<s>
In	O
telephony	O
,	O
pair	B-Protocol
gain	I-Protocol
is	O
the	O
transmitting	O
of	O
multiple	O
POTS	O
signals	O
over	O
the	O
twisted	O
pairs	O
traditionally	O
used	O
for	O
a	O
single	O
traditional	O
subscriber	B-Architecture
line	I-Architecture
in	O
telephone	O
systems	O
.	O
</s>
<s>
Pair	B-Protocol
gain	I-Protocol
has	O
the	O
effect	O
of	O
creating	O
additional	O
subscriber	B-Architecture
lines	I-Architecture
.	O
</s>
<s>
This	O
is	O
typically	O
used	O
as	O
an	O
expedient	O
way	O
to	O
solve	O
subscriber	B-Architecture
line	I-Architecture
shortage	O
problems	O
by	O
using	O
existing	O
wiring	O
,	O
instead	O
of	O
installing	O
new	O
wires	O
from	O
the	O
central	O
office	O
to	O
the	O
customer	O
premises	O
.	O
</s>
<s>
The	O
term	O
was	O
invented	O
in	O
the	O
middle	O
20th	O
century	O
by	O
analogy	O
with	O
earlier	O
use	O
of	O
gain	O
to	O
extend	O
telephone	O
local	B-Architecture
loops	I-Architecture
far	O
from	O
the	O
telephone	O
exchange	O
.	O
</s>
<s>
A	O
pair	B-Protocol
gain	I-Protocol
system	I-Protocol
consists	O
of	O
concentrators	O
or	O
multiplexers	B-Protocol
which	O
combine	O
the	O
separate	O
signals	O
into	O
a	O
single	O
signal	O
which	O
is	O
transmitted	O
through	O
the	O
existing	O
copper	O
cable	O
pair	O
.	O
</s>
<s>
The	O
signals	O
are	O
then	O
separated	O
into	O
individual	O
subscriber	B-Architecture
lines	I-Architecture
at	O
the	O
customer	O
premises	O
.	O
</s>
<s>
The	O
pair	B-Protocol
gain	I-Protocol
unit	O
which	O
performs	O
the	O
multiplexing	O
can	O
simply	O
provide	O
a	O
second	O
telephone	O
connection	O
over	O
a	O
single	O
subscriber	B-Architecture
line	I-Architecture
(	O
called	O
an	O
Analog	O
Multi-Line	O
Carrier	O
or	O
AML	O
)	O
in	O
circumstances	O
where	O
a	O
customer	O
wants	O
to	O
add	O
a	O
new	O
phone	O
line	O
for	O
a	O
fax	O
machine	O
or	O
dial-up	O
internet	O
connection	O
.	O
</s>
<s>
A	O
larger	O
analog	O
pair	B-Protocol
gain	I-Protocol
system	I-Protocol
made	O
by	O
Anaconda	O
in	O
the	O
1960s	O
provided	O
seven	O
lines	O
.	O
</s>
<s>
Some	O
pair	B-Protocol
gain	I-Protocol
units	O
can	O
expand	O
the	O
number	O
of	O
subscriber	B-Architecture
lines	I-Architecture
available	O
over	O
a	O
single	O
copper	O
pair	O
to	O
as	O
many	O
as	O
sixty	O
.	O
</s>
<s>
Large	O
pair	B-Protocol
gain	I-Protocol
units	O
are	O
stored	O
in	O
serving	O
area	O
interfaces	O
or	O
metal	O
cabinets	O
typically	O
resembling	O
small	O
apartment-sized	O
refrigerators	O
alongside	O
or	O
near	O
roadways	O
that	O
overlie	O
communications	O
rights-of-way	O
.	O
</s>
<s>
DACS	O
(	O
Digital	O
Access	O
Carrier	O
System	O
)	O
is	O
a	O
form	O
of	O
pair	B-Protocol
gain	I-Protocol
used	O
in	O
the	O
United	O
Kingdom	O
.	O
</s>
<s>
It	O
uses	O
a	O
form	O
of	O
time-division	O
multiple	O
access	O
called	O
ISDN	B-Protocol
.	O
</s>
<s>
Analog	O
pair	B-Protocol
gain	I-Protocol
came	O
into	O
disfavor	O
in	O
the	O
21st	O
century	O
,	O
as	O
it	O
is	O
detrimental	O
to	O
high	O
speed	O
dial-up	O
modem	O
connections	O
,	O
does	O
not	O
support	O
56k	O
and	O
is	O
incompatible	O
with	O
digital	O
subscriber	B-Architecture
line	I-Architecture
(	O
DSL	O
)	O
systems	O
.	O
</s>
<s>
20th	O
century	O
Subscriber	B-Application
Loop	I-Application
Carrier	I-Application
systems	O
had	O
similar	O
problems	O
.	O
</s>
<s>
More	O
recent	O
digital	B-Protocol
pair	I-Protocol
gain	I-Protocol
systems	O
,	O
however	O
,	O
restore	O
56k	O
and	O
DSL	O
capabilities	O
by	O
performing	O
the	O
functions	O
of	O
a	O
DSLAM	B-Protocol
at	O
the	O
pair	B-Protocol
gain	I-Protocol
device	O
.	O
</s>
<s>
More	O
recently	O
,	O
the	O
term	O
pair	B-Protocol
gain	I-Protocol
has	O
been	O
used	O
to	O
refer	O
to	O
any	O
multiplex/demultiplex	O
unit	O
used	O
between	O
the	O
central	O
office	O
and	O
end	O
users	O
,	O
not	O
just	O
equipment	O
used	O
with	O
copper	O
twisted	O
pair	O
.	O
</s>
<s>
Where	O
digital	B-Application
loop	I-Application
carrier	I-Application
(	O
called	O
Remote	O
Integrated	O
Multiplexer	B-Protocol
in	O
Australia	O
)	O
is	O
installed	O
,	O
broadband	O
Internet	O
subscribers	O
may	O
be	O
disappointed	O
when	O
informed	O
that	O
although	O
initial	O
line	O
checks	O
were	O
successful	O
,	O
high	O
speed	O
connections	O
such	O
as	O
ADSL	O
are	O
unavailable	O
due	O
to	O
infrastructure	O
issues	O
.	O
</s>
<s>
The	O
problem	O
is	O
that	O
pair	B-Protocol
gain	I-Protocol
lines	O
cannot	O
carry	O
high	O
speed	O
data	O
.	O
</s>
