<s>
The	O
Number	B-Device
One	I-Device
Crossbar	I-Device
Switching	I-Device
System	I-Device
(	O
1XB	B-Device
)	O
,	O
was	O
the	O
primary	O
technology	O
for	O
urban	O
telephone	O
exchanges	O
served	O
by	O
the	O
Bell	O
System	O
in	O
the	O
mid-20th	O
century	O
.	O
</s>
<s>
Each	O
section	O
had	O
its	O
own	O
central	O
control	O
elements	O
known	O
as	O
markers	B-Device
.	O
</s>
<s>
The	O
originating	O
marker	B-Device
handled	O
call	O
routing	O
up	O
to	O
the	O
outgoing	O
trunk	O
,	O
and	O
the	O
terminating	O
marker	B-Device
routed	O
calls	O
from	O
the	O
incoming	O
trunk	O
all	O
the	O
way	O
up	O
to	O
the	O
termination	O
point	O
at	O
the	O
line	O
link	O
frame	O
(	O
LLF	O
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
Line	O
Link	O
Frame	O
,	O
the	O
1XB	B-Device
consisted	O
of	O
a	O
series	O
of	O
additional	O
crossbar	O
frames	O
and	O
junctors	O
that	O
were	O
used	O
in	O
call	O
completion	O
.	O
</s>
<s>
Examples	O
of	O
these	O
frames	O
included	O
district	O
junctors	O
,	O
subscriber	O
senders	O
,	O
and	O
originating	O
and	O
terminating	O
markers	B-Device
.	O
</s>
<s>
Unlike	O
the	O
motor	O
driven	O
,	O
clutch	O
controlled	O
panel	O
switch	O
selectors	O
,	O
crossbar	O
switches	O
using	O
the	O
link	O
principle	O
required	O
originating	O
and	O
terminating	O
markers	B-Device
to	O
find	O
an	O
idle	O
path	O
and	O
set	O
up	O
the	O
switch	O
train	O
for	O
each	O
call	O
.	O
</s>
<s>
A	O
marker	B-Device
,	O
being	O
a	O
complex	O
control	O
instrument	O
with	O
a	O
short	O
holding	O
time	O
,	O
had	O
the	O
task	O
of	O
decoding	O
the	O
digits	O
of	O
the	O
seven-digit	O
telephone	O
number	O
to	O
determine	O
the	O
routing	O
required	O
to	O
set	O
up	O
the	O
switching	O
fabric	O
for	O
call	O
completion	O
.	O
</s>
<s>
In	O
an	O
originating	O
marker	B-Device
,	O
a	O
cross	O
connect	O
field	O
had	O
a	O
terminal	O
for	O
each	O
two	O
-	O
or	O
three-digit	O
office	O
code	O
.	O
</s>
<s>
These	O
cross	O
connects	O
were	O
in	O
turn	O
used	O
to	O
activate	O
relays	O
in	O
the	O
marker	B-Device
that	O
controlled	O
the	O
treatment	O
,	O
or	O
handling	O
for	O
the	O
office	O
code	O
dialed	O
.	O
</s>
<s>
Using	O
the	O
output	O
provided	O
by	O
its	O
decoding	O
stage	O
,	O
the	O
originating	O
marker	B-Device
could	O
select	O
two	O
office	O
link	O
frames	O
to	O
search	O
for	O
idle	O
trunks	O
to	O
the	O
destination	O
.	O
</s>
<s>
Once	O
the	O
originating	O
marker	B-Device
established	O
a	O
path	O
to	O
the	O
called	O
office	O
,	O
it	O
returned	O
pulsing	O
information	O
to	O
the	O
subscriber	O
sender	O
.	O
</s>
<s>
In	O
a	O
terminating	O
marker	B-Device
,	O
there	O
was	O
also	O
a	O
cross-connect	O
field	O
which	O
was	O
used	O
to	O
declare	O
which	O
frames	O
the	O
marker	B-Device
had	O
to	O
access	O
in	O
order	O
to	O
complete	O
the	O
call	O
to	O
the	O
desired	O
line	O
.	O
</s>
<s>
Subscriber	O
lines	O
were	O
terminated	O
in	O
arbitrary	O
locations	O
on	O
the	O
line	O
link	O
frame	O
,	O
and	O
it	O
was	O
the	O
task	O
of	O
the	O
terminating	O
marker	B-Device
to	O
locate	O
the	O
line	O
,	O
and	O
close	O
the	O
required	O
crosspoints	O
to	O
connect	O
the	O
call	O
to	O
its	O
destination	O
.	O
</s>
<s>
An	O
outstanding	O
feature	O
of	O
the	O
originating	O
marker	B-Device
was	O
route	O
advance	O
,	O
where	O
if	O
all	O
trunks	O
were	O
busy	O
,	O
the	O
marker	B-Device
would	O
operate	O
a	O
different	O
route	O
relay	O
to	O
select	O
an	O
alternate	O
route	O
via	O
a	O
tandem	O
.	O
</s>
<s>
This	O
