<s>
The	O
plesiochronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
(	O
PDH	O
)	O
is	O
a	O
technology	O
used	O
in	O
telecommunications	O
networks	O
to	O
transport	O
large	O
quantities	O
of	O
data	O
over	O
digital	O
transport	O
equipment	O
such	O
as	O
fibre	B-Architecture
optic	I-Architecture
and	O
microwave	B-Application
radio	I-Application
systems	O
.	O
</s>
<s>
The	O
term	O
plesiochronous	B-Architecture
is	O
derived	O
from	O
Greek	O
plēsios	O
,	O
meaning	O
near	O
,	O
and	O
chronos	O
,	O
time	O
,	O
and	O
refers	O
to	O
the	O
fact	O
that	O
PDH	O
networks	O
run	O
in	O
a	O
state	O
where	O
different	O
parts	O
of	O
the	O
network	O
are	O
nearly	O
,	O
but	O
not	O
quite	O
perfectly	O
,	O
synchronized	O
.	O
</s>
<s>
Backbone	B-General_Concept
transport	I-General_Concept
networks	I-General_Concept
replaced	O
PDH	O
networks	O
with	O
synchronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
(	O
SDH	O
)	O
or	O
synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
(	O
SONET	B-Protocol
)	O
equipment	O
over	O
the	O
ten	O
years	O
ending	O
around	O
the	O
turn	O
of	O
the	O
millennium	O
(	O
2000	O
)	O
,	O
whose	O
floating	O
payloads	O
relaxed	O
the	O
more	O
stringent	O
timing	O
requirements	O
of	O
PDH	O
network	O
technology	O
.	O
</s>
<s>
Typically	O
the	O
multiplexer	O
takes	O
the	O
data	O
from	O
the	O
4	O
incoming	O
2.048	O
Mbit/s	O
data	O
streams	O
and	O
feeds	O
each	O
into	O
a	O
2.112	O
Mbit/s	O
stream	O
via	O
a	O
buffer	B-General_Concept
store	O
leaving	O
a	O
series	O
of	O
fixed	O
gaps	O
in	O
each	O
frame	O
.	O
</s>
<s>
In	O
a	O
practical	O
system	O
however	O
,	O
the	O
actual	O
decision	O
to	O
stuff	O
or	O
not	O
may	O
be	O
made	O
by	O
comparing	O
the	O
read	O
address	O
and	O
write	O
address	O
of	O
the	O
input	O
buffer	B-General_Concept
store	O
so	O
the	O
position	O
in	O
the	O
frame	O
when	O
the	O
decision	O
is	O
made	O
varies	O
and	O
adds	O
a	O
second	O
variable	O
dependent	O
on	O
the	O
length	O
of	O
the	O
store	O
.	O
</s>
<s>
Similar	O
techniques	O
are	O
used	O
to	O
combine	O
four	O
×	O
8Mbit/s	O
together	O
,	O
plus	O
bit	B-Protocol
stuffing	I-Protocol
and	O
frame	O
alignment	O
,	O
giving	O
34Mbit/s	O
.	O
</s>
<s>
In	O
telecommunications	O
networks	O
,	O
independent	B-Protocol
clocks	I-Protocol
are	O
free-running	O
precision	O
clocks	O
located	O
at	O
the	O
nodes	O
which	O
are	O
used	O
for	O
synchronization	O
.	O
</s>
