<s>
A	O
digital	O
subscriber	B-Architecture
line	I-Architecture
(	O
DSL	O
)	O
modem	B-Application
is	O
a	O
device	O
used	O
to	O
connect	O
a	O
computer	O
or	O
router	B-Protocol
to	O
a	O
telephone	O
line	O
which	O
provides	O
the	O
digital	O
subscriber	B-Architecture
line	I-Architecture
(	O
DSL	O
)	O
service	O
for	O
connection	O
to	O
the	O
Internet	O
,	O
which	O
is	O
often	O
called	O
DSL	O
broadband	O
.	O
</s>
<s>
The	O
modem	B-Application
connects	O
to	O
a	O
single	O
computer	O
or	O
router	B-Protocol
,	O
through	O
an	O
Ethernet	O
port	O
,	O
USB	B-Protocol
port	O
,	O
or	O
is	O
installed	O
in	O
a	O
computer	O
PCI	B-Protocol
slot	I-Protocol
.	O
</s>
<s>
The	O
more	O
common	O
DSL	B-Application
router	I-Application
is	O
a	O
standalone	O
device	O
that	O
combines	O
the	O
function	O
of	O
a	O
DSL	B-Device
modem	I-Device
and	O
a	O
router	B-Protocol
,	O
and	O
can	O
connect	O
multiple	O
computers	O
through	O
multiple	O
Ethernet	O
ports	O
or	O
an	O
integral	O
wireless	B-Device
access	I-Device
point	I-Device
.	O
</s>
<s>
Also	O
called	O
a	O
residential	B-Application
gateway	I-Application
,	O
a	O
DSL	B-Application
router	I-Application
usually	O
manages	O
the	O
connection	O
and	O
sharing	O
of	O
the	O
DSL	O
service	O
in	O
a	O
home	O
or	O
small	O
office	O
network	O
.	O
</s>
<s>
Different	O
DSL	B-Application
routers	I-Application
and	O
modems	B-Application
support	O
different	O
DSL	O
technology	O
variants	O
:	O
VDSL	O
,	O
SDSL	O
,	O
and	O
ADSL	O
.	O
</s>
<s>
A	O
DSL	B-Application
router	I-Application
consists	O
of	O
a	O
box	O
with	O
an	O
RJ11	O
jack	O
to	O
connect	O
to	O
a	O
standard	O
subscriber	O
telephone	O
line	O
.	O
</s>
<s>
It	O
has	O
several	O
RJ45	O
jacks	O
for	O
Ethernet	O
cables	O
to	O
connect	O
it	O
to	O
computers	O
or	O
printers	O
,	O
creating	O
a	O
local	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
It	O
usually	O
also	O
has	O
a	O
USB	B-Protocol
jack	O
which	O
can	O
be	O
used	O
to	O
connect	O
to	O
computers	O
via	O
a	O
USB	B-Protocol
cable	O
,	O
to	O
allow	O
connection	O
to	O
computers	O
without	O
an	O
Ethernet	O
port	O
.	O
</s>
<s>
A	O
wireless	O
DSL	B-Application
router	I-Application
also	O
has	O
antennas	O
to	O
allow	O
it	O
to	O
act	O
as	O
a	O
wireless	B-Device
access	I-Device
point	I-Device
,	O
so	O
computers	O
can	O
connect	O
to	O
it	O
forming	O
a	O
wireless	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
Many	O
routers	B-Protocol
provide	O
an	O
internal	O
web	O
page	O
to	O
the	O
local	B-General_Concept
network	I-General_Concept
for	O
device	O
configuration	O
and	O
status	O
reporting	O
.	O
</s>
<s>
Most	O
DSL	B-Application
routers	I-Application
are	O
designed	O
to	O
be	O
installed	O
by	O
the	O
customer	O
for	O
which	O
a	O
CD	O
