<s>
The	O
5ESS	B-Device
Switching	I-Device
System	I-Device
is	O
a	O
Class	B-Device
5	I-Device
telephone	O
electronic	B-Device
switching	I-Device
system	I-Device
developed	O
by	O
Western	O
Electric	O
for	O
the	O
American	O
Telephone	O
and	O
Telegraph	O
Company	O
(	O
AT&T	O
)	O
and	O
the	O
Bell	O
System	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
The	O
5ESS	B-Device
came	O
to	O
market	O
as	O
the	O
Western	O
Electric	O
No	O
.	O
</s>
<s>
5	O
ESS	B-Device
.	O
</s>
<s>
It	O
commenced	O
service	O
in	O
Seneca	O
,	O
Illinois	O
on	O
25	O
March	O
1982	O
,	O
and	O
was	O
destined	O
to	O
replace	O
the	O
Number	B-Device
One	I-Device
Electronic	I-Device
Switching	I-Device
System	I-Device
(	O
1ESS	B-Device
and	O
1AESS	B-Device
)	O
and	O
other	O
electromechanical	O
systems	O
in	O
the	O
1980s	O
and	O
1990s	O
.	O
</s>
<s>
The	O
5ESS	B-Device
was	O
also	O
used	O
as	O
a	O
Class-4	B-Device
telephone	I-Device
switch	I-Device
or	O
as	O
a	O
hybrid	O
Class	O
4/Class	O
5	O
switch	O
in	O
markets	O
too	O
small	O
for	O
the	O
4ESS	B-Device
.	O
</s>
<s>
Approximately	O
half	O
of	O
all	O
US	O
central	O
offices	O
are	O
served	O
by	O
5ESS	B-Device
switches	I-Device
.	O
</s>
<s>
The	O
5ESS	B-Device
is	O
also	O
exported	O
internationally	O
,	O
and	O
manufactured	O
outside	O
the	O
US	O
under	O
license	O
.	O
</s>
<s>
The	O
5ESS	B-Device
–	O
2000	O
version	O
,	O
introduced	O
in	O
the	O
1990s	O
,	O
increased	O
the	O
capacity	O
of	O
the	O
switching	O
module	O
(	O
SM	O
)	O
,	O
with	O
more	O
peripheral	O
modules	O
and	O
more	O
optical	O
links	O
per	O
SM	O
to	O
the	O
communications	O
module	O
(	O
CM	O
)	O
.	O
</s>
<s>
A	O
follow-on	O
version	O
,	O
the	O
5ESS	B-Device
–	O
R/E	O
,	O
was	O
in	O
development	O
during	O
the	O
late	O
1990s	O
but	O
did	O
not	O
reach	O
market	O
.	O
</s>
<s>
The	O
5ESS	B-Device
technology	O
was	O
transferred	O
to	O
the	O
AT&T	O
Network	O
Systems	O
division	O
upon	O
the	O
1984	O
breakup	O
of	O
the	O
Bell	O
System	O
.	O
</s>
<s>
5ESS	B-Device
switches	I-Device
in	O
service	O
in	O
2021	O
included	O
several	O
operated	O
by	O
the	O
United	O
States	O
Navy	O
.	O
</s>
<s>
The	O
5ESS	B-Device
switch	I-Device
has	O
three	O
main	O
types	O
of	O
modules	O
:	O
the	O
Administrative	O
Module	O
(	O
AM	O
)	O
contains	O
the	O
central	O
computers	O
;	O
the	O
Communications	O
Module	O
(	O
CM	O
)	O
is	O
the	O
central	O
time-divided	O
switch	O
of	O
the	O
system	O
;	O
and	O
the	O
Switching	O
Module	O
(	O
SM	O
)	O
makes	O
up	O
the	O
majority	O
of	O
the	O
equipment	O
in	O
most	O
exchanges	O
.	O
</s>
<s>
Each	O
has	O
a	O
controller	O
,	O
a	O
small	O
computer	O
with	O
duplicated	O
CPUs	B-Device
and	O
memories	O
,	O
like	O
most	O
common	O
equipment	O
of	O
the	O
exchange	O
,	O
for	O
