<s>
A	O
cable	B-Device
modem	I-Device
termination	I-Device
system	I-Device
(	O
CMTS	O
,	O
also	O
called	O
a	O
CMTS	O
Edge	B-Protocol
Router	I-Protocol
)	O
is	O
a	O
piece	O
of	O
equipment	O
,	O
typically	O
located	O
in	O
a	O
cable	O
company	O
's	O
headend	O
or	O
hubsite	O
,	O
which	O
is	O
used	O
to	O
provide	O
high	O
speed	O
data	O
services	O
,	O
such	O
as	O
cable	B-Device
Internet	I-Device
or	O
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
,	O
to	O
cable	O
subscribers	O
.	O
</s>
<s>
A	O
CMTS	O
provides	O
many	O
of	O
the	O
same	O
functions	O
provided	O
by	O
the	O
DSLAM	B-Protocol
in	O
a	O
DSL	O
system	O
.	O
</s>
<s>
On	O
the	O
subscriber	O
side	O
of	O
the	O
headend	O
,	O
the	O
CMTS	O
enables	O
communication	O
with	O
subscribers	O
 '	O
cable	B-Device
modems	I-Device
.	O
</s>
<s>
Different	O
CMTSs	O
are	O
capable	O
of	O
serving	O
different	O
cable	B-Device
modem	I-Device
population	O
sizes	O
—	O
ranging	O
from	O
4,000	O
cable	B-Device
modems	I-Device
to	O
150,000	O
or	O
more	O
,	O
depending	O
in	O
part	O
on	O
traffic	O
.	O
</s>
<s>
A	O
given	O
headend	O
may	O
have	O
between	O
1-12	O
CMTSs	O
to	O
service	O
the	O
cable	B-Device
modem	I-Device
population	O
served	O
by	O
that	O
headend	O
or	O
HFC	B-Architecture
hub	I-Architecture
.	O
</s>
<s>
One	O
way	O
to	O
think	O
of	O
a	O
CMTS	O
is	O
to	O
imagine	O
a	O
router	B-Protocol
with	O
Ethernet	O
interfaces	B-Protocol
(	O
connections	O
)	O
on	O
one	O
side	O
and	O
coaxial	O
cable	O
RF	O
interfaces	B-Protocol
on	O
the	O
other	O
side	O
.	O
</s>
<s>
A	O
CMTS	O
has	O
separate	O
RF	O
interfaces	B-Protocol
and	O
connectors	O
for	O
downlink	O
and	O
uplink	O
signals	O
.	O
</s>
<s>
The	O
RF/coax	O
interfaces	B-Protocol
carry	O
RF	O
signals	O
to	O
and	O
from	O
coaxial	O
"	O
trunks	O
"	O
connected	O
to	O
subscribers	O
 '	O
cable	B-Device
modems	I-Device
,	O
using	O
one	O
pair	O
of	O
connectors	O
per	O
trunk	O
,	O
one	O
for	O
downlink	O
and	O
the	O
other	O
for	O
uplink	O
.	O
</s>
<s>
The	O
RF	O
signals	O
from	O
a	O
CMTS	O
,	O
are	O
connected	O
via	O
coaxial	O
cable	O
to	O
an	O
"	O
optics	O
platform	O
"	O
or	O
headend	O
platform	O
,	O
which	O
has	O
transmitter	O
and	O
receiver	O
modules	O
that	O
turn	O
the	O
RF	O
signals	O
into	O
light	O
pulses	O
for	O
delivery	O
over	O
fiber	O
optics	O
through	O
an	O
HFC	B-Architecture
network	I-Architecture
.	O
</s>
<s>
In	O
fact	O
,	O
most	O
CMTSs	O
have	O
both	O
Ethernet	O
interfaces	B-Protocol
(	O
or	O
other	O
more	O
traditional	O
high-speed	O
data	O
interfaces	B-Protocol
like	O