or	O
DVD	O
containing	O
an	O
installation	O
program	O
is	O
supplied	O
.	O
</s>
<s>
Upon	O
powering	O
the	O
router	B-Protocol
it	O
may	O
take	O
several	O
minutes	O
for	O
the	O
local	B-General_Concept
network	I-General_Concept
and	O
DSL	O
link	O
to	O
initialize	O
,	O
usually	O
indicated	O
by	O
the	O
status	O
lights	O
turning	O
green	O
.	O
</s>
<s>
There	O
are	O
also	O
PCI	B-Protocol
DSL	B-Device
modems	I-Device
,	O
which	O
plug	O
into	O
an	O
available	O
PCI	B-Protocol
card	I-Protocol
slot	O
on	O
a	O
computer	O
.	O
</s>
<s>
The	O
public	O
switched	O
telephone	O
network	O
,	O
the	O
network	O
of	O
switching	O
centers	O
,	O
trunk	O
lines	O
,	O
amplifiers	O
and	O
switches	O
which	O
transmits	O
telephone	O
calls	O
from	O
one	O
phone	O
to	O
another	O
,	O
is	O
designed	O
to	O
carry	O
voice	O
frequency	O
signals	O
,	O
and	O
is	O
therefore	O
limited	O
to	O
a	O
bandwidth	B-Algorithm
of	O
3.4kHz	O
.	O
</s>
<s>
Before	O
DSL	O
,	O
voice-band	O
modems	B-Application
transmitted	O
information	O
through	O
the	O
telephone	O
network	O
with	O
audio	O
frequencies	O
within	O
that	O
bandwidth	B-Algorithm
,	O
which	O
limited	O
them	O
to	O
a	O
data	O
rate	O
of	O
about	O
56	O
kbit/s	O
.	O
</s>
<s>
However	O
,	O
the	O
copper	O
wires	O
that	O
connect	O
telephones	O
with	O
the	O
local	O
switching	O
center	O
(	O
telephone	O
exchange	O
)	O
,	O
called	O
the	O
subscriber	B-Architecture
loop	I-Architecture
,	O
are	O
actually	O
able	O
to	O
carry	O
a	O
much	O
wider	O
band	O
of	O
frequencies	O
,	O
up	O
to	O
several	O
megahertz	O
.	O
</s>
<s>
DSL	O
uses	O
these	O
higher	O
frequencies	O
to	O
send	O
digital	B-General_Concept
data	I-General_Concept
between	O
the	O
DSL	B-Device
modem	I-Device
and	O
the	O
local	O
switching	O
center	O
,	O
without	O
interfering	O
with	O
normal	O
telephone	O
service	O
.	O
</s>
<s>
It	O
is	O
not	O
necessary	O
to	O
dial	O
a	O
telephone	O
number	O
to	O
initiate	O
a	O
connection	O
;	O
the	O
DSL	O
connection	O
is	O
"	O
on	O
"	O
whenever	O
the	O
modem	B-Application
is	O
on	O
.	O
</s>
<s>
The	O
device	O
at	O
the	O
local	O
switching	O
center	O
which	O
communicates	O
with	O
the	O
DSL	B-Device
modem	I-Device
is	O
called	O
a	O
Digital	B-Protocol
Subscriber	I-Protocol
Line	I-Protocol
Access	I-Protocol
Multiplexer	I-Protocol
(	O
DSLAM	B-Protocol
)	O
,	O
which	O
is	O
connected	O
directly	O
to	O
the	O
Internet	O
.	O
</s>
<s>
With	O
ADSL	O
,	O
the	O
modem	B-Application
and	O
the	O
DSLAM	B-Protocol
communicate	O
by	O
a	O
protocol	O
called	O
discrete	B-Protocol
multitone	I-Protocol
modulation	I-Protocol
(	O
DMT	O
)	O
,	O
which	O
is	O
a	O
form	O
of	O
frequency	B-Protocol
division	I-Protocol
multiplexing	I-Protocol
.	O
</s>
<s>
The	O
modem	B-Application
only	O
uses	O
