<s>
The	O
term	O
is	O
also	O
used	O
frequently	O
to	O
refer	O
to	O
computer	O
hardware	O
,	O
software	O
,	O
and	O
computer	B-Architecture
network	I-Architecture
systems	O
,	O
that	O
perform	O
functions	O
traditionally	O
performed	O
by	O
telephone	O
equipment	O
.	O
</s>
<s>
In	O
this	O
context	O
the	O
technology	O
is	O
specifically	O
referred	O
to	O
as	O
Internet	B-Application
telephony	I-Application
,	O
or	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
.	O
</s>
<s>
Each	O
telephone	O
was	O
connected	O
to	O
the	O
exchange	O
at	O
first	O
with	O
one	O
wire	O
,	O
later	O
one	O
wire	O
pair	O
,	O
the	O
local	B-Architecture
loop	I-Architecture
.	O
</s>
<s>
Later	O
,	O
conversion	O
to	O
installation	O
of	O
jacks	O
that	O
terminated	O
the	O
inside	B-Protocol
wiring	I-Protocol
permitted	O
simple	O
exchange	O
of	O
telephone	O
sets	O
with	O
telephone	B-Protocol
plugs	I-Protocol
and	O
allowed	O
portability	O
of	O
the	O
set	O
to	O
multiple	O
locations	O
in	O
the	O
premises	O
where	O
jacks	O
were	O
installed	O
.	O
</s>
<s>
The	O
inside	B-Protocol
wiring	I-Protocol
to	O
all	O
jacks	O
was	O
connected	O
in	O
one	O
place	O
to	O
the	O
wire	O
drop	O
which	O
connects	O
the	O
building	O
to	O
a	O
cable	O
.	O
</s>
<s>
Most	O
of	O
the	O
exchanges	O
in	O
the	O
world	O
are	O
interconnected	O
through	O
a	O
system	O
of	O
larger	O
switching	B-Device
systems	I-Device
,	O
forming	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
In	O
the	O
second	O
half	O
of	O
the	O
20th	O
century	O
,	O
fax	O
and	O
data	O
became	O
important	O
secondary	O
applications	O
of	O
the	O
network	O
created	O
to	O
carry	O
voices	O
,	O
and	O
late	O
in	O
the	O
century	O
,	O
parts	O
of	O
the	O
network	O
were	O
upgraded	O
with	O
ISDN	B-Protocol
and	O
DSL	O
to	O
improve	O
handling	O
of	O
such	O
traffic	O
.	O
</s>
<s>
Today	O
,	O
telephony	O
uses	O
digital	O
technology	O
(	O
digital	B-General_Concept
telephony	I-General_Concept
)	O
in	O
the	O
provisioning	O
of	O
telephone	O
services	O
and	O
systems	O
.	O
</s>
<s>
Telephone	O
calls	O
can	O
be	O
provided	O
digitally	O
,	O
but	O
may	O
be	O
restricted	O
to	O
cases	O
in	O
which	O
the	O
last	B-Protocol
mile	I-Protocol
is	O
digital	O
,	O
or	O
where	O
the	O
conversion	O
between	O
digital	O
and	O
analog	O
signals	O
takes	O
place	O
inside	O
the	O
telephone	O
.	O
</s>
<s>
The	O
first	O
implementation	O
of	O
this	O
,	O
ISDN	B-Protocol
,	O
permitted	O
all	O
data	O
transport	O
from	O
end-to-end	O
speedily	O
over	O
telephone	O
lines	O
.	O
</s>
<s>
This	O
service	O
was	O
later	O
made	O
much	O
less	O
important	O
due	O
to	O
the	O
ability	O
to	O
provide	O
digital	O
services	O
based	O
on	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
integration	O
of	O
telephony	O
software	O
and	O
computer	O
systems	O
is	O
a	O
major	O
development	O
in	O
the	O
evolution	O
of	O
office	O
automation	B-Application
.	O
</s>
<s>
Digital	B-General_Concept
telephony	I-General_Concept
is	O
the	O
use	O
of	O
digital	O
electronics	O
in	O
the	O
operation	O
and	O
provisioning	O
of	O
telephony	O
