<s>
The	O
Number	B-Device
Five	I-Device
Crossbar	I-Device
Switching	I-Device
System	I-Device
(	O
5XB	B-Device
switch	I-Device
)	O
is	O
a	O
telephone	O
switch	O
for	O
telephone	O
exchanges	O
designed	O
by	O
Bell	O
Labs	O
and	O
manufactured	O
by	O
Western	O
Electric	O
starting	O
in	O
1947	O
.	O
</s>
<s>
It	O
was	O
used	O
in	O
the	O
Bell	O
System	O
principally	O
as	O
a	O
Class	B-Device
5	I-Device
telephone	I-Device
switch	I-Device
in	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
until	O
the	O
early	O
1990s	O
,	O
when	O
it	O
was	O
replaced	O
with	O
electronic	B-Device
switching	I-Device
systems	I-Device
.	O
</s>
<s>
5XB	B-Device
was	O
originally	O
intended	O
to	O
bring	O
the	O
benefits	O
of	O
crossbar	O
switching	O
to	O
towns	O
and	O
small	O
cities	O
with	O
only	O
a	O
few	O
thousand	O
telephone	O
lines	O
.	O
</s>
<s>
The	O
earlier	O
1XB	B-Device
urban	O
crossbar	O
was	O
impractically	O
expensive	O
in	O
small	O
installations	O
,	O
and	O
had	O
difficulties	O
handling	O
large	O
trunk	O
groups	O
.	O
</s>
<s>
5XB	B-Device
was	O
converted	O
to	O
wire	B-Algorithm
spring	I-Algorithm
relays	I-Algorithm
in	O
the	O
1950s	O
and	O
otherwise	O
upgraded	O
in	O
the	O
1960s	O
to	O
serve	O
exchanges	O
with	O
tens	O
of	O
thousands	O
of	O
lines	O
.	O
</s>
<s>
5XB	B-Device
introduced	O
the	O
call-back	O
principle	O
,	O
in	O
which	O
the	O
initial	O
concentrating	O
switch	O
train	O
from	O
the	O
line	O
to	O
the	O
digit	O
receiver	O
was	O
entirely	O
dropped	O
during	O
call	O
completion	O
so	O
its	O
links	O
could	O
immediately	O
be	O
reused	O
for	O
this	O
or	O
another	O
call	O
.	O
</s>
<s>
The	O
line	O
circuit	O
is	O
much	O
like	O
that	O
in	O
1XB	B-Device
with	O
a	O
line	O
relay	O
for	O
alerting	O
the	O
exchange	O
to	O
a	O
trip	O
condition	O
,	O
and	O
the	O
vertical	O
off-normal	O
contacts	O
of	O
the	O
switch	O
vertical	O
serving	O
as	O
cutoff	O
relay	O
.	O
</s>
<s>
Each	O
vertical	O
group	O
is	O
divided	O
into	O
five	O
vertical	O
files	O
of	O
ten	O
lines	O
,	O
important	O
because	O
class	B-Device
of	I-Device
service	I-Device
,	O
or	O
customer	O
group	O
identification	O
in	O
later	O
Centrex	B-Device
offices	O
,	O
is	O
shared	O
by	O
all	O
ten	O
lines	O
in	O
the	O
vertical	O
file	O
.	O
</s>
<s>
Staff	O
in	O
Centrex	B-Device
offices	O
spent	O
much	O
time	O
standing	O
on	O
ladders	O
,	O
rewiring	O
the	O
Class	B-Device
of	I-Device
Service	I-Device
data	O
fields	O
at	O
the	O
top	O
of	O
LLF	O
.	O
</s>
<s>
Late	O
in	O
the	O
career	O
of	O
5XB	B-Device
,	O
junctor	O
group	O
size	O
and	O
thus	O
link	O
efficiency	O
of	O
the	O
largest	O
offices	O
was	O
increased	O
by	O
the	O
use	O
of	O
auxiliary	O
line	O
link	O
(	O
ALL	O
)	O
frames	O
.	O
</s>
<s>
By	O
this	O
means	O
,	O
each	O
LLF	O
can	O
use	O
the	O
junctors	O
of	O
