<s>
Time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
is	O
a	O
method	O
of	O
transmitting	O
and	O
receiving	O
independent	O
signals	O
over	O
a	O
common	O
signal	O
path	O
by	O
means	O
of	O
synchronized	O
switches	O
at	O
each	O
end	O
of	O
the	O
transmission	O
line	O
so	O
that	O
each	O
signal	O
appears	O
on	O
the	O
line	O
only	O
a	O
fraction	O
of	O
time	O
in	O
an	O
alternating	O
pattern	O
.	O
</s>
<s>
This	O
method	O
transmits	O
two	O
or	O
more	O
digital	B-General_Concept
signals	O
or	O
analog	B-Device
signals	O
over	O
a	O
common	O
channel	O
.	O
</s>
<s>
This	O
form	O
of	O
signal	O
multiplexing	B-Architecture
was	O
developed	O
in	O
telecommunications	O
for	O
telegraphy	O
systems	O
in	O
the	O
late	O
19th	O
century	O
,	O
but	O
found	O
its	O
most	O
common	O
application	O
in	O
digital	B-General_Concept
telephony	O
in	O
the	O
second	O
half	O
of	O
the	O
20th	O
century	O
.	O
</s>
<s>
Time-division	B-Protocol
multiplexing	I-Protocol
was	O
first	O
developed	O
for	O
applications	O
in	O
telegraphy	O
to	O
route	O
multiple	O
transmissions	O
simultaneously	O
over	O
a	O
single	O
transmission	O
line	O
.	O
</s>
<s>
In	O
the	O
1870s	O
,	O
Émile	O
Baudot	O
developed	O
a	O
time-multiplexing	B-Protocol
system	O
of	O
multiple	O
Hughes	O
telegraph	O
machines	O
.	O
</s>
<s>
10	O
to	O
multiplex	O
10	O
telephone	O
conversations	O
over	O
a	O
microwave	B-Application
relay	I-Application
as	O
far	O
as	O
50	O
miles	O
.	O
</s>
<s>
A	O
channel	O
bank	O
sliced	O
a	O
1.544	O
Mbit/s	O
digital	B-General_Concept
signal	O
into	O
8,000	O
separate	O
frames	O
,	O
each	O
composed	O
of	O
24	O
contiguous	O
bytes	O
.	O
</s>
<s>
Channel	O
banks	O
used	O
the	O
fixed	O
position	O
(	O
temporal	O
alignment	O
)	O
of	O
one	O
byte	O
in	O
the	O
frame	B-Protocol
to	O
identify	O
the	O
call	O
it	O
belonged	O
to	O
.	O
</s>
<s>
Time-division	B-Protocol
multiplexing	I-Protocol
is	O
used	O
primarily	O
for	O
digital	B-General_Concept
signals	O
,	O
but	O
may	O
be	O
applied	O
in	O
analog	B-Device
multiplexing	B-Architecture
in	O
which	O
two	O
or	O
more	O
signals	O
or	O
bit	O
streams	O
are	O
transferred	O
appearing	O
simultaneously	O
as	O
sub-channels	O
in	O
one	O
communication	O
channel	O
,	O
but	O
are	O
physically	O
taking	O
turns	O
on	O
the	O
channel	O
.	O
</s>
<s>
One	O
TDM	O
frame	B-Protocol
consists	O
of	O
one	O
time	O
slot	O
per	O
sub-channel	O
plus	O
a	O
synchronization	O
channel	O
and	O
sometimes	O
error	O
correction	O
channel	O
before	O
the	O
synchronization	O
.	O
</s>
<s>
After	O
the	O
last	O
sub-channel	O
,	O
error	O
correction	O
,	O
and	O
synchronization	O
,	O
the	O
cycle	O
starts	O
all	O
over	O
again	O
with	O
a	O
new	O
frame	B-Protocol
,	O
starting	O
with	O
the	O
second	O
sample	O
,	O
byte	O
or	O
data	O
block	O
from	O
sub-channel	O
1	O
,	O
etc	O
.	O
</s>
<s>
The	O
synchronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
(	O
SDH	O
)	O
/synchronous	O
optical	O
networking	O
(	O