frequencies	O
above	O
8kHz	O
,	O
to	O
avoid	O
interfering	O
with	O
normal	O
phone	O
service	O
.	O
</s>
<s>
The	O
bandwidth	B-Algorithm
of	O
the	O
line	O
between	O
8kHz	O
and	O
about	O
1MHz	O
is	O
divided	O
into	O
247	O
separate	O
channels	O
,	O
each	O
4kHz	O
wide	O
.	O
</s>
<s>
Thus	O
the	O
system	O
acts	O
like	O
247	O
separate	O
modems	B-Application
operating	O
simultaneously	O
.	O
</s>
<s>
The	O
bits	O
of	O
the	O
incoming	O
digital	B-General_Concept
data	I-General_Concept
are	O
split	O
up	O
and	O
sent	O
in	O
parallel	O
over	O
the	O
channels	O
.	O
</s>
<s>
Each	O
data	O
stream	O
is	O
sent	O
using	O
an	O
error-correcting	B-Error_Name
code	I-Error_Name
to	O
allow	O
minor	O
bit	O
errors	O
due	O
to	O
noise	O
to	O
be	O
corrected	O
at	O
the	O
receiving	O
end	O
.	O
</s>
<s>
Most	O
of	O
the	O
channels	O
are	O
unidirectional	O
,	O
carrying	O
download	O
data	O
from	O
the	O
DSLAM	B-Protocol
to	O
the	O
modem	B-Application
,	O
but	O
some	O
on	O
the	O
low	O
frequency	O
end	O
are	O
bidirectional	O
,	O
to	O
carry	O
the	O
smaller	O
quantity	O
of	O
upload	O
traffic	O
.	O
</s>
<s>
The	O
modem	B-Application
constantly	O
monitors	O
the	O
transmission	O
quality	O
on	O
each	O
channel	O
,	O
and	O
if	O
it	O
is	O
too	O
impaired	O
it	O
will	O
shift	O
the	O
signal	O
to	O
other	O
channels	O
.	O
</s>
<s>
The	O
modem	B-Application
is	O
constantly	O
shifting	O
data	O
between	O
channels	O
searching	O
for	O
the	O
best	O
transmission	O
rates	O
.	O
</s>
<s>
Thus	O
interference	O
or	O
poor	O
quality	O
lines	O
will	O
generally	O
not	O
interrupt	O
transmission	O
,	O
but	O
only	O
cause	O
the	O
data	O
rate	O
of	O
the	O
modem	B-Application
to	O
degrade	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
downloading	O
a	O
web	O
page	O
,	O
the	O
packets	O
of	O
web	O
page	O
data	O
travel	O
over	O
optical	B-Architecture
fiber	I-Architecture
Internet	O
lines	O
from	O
the	O
server	B-Application
computer	I-Application
directly	O
to	O
the	O
DSLAM	B-Protocol
at	O
the	O
neighborhood	O
telephone	O
exchange	O
.	O
</s>
<s>
At	O
the	O
DSLAM	B-Protocol
they	O
are	O
split	O
into	O
as	O
many	O
as	O
247	O
parallel	O
data	O
streams	O
.	O
</s>
<s>
Each	O
is	O
modulated	B-Application
onto	O
a	O
separate	O
carrier	O
signal	O
and	O
sent	O
through	O
a	O
separate	O
frequency	O
channel	O
over	O
the	O
subscriber	O
's	O
telephone	O
line	O
to	O
the	O
DSL	B-Device
modem	I-Device
.	O
</s>
<s>
The	O
modem	B-Application
demodulates	B-Device
the	O
carrier	O
,	O
extracting	O
the	O
data	O
stream	O
from	O
each	O
carrier	O
signal	O
,	O
performs	O
error	B-Error_Name
correction	I-Error_Name
,	O
puts	O
the	O
data	O
together	O
again	O
in	O
the	O
proper	O
order	O
,	O
and	O
sends	O
it	O
to	O
the	O
computer	O
over	O
the	O