its	O
mate	O
,	O
if	O
the	O
marker	B-Device
failed	O
to	O
find	O
an	O
idle	O
path	O
on	O
the	O
first	O
try	O
.	O
</s>
<s>
The	O
final	O
1970s	O
5XB	B-Device
offices	O
had	O
type	O
C	O
trunk	O
switches	O
with	O
twelve	O
levels	O
,	O
using	O
two	O
for	O
discrimination	O
,	O
leaving	O
a	O
TCR	O
of	O
unity	O
.	O
</s>
<s>
Development	O
stopped	O
at	O
that	O
point	O
because	O
the	O
four-stage	O
layout	O
was	O
becoming	O
progressively	O
less	O
efficient	O
at	O
greater	O
sizes	O
,	O
and	O
because	O
the	O
1ESS	B-Device
switch	I-Device
with	O
eight	O
stages	O
was	O
under	O
development	O
.	O
</s>
<s>
Logic	O
in	O
the	O
marker	B-Device
compares	O
the	O
ten	O
links	O
of	O
each	O
kind	O
to	O
obtain	O
a	O
clear	O
channel	O
.	O
</s>
<s>
Since	O
different	O
outgoing	O
trunks	O
are	O
connected	O
to	O
different	O
places	O
and	O
are	O
used	O
for	O
different	O
calls	O
,	O
their	O
relay	O
sets	O
can	O
be	O
specialized	O
for	O
a	O
particular	O
kind	O
of	O
signalling	O
or	O
call	O
metering	O
(	O
see	O
automatic	B-Application
message	I-Application
accounting	I-Application
)	O
or	O
other	O
peculiarity	O
.	O
</s>
<s>
Thus	O
a	O
TSPS	B-Algorithm
trunk	O
can	O
give	O
complete	O
control	O
to	O
an	O
operator	O
,	O
while	O
an	O
E	O
and	O
M	O
signaling	O
trunk	O
can	O
do	O
the	O
kind	O
of	O
signaling	O
required	O
of	O
a	O
private	O
long-distance	O
line	O
,	O
while	O
a	O
local	O
outgoing	O
trunk	O
can	O
be	O
simpler	O
.	O
</s>
<s>
The	O
route	O
relay	O
in	O
the	O
completing	O
marker	B-Device
connects	O
sensor	O
relays	O
to	O
all	O
the	O
trunk	O
link	O
frames	O
,	O
allowing	O
the	O
marker	B-Device
to	O
choose	O
a	O
TLF	O
that	O
has	O
an	O
idle	O
trunk	O
and	O
then	O
connect	O
to	O
that	O
trunk	O
through	O
the	O
trunk	O
link	O
connector	O
(	O
TLC	O
)	O
to	O
choose	O
one	O
of	O
those	O
idle	O
trunks	O
.	O
</s>
<s>
Where	O
the	O
5XB	B-Device
was	O
the	O
only	O
exchange	O
,	O
a	O
number	O
of	O
work-around	O
methods	O
were	O
devised	O
.	O
</s>
<s>
Regular	O
and	O
coin	O
phones	O
shared	O
the	O
more	O
complex	O
and	O
expensive	O
coin	O
trunks	O
,	O
or	O
else	O
separate	O
routes	O
were	O
established	O
,	O
or	O
coin	O
trunks	O
connected	O
via	O
tandem	B-Device
switches	I-Device
including	O
the	O
5XB	B-Device
itself	O
acting	O
as	O
its	O
own	O
tandem	O
.	O
</s>
<s>
This	O
complication	O
was	O
avoided	O
in	O
places	O
big	O
enough	O
to	O
pay	O
for	O
a	O
separate	O
tandem	B-Device
switch	I-Device
.	O
</s>
<s>
5XB	B-Device
uses	O
a	O
ringing	O
selection	O
switch	O
(	O
RSS	O
)	O
:	O
a	O
crossbar	O
switch	O
with	O
ten	O
verticals	O
,	O
serving	O
ten	O
trunks	O
.	O
</s>
<s>
The	O
various	O
levels	O
provide	O
various	O
tones	O
,	O
and	O
ringing	O
current	O
of	O
various	O
durations	O
and	O
cadences	O
(	O
especially	O
valuable	O
for	O
