<s>
The	O
U	B-Protocol
interface	I-Protocol
or	O
U	O
reference	O
point	O
is	O
a	O
Basic	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
BRI	O
)	O
in	O
the	O
local	B-Architecture
loop	I-Architecture
of	O
an	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
(	O
ISDN	B-Protocol
)	O
,	O
connecting	O
the	O
network	O
terminator	O
(	O
NT1/2	O
)	O
on	O
the	O
customer	O
's	O
premises	O
to	O
the	O
line	O
termination	O
(	O
LT	O
)	O
in	O
the	O
carrier	O
's	O
local	O
exchange	O
,	O
in	O
other	O
words	O
providing	O
the	O
connection	O
from	O
subscriber	O
to	O
central	O
office	O
.	O
</s>
<s>
Unlike	O
the	O
ISDN	B-Protocol
S/T	O
interfaces	O
,	O
the	O
U	B-Protocol
interface	I-Protocol
was	O
not	O
originally	O
electrically	O
defined	O
by	O
the	O
ITU	O
ISDN	B-Protocol
specifications	O
,	O
but	O
left	O
up	O
to	O
network	O
operators	O
to	O
implement	O
,	O
although	O
the	O
ITU	O
has	O
issued	O
recommendations	O
G.960	O
and	O
G.961	O
to	O
formalize	O
the	O
standards	O
adopted	O
in	O
the	O
US	O
and	O
EU	O
.	O
</s>
<s>
In	O
the	O
US	O
,	O
the	O
U	B-Protocol
interface	I-Protocol
is	O
originally	O
defined	O
by	O
the	O
ANSI	O
T1.601	O
specification	O
as	O
a	O
2-wire	O
connection	O
using	O
2B1Q	B-Protocol
line	O
coding	O
.	O
</s>
<s>
It	O
is	O
not	O
as	O
distance	O
sensitive	O
as	O
the	O
S	B-Protocol
interface	I-Protocol
or	O
T	B-Protocol
interface	I-Protocol
,	O
and	O
can	O
operate	O
at	O
distances	O
up	O
to	O
18,000	O
feet	O
.	O
</s>
<s>
Typically	O
the	O
U	B-Protocol
interface	I-Protocol
does	O
not	O
connect	O
to	O
terminal	O
equipment	O
(	O
which	O
typically	O
has	O
an	O
S/T	O
interface	O
)	O
but	O
to	O
an	O
NT1	O
or	O
NT2	O
(	O
network	O
terminator	O
type	B-Protocol
1	I-Protocol
or	O
2	O
.	O
)	O
</s>
<s>
An	O
NT1	O
is	O
a	O
discrete	O
device	O
that	O
converts	O
the	O
U	B-Protocol
interface	I-Protocol
to	O
an	O
S/T	O
interface	O
,	O
which	O
is	O
then	O
connected	O
to	O
terminal	O
equipment	O
(	O
TE	O
)	O
having	O
an	O
S/T	O
interface	O
.	O
</s>
<s>
However	O
,	O
some	O
TE	O
devices	O
integrate	O
an	O
NT1	O
,	O
and	O
therefore	O
have	O
a	O
direct	O
U	B-Protocol
interface	I-Protocol
suitable	O
for	O
connection	O
directly	O
to	O
the	O
loop	O
.	O
</s>
<s>
An	O
NT2	O
is	O
a	O
more	O
sophisticated	O
local	O
switching	O
device	O
such	O
as	O
a	O
PBX	O
,	O
that	O
may	O
convert	O
the	O
signal	O
to	O
a	O
different	O
format	O
or	O
hand	O
it	O
off	O
as	O
S/T	O
to	O
terminal	O
equipment	O
.	O
</s>
<s>
In	O
America	O
,	O
the	O
NT1	O
is	O
customer	O
premises	O
equipment	O
(	O
CPE	O
)	O
which	O
is	O
purchased	O
and	O
maintained	O
by	O
the	O
user	O
,	O
which	O
makes	O
the	O
U	B-Protocol
interface	I-Protocol
a	O
User	O
–	O
network	O