Ethernet	O
line	O
,	O
or	O
for	O
a	O
wireless	O
(	O
Wi-Fi	O
)	O
network	O
by	O
radio	O
signals	O
.	O
</s>
<s>
The	O
"	O
asymmetric	O
"	O
means	O
that	O
more	O
of	O
the	O
bandwidth	B-Algorithm
of	O
the	O
line	O
is	O
dedicated	O
to	O
downstream	O
(	O
download	O
)	O
data	O
than	O
upstream	O
(	O
upload	O
)	O
data	O
,	O
so	O
,	O
download	O
rates	O
are	O
faster	O
than	O
upload	O
rates	O
,	O
because	O
most	O
users	O
download	O
much	O
larger	O
quantities	O
of	O
data	O
than	O
they	O
upload	O
.	O
</s>
<s>
The	O
farther	O
away	O
from	O
the	O
switching	O
center	O
the	O
modem	B-Application
is	O
,	O
the	O
longer	O
the	O
telephone	O
wires	O
,	O
the	O
weaker	O
the	O
signal	O
,	O
and	O
the	O
lower	O
the	O
data	O
rate	O
that	O
the	O
modem	B-Application
can	O
achieve	O
.	O
</s>
<s>
However	O
,	O
other	O
devices	O
installed	O
in	O
telephone	O
lines	O
by	O
the	O
phone	O
company	O
,	O
such	O
as	O
loading	O
coils	O
and	O
bridge	B-Architecture
taps	I-Architecture
,	O
block	O
the	O
signal	O
,	O
and	O
may	O
disqualify	O
a	O
given	O
phone	O
line	O
from	O
DSL	O
service	O
.	O
</s>
<s>
To	O
prevent	O
the	O
DSL	O
signal	O
from	O
entering	O
the	O
phone	O
lines	O
into	O
telephones	O
,	O
answering	O
machines	O
,	O
faxes	O
and	O
other	O
devices	O
where	O
it	O
could	O
cause	O
interference	O
,	O
DSL	B-Device
modems	I-Device
come	O
with	O
low	B-Algorithm
pass	I-Algorithm
filters	I-Algorithm
which	O
must	O
be	O
plugged	O
into	O
the	O
phone	O
lines	O
going	O
to	O
all	O
voiceband	O
devices	O
on	O
the	O
same	O
line	O
.	O
</s>
<s>
The	O
filter	B-Architecture
blocks	O
all	O
frequencies	O
above	O
4kHz	O
,	O
so	O
it	O
blocks	O
the	O
DSL	O
signal	O
while	O
allowing	O
voice	O
frequency	O
signals	O
through	O
.	O
</s>
<s>
A	O
filter	B-Architecture
must	O
not	O
be	O
inserted	O
in	O
the	O
phone	O
line	O
going	O
to	O
the	O
DSL	B-Device
modem	I-Device
,	O
because	O
it	O
would	O
block	O
the	O
communication	O
between	O
the	O
modem	B-Application
and	O
switching	O
center	O
.	O
</s>
<s>
A	O
DSL	B-Device
modem	I-Device
modulates	O
high-frequency	O
tones	O
for	O
transmission	O
to	O
a	O
digital	B-Protocol
subscriber	I-Protocol
line	I-Protocol
access	I-Protocol
multiplexer	I-Protocol
(	O
DSLAM	B-Protocol
)	O
,	O
and	O
receives	O
and	O
demodulates	B-Device
them	O
from	O
the	O
DSLAM	B-Protocol
.	O
</s>
<s>
It	O
serves	O
fundamentally	O
the	O
same	O
purpose	O
as	O
the	O
voice-band	B-Application
modem	I-Application
that	O
was	O
a	O
mainstay	O
in	O
the	O
late	O
20th	O
century	O
,	O
but	O
differs	O
from	O
it	O
in	O
important	O
ways	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
transfer	O
data	O
at	O
a	O
rate	O
which	O
is	O
at	O
least	O
10	O
to	O
20	O
times	O
that	O
of	O
a	O
voice-band	B-Application