systems	O
and	O
services	O
.	O
</s>
<s>
Since	O
the	O
late	O
20th	O
century	O
,	O
a	O
digital	O
core	B-General_Concept
network	I-General_Concept
has	O
replaced	O
the	O
traditional	O
analog	B-Protocol
transmission	I-Protocol
and	O
signaling	O
systems	O
,	O
and	O
much	O
of	O
the	O
access	O
network	O
has	O
also	O
been	O
digitized	O
.	O
</s>
<s>
Starting	O
with	O
the	O
development	O
of	O
transistor	B-Application
technology	O
,	O
originating	O
from	O
Bell	O
Telephone	O
Laboratories	O
in	O
1947	O
,	O
to	O
amplification	O
and	O
switching	O
circuits	O
in	O
the	O
1950s	O
,	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
has	O
gradually	O
moved	O
towards	O
solid-state	O
electronics	O
and	O
automation	B-Application
.	O
</s>
<s>
Following	O
the	O
development	O
of	O
computer-based	O
electronic	B-Device
switching	I-Device
systems	I-Device
incorporating	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	O
)	O
and	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
technologies	O
,	O
the	O
PSTN	O
gradually	O
evolved	O
towards	O
the	O
digitization	O
of	O
signaling	O
and	O
audio	O
transmissions	O
.	O
</s>
<s>
Digital	B-General_Concept
telephony	I-General_Concept
has	O
since	O
dramatically	O
improved	O
the	O
capacity	O
,	O
quality	O
and	O
cost	O
of	O
the	O
network	O
.	O
</s>
<s>
Digitization	O
allows	O
wideband	B-Algorithm
voice	I-Algorithm
on	O
the	O
same	O
channel	O
,	O
with	O
improved	O
quality	O
of	O
a	O
wider	O
analog	O
voice	O
channel	O
.	O
</s>
<s>
The	O
earliest	O
end-to-end	O
analog	O
telephone	O
networks	O
to	O
be	O
modified	O
and	O
upgraded	O
to	O
transmission	O
networks	O
with	O
Digital	B-Protocol
Signal	I-Protocol
1	I-Protocol
(	O
DS1/T1	O
)	O
carrier	O
systems	O
date	O
back	O
to	O
the	O
early	O
1960s	O
.	O
</s>
<s>
They	O
were	O
designed	O
to	O
support	O
the	O
basic	O
3kHz	O
voice	O
channel	O
by	O
sampling	O
the	O
bandwidth-limited	O
analog	O
voice	O
signal	O
and	O
encoding	O
using	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
.	O
</s>
<s>
Early	O
PCM	B-Algorithm
codec-filters	O
were	O
implemented	O
as	O
passive	O
resistorcapacitorinductor	O
filter	O
circuits	O
,	O
with	O
analog-to-digital	O
conversion	O
(	O
for	O
digitizing	O
voices	O
)	O
and	O
digital-to-analog	O
conversion	O
(	O
for	O
reconstructing	O
voices	O
)	O
handled	O
by	O
discrete	O
devices	O
.	O
</s>
<s>
Early	O
digital	B-General_Concept
telephony	I-General_Concept
was	O
impractical	O
due	O
to	O
the	O
low	O
performance	O
and	O
high	O
costs	O
of	O
early	O
PCM	B-Algorithm
codec-filters	O
.	O
</s>
<s>
Practical	O
digital	O
telecommunication	O
was	O
enabled	O
by	O
the	O
invention	O
of	O
the	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
field-effect	I-Architecture
transistor	I-Architecture
(	O
MOSFET	B-Architecture
)	O
,	O
which	O
led	O
to	O
the	O
rapid	O
development	O
and	O
wide	O
adoption	O
of	O
PCM	B-Algorithm
digital	B-General_Concept
telephony	I-General_Concept
.	O
</s>
<s>
The	O
MOSFET	B-Architecture
was	O
invented	O
by	O
Mohamed	O
M	O
.	O
Atalla	O
and	O
Dawon	O
