<s>
Synchronous	B-Protocol
Ethernet	I-Protocol
,	O
also	O
referred	O
as	O
SyncE	B-Protocol
,	O
is	O
an	O
ITU-T	O
standard	O
for	O
computer	B-Architecture
networking	I-Architecture
that	O
facilitates	O
the	O
transference	O
of	O
clock	O
signals	O
over	O
the	O
Ethernet	B-Protocol
physical	I-Protocol
layer	I-Protocol
.	O
</s>
<s>
The	O
aim	O
of	O
Synchronous	B-Protocol
Ethernet	I-Protocol
is	O
to	O
provide	O
a	O
synchronization	O
signal	O
to	O
those	O
network	O
resources	O
that	O
may	O
eventually	O
require	O
such	O
a	O
type	O
of	O
signal	O
.	O
</s>
<s>
The	O
Synchronous	B-Protocol
Ethernet	I-Protocol
signal	O
transmitted	O
over	O
the	O
Ethernet	B-Protocol
physical	I-Protocol
layer	I-Protocol
should	O
be	O
traceable	O
to	O
an	O
external	O
clock	O
,	O
ideally	O
a	O
master	O
and	O
unique	O
clock	O
for	O
the	O
whole	O
network	O
.	O
</s>
<s>
Applications	O
include	O
cellular	O
networks	O
,	O
access	O
technologies	O
such	O
as	O
Ethernet	O
passive	O
optical	O
network	O
,	O
and	O
applications	O
such	O
as	O
IPTV	O
or	O
VoIP	B-Application
,	O
as	O
well	O
as	O
CERN	O
's	O
White	O
Rabbit	O
Project	O
for	O
sub-nanosecond	O
time	O
synchronization	O
of	O
data	O
acquisition	O
equipment	O
for	O
their	O
high-energy	O
experiments	O
.	O
</s>
<s>
Unlike	O
time-division	B-Protocol
multiplexing	I-Protocol
networks	O
,	O
the	O
Ethernet	O
family	O
of	O
computer	B-Architecture
networks	I-Architecture
do	O
not	O
carry	O
clock	O
synchronization	O
information	O
.	O
</s>
<s>
IETF	O
’s	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
,	O
IEEE	O
's	O
1588-2008	O
Precision	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
are	O
some	O
of	O
them	O
.	O
</s>
<s>
SyncE	B-Protocol
was	O
standardized	O
by	O
the	O
ITU-T	O
,	O
in	O
cooperation	O
with	O
IEEE	O
,	O
as	O
three	O
recommendations	O
:	O
</s>
<s>
SyncE	B-Protocol
architecture	O
minimally	O
requires	O
replacement	O
of	O
the	O
internal	O
clock	O
of	O
the	O
Ethernet	O
card	O
by	O
a	O
phase	O
locked	O
loop	O
in	O
order	O
to	O
feed	O
the	O
Ethernet	O
PHY	O
.	O
</s>
<s>
This	O
enables	O
Synchronous	B-Protocol
Ethernet	I-Protocol
network	O
equipment	O
to	O
be	O
connected	O
to	O
the	O
same	O
synchronization	O
network	O
as	O
Synchronous	B-Protocol
Digital	I-Protocol
Hierarchy	I-Protocol
(	O
SDH	O
)	O
.	O
</s>
<s>
ITU-T	B-Protocol
G.8262	I-Protocol
defines	O
Synchronous	B-Protocol
Ethernet	I-Protocol
clocks	O
compatible	O
with	O
SDH	O
clocks	O
.	O
</s>
<s>
Synchronous	B-Protocol
Ethernet	I-Protocol
clocks	O
,	O
based	O
on	O
ITU-T	O
G.813	O
clocks	O
,	O
are	O
defined	O
in	O
terms	O
of	O
accuracy	O
,	O
noise	O
transfer	O
,	O
holdover	O
performance	O
,	O
noise	O
tolerance	O
and	O
noise	O
generation	O
.	O
</s>
<s>
In	O
SDH	O
,	O
the	O
Synchronization	O
Status	O
Message	O
(	O
SSM	O
)	O
provides	O
traceability	O
of	O
synchronization	O
signals	O
and	O
it	O
is	O
therefore	O
required	O
to	O
extend	O
the	O
SSM	O
functionality	O
to	O
Synchronous	B-Protocol
Ethernet	I-Protocol
to	O
achieve	O
full	O
interoperability	O
with	O
SDH	O
equipment	O
.	O
</s>
<s>
The	O
mechanisms	O
needed	O
to	O
transport	O
the	O
SSM	O
over	O
Synchronous	B-Protocol
Ethernet	I-Protocol
are	O
defined	O
by	O
the	O
ITU-T	O
in	O
G.8264	O
in	O
cooperation	O
with	O
IEEE	O
.	O
</s>
<s>
A	O
general	O
requirement	O
for	O
SyncE	B-Protocol
was	O
that	O
any	O
network	O
element	O
(	O
NE	O
)	O
should	O
have	O
at	O
least	O
two	O
reference	O
clocks	O
,	O
and	O
in	O
addition	O
,	O
Ethernet	O
interfaces	O
must	O
be	O
able	O
to	O
generate	O
their	O
own	O
synchronization	O
signal	O
in	O
case	O
they	O
lose	O
their	O
external	O
reference	O
.	O
</s>
<s>
SyncE	B-Protocol
networks	O
do	O
not	O
usually	O
have	O
only	O
one	O
topology	O
,	O
but	O
rather	O
a	O
combination	O
of	O
all	O
of	O
them	O
.	O
</s>
<s>
T1/E1	O
,	O
SONET/SDH	B-Protocol
and	O
any	O
rate	O
,	O
and	O
SyncE	B-Protocol
.	O
</s>
<s>
In	O
SyncE	B-Protocol
,	O
there	O
are	O
several	O
ways	O
to	O
synchronize	O
nodes	O
:	O
</s>