modem	I-Application
.	O
</s>
<s>
A	O
voice-band	B-Application
modem	I-Application
dials	O
a	O
telephone	O
number	O
to	O
initiate	O
a	O
connection	O
,	O
and	O
while	O
it	O
is	O
connected	O
the	O
telephone	O
line	O
cannot	O
be	O
used	O
for	O
normal	O
telephone	O
service	O
.	O
</s>
<s>
DSL	B-Application
routers	I-Application
,	O
the	O
most	O
common	O
form	O
of	O
DSL	B-Device
modem	I-Device
,	O
are	O
external	O
to	O
the	O
computer	O
and	O
wired	O
to	O
the	O
computer	O
's	O
Ethernet	O
port	O
or	O
its	O
USB	B-Protocol
port	O
,	O
whereas	O
voice-band	O
modems	B-Application
are	O
usually	O
internal	O
devices	O
installed	O
in	O
the	O
computer	O
itself	O
in	O
a	O
PCI	B-Protocol
interface	I-Protocol
slot	O
in	O
the	O
back	O
.	O
</s>
<s>
Internal	O
DSL	B-Device
modems	I-Device
are	O
rare	O
but	O
available	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
and	O
other	O
operating	B-General_Concept
systems	I-General_Concept
regard	O
voice-band	O
modems	B-Application
as	O
part	O
of	O
the	O
hardware	O
of	O
the	O
computer	O
,	O
and	O
similarly	O
to	O
other	O
parts	O
of	O
the	O
computer	O
's	O
hardware	O
such	O
as	O
the	O
mouse	B-Device
or	O
hard	B-Device
disk	I-Device
are	O
configured	O
through	O
the	O
Windows	O
Control	O
Panel	O
.	O
</s>
<s>
In	O
contrast	O
,	O
DSL	B-Application
routers	I-Application
are	O
regarded	O
as	O
separate	O
nodes	O
in	O
the	O
LAN	B-General_Concept
(	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
)	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
rarely	O
require	O
manual	O
configuration	O
or	O
attention	O
,	O
but	O
when	O
they	O
do	O
they	O
can	O
be	O
accessed	O
using	O
the	O
internet	B-Application
browser	I-Application
.	O
</s>
<s>
Routers	B-Protocol
usually	O
have	O
a	O
webpage	O
,	O
accessed	O
by	O
typing	O
an	O
IP	B-Protocol
address	I-Protocol
given	O
in	O
the	O
router	B-Protocol
's	O
manual	O
into	O
the	O
browser	B-Application
's	O
address	O
bar	O
,	O
with	O
which	O
various	O
technical	O
changes	O
can	O
be	O
made	O
,	O
such	O
as	O
changing	O
the	O
wireless	B-General_Concept
network	I-General_Concept
's	O
password	O
,	O
and	O
adjusting	O
the	O
router	B-Protocol
's	O
firewall	B-Application
.	O
</s>
<s>
For	O
external	O
DSL	B-Device
modems	I-Device
connected	O
by	O
USB	B-Protocol
,	O
Microsoft	B-Application
Windows	I-Application
and	O
other	O
operating	B-General_Concept
systems	I-General_Concept
generally	O
recognize	O
these	O
as	O
a	O
Network	B-Protocol
interface	I-Protocol
controller	I-Protocol
.	O
</s>
<s>
For	O
internal	O
DSL	B-Device
modems	I-Device
,	O
Microsoft	B-Application
Windows	I-Application
and	O
other	O
operating	B-General_Concept
systems	I-General_Concept
provide	O
interfaces	O
similar	O
to	O
