<s>
A	O
network	B-Device
tap	I-Device
is	O
a	O
system	O
that	O
monitors	O
events	O
on	O
a	O
local	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
A	O
tap	O
is	O
typically	O
a	O
dedicated	O
hardware	O
device	O
,	O
which	O
provides	O
a	O
way	O
to	O
access	O
the	O
data	O
flowing	O
across	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
The	O
network	B-Device
tap	I-Device
has	O
(	O
at	O
least	O
)	O
three	O
ports	O
:	O
an	O
A	O
port	O
,	O
a	O
B	O
port	O
,	O
and	O
a	O
monitor	O
port	O
.	O
</s>
<s>
Network	B-Device
taps	I-Device
are	O
commonly	O
used	O
for	O
network	O
intrusion	O
detection	O
systems	O
,	O
VoIP	B-Protocol
recording	I-Protocol
,	O
network	O
probes	O
,	O
RMON	O
probes	O
,	O
packet	B-Protocol
sniffers	I-Protocol
,	O
and	O
other	O
monitoring	O
and	O
collection	O
devices	O
and	O
software	O
that	O
require	O
access	O
to	O
a	O
network	B-Protocol
segment	I-Protocol
.	O
</s>
<s>
The	O
term	O
network	B-Device
tap	I-Device
is	O
analogous	O
to	O
phone	O
tap	O
or	O
vampire	B-Protocol
tap	I-Protocol
.	O
</s>
<s>
A	O
distributed	B-Device
tap	I-Device
is	O
a	O
set	O
of	O
network	B-Device
taps	I-Device
that	O
report	O
to	O
a	O
centralized	O
monitoring	O
system	O
or	O
packet	B-Protocol
analyzer	I-Protocol
.	O
</s>
<s>
However	O
,	O
it	O
does	B-Architecture
not	I-Architecture
scale	I-Architecture
well	O
to	O
large	O
networks	O
.	O
</s>
<s>
Another	O
way	O
to	O
monitor	O
devices	O
is	O
to	O
use	O
a	O
remote	O
management	O
protocol	O
such	O
as	O
SNMP	B-Protocol
to	O
ask	O
devices	O
about	O
their	O
performance	O
.	O
</s>
<s>
This	O
scales	B-Architecture
well	O
,	O
but	O
is	O
not	O
necessarily	O
appropriate	O
for	O
all	O
types	O
of	O
monitoring	O
.	O
</s>
<s>
The	O
inherent	O
problems	O
with	O
SNMP	B-Protocol
are	O
the	O
polling	O
effect	O
.	O
</s>
<s>
Another	O
method	O
to	O
monitor	O
networks	O
is	O
to	O
use	O
port	B-Protocol
mirroring	I-Protocol
(	O
called	O
"	O
SPAN	O
"	O
,	O
for	O
Switched	B-Protocol
Port	I-Protocol
Analyzer	I-Protocol
,	O
by	O
vendors	O
such	O
as	O
Cisco	O
,	O
and	O
given	O
other	O
names	O
,	O
such	O
MLXe	O
telemetry	O
by	O
Brocade	O
Communications	O
and	O
other	O
vendors	O
)	O
(	O
also	O
known	O
as	O
MIRROR	O
port	O
)	O
or	O
a	O
monitoring	O
protocol	O
such	O
as	O
TZSP	B-Protocol
on	O
routers	B-Protocol
and	O
switches	B-Protocol
.	O
</s>
<s>
This	O
is	O
a	O
low-cost	O
alternative	O
to	O
network	B-Device
taps	I-Device
and	O
solves	O
many	O
of	O
the	O
same	O
problems	O
.	O
</s>
<s>
However	O
,	O
not	O
all	O
routers	B-Protocol
and	O
switches	B-Protocol
support	O
port	B-Protocol
mirroring	I-Protocol
,	O
and	O
,	O
on	O
those	O
that	O
do	O
,	O
using	O
port	B-Protocol
mirroring	I-Protocol
can	O
affect	O
the	O
performance	O
of	O
the	O
router	B-Protocol
or	O
switch	O
.	O
</s>
<s>
These	O
technologies	O
may	O
also	O
be	O
subject	O
to	O
the	O
problem	O
with	O
full-duplex	O
described	O
elsewhere	O
in	O
this	O
article	O
,	O
and	O
there	O
are	O
often	O
limits	O
for	O
the	O
router	B-Protocol
or	O
switch	O
on	O