SONET	B-Protocol
)	O
as	O
well	O
as	O
RF	O
interfaces	B-Protocol
.	O
</s>
<s>
In	O
this	O
way	O
,	O
traffic	O
that	O
is	O
coming	O
from	O
the	O
Internet	O
can	O
be	O
routed	O
(	O
or	O
bridged	O
)	O
through	O
the	O
Ethernet	O
interface	O
,	O
through	O
the	O
CMTS	O
and	O
then	O
onto	O
the	O
RF	O
interfaces	B-Protocol
that	O
are	O
connected	O
to	O
the	O
cable	O
company	O
's	O
hybrid	B-Architecture
fiber	I-Architecture
coax	I-Architecture
(	O
HFC	B-Architecture
)	O
.	O
</s>
<s>
The	O
traffic	O
winds	O
its	O
way	O
through	O
the	O
HFC	B-Architecture
to	O
end	O
up	O
at	O
the	O
cable	B-Device
modem	I-Device
in	O
the	O
subscriber	O
's	O
home	O
.	O
</s>
<s>
Traffic	O
from	O
a	O
subscriber	O
's	O
home	O
system	O
goes	O
through	O
the	O
cable	B-Device
modem	I-Device
and	O
out	O
to	O
the	O
Internet	O
in	O
the	O
opposite	O
direction	O
.	O
</s>
<s>
CMTSs	O
typically	O
carry	O
only	O
IP	B-Protocol
traffic	O
.	O
</s>
<s>
Traffic	O
destined	O
for	O
the	O
cable	B-Device
modem	I-Device
from	O
the	O
Internet	O
,	O
known	O
as	O
downstream	O
traffic	O
,	O
is	O
carried	O
in	O
IP	B-Protocol
packets	O
encapsulated	O
according	O
to	O
DOCSIS	B-Protocol
standard	O
.	O
</s>
<s>
These	O
packets	O
are	O
carried	O
on	O
data	O
streams	O
that	O
are	O
typically	O
modulated	O
onto	O
a	O
TV	O
channel	O
using	O
either	O
64-QAM	B-Device
or	O
256-QAM	B-Device
versions	O
of	O
quadrature	B-Device
amplitude	I-Device
modulation	I-Device
.	O
</s>
<s>
Upstream	O
data	O
(	O
data	O
from	O
cable	B-Device
modems	I-Device
to	O
the	O
headend	O
or	O
Internet	O
)	O
is	O
carried	O
in	O
Ethernet	O
frames	O
encapsulated	O
inside	O
DOCSIS	B-Protocol
frames	O
modulated	O
with	O
QPSK	O
,	O
16-QAM	B-Device
,	O
32-QAM	B-Device
,	O
64-QAM	B-Device
or	O
128-QAM	B-Device
using	O
TDMA	O
,	O
ATDMA	O
or	O
S-CDMA	B-Protocol
frequency	O
sharing	O
mechanisms	O
.	O
</s>
<s>
This	O
is	O
usually	O
done	O
at	O
the	O
"	O
subband	O
"	O
or	O
"	O
return	O
"	O
portion	O
of	O
the	O
cable	O
TV	O
spectrum	O
(	O
also	O
known	O
as	O
the	O
"	O
T	O
"	O
channels	O
)	O
,	O
a	O
much	O
lower	O
part	O
of	O
the	O
frequency	O
spectrum	O
than	O
the	O
downstream	O
signal	O
,	O
usually	O
5	O
–	O
42MHz	O
in	O
DOCSIS	B-Protocol
2.0	I-Protocol
or	O
5	O
–	O
65MHz	O
in	O
EuroDOCSIS	B-Protocol
.	O
</s>
<s>
A	O
typical	O
CMTS	O
allows	O
a	O
subscriber	O
's	O
computer	O
to	O
obtain	O
an	O
IP	B-Protocol
address	I-Protocol
by	O
forwarding	O
DHCP	B-Protocol
requests	O
to	O
the	O
relevant	O
servers	O
.	O
</s>
<s>
This	O
DHCP	B-Protocol
server	I-Protocol
returns	O
,	O