sender	O
received	O
and	O
registered	O
the	O
dialed	O
digits	O
,	O
communicated	O
with	O
an	O
originating	O
marker	B-Device
to	O
establish	O
the	O
path	O
for	O
the	O
call	O
,	O
and	O
then	O
pulsed	O
the	O
dialed	O
digits	O
forward	O
to	O
the	O
terminating	O
office	O
in	O
whatever	O
format	O
was	O
required	O
.	O
</s>
<s>
Once	O
the	O
terminating	O
sender	O
received	O
and	O
stored	O
the	O
digits	O
of	O
the	O
called	O
telephone	O
number	O
,	O
it	O
activated	O
a	O
terminating	O
marker	B-Device
,	O
which	O
then	O
used	O
a	O
Number	O
Group	O
Circuit	O
to	O
find	O
the	O
line	O
,	O
marked	O
an	O
idle	O
path	O
,	O
and	O
operated	O
the	O
crossbar	O
switches	O
to	O
use	O
the	O
links	O
to	O
connect	O
the	O
incoming	O
trunk	O
to	O
the	O
line	O
.	O
</s>
<s>
The	O
revertive	O
pulse	O
system	O
as	O
used	O
in	O
1XB	B-Device
had	O
a	O
"	O
High	O
Five	O
"	O
feature	O
by	O
which	O
the	O
"	O
incoming	O
brush	O
"	O
selection	O
could	O
be	O
incremented	O
by	O
five	O
.	O
</s>
<s>
This	O
allowed	O
each	O
1XB	B-Device
incoming	O
section	O
to	O
handle	O
twenty	O
thousand	O
lines	O
,	O
whereas	O
a	O
panel	O
office	O
using	O
the	O
revertive	O
pulse	O
system	O
could	O
handle	O
a	O
maximum	O
of	O
10,000	O
lines	O
per	O
office	O
.	O
</s>
<s>
The	O
originating	O
marker	B-Device
told	O
the	O
sender	O
to	O
delete	O
the	O
first	O
two	O
of	O
seven	O
digits	O
in	O
these	O
cases	O
.	O
</s>
<s>
This	O
decision	O
was	O
also	O
driven	O
by	O
the	O
use	O
of	O
markers	B-Device
,	O
which	O
carried	O
out	O
their	O
individual	O
tasks	O
quickly	O
,	O
and	O
interfaced	O
with	O
several	O
other	O
circuits	O
while	O
doing	O
so	O
.	O
</s>
<s>
Terminating	O
Trouble	O
Indicator	O
(	O
TTI	O
)	O
-	O
Used	O
both	O
to	O
indicate	O
trouble	O
as	O
it	O
occurred	O
in	O
the	O
terminating	O
portion	O
of	O
the	O
machine	O
,	O
and	O
to	O
test	O
the	O
operation	O
of	O
the	O
terminating	O
marker	B-Device
.	O
</s>
<s>
Originating	O
Trouble	O
Indicator	O
(	O
OTI	O
)	O
-	O
Used	O
to	O
indicate	O
trouble	O
as	O
it	O
occurred	O
in	O
the	O
originating	O
portion	O
of	O
the	O
switch	O
,	O
and	O
to	O
test	O
the	O
operation	O
of	O
the	O
originating	O
marker	B-Device
.	O
</s>
<s>
A	O
tandem	O
version	O
of	O
the	O
1XB	B-Device
omitted	O
the	O
incoming	O
section	O
and	O
Line	O
Link	O
Frames	O
and	O
replaced	O
the	O
junctor	O
circuits	O
with	O
incoming	O
trunks	O
,	O
leaving	O
only	O
the	O
ability	O
to	O
connect	O
those	O
incoming	O
trunks	O
to	O
outgoing	O
trunks	O
.	O
</s>
<s>
In	O
many	O
cases	O
the	O
resulting	O
Crossbar	B-Device
tandem	I-Device
switch	I-Device
(	O
XBT	O
)	O
replaced	O
a	O
Panel	O
Sender	O
Tandem	O
,	O
because	O
its	O
multi-frequency	O
senders	O
were	O
able	O
to	O
receive	O
seven	O
digits	O
and	O
some	O
were	O
modified	O
to	O
accept	O
ten	O
.	O
</s>
<s>
In	O
big	O
cities	O
,	O
some	O
XBT	O
were	O
strictly	O
incoming	O
Class	B-Device
4	I-Device
telephone	I-Device
switches	I-Device
,	O
some	O
outgoing	O
,	O
a	O
few	O
both-way	O
,	O
and	O
some	O
only	O
for	O
tandem	O
traffic	O
within	O
the	O
metropolitan	O
area	O
.	O
</s>
<s>
XBT	O
served	O
local	O
telephone	O
companies	O
until	O
the	O
late	O
20th	O
Century	O
when	O
they	O
were	O
replaced	O
by	O
4ESS	B-Device
switches	I-Device
or	O
other	O
digital	O
switches	O
.	O
</s>
<s>
1	O
Crossbar	O
switches	O
were	O
replaced	O
in	O
the	O
1970s	O
by	O
the	O
1ESS	B-Device
switch	I-Device
generation	O
with	O
stored	B-Device
program	I-Device
control	I-Device
.	O
</s>