party	B-Architecture
lines	I-Architecture
)	O
.	O
</s>
<s>
5XB	B-Device
also	O
separates	O
the	O
registers	O
for	O
receiving	O
digits	O
from	O
the	O
senders	O
for	O
sending	O
them	O
.	O
</s>
<s>
In	O
the	O
original	O
5XB	B-Device
,	O
a	O
marker	B-Device
,	O
once	O
alerted	O
to	O
a	O
trip	O
condition	O
,	O
picks	O
an	O
OR	O
by	O
the	O
same	O
mechanism	O
it	O
uses	O
to	O
pick	O
a	O
trunk	O
,	O
identifies	O
a	O
clear	O
path	O
between	O
line	O
and	O
OR	O
,	O
loads	O
the	O
OR	O
with	O
any	O
information	O
necessary	O
for	O
later	O
processing	O
(	O
such	O
as	O
line	O
equipment	O
and	O
class	B-Device
of	I-Device
service	I-Device
)	O
and	O
releases	O
itself	O
.	O
</s>
<s>
The	O
OR	O
then	O
receives	O
the	O
digits	O
(	O
rotary	O
or	O
tone	O
)	O
,	O
stores	O
them	O
in	O
reed	O
relay	O
packs	O
,	O
and	O
used	O
the	O
pretranslator	O
to	O
determine	O
how	O
many	O
digits	O
to	O
receive	O
before	O
calling	O
in	O
the	O
marker	B-Device
again	O
to	O
complete	O
the	O
call	O
.	O
</s>
<s>
Larger	O
5XB	B-Device
systems	O
were	O
built	O
in	O
the	O
1960s	O
with	O
more	O
markers	B-Device
.	O
</s>
<s>
To	O
save	O
money	O
the	O
markers	B-Device
were	O
separated	O
into	O
two	O
kinds	O
:	O
simple	O
dial	O
tone	O
markers	B-Device
(	O
DTM	O
)	O
just	O
to	O
connect	O
the	O
line	O
to	O
the	O
OR	O
,	O
and	O
completing	O
markers	B-Device
(	O
CM	O
)	O
,	O
many	O
times	O
more	O
complex	O
and	O
expensive	O
,	O
to	O
complete	O
the	O
call	O
to	O
or	O
from	O
a	O
trunk	O
.	O
</s>
<s>
Connectors	O
,	O
similar	O
in	O
purpose	O
to	O
the	O
data	O
buses	O
inside	O
a	O
computer	O
CPU	O
,	O
connect	O
the	O
markers	B-Device
to	O
the	O
peripheral	O
equipment	O
.	O
</s>
<s>
each	O
connector	O
is	O
made	O
up	O
of	O
large	O
relays	O
of	O
30	O
contacts	O
each	O
,	O
to	O
connect	O
all	O
the	O
leads	O
by	O
which	O
the	O
marker	B-Device
would	O
exchange	O
information	O
and	O
control	O
signals	O
.	O
</s>
<s>
For	O
example	O
,	O
each	O
of	O
the	O
oblong	O
reed	O
packs	O
in	O
an	O
OR	O
would	O
have	O
to	O
be	O
connected	O
by	O
five	O
leads	O
through	O
the	O
originating	O
register	O
marker	B-Device
connector	O
to	O
transmit	O
the	O
two-out-of-five	O
code	O
representing	O
one	O
dialed	O
digit	O
.	O
</s>
<s>
Connectors	O
that	O
respond	O
to	O
a	O
peripheral	O
circuit	O
's	O
request	O
for	O
action	O
are	O
given	O
the	O
name	O
of	O
the	O
requesting	O
circuit	O
and	O
"	O
marker	B-Device
"	O
,	O
as	O
in	O
ORMC	O
or	O
IRMC	O
.	O
</s>
<s>
Connectors	O
whose	O
use	O
was	O
requested	O
by	O
a	O
marker	B-Device
are	O
named	O
only	O
for	O
the	O
circuit	O
to	O
which	O
they	O
connect	O
,	O
as	O
in	O
outsender	O
connector	O
,	O
line	O
link	O
connector	O
,	O
and	O
trouble	O
recorder	O
connector	O
.	O
</s>
<s>
5XB	B-Device
markers	B-Device
were	O
designed	O
to	O
rotate	O