redundancy	O
.	O
</s>
<s>
Distributed	B-Architecture
systems	I-Architecture
lessen	O
the	O
load	O
on	O
the	O
Central	O
Administrative	O
Module	O
(	O
AM	O
)	O
or	O
main	O
computer	O
.	O
</s>
<s>
Each	O
has	O
its	O
own	O
processors	B-Device
,	O
also	O
called	O
Module	O
Controllers	O
,	O
which	O
perform	O
most	O
call	O
handling	O
processes	O
,	O
using	O
their	O
own	O
memory	B-General_Concept
boards	O
.	O
</s>
<s>
Originally	O
the	O
peripheral	O
processors	B-Device
were	O
to	O
be	O
Intel	B-General_Concept
8086	I-General_Concept
,	O
but	O
those	O
proved	O
inadequate	O
and	O
the	O
system	O
was	O
introduced	O
with	O
Motorola	B-Device
68000	I-Device
series	O
processors	B-Device
.	O
</s>
<s>
The	O
Time	O
Slot	O
Interchanger	O
(	O
TSI	O
)	O
in	O
the	O
SM	O
uses	O
random-access	O
memory	B-General_Concept
to	O
delay	O
each	O
speech	O
sample	O
to	O
fit	O
into	O
a	O
time	O
slot	O
which	O
will	O
carry	O
its	O
call	O
through	O
the	O
exchange	O
to	O
another	O
or	O
,	O
in	O
some	O
cases	O
,	O
the	O
same	O
SM	O
.	O
</s>
<s>
These	O
may	O
serve	O
either	O
interoffice	O
trunks	O
or	O
,	O
using	O
Integrated	O
Subscriber	B-Application
Loop	I-Application
Carrier	I-Application
,	O
subscriber	O
lines	O
.	O
</s>
<s>
Higher-capacity	O
DS3	O
signals	O
can	O
also	O
have	O
their	O
DS0	O
signals	O
switched	O
in	O
Digital	O
Network	O
Unit	O
SONET	B-Protocol
(	O
DNUS	O
)	O
units	O
,	O
without	O
demultiplexing	O
them	O
into	O
DS1	B-Protocol
.	O
</s>
<s>
SMs	O
have	O
Dual	O
Link	O
Interface	O
(	O
DLI	O
)	O
cards	O
to	O
connect	O
them	O
by	O
multi-mode	B-Device
optical	I-Device
fibers	I-Device
to	O
the	O
Communications	O
Modules	O
for	O
time-divided	O
switching	O
to	O
other	O
SMs	O
.	O
</s>
<s>
Calls	O
among	O
the	O
lines	O
and	O
trunks	O
of	O
a	O
particular	O
SM	O
need	O
n't	O
go	O
through	O
CM	O
,	O
and	O
an	O
SM	O
located	O
remotely	O
can	O
act	O
as	O
distributed	B-Device
switching	I-Device
,	O
administered	O
from	O
the	O
central	O
AM	O
.	O
</s>
<s>
In	O
contrast	O
to	O
Nortel	O
's	O
DMS-100	B-Protocol
which	O
uses	O
individual	O
line	O
cards	O
with	O
a	O
codec	B-General_Concept
,	O
most	O
lines	O
are	O
on	O
two-stage	O
analog	O
space-division	O
concentrators	O
or	O
Line	O
Units	O
,	O
which	O
connect	O
as	O
many	O
as	O
512	O
lines	O
,	O
as	O
needed	O
,	O
to	O
the	O
8	O
Channel	O
cards	O
that	O
each	O
contain	O
8	O
codecs	B-General_Concept
,	O
and	O
to	O
high-level	O
service	O
circuits	O
for	O
ringing	O
and	O
testing	O
.	O
</s>
<s>
Limited	B-Device
availability	I-Device
saves	O
money	O
with	O
incompletely	O
filled	O
matrixes	O
.	O
</s>
<s>
ISDN	B-Protocol
lines	I-Protocol
are	O
served	O
by	O
individual	O
line	O
cards	O
in	O
an	O
ISLU	O
(	O
Integrated	O
Services	O
Line	O
Unit	O