Kahng	O
at	O
Bell	O
Telephone	O
Laboratories	O
in	O
1959	O
,	O
and	O
the	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	O
)	O
integrated	O
circuit	O
(	O
IC	O
)	O
chip	O
was	O
proposed	O
soon	O
after	O
,	O
but	O
MOS	B-Architecture
technology	I-Architecture
was	O
initially	O
overlooked	O
by	O
Bell	O
because	O
they	O
did	O
not	O
find	O
it	O
practical	O
for	O
analog	O
telephone	O
applications	O
,	O
before	O
it	O
was	O
commercialized	O
by	O
Fairchild	O
and	O
RCA	O
for	O
digital	O
electronics	O
such	O
as	O
computers	O
.	O
</s>
<s>
MOS	B-Architecture
technology	I-Architecture
eventually	O
became	O
practical	O
for	O
telephone	O
applications	O
with	O
the	O
MOS	O
mixed-signal	O
integrated	O
circuit	O
,	O
which	O
combines	O
analog	O
and	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
on	O
a	O
single	O
chip	O
,	O
developed	O
by	O
former	O
Bell	O
engineer	O
David	O
A	O
.	O
Hodges	O
with	O
Paul	O
R	O
.	O
Gray	O
at	O
UC	O
Berkeley	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
Suarez	O
to	O
develop	O
MOS	O
switched	B-Algorithm
capacitor	I-Algorithm
(	O
SC	O
)	O
circuit	O
technology	O
,	O
which	O
they	O
used	O
to	O
develop	O
a	O
digital-to-analog	O
converter	O
(	O
DAC	O
)	O
chip	O
,	O
using	O
MOS	O
capacitors	O
and	O
MOSFET	B-Architecture
switches	O
for	O
data	O
conversion	O
.	O
</s>
<s>
MOS	O
SC	O
circuits	O
led	O
to	O
the	O
development	O
of	O
PCM	B-Algorithm
codec-filter	O
chips	O
in	O
the	O
late	O
1970s	O
.	O
</s>
<s>
The	O
silicon-gate	O
CMOS	B-Device
(	O
complementary	O
MOS	O
)	O
PCM	B-Algorithm
codec-filter	O
chip	O
,	O
developed	O
by	O
Hodges	O
and	O
W.C.	O
Black	O
in	O
1980	O
,	O
has	O
since	O
been	O
the	O
industry	O
standard	O
for	O
digital	B-General_Concept
telephony	I-General_Concept
.	O
</s>
<s>
By	O
the	O
1990s	O
,	O
telecommunication	O
networks	O
such	O
as	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
had	O
been	O
largely	O
digitized	O
with	O
very-large-scale	O
integration	O
(	O
VLSI	O
)	O
CMOS	B-Device
PCM	B-Algorithm
codec-filters	O
,	O
widely	O
used	O
in	O
electronic	B-Device
switching	I-Device
systems	I-Device
for	O
telephone	O
exchanges	O
,	O
private	B-Device
branch	I-Device
exchanges	I-Device
(	O
PBX	O
)	O
and	O
key	O
telephone	O
systems	O
(	O
KTS	O
)	O
;	O
user-end	O
modems	B-Application
;	O
data	O
transmission	O
applications	O
such	O
as	O
digital	B-Application
loop	I-Application
carriers	I-Application
,	O
pair	B-Protocol
gain	I-Protocol
multiplexers	B-Protocol
,	O
telephone	O
loop	O
extenders	O
,	O
integrated	B-Protocol
services	I-Protocol
digital	I-Protocol
network	I-Protocol
(	O
ISDN	B-Protocol
)	O
terminals	O
,	O
digital	O
cordless	O
telephones	O
and	O
digital	O
cell	O
phones	O
;	O
and	O
applications	O
such	O
as	O
speech	B-Application
recognition	I-Application
equipment	O
,	O
voice	O
data	B-General_Concept
storage	I-General_Concept
,	O
voice	O
mail	O
and	O
digital	O
tapeless	O
answering	O
machines	O
.	O
</s>
<s>
The	O
bandwidth	O
of	O
digital	O
telecommunication	O
networks	O
has	O
been	O
rapidly	O
increasing	O