interface	O
(	O
UNI	O
)	O
.	O
</s>
<s>
In	O
Europe	O
,	O
the	O
NT1	O
belongs	O
to	O
the	O
network	O
operator	O
,	O
so	O
the	O
user	O
does	O
n't	O
have	O
direct	O
access	O
to	O
the	O
U	B-Protocol
interface	I-Protocol
.	O
</s>
<s>
The	O
ITU-T	O
has	O
issued	O
recommendations	O
G.960	O
and	O
G.961	O
with	O
world-wide	O
scope	O
,	O
encompassing	O
both	O
the	O
European	O
and	O
American	O
variants	O
of	O
the	O
U	B-Protocol
interface	I-Protocol
.	O
</s>
<s>
Like	O
all	O
other	O
ISDN	B-Protocol
basic	B-Protocol
rate	I-Protocol
interfaces	I-Protocol
,	O
the	O
U	B-Protocol
interface	I-Protocol
carries	O
two	O
B	O
(	O
bearer	O
)	O
channels	O
at	O
64kbit/s	O
and	O
one	O
D	O
(	O
data	O
)	O
channel	O
at	O
16kbit/s	O
for	O
a	O
combined	O
bitrate	O
of	O
144kbit/s	O
(	O
2B+D	B-Protocol
)	O
.	O
</s>
<s>
While	O
in	O
a	O
four-wire	O
interface	O
such	O
as	O
the	O
ISDN	B-Protocol
S	B-Protocol
and	O
T-interfaces	B-Protocol
one	O
wire	O
pair	O
is	O
available	O
for	O
each	O
direction	O
of	O
transmission	O
,	O
a	O
two-wire	O
interface	O
needs	O
to	O
implement	O
both	O
directions	O
on	O
a	O
single	O
wire	O
pair	O
.	O
</s>
<s>
To	O
that	O
end	O
,	O
ITU-T	O
recommendation	O
G.961	O
specifies	O
two	O
duplex	O
transmission	O
technologies	O
for	O
the	O
ISDN	B-Protocol
U	B-Protocol
interface	I-Protocol
,	O
either	O
of	O
which	O
shall	O
be	O
used	O
:	O
Echo	O
cancellation	O
(	O
ECH	O
)	O
and	O
Time	O
Compression	O
Multiplex	O
(	O
TCM	O
)	O
.	O
</s>
<s>
ITU-T	O
G.961	O
specifies	O
four	O
line	O
systems	O
for	O
the	O
ISDN	B-Protocol
U	B-Protocol
interface	I-Protocol
:	O
MMS43	B-Protocol
,	O
2B1Q	B-Protocol
,	O
TCM	O
,	O
and	O
SU32	O
.	O
</s>
<s>
The	O
American	O
standard	O
ANSI	O
T1.601	O
specifies	O
the	O
2B1Q	B-Protocol
line	O
system	O
,	O
the	O
European	O
ETSI	O
TR	O
080	O
recommendation	O
specifies	O
2B1Q	B-Protocol
and	O
MMS43	B-Protocol
.	O
</s>
<s>
The	O
Modified	O
Monitoring	O
State	O
Code	O
mapping	O
4	O
bits	O
into	O
3	O
ternary	O
symbols	O
(	O
MMS43	B-Protocol
)	O
,	O
which	O
is	O
also	O
referred	O
to	O
as	O
4B3T	B-Protocol
(	O
four	O
binary	O
,	O
three	O
ternary	O
)	O
is	O
a	O
line	O
system	O
used	O
in	O
Europe	O
and	O
elsewhere	O
in	O
the	O
world	O
.	O
</s>
<s>
4B3T	B-Protocol
is	O
a	O
"	O
block	O
code	O
"	O
that	O
uses	O
Return-to-Zero	O
states	O
on	O
the	O
line	O
.	O
</s>
<s>
4B3T	B-Protocol
converts	O
each	O
group	O
of	O
4	O
data	O
bits	O
into	O
3	O
"	O
ternary	O
"	O
line	O
signal	O
states	O
(	O
3	O
symbols	O
)	O
.	O
</s>
<s>
MMS43	B-Protocol
is	O
defined	O
in	O
Appendix	O
I	O
of	O
G.961	O
,	O
Annex	O
B	O
of	O
ETR	O
080	O
,	O
and	O
other	O
national	O
standards	O
,	O
like	O
Germany	O
's	O
1TR220	O
.	O
</s>
<s>
4B3T	B-Protocol