preferences	O
in	O
such	O
a	O
way	O
that	O
it	O
is	O
highly	O
unlikely	O
that	O
the	O
same	O
circuit	O
elements	O
will	O
be	O
used	O
in	O
the	O
next	O
call	O
.	O
</s>
<s>
Partly	O
because	O
of	O
this	O
deliberate	O
design	O
decision	O
to	O
help	O
shield	O
users	O
from	O
component	O
failures	O
,	O
the	O
few	O
shared	O
markers	B-Device
contain	O
a	O
great	O
deal	O
of	O
self-checking	O
circuitry	O
.	O
</s>
<s>
This	O
is	O
feasible	O
because	O
there	O
are	O
only	O
a	O
few	O
markers	B-Device
,	O
and	O
beneficial	O
because	O
their	O
correct	O
function	O
was	O
critical	O
.	O
</s>
<s>
When	O
the	O
built-in	O
self-test	O
circuits	O
of	O
a	O
marker	B-Device
detect	O
an	O
error	O
,	O
a	O
large	O
punch	O
card	O
is	O
produced	O
at	O
the	O
test	O
station	O
recording	O
the	O
failure	O
in	O
order	O
to	O
assist	O
the	O
switchmen	O
in	O
detecting	O
it	O
and	O
diagnosing	O
the	O
source	O
.	O
</s>
<s>
The	O
MTF	O
can	O
select	O
particular	O
outgoing	O
trunks	O
,	O
including	O
testing	O
the	O
ability	O
of	O
the	O
markers	B-Device
to	O
select	O
different	O
lines	O
to	O
particular	O
trunks	O
.	O
</s>
<s>
This	O
test	O
facility	O
became	O
more	O
valuable	O
as	O
Centrex	B-Device
,	O
direct	O
distance	O
dialing	O
,	O
and	O
other	O
innovations	O
brought	O
more	O
complications	O
to	O
the	O
tasks	O
of	O
translation	O
and	O
trunk	O
selection	O
.	O
</s>
<s>
Most	O
large	O
new	O
urban	O
5XB	B-Device
in	O
subsequent	O
years	O
had	O
IDDD	O
,	O
and	O
it	O
was	O
retrofitted	O
to	O
some	O
existing	O
ones	O
,	O
but	O
most	O
omitted	O
the	O
dual	O
outpulse	O
capability	O
,	O
that	O
job	O
being	O
handled	O
by	O
TSPS	B-Algorithm
.	O
</s>
<s>
Also	O
taking	O
advantage	O
of	O
the	O
superior	O
versatility	O
of	O
5XB	B-Device
,	O
Centrex	B-Device
was	O
invented	O
as	O
a	O
service	O
package	O
.	O
</s>
<s>
Later	O
stored	B-Device
program	I-Device
control	I-Device
exchanges	I-Device
allowed	O
more	O
extensive	O
service	O
features	O
.	O
</s>
<s>
Autovon	O
originally	O
used	O
a	O
four-wire	O
version	O
of	O
5XB	B-Device
,	O
with	O
a	O
more	O
complex	O
marker	B-Device
to	O
implement	O
its	O
nonhierarchical	O
polygrid	O
routing	O
system	O
.	O
</s>
<s>
1ESS	B-Device
switch	I-Device
was	O
already	O
going	O
into	O
service	O
and	O
provided	O
a	O
more	O
sophisticated	O
basis	O
for	O
advanced	O
services	O
,	O
but	O
was	O
not	O
yet	O
as	O
widely	O
available	O
,	O
so	O
5XB	B-Device
was	O
designated	O
as	O
the	O
switching	O
vehicle	O
.	O
</s>
<s>
When	O
completing	O
a	O
Picturephone	O
call	O
,	O
the	O
completing	O
marker	B-Device
first	O
picked	O
the	O
line	O
or	O
trunk	O
to	O
which	O
the	O
audio	O
portion	O
of	O
the	O
call	O
would	O
be	O
completed	O
,	O
and	O
then	O
set	O
up	O
both	O
the	O
audio	O
switches	O
and	O
the	O
video	O
ones	O
.	O
</s>
