<s>
Trellis-coded	B-Device
pulse-amplitude	I-Device
modulation	I-Device
(	O
TC-PAM	B-Device
)	O
is	O
the	O
modulation	B-Application
format	O
that	O
is	O
used	O
in	O
HDSL2	O
and	O
G.SHDSL	O
.	O
</s>
<s>
It	O
is	O
a	O
variant	O
of	O
trellis	O
coded	O
modulation	B-Application
(	O
TCM	O
)	O
which	O
uses	O
a	O
one-dimensional	O
pulse-amplitude	B-Device
modulation	I-Device
(	O
PAM	O
)	O
symbol	O
space	O
,	O
as	O
opposed	O
to	O
a	O
two-dimensional	O
quadrature	B-Device
amplitude	I-Device
modulation	I-Device
(	O
QAM	B-Device
)	O
symbol	O
space	O
.	O
</s>
<s>
Compared	O
to	O
the	O
2B1Q	B-Protocol
scheme	O
used	O
in	O
the	O
older	O
HDSL	O
and	O
SDSL	O
standards	O
,	O
TC-PAM	B-Device
improves	O
range	O
at	O
a	O
given	O
bit-rate	O
and	O
provides	O
enhanced	O
spectral	O
compatibility	O
with	O
ADSL	O
.	O
</s>
<s>
TC-PAM	B-Device
is	O
also	O
known	O
as	O
4B1H	O
,	O
because	O
it	O
uses	O
16	O
levels	O
to	O
represents	O
a	O
4	O
digit	O
binary	O
,	O
4	O
Binary	O
1	O
Hexadecimal	O
.	O
</s>
