<s>
A	O
"	O
Community	B-Device
Dial	I-Device
Office	I-Device
"	O
(	O
CDO	O
)	O
was	O
a	O
small	O
Class	B-Device
5	I-Device
telephone	O
exchange	O
in	O
a	O
rural	O
area	O
.	O
</s>
<s>
CDOs	O
could	O
be	O
step	B-Device
by	I-Device
step	I-Device
,	O
all	O
relay	O
or	O
crossbar	O
.	O
</s>
<s>
Initial	O
deployments	O
were	O
based	O
on	O
step-by-step	B-Device
equipment	O
such	O
as	O
the	O
Western	O
Electric	O
350A	O
(	O
first	O
deployed	O
on	O
May	O
27	O
,	O
1928	O
,	O
in	O
Arcadia	O
,	O
California	O
)	O
and	O
its	O
successor	O
models	O
360A	O
,	O
355A	O
and	O
356A	O
.	O
</s>
<s>
1	O
step-by-step	B-Device
systems	O
already	O
in	O
use	O
in	O
large	O
offices	O
(	O
10,000	O
subscribers	O
or	O
more	O
)	O
but	O
were	O
of	O
a	O
simpler	O
design	O
(	O
line	O
finders	O
would	O
only	O
need	O
to	O
scan	O
hundreds	O
of	O
lines	O
instead	O
of	O
the	O
thousands	O
in	O
use	O
in	O
the	O
cities	O
)	O
and	O
not	O
designed	O
for	O
expandability	O
.	O
</s>
<s>
In	O
the	O
1960s	O
,	O
Northern	O
Electric	O
developed	O
the	O
SA-1	O
community	B-Device
dial	I-Device
office	I-Device
as	O
a	O
small	O
Class	B-Device
5	I-Device
crossbar	O
switch	O
for	O
local	O
use	O
in	O
rural	O
areas	O
.	O
</s>
<s>
In	O
the	O
late	O
1970s	O
,	O
Western	O
Electric	O
's	O
number	O
3	O
Electronic	O
Switching	O
System	O
(	O
ESS	O
)	O
was	O
installed	O
in	O
small	O
communities	O
with	O
up	O
to	O
4,500	O
telephone	O
lines	O
;	O
it	O
also	O
served	O
as	O
workhorse	O
in	O
some	O
large	O
private	B-Device
branch	I-Device
exchange	I-Device
installations	O
.	O
</s>
<s>
Late	O
in	O
the	O
20th	O
century	O
the	O
smallest	O
offices	O
were	O
replaced	O
by	O
remote	B-Architecture
concentrators	I-Architecture
connected	O
to	O
a	O
host	O
office	O
in	O
an	O
adjacent	O
exchange	O
area	O
,	O
which	O
may	O
be	O
several	O
miles	O
away	O
.	O
</s>
<s>
Others	O
were	O
replaced	O
with	O
distributed	B-Device
switching	I-Device
hardware	O
(	O
a	O
remote	B-Device
switching	I-Device
centre	I-Device
controlled	O
by	O
a	O
digital	O
switch	O
in	O
the	O
city	O
)	O
or	O
with	O
small	O
digital	O
switches	O
such	O
as	O
Nortel	O
's	O
DMS-10	B-Protocol
(	O
a	O
self-contained	O
exchange	O
for	O
up	O
to	O
8,000	O
subscribers	O
)	O
.	O
</s>
