<s>
In	O
telecommunications	O
,	O
a	O
plesiochronous	B-Architecture
system	I-Architecture
is	O
one	O
where	O
different	O
parts	O
of	O
the	O
system	O
are	O
almost	O
,	O
but	O
not	O
quite	O
,	O
perfectly	O
synchronised	O
.	O
</s>
<s>
According	O
to	O
ITU-T	O
standards	O
,	O
a	O
pair	O
of	O
signals	O
are	O
plesiochronous	B-Architecture
if	O
their	O
significant	O
instants	O
occur	O
at	O
nominally	O
the	O
same	O
rate	O
,	O
with	O
any	O
variation	O
in	O
rate	O
being	O
constrained	O
within	O
specified	O
limits	O
.	O
</s>
<s>
A	O
sender	O
and	O
receiver	O
operate	O
plesiosynchronously	B-Architecture
if	O
they	O
operate	O
at	O
the	O
same	O
nominal	O
clock	O
frequency	O
but	O
may	O
have	O
a	O
slight	O
clock	O
frequency	O
mismatch	O
,	O
which	O
leads	O
to	O
a	O
drifting	O
phase	O
.	O
</s>
<s>
The	O
mismatch	O
between	O
the	O
two	O
systems	O
 '	O
clocks	O
is	O
known	O
as	O
the	O
plesiochronous	B-Architecture
difference	I-Architecture
.	O
</s>
<s>
In	O
general	O
,	O
plesiochronous	B-Architecture
systems	I-Architecture
behave	O
similarly	O
to	O
synchronous	B-Application
systems	I-Application
,	O
except	O
they	O
must	O
employ	O
some	O
means	O
in	O
order	O
to	O
cope	O
with	O
"	O
sync	O
slips	O
"	O
,	O
which	O
will	O
happen	O
at	O
intervals	O
due	O
to	O
the	O
plesiochronous	B-Architecture
nature	O
of	O
the	O
system	O
.	O
</s>
<s>
The	O
most	O
common	O
example	O
of	O
a	O
plesiochronous	B-Architecture
system	I-Architecture
design	O
is	O
the	O
plesiochronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
networking	O
standard	O
.	O
</s>
<s>
The	O
asynchronous	B-Application
serial	O
communication	O
protocol	O
is	O
asynchronous	B-Application
on	O
the	O
byte	O
level	O
,	O
but	O
plesiochronous	B-Architecture
on	O
the	O
bit	O
level	O
.	O
</s>
<s>
The	O
indefinite	O
wait	O
and	O
lack	O
of	O
external	O
synchronization	O
signals	O
makes	O
byte	O
detection	O
asynchronous	B-Application
.	O
</s>
<s>
Then	O
the	O
receiver	O
samples	O
at	O
predefined	O
intervals	O
to	O
determine	O
the	O
values	O
of	O
the	O
bits	O
in	O
the	O
byte	O
;	O
this	O
is	O
plesiochronous	B-Architecture
since	O
it	O
depends	O
on	O
the	O
transmitter	O
to	O
transmit	O
at	O
roughly	O
the	O
same	O
rate	O
the	O
receiver	O
expects	O
,	O
without	O
coordination	O
of	O
the	O
rate	O
while	O
the	O
bits	O
are	O
being	O
transmitted	O
.	O
</s>
<s>
The	O
modern	O
tendency	O
in	O
systems	O
engineering	O
is	O
towards	O
using	O
systems	O
that	O
are	O
either	O
fundamentally	O
asynchronous	B-Application
(	O
such	O
as	O
Ethernet	O
)	O
,	O
or	O
fundamentally	O
synchronous	O
(	O
such	O
as	O
synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
)	O
,	O
and	O
layering	O
these	O
where	O
necessary	O
,	O
rather	O
than	O
using	O
a	O
mixture	O
between	O
the	O
two	O
in	O
a	O
single	O
technology	O
.	O
</s>
<s>
The	O
term	O
plesiochronous	B-Architecture
comes	O
from	O
the	O
Greek	O
πλησίος	O
plesios	O
(	O
"	O
near	O
"	O
)	O
and	O
χρόνος	O
chrónos	O
(	O
"	O
time	O
"	O
)	O
.	O
</s>