can	O
be	O
transmitted	O
reliably	O
at	O
up	O
to	O
over	O
cable	O
or	O
up	O
to	O
over	O
cable	O
.	O
</s>
<s>
A	O
1ms	O
frame	O
carrying	O
144bits	O
of	O
2B+D	B-Protocol
data	O
is	O
mapped	O
to	O
108	O
ternary	O
symbols	O
.	O
</s>
<s>
In	O
4B3T	B-Protocol
coding	O
,	O
there	O
are	O
three	O
states	O
presented	O
to	O
line	O
:	O
a	O
positive	O
pulse	O
( +	O
)	O
,	O
a	O
negative	O
pulse	O
(	O
-	O
)	O
,	O
or	O
a	O
zero-state	O
(	O
no	O
pulse	O
:	O
0	O
)	O
.	O
</s>
<s>
An	O
analogy	O
here	O
is	O
that	O
operation	O
is	O
similar	O
to	O
B8ZS	B-Protocol
or	O
HDB3	B-Protocol
in	O
T1/E1	O
systems	O
,	O
except	O
that	O
there	O
is	O
an	O
actual	O
gain	O
in	O
the	O
information	O
rate	O
by	O
coding	O
24	O
=	O
16	O
possible	O
binary	O
states	O
to	O
one	O
of	O
33	O
=	O
27	O
ternary	O
states	O
.	O
</s>
<s>
2B1Q	B-Protocol
coding	O
is	O
the	O
standard	O
used	O
in	O
North	O
America	O
,	O
Italy	O
,	O
and	O
Switzerland	O
.	O
</s>
<s>
2B1Q	B-Protocol
means	O
that	O
two	O
bits	O
are	O
combined	O
to	O
form	O
a	O
single	O
Quaternary	O
line	O
state	O
(	O
symbol	O
)	O
.	O
</s>
<s>
2B1Q	B-Protocol
combines	O
two	O
bits	O
at	O
a	O
time	O
to	O
be	O
represented	O
by	O
one	O
of	O
four	O
signal	O
levels	O
on	O
the	O
line	O
.	O
</s>
<s>
2B1Q	B-Protocol
coding	O
is	O
defined	O
in	O
Appendix	O
II	O
of	O
G.961	O
,	O
ANSI	O
T1.601	O
,	O
and	O
Annex	O
A	O
of	O
ETR	O
080	O
.	O
</s>
<s>
A	O
1.5ms	O
frame	O
carrying	O
216	O
scrambled	O
bits	O
of	O
2B+D	B-Protocol
data	O
is	O
mapped	O
to	O
108	O
quaternary	O
symbols	O
.	O
</s>
<s>
The	O
TCM	O
/	O
AMI	O
ISDN	B-Protocol
line	I-Protocol
system	O
,	O
also	O
referred	O
to	O
as	O
TCM-ISDN	O
,	O
is	O
used	O
by	O
Nippon	O
Telegraph	O
and	O
Telephone	O
in	O
its	O
"	O
INS-Net	O
64	O
"	O
service	O
.	O
</s>
<s>
Like	O
with	O
MMS43	B-Protocol
,	O
the	O
ternary	O
symbol	O
can	O
either	O
be	O
a	O
positive	O
( +	O
)	O
,	O
zero	O
(	O
0	O
)	O
,	O
or	O
negative	O
(	O
-	O
)	O
voltage	O
.	O
</s>
<s>
In	O
a	O
2.5ms	O
interval	O
,	O
each	O
side	O
can	O
send	O
a	O
1.178ms	O
frame	O
representing	O
360bits	O
of	O
2B+D	B-Protocol
data	O
.	O
</s>
<s>
To	O
the	O
2B+D	B-Protocol
data	O
,	O
an	O
8-bit	O
preamble	O
,	O
8	O
bits	O
from	O
the	O
CL	O
channel	O
,	O
as	O
well	O
as	O
a	O
parity	O
bit	O
are	O
added	O
,	O
yielding	O
a	O
frame	O
size	O
of	O
377bits	O
and	O
a	O
baud	O
rate	O
of	O
320kilobaud	O
.	O
</s>
<s>
As	O
with	O
MMS43	B-Protocol
and	O
AMI	O
,	O
the	O
ternary	O
symbol	O
can	O
either	O
be	O
a	O
positive	O
( +	O
)	O
,	O
zero	O
(	O
0	O
)	O
,	O
or	O
negative	O
(	O
-	O
)	O
voltage	O
.	O
</s>
<s>
A	O
0.75ms	O
frame	O
carrying	O
108	O
bits	O
of	O
2B+D	B-Protocol
data	O
is	O
mapped	O
to	O
72	O
ternary	O
symbols	O
.	O
</s>