)	O
.	O
</s>
<s>
The	O
Administrative	O
Module	O
(	O
AM	O
)	O
is	O
a	O
dual-processor	O
mini	O
main	B-Architecture
frame	I-Architecture
computer	I-Architecture
of	O
the	O
AT&T	O
3B	B-Device
series	I-Device
,	O
running	O
UNIX-RTR	B-Operating_System
.	O
</s>
<s>
AM	O
contains	O
the	O
hard	O
drives	O
and	O
tape	B-Architecture
drives	I-Architecture
used	O
to	O
load	O
and	O
backup	O
the	O
central	O
and	O
peripheral	O
processor	O
software	O
and	O
translations	O
.	O
</s>
<s>
Disk	O
drives	O
were	O
originally	O
several	O
300	O
megabyte	O
SMD	B-Architecture
multi-platter	O
units	O
in	O
a	O
separate	O
frame	O
.	O
</s>
<s>
Now	O
they	O
consist	O
of	O
several	O
redundant	O
multi-gigabyte	O
SCSI	B-Architecture
drives	O
that	O
each	O
reside	O
on	O
a	O
card	O
.	O
</s>
<s>
Tape	B-Architecture
drives	I-Architecture
were	O
originally	O
half	O
inch	O
open	O
reel	O
at	O
6250	O
bits	O
per	O
inch	O
,	O
which	O
were	O
replaced	O
in	O
the	O
early	O
1990s	O
with	O
4mm	O
Digital	O
Audio	O
Tape	O
cassettes	O
.	O
</s>
<s>
The	O
Administrative	O
Module	O
is	O
built	O
on	O
the	O
3B21D	B-Device
platform	O
and	O
is	O
used	O
to	O
load	O
software	O
to	O
the	O
many	O
microprocessors	B-Architecture
throughout	O
the	O
switch	O
and	O
to	O
provide	O
high	O
speed	O
control	O
functions	O
.	O
</s>
<s>
The	O
AM	O
of	O
a	O
5ESS	B-Device
consists	O
of	O
the	O
3B20x	O
or	O
3B21D	B-Device
processor	O
unit	O
,	O
including	O
I/O	O
,	O
disks	O
,	O
and	O
tape	O
drive	O
units	O
.	O
</s>
<s>
Once	O
the	O
3B21D	B-Device
has	O
loaded	O
the	O
software	O
into	O
the	O
5ESS	B-Device
and	O
the	O
switch	O
is	O
activated	O
,	O
packet	B-Protocol
switching	I-Protocol
takes	O
place	O
without	O
further	O
action	O
by	O
the	O
3B21D	B-Device
,	O
except	O
for	O
billing	O
functions	O
requiring	O
records	O
to	O
be	O
transferred	O
to	O
disk	O
for	O
storage	O
.	O
</s>
<s>
5ESS	B-Device
uses	O
a	O
time-space-time	O
(	O
TST	O
)	O
topology	O
in	O
which	O
the	O
Time-Slot-Interchangers	B-Device
(	O
TSI	O
)	O
in	O
the	O
Switching	O
Modules	O
assign	O
each	O
phone	O
call	O
to	O
a	O
time	O
slot	O
for	O
routing	O
through	O
the	O
CM	O
.	O
</s>
<s>
Each	O
optical	O
link	O
consists	O
of	O
two	O
multimode	B-Device
optical	I-Device
fibers	I-Device
with	O
ST	O
connectors	O
to	O
plug	O
into	O
transceivers	O
plugged	O
into	O
backplane	O
wiring	O
at	O
each	O
end	O
.	O
</s>
<s>
The	O
Very	O
Compact	O
Digital	O
Exchange	O
(	O
VCDX	O
)	O
was	O
developed	O
with	O
the	O
5ESS-2000	O
,	O
and	O
marketed	O
to	O
mostly	O
non-Bell	O
telephone	O
companies	O
as	O
an	O
inexpensive	O
,	O
effective	O
way	O
to	O
offer	O
ISDN	B-Protocol
and	O
other	O
digital	O
services	O
in	O
an	O
analog	O
switching	O
center	O
.	O
</s>
<s>
An	O
example	O
would	O
be	O