for	O
the	O
most	O
part	O
,	O
what	O
looks	O
like	O
a	O
typical	O
response	O
including	O
an	O
assigned	O
IP	B-Protocol
address	I-Protocol
for	O
the	O
computer	O
,	O
gateway/router	O
addresses	O
to	O
use	O
,	O
DNS	O
servers	O
,	O
etc	O
.	O
</s>
<s>
Traffic	O
shaping	O
is	O
sometimes	O
performed	O
to	O
prioritize	O
application	O
traffic	O
,	O
perhaps	O
based	O
upon	O
subscribed	O
plan	O
or	O
download	O
usage	O
and	O
also	O
to	O
provide	O
guaranteed	O
Quality	O
of	O
service	O
(	O
QoS	O
)	O
for	O
the	O
cable	O
operator	O
's	O
own	O
PacketCable-based	O
VOIP	B-Application
service	O
.	O
</s>
<s>
However	O
,	O
the	O
function	O
of	O
traffic	O
shaping	O
is	O
more	O
likely	O
done	O
by	O
a	O
Cable	B-Device
Modem	I-Device
or	O
policy	O
traffic	O
switch	O
.	O
</s>
<s>
A	O
CMTS	O
may	O
also	O
act	O
as	O
a	O
bridge	B-Protocol
or	O
router	B-Protocol
.	O
</s>
<s>
A	O
customer	O
's	O
cable	B-Device
modem	I-Device
cannot	O
communicate	O
directly	O
with	O
other	O
modems	O
on	O
the	O
line	O
.	O
</s>
<s>
In	O
general	O
,	O
cable	B-Device
modem	I-Device
traffic	O
is	O
routed	O
to	O
other	O
cable	B-Device
modems	I-Device
or	O
to	O
the	O
Internet	O
through	O
a	O
series	O
of	O
CMTSs	O
and	O
traditional	O
routers	B-Protocol
.	O
</s>
<s>
A	O
CCAP	O
(	O
Converged	O
Cable	O
Access	O
Platform	O
)	O
combines	O
CMTS	O
and	O
Edge	O
QAM	B-Device
functionality	O
in	O
a	O
single	O
device	O
so	O
that	O
it	O
can	O
provide	O
both	O
data	O
(	O
internet	O
)	O
with	O
CMTS	O
functionality	O
,	O
and	O
video	O
(	O
TV	O
channels	O
)	O
with	O
Edge	O
QAM	B-Device
functionality	O
.	O
</s>
<s>
Edge	O
QAM	B-Device
(	O
Quadrature	O
Amplitude	O
Modulator/Modulation	O
)	O
converts	O
video	O
sent	O
via	O
IP	B-Protocol
(	O
internet	B-Protocol
protocol	I-Protocol
)	O
or	O
otherwise	O
,	O
into	O
a	O
QAM	B-Device
signal	O
for	O
delivery	O
over	O
a	O
cable	O
network	O
.	O
</s>
<s>
Edge	O
QAMs	B-Device
are	O
normally	O
standalone	O
devices	O
placed	O
at	O
the	O
"	O
edge	O
"	O
of	O
a	O
network	O
.	O
</s>
<s>
The	O
first	O
part	O
is	O
the	O
Physical	O
Downstream	O
component	O
(	O
PHY	O
)	O
which	O
is	O
known	O
as	O
the	O
Edge	O
QAM	B-Device
(	O
EQAM	O
)	O
.	O
</s>
<s>
The	O
second	O
part	O
is	O
the	O
IP	B-Protocol
networking	O
and	O
DOCSIS	B-Protocol
MAC	O
Component	O
which	O
is	O
referred	O
to	O
as	O
the	O
M-CMTS	O
Core	O
.	O
</s>
<s>
One	O
is	O
the	O
DOCSIS	B-Protocol
Timing	O
Interface	O
,	O
which	O
provides	O
a	O
reference	O
frequency	O
between	O
the	O
EQAM	O
and	O
M-CMTS	O
Core	O
via	O
a	O
DTI	O
Server	O
.	O
</s>