how	O
many	O
pass-through	O
sessions	O
can	O
be	O
monitored	O
,	O
or	O
how	O
many	O
monitor	O
ports	O
(	O
generally	O
two	O
)	O
can	O
monitor	O
a	O
given	O
session	O
.	O
</s>
<s>
Often	O
,	O
when	O
the	O
SPAN	B-Protocol
port	I-Protocol
is	O
overloaded	O
,	O
packets	O
will	O
be	O
dropped	O
before	O
reaching	O
the	O
monitoring	O
device	O
.	O
</s>
<s>
This	O
tapping	O
method	O
consists	O
in	O
enabling	O
promiscuous	B-Protocol
mode	I-Protocol
on	O
the	O
device	O
that	O
is	O
used	O
for	O
the	O
monitoring	O
and	O
attaching	O
it	O
to	O
a	O
network	O
hub	O
.	O
</s>
<s>
This	O
works	O
well	O
with	O
older	O
LAN	B-General_Concept
technologies	O
such	O
as	O
10BASE2	O
,	O
FDDI	B-Protocol
,	O
and	O
Token	B-Device
Ring	I-Device
.	O
</s>
<s>
On	O
such	O
networks	O
,	O
any	O
host	O
can	O
automatically	O
see	O
what	O
all	O
other	O
hosts	O
were	O
doing	O
by	O
enabling	O
promiscuous	B-Protocol
mode	I-Protocol
.	O
</s>
<s>
However	O
,	O
modern	O
switched	B-Protocol
network	O
technologies	O
create	O
point-to-point	O
links	O
between	O
pairs	O
of	O
devices	O
,	O
making	O
it	O
impossible	O
to	O
tap	O
network	O
traffic	O
with	O
this	O
method	O
.	O
</s>
<s>
This	O
method	O
always	O
passes	O
every	O
packet	O
,	O
even	O
error	O
packets	O
that	O
a	O
SPAN	B-Protocol
port	I-Protocol
may	O
drop	O
,	O
to	O
the	O
monitoring	O
device	O
.	O
</s>
<s>
Therefore	O
,	O
network	B-Device
taps	I-Device
for	O
full-duplex	O
technologies	O
usually	O
have	O
two	O
monitor	O
ports	O
,	O
one	O
for	O
each	O
half	O
of	O
the	O
connection	O
.	O
</s>
<s>
The	O
listener	O
must	O
use	O
channel	B-Architecture
bonding	I-Architecture
or	O
link	B-Architecture
aggregation	I-Architecture
to	O
merge	O
the	O
two	O
connections	O
into	O
one	O
aggregate	O
interface	O
to	O
see	O
both	O
halves	O
of	O
the	O
traffic	O
.	O
</s>
<s>
Other	O
monitoring	O
technologies	O
,	O
such	O
as	O
passive	O
fiber	O
network	B-Device
TAPs	I-Device
do	O
not	O
deal	O
well	O
with	O
the	O
full-duplex	O
traffic	O
.	O
</s>
<s>
Once	O
a	O
network	B-Device
tap	I-Device
is	O
in	O
place	O
,	O
the	O
network	O
can	O
be	O
monitored	O
without	O
interfering	O
with	O
the	O
network	O
itself	O
.	O
</s>
<s>
This	O
scenario	O
is	O
for	O
active	O
,	O
inline	O
security	O
tools	O
,	O
such	O
as	O
next-generation	O
fire	O
walls	O
,	O
intrusion	O
prevention	O
systems	O
and	O
web	B-Protocol
application	I-Protocol
firewalls	I-Protocol
.	O
</s>
<s>
Some	O
taps	O
operate	O
at	O
the	O
physical	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
rather	O
than	O
the	O
data	O
link	O
layer	O
.	O
</s>
<s>
For	O
example	O
,	O
they	O
work	O
with	O
multi-mode	B-Device
fiber	I-Device
rather	O
than	O
1000BASE-SX	O
.	O
</s>
<s>
Network	B-Device
taps	I-Device
that	O
act	O
as	O
simple	O
optical	O
splitters	O
,	O
sometimes	O
called	O
passive	O
taps	O
(	O
although	O
that	O
term	O
is	O
not	O
used	O
consistently	O
)	O
can	O
have	O
this	O
property	O
.	O
</s>
<s>
Some	O
network	B-Device
taps	I-Device
offer	O
both	O
duplication	O
of	O
network	O
traffic	O