SONET	B-Protocol
)	O
network	O
transmission	O
standards	O
that	O
have	O
replaced	O
PDH	O
.	O
</s>
<s>
The	O
Basic	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
and	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
for	O
the	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
(	O
ISDN	B-Protocol
)	O
.	O
</s>
<s>
TDM	O
can	O
be	O
further	O
extended	O
into	O
the	O
time-division	B-Protocol
multiple	O
access	O
(	O
TDMA	O
)	O
scheme	O
,	O
where	O
several	O
stations	O
connected	O
to	O
the	O
same	O
physical	O
medium	O
,	O
for	O
example	O
sharing	O
the	O
same	O
frequency	B-Protocol
channel	O
,	O
can	O
communicate	O
.	O
</s>
<s>
In	O
circuit-switched	B-Protocol
networks	I-Protocol
,	O
such	O
as	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
,	O
it	O
is	O
desirable	O
to	O
transmit	O
multiple	O
subscriber	O
calls	O
over	O
the	O
same	O
transmission	O
medium	O
to	O
effectively	O
utilize	O
the	O
bandwidth	O
of	O
the	O
medium	O
.	O
</s>
<s>
A	O
TDM	O
circuit	O
runs	O
at	O
a	O
much	O
higher	O
signal	O
bandwidth	O
,	O
permitting	O
the	O
bandwidth	O
to	O
be	O
divided	O
into	O
time	O
frames	O
(	O
time	O
slots	O
)	O
for	O
each	O
voice	O
signal	O
which	O
is	O
multiplexed	B-Architecture
onto	O
the	O
line	O
by	O
the	O
transmitter	O
.	O
</s>
<s>
If	O
the	O
TDM	O
frame	B-Protocol
consists	O
of	O
n	O
voice	O
frames	O
,	O
the	O
line	O
bandwidth	O
is	O
n*64	O
kbit/s	O
.	O
</s>
<s>
Each	O
voice	O
time	O
slot	O
in	O
the	O
TDM	O
frame	B-Protocol
is	O
called	O
a	O
channel	O
.	O
</s>
<s>
In	O
European	O
systems	O
,	O
standard	O
TDM	O
frames	O
contain	O
30	O
digital	B-General_Concept
voice	O
channels	O
(	O
E1	O
)	O
,	O
and	O
in	O
American	O
systems	O
(	O
T1	O
)	O
,	O
they	O
contain	O
24	O
channels	O
.	O
</s>
<s>
Multiplexing	B-Architecture
more	O
than	O
24	O
or	O
30	O
digital	B-General_Concept
voice	O
channels	O
is	O
called	O
higher	O
order	O
multiplexing	B-Architecture
.	O
</s>
<s>
Higher	O
order	O
multiplexing	B-Architecture
is	O
accomplished	O
by	O
multiplexing	B-Architecture
the	O
standard	O
TDM	O
frames	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
European	O
120	O
channel	O
TDM	O
frame	B-Protocol
is	O
formed	O
by	O
multiplexing	B-Architecture
four	O
standard	O
30	O
channel	O
TDM	O
frames	O
.	O
</s>
<s>
At	O
each	O
higher	O
order	O
multiplex	O
,	O
four	O
TDM	O
frames	O
from	O
the	O
immediate	O
lower	O
order	O
are	O
combined	O
,	O
creating	O
multiplexes	B-Architecture
with	O
a	O
bandwidth	O
of	O
n*64	O
kbit/s	O
,	O
where	O
n	O
=	O
120	O
,	O
480	O
,	O
1920	O
,	O
etc	O
.	O
</s>
<s>
There	O
are	O
three	O
types	O
of	O
synchronous	O
TDM	O
:	O
T1	O
,	O
SONET/SDH	B-Protocol
,	O
and	O
ISDN	B-Protocol
.	O
</s>
<s>
Plesiochronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
(	O
PDH	O
)	O
was	O
developed	O
as	O
a	O
standard	O
for	O
multiplexing	B-Architecture
higher	O
order	O
frames	O
.	O
</s>
<s>
PDH	O
created	O
larger	O
numbers	O
of	O
channels	O