at	O
an	O
exponential	O
rate	O
,	O
as	O
observed	O
by	O
Edholm	B-Algorithm
's	I-Algorithm
law	I-Algorithm
,	O
largely	O
driven	O
by	O
the	O
rapid	O
scaling	O
and	O
miniaturization	O
of	O
MOS	B-Architecture
technology	I-Architecture
.	O
</s>
<s>
Uncompressed	B-General_Concept
PCM	B-Algorithm
digital	O
audio	O
with	O
8-bit	O
depth	O
and	O
8kHz	O
sample	O
rate	O
requires	O
a	O
bit	O
rate	O
of	O
64kbit/s	O
,	O
which	O
was	O
impractical	O
for	O
early	O
digital	O
telecommunication	O
networks	O
with	O
limited	O
network	O
bandwidth	O
.	O
</s>
<s>
A	O
solution	O
to	O
this	O
issue	O
was	O
linear	B-Algorithm
predictive	I-Algorithm
coding	I-Algorithm
(	O
LPC	O
)	O
,	O
a	O
speech	B-Application
coding	I-Application
data	B-General_Concept
compression	I-General_Concept
algorithm	I-General_Concept
that	O
was	O
first	O
proposed	O
by	O
Fumitada	O
Itakura	O
of	O
Nagoya	O
University	O
and	O
Shuzo	O
Saito	O
of	O
Nippon	O
Telegraph	O
and	O
Telephone	O
(	O
NTT	O
)	O
in	O
1966	O
.	O
</s>
<s>
LPC	O
was	O
capable	O
of	O
audio	O
data	B-General_Concept
compression	I-General_Concept
down	O
to	O
2.4kbit/s	O
,	O
leading	O
to	O
the	O
first	O
successful	O
real-time	O
conversations	O
over	O
digital	B-Architecture
networks	I-Architecture
in	O
the	O
1970s	O
.	O
</s>
<s>
LPC	O
has	O
since	O
been	O
the	O
most	O
widely	O
used	O
speech	B-Application
coding	I-Application
method	O
.	O
</s>
<s>
Another	O
audio	O
data	B-General_Concept
compression	I-General_Concept
method	O
,	O
a	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
algorithm	O
called	O
the	O
modified	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
MDCT	O
)	O
,	O
has	O
been	O
widely	O
adopted	O
for	O
speech	B-Application
coding	I-Application
in	O
voice-over-IP	B-Application
(	O
VoIP	B-Application
)	O
applications	O
since	O
the	O
late	O
1990s	O
.	O
</s>
<s>
The	O
development	O
of	O
transmission	O
methods	O
such	O
as	O
SONET	B-Protocol
and	O
fiber	B-Architecture
optic	I-Architecture
transmission	O
further	O
advanced	O
digital	O
transmission	O
.	O
</s>
<s>
Although	O
analog	O
carrier	O
systems	O
existed	O
that	O
multiplexed	B-Architecture
multiple	O
analog	O
voice	O
channels	O
onto	O
a	O
single	O
transmission	O
medium	O
,	O
digital	O
transmission	O
allowed	O
lower	O
cost	O
and	O
more	O
channels	O
multiplexed	B-Architecture
on	O
the	O
transmission	O
medium	O
.	O
</s>
<s>
Digital	B-Application
loop	I-Application
carriers	I-Application
(	O
DLC	O
)	O
and	O
fiber	B-Architecture
to	I-Architecture
the	I-Architecture
x	I-Architecture
place	O
the	O
digital	B-Architecture
network	I-Architecture
ever	O
closer	O
to	O
the	O
customer	O
premises	O
,	O
relegating	O
the	O
analog	O
local	B-Architecture
loop	I-Architecture
to	O
legacy	O
status	O
.	O
</s>
<s>
Telephony	O
now	O
includes	O
the	O
technologies	O
of	O
Internet	B-Protocol
services	I-Protocol
and	O
mobile	O
communication	O
,	O
including	O
video	B-Application
conferencing	I-Application
.	O
</s>
<s>
The	O
new	O
technologies	O
based	O
on	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