those	O
provided	O
for	O
voice-band	O
modems	B-Application
.	O
</s>
<s>
This	O
is	O
based	O
on	O
the	O
assumption	O
that	O
in	O
the	O
future	O
,	O
as	O
CPU	O
speeds	O
increase	O
,	O
internal	O
DSL	B-Device
modems	I-Device
may	O
become	O
more	O
mainstream	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
use	O
frequencies	O
from	O
25kHz	O
to	O
above	O
1MHz	O
(	O
see	O
Asymmetric	O
Digital	O
Subscriber	B-Architecture
Line	I-Architecture
)	O
,	O
in	O
order	O
not	O
to	O
interfere	O
with	O
voice	O
service	O
which	O
is	O
primarily	O
0	O
–	O
4kHz	O
.	O
</s>
<s>
Voice-band	O
modems	B-Application
use	O
the	O
same	O
frequency	O
spectrum	O
as	O
ordinary	O
telephones	O
,	O
and	O
will	O
interfere	O
with	O
voice	O
service	O
-	O
it	O
is	O
usually	O
impossible	O
to	O
make	O
a	O
telephone	O
call	O
on	O
a	O
line	O
which	O
is	O
being	O
used	O
by	O
a	O
voice-band	B-Application
modem	I-Application
.	O
</s>
<s>
Because	O
a	O
single	O
phone	O
line	O
commonly	O
carries	O
DSL	O
and	O
voice	O
,	O
DSL	B-Algorithm
filters	I-Algorithm
are	O
used	O
to	O
separate	O
the	O
two	O
uses	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
vary	O
in	O
data	O
speed	O
from	O
hundreds	O
of	O
kilobits	O
per	O
second	O
to	O
many	O
megabits	O
,	O
while	O
voice-band	O
modems	B-Application
are	O
nominally	O
56K	O
modems	B-Application
and	O
actually	O
limited	O
to	O
approximately	O
50	O
kbit/s	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
exchange	O
data	O
with	O
only	O
the	O
DSLAM	B-Protocol
to	O
which	O
they	O
are	O
wired	O
,	O
which	O
in	O
turn	O
connects	O
them	O
to	O
the	O
Internet	O
,	O
while	O
most	O
voice-band	O
modems	B-Application
can	O
dial	O
directly	O
anywhere	O
in	O
the	O
world	O
.	O
</s>
<s>
DSL	B-Device
modems	I-Device
are	O
intended	O
for	O
particular	O
protocols	O
and	O
sometimes	O
wo	O
n't	O
work	O
on	O
another	O
line	O
even	O
from	O
the	O
same	O
company	O
,	O
while	O
most	O
voice-band	O
modems	B-Application
use	O
international	O
standards	O
and	O
can	O
"	O
fall	O
back	O
"	O
to	O
find	O
a	O
standard	O
that	O
will	O
work	O
.	O
</s>
<s>
A	O
DSL	B-Device
modem	I-Device
requires	O
the	O
following	O
for	O
its	O
operation	O
;	O
exactly	O
what	O
is	O
on	O
the	O
circuit	O
card	O
and	O
how	O
it	O
is	O
arranged	O
can	O
change	O
as	O
technology	O
improves	O
:	O
</s>
<s>
Apart	O
from	O
connecting	O
to	O
a	O
DSL	O
service	O
,	O
many	O
modems	B-Application
offer	O
additional	O
integrated	O
features	O
,	O
forming	O
a	O
residential	B-Application
gateway	I-Application
:	O
</s>
<s>
Router	B-Protocol
functionality	O
that	O
includes	O
Network	B-Protocol
Address	I-Protocol
Translation	I-Protocol
(	O
NAT	O
)	O
to	O
share	O
a	O
single	O
IPv4	B-Protocol
address	O
.	O
</s>