for	O
monitoring	O
devices	O
and	O
SNMP	B-Protocol
services	O
.	O
</s>
<s>
Most	O
major	O
network	B-Device
tap	I-Device
manufacturers	O
offer	O
taps	O
with	O
remote	O
management	O
through	O
Telnet	O
,	O
HTTP	O
,	O
or	O
SNMP	B-Protocol
interfaces	O
.	O
</s>
<s>
Such	O
network	B-Device
tap	I-Device
hybrids	O
can	O
be	O
helpful	O
to	O
network	O
managers	O
who	O
wish	O
to	O
view	O
baseline	O
performance	O
statistics	O
without	O
diverting	O
existing	O
tools	O
.	O
</s>
<s>
Alternatively	O
,	O
SNMP	B-Protocol
alarms	O
generated	O
by	O
managed	O
taps	O
can	O
alert	O
network	O
managers	O
to	O
link	O
conditions	O
that	O
merit	O
examination	O
by	O
analyzers	O
to	O
intrusion	O
detection	O
systems	O
.	O
</s>
<s>
Here	O
are	O
some	O
advantages	O
of	O
a	O
network	B-Device
tap	I-Device
over	O
port	B-Protocol
mirroring	I-Protocol
or	O
SPAN	O
:	O
</s>
<s>
Because	O
network	B-Device
taps	I-Device
require	O
additional	O
hardware	O
,	O
they	O
are	O
not	O
as	O
cheap	O
as	O
technologies	O
that	O
use	O
capabilities	O
that	O
are	O
built	O
into	O
the	O
network	O
.	O
</s>
<s>
However	O
,	O
network	B-Device
taps	I-Device
are	O
easier	O
to	O
manage	O
and	O
normally	O
provide	O
more	O
data	O
than	O
some	O
network	O
devices	O
.	O
</s>
<s>
Network	B-Device
taps	I-Device
can	O
require	O
channel	B-Architecture
bonding	I-Architecture
on	O
monitoring	O
devices	O
to	O
get	O
around	O
the	O
problem	O
with	O
full-duplex	O
discussed	O
above	O
.	O
</s>
<s>
Putting	O
a	O
network	B-Device
tap	I-Device
into	O
place	O
will	O
disrupt	O
the	O
network	O
being	O
monitored	O
for	O
a	O
short	O
time	O
.	O
</s>
<s>
Establishing	O
good	O
guidelines	O
for	O
the	O
placement	O
of	O
network	B-Device
taps	I-Device
is	O
recommended	O
.	O
</s>
<s>
Monitoring	O
large	O
networks	O
using	O
network	B-Device
taps	I-Device
can	O
require	O
a	O
lot	O
of	O
monitoring	O
devices	O
.	O
</s>
<s>
High-end	O
networking	O
devices	O
often	O
allow	O
ports	O
to	O
be	O
enabled	O
as	O
mirror	B-Protocol
ports	I-Protocol
,	O
which	O
is	O
a	O
software	O
network	B-Device
tap	I-Device
.	O
</s>
<s>
Even	O
fully	O
passive	O
network	B-Device
taps	I-Device
introduce	O
new	O
points	O
of	O
failure	O
into	O
the	O
network	O
.	O
</s>
<s>
Consider	O
non-powered	O
taps	O
for	O
optical-only	O
environments	O
or	O
throwing	O
star	O
network	B-Device
tap	I-Device
for	O
copper	O
100BASE-TX	O
.	O
</s>
<s>
Countermeasures	O
for	O
network	B-Device
taps	I-Device
include	O
encryption	O
and	O
alarm	O
systems	O
.	O
</s>
<s>
In	O
the	O
alarmed	O
cable	O
scenario	O
,	O
the	O
sensing	O
mechanism	O
uses	O
optical	O
interferometry	O
in	O
which	O
modally	O
dispersive	O
coherent	O
light	O
traveling	O
through	O
the	O
multi-mode	B-Device
fiber	I-Device
mixes	O
at	O
the	O
fiber	O
's	O
terminus	O
,	O
resulting	O
in	O
a	O
characteristic	O
pattern	O
of	O
light	O
and	O
dark	O
splotches	O
called	O
speckle	O
.	O
</s>
<s>
Some	O
layer	B-Application
1	I-Application
information	O
is	O
not	O
transported	O
over	O
a	O
copper	O
tap	O
(	O
e.g.	O
</s>