concepts	O
are	O
often	O
referred	O
to	O
separately	O
as	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
telephony	O
,	O
also	O
commonly	O
referred	O
to	O
as	O
IP	B-Application
telephony	I-Application
or	O
Internet	B-Application
telephony	I-Application
.	O
</s>
<s>
Unlike	O
traditional	O
phone	O
service	O
,	O
IP	B-Application
telephony	I-Application
service	O
is	O
relatively	O
unregulated	O
by	O
government	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
the	O
Federal	O
Communications	O
Commission	O
(	O
FCC	O
)	O
regulates	O
phone-to-phone	O
connections	O
,	O
but	O
says	O
they	O
do	O
not	O
plan	O
to	O
regulate	O
connections	O
between	O
a	O
phone	O
user	O
and	O
an	O
IP	B-Application
telephony	I-Application
service	O
provider	O
.	O
</s>
<s>
A	O
specialization	O
of	O
digital	B-General_Concept
telephony	I-General_Concept
,	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
telephony	O
involves	O
the	O
application	O
of	O
digital	O
networking	O
technology	O
that	O
was	O
the	O
foundation	O
to	O
the	O
Internet	O
to	O
create	O
,	O
transmit	O
,	O
and	O
receive	O
telecommunications	O
sessions	O
over	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Internet	B-Application
telephony	I-Application
is	O
commonly	O
known	O
as	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
,	O
reflecting	O
the	O
principle	O
,	O
but	O
it	O
has	O
been	O
referred	O
with	O
many	O
other	O
terms	O
.	O
</s>
<s>
VoIP	B-Application
has	O
proven	O
to	O
be	O
a	O
disruptive	O
technology	O
that	O
is	O
rapidly	O
replacing	O
traditional	O
telephone	O
infrastructure	O
technologies	O
.	O
</s>
<s>
A	O
January	O
2005	O
Newsweek	O
article	O
suggested	O
that	O
Internet	B-Application
telephony	I-Application
may	O
be	O
"	O
the	O
next	O
big	O
thing	O
"	O
.	O
</s>
<s>
As	O
of	O
2006	O
,	O
many	O
VoIP	B-Application
companies	O
offer	O
service	O
to	O
consumers	O
and	O
businesses	O
.	O
</s>
<s>
IP	B-Application
telephony	I-Application
uses	O
an	O
Internet	O
connection	O
and	O
hardware	O
IP	B-Device
phones	I-Device
,	O
analog	O
telephone	O
adapters	O
,	O
or	O
softphone	B-Protocol
computer	O
applications	O
to	O
transmit	O
conversations	O
encoded	O
as	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
replacing	O
plain	O
old	O
telephone	O
service	O
(	O
POTS	O
)	O
,	O
IP	B-Application
telephony	I-Application
services	O
compete	O
with	O
mobile	O
phone	O
services	O
by	O
offering	O
free	O
or	O
lower	O
cost	O
connections	O
via	O
WiFi	B-Application
hotspots	I-Application
.	O
</s>
<s>
VoIP	B-Application
is	O
also	O
used	O
on	O
private	O
networks	O
which	O
may	O
or	O
may	O
not	O
have	O
a	O
connection	O
to	O
the	O
global	O
telephone	O
network	O
.	O
</s>
<s>
Video	B-Application
telephony	I-Application
restores	O
such	O
interactions	O
to	O
varying	O
degrees	O
.	O
</s>
<s>
The	O
expansion	O
of	O
communication	O
to	O
mobile	O
telephone	O
service	O
has	O
created	O
a	O
different	O
filter	O
of	O
the	O
social	O
cues	O
than	O
the	O
land-line	B-Application
telephone	I-Application
.	O
</s>