by	O
multiplexing	B-Architecture
the	O
standard	O
Europeans	O
30	O
channel	O
TDM	O
frames	O
.	O
</s>
<s>
This	O
solution	O
worked	O
for	O
a	O
while	O
;	O
however	O
PDH	O
suffered	O
from	O
several	O
inherent	O
drawbacks	O
which	O
ultimately	O
resulted	O
in	O
the	O
development	O
of	O
the	O
Synchronous	B-Protocol
Digital	I-Protocol
Hierarchy	I-Protocol
(	O
SDH	O
)	O
.	O
</s>
<s>
Allow	O
frames	O
of	O
any	O
size	O
to	O
be	O
removed	O
or	O
inserted	O
into	O
an	O
SDH	O
frame	B-Protocol
of	O
any	O
size	O
.	O
</s>
<s>
Provide	O
high	O
data	O
rates	O
by	O
multiplexing	B-Architecture
any	O
size	O
frame	B-Protocol
,	O
limited	O
only	O
by	O
technology	O
.	O
</s>
<s>
It	O
was	O
developed	O
to	O
allow	O
streams	O
1.544	O
Mbit/s	O
and	O
above	O
to	O
be	O
multiplexed	B-Architecture
,	O
in	O
order	O
to	O
create	O
larger	O
SDH	O
frames	O
known	O
as	O
Synchronous	O
Transport	O
Modules	O
(	O
STM	O
)	O
.	O
</s>
<s>
The	O
STM-1	O
frame	B-Protocol
consists	O
of	O
smaller	O
streams	O
that	O
are	O
multiplexed	B-Architecture
to	O
create	O
a	O
155.52	O
Mbit/s	O
frame	B-Protocol
.	O
</s>
<s>
SDH	O
Add-Drop	O
Multiplexer	O
The	O
SDH	O
Add-Drop	O
Multiplexer	O
(	O
ADM	O
)	O
can	O
add	O
or	O
remove	O
any	O
multiplexed	B-Architecture
frame	B-Protocol
down	O
to	O
1.544Mb	O
.	O
</s>
<s>
Modern	O
optic	O
fibre	O
transmission	O
makes	O
use	O
of	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
(	O
WDM	O
)	O
where	O
signals	O
transmitted	O
across	O
the	O
fibre	O
are	O
transmitted	O
at	O
different	O
wavelengths	O
,	O
creating	O
additional	O
channels	O
for	O
transmission	O
.	O
</s>
<s>
Statistical	B-Algorithm
time-division	I-Algorithm
multiplexing	I-Algorithm
(	O
STDM	O
)	O
is	O
an	O
advanced	O
version	O
of	O
TDM	O
in	O
which	O
both	O
the	O
address	O
of	O
the	O
terminal	O
and	O
the	O
data	O
itself	O
are	O
transmitted	O
together	O
for	O
better	O
routing	O
.	O
</s>
<s>
In	O
its	O
primary	O
form	O
,	O
TDM	O
is	O
used	O
for	O
circuit	B-Protocol
mode	I-Protocol
communication	O
with	O
a	O
fixed	O
number	O
of	O
channels	O
and	O
constant	O
bandwidth	O
per	O
channel	O
.	O
</s>
<s>
Bandwidth	O
reservation	O
distinguishes	O
time-division	B-Protocol
multiplexing	I-Protocol
from	O
statistical	B-Algorithm
multiplexing	I-Algorithm
such	O
as	O
statistical	B-Algorithm
time-division	I-Algorithm
multiplexing	I-Algorithm
.	O
</s>
<s>
In	O
dynamic	O
TDMA	O
,	O
a	O
scheduling	O
algorithm	O
dynamically	O
reserves	O
a	O
variable	O
number	O
of	O
time	O
slots	O
in	O
each	O
frame	B-Protocol
to	O
variable	O
bit-rate	O
data	O
streams	O
,	O
based	O
on	O
the	O
traffic	O
demand	O
of	O
each	O
data	O
stream	O
.	O
</s>
<s>
Asynchronous	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
ATDM	O
)	O
,	O
is	O
an	O
alternative	O
nomenclature	O
in	O
which	O
STDM	O
designates	O
synchronous	O
time-division	B-Protocol
multiplexing	I-Protocol
,	O
the	O
older	O
method	O
that	O
uses	O
fixed	O
time	O
slots	O
.	O
</s>
