection	O
system	O
for	O
detecting	O
both	O
network	O
and	O
computer	O
intrusions	O
and	O
misuse	O
by	O
monitoring	O
system	O
activity	O
and	O
classifying	O
it	O
as	O
either	O
normal	O
or	O
anomalous	O
.	O
</s>
<s>
The	O
classification	O
is	O
based	O
on	O
heuristics	B-Algorithm
or	O
rules	O
,	O
rather	O
than	O
patterns	O
or	O
signatures	O
,	O
and	O
attempts	O
to	O
detect	O
any	O
type	O
of	O
misuse	O
that	O
falls	O
out	O
of	O
normal	O
system	O
operation	O
.	O
</s>
<s>
Anomalies	O
are	O
detected	O
in	O
several	O
ways	O
,	O
most	O
often	O
with	O
artificial	B-Application
intelligence	I-Application
type	O
techniques	O
.	O
</s>
<s>
Systems	O
using	O
artificial	O
neural	B-Architecture
networks	I-Architecture
have	O
been	O
used	O
to	O
great	O
effect	O
.	O
</s>
<s>
Other	O
techniques	O
used	O
to	O
detect	O
anomalies	O
include	O
data	B-Application
mining	I-Application
methods	O
,	O
grammar	O
based	O
methods	O
,	O
and	O
Artificial	B-Algorithm
Immune	I-Algorithm
System	I-Algorithm
.	O
</s>
<s>
Network-based	O
anomalous	O
intrusion	O
detection	O
systems	O
often	O
provide	O
a	O
second	O
line	O
of	O
defense	O
to	O
detect	O
anomalous	O
traffic	O
at	O
the	O
physical	O
and	O
network	O
layers	O
after	O
it	O
has	O
passed	O
through	O
a	O
firewall	B-Application
or	O
other	O
security	O
appliance	O
on	O
the	O
border	O
of	O
a	O
network	O
.	O
</s>