the	O
(	O
former	O
)	O
GTE/Verizon	O
Class-5	B-Device
telephone	I-Device
switch	I-Device
,	O
the	O
GTD-5	B-Device
EAX	I-Device
.	O
</s>
<s>
Like	O
the	O
Western	O
Electric	O
1ESS/1AESS	O
,	O
it	O
served	O
mostly	O
medium	O
to	O
large	O
wire	O
centers	O
.	O
</s>
<s>
The	O
standalone	O
VCDX	O
was	O
also	O
capable	O
of	O
serving	O
as	O
a	O
switch	O
for	O
very	O
small	O
wire	O
centers	O
(	O
a	O
CDX	O
-	O
Community	B-Device
dial	I-Device
office	I-Device
)	O
of	O
fewer	O
than	O
~	O
400	O
lines	O
.	O
</s>
<s>
However	O
,	O
for	O
small	O
wire	O
centers	O
,	O
400-4000	O
lines	O
,	O
that	O
function	O
was	O
usually	O
served	O
by	O
RSM	O
's	O
,	O
a	O
5ESS	B-Device
"	O
Remote	O
SM	O
"	O
,	O
ORM	O
's	O
or	O
Wired	O
ORM	O
's	O
.	O
</s>
<s>
The	O
RSM	O
is	O
controlled	O
by	O
T1	B-Protocol
lines	I-Protocol
connected	O
to	O
a	O
DLTU	O
unit	O
.	O
</s>
<s>
An	O
RSM	O
or	O
ORM	O
can	O
have	O
many	O
of	O
the	O
same	O
peripheral	O
units	O
that	O
are	O
part	O
of	O
a	O
full	O
5ESS	B-Device
switch	I-Device
.	O
</s>
<s>
Both	O
the	O
RSM	O
and	O
ORM	O
is	O
often	O
used	O
as	O
a	O
Class-5	O
wire	O
center	O
for	O
small	O
to	O
medium	O
towns	O
hosted	O
from	O
a	O
5ESS	B-Device
located	O
in	O
a	O
larger	O
city	O
.	O
</s>
<s>
ORM	O
's	O
are	O
linked	O
with	O
DS3	O
,	O
RSM	O
's	O
are	O
linked	O
with	O
T1	B-Protocol
lines	I-Protocol
.	O
</s>
<s>
The	O
VCDX	O
was	O
also	O
used	O
as	O
a	O
large	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
.	O
</s>
<s>
Its	O
Sun	O
Microsystems	O
SPARC	B-Architecture
workstation	B-Device
runs	O
the	O
UNIX-based	O
Solaris	B-Application
(	O
operating	B-General_Concept
system	I-General_Concept
)	O
that	O
executes	O
a	O
3B20/21D	B-Device
processor	O
MERT	B-Operating_System
OS	B-General_Concept
emulation	B-Application
system	O
,	O
acting	O
as	O
the	O
VCDX	O
's	O
Administrative	O
Module	O
.	O
</s>
<s>
The	O
VCDX	O
uses	O
the	O
CO	O
's	O
normal	O
telephone	O
power	O
sources	O
(	O
which	O
are	O
very	O
large	O
uninterruptible	O
power	O
supplies	O
)	O
,	O
and	O
has	O
connections	O
to	O
the	O
CO	O
Digital	B-Architecture
cross	I-Architecture
connect	I-Architecture
system	I-Architecture
for	O
T1	O
access	O
,	O
etc	O
.	O
</s>
<s>
The	O
5ESS	B-Device
has	O
two	O
different	O
signaling	O
architectures	O
:	O
Common	O
Network	O
Interface	O
(	O
CNI	O
)	O
Ring	O
and	O
Packet	B-Protocol
Switching	I-Protocol
Unit	O
(	O
PSU	O
)	O
-based	O
SS7	O
Signaling	O
.	O
</s>
<s>
The	O
development	O
effort	O
for	O
5ESS	B-Device
required	O
five	O
thousand	O
employees	O
,	O
producing	O
100	O
million	O
lines	O
of	O
system	O
source	O
code	O
,	O
mostly	O
in	O
the	O
C	B-Language
language	I-Language
,	O
with	O
100	O
million	O
lines	O
of	O
header	B-Language
files	I-Language
and	O
makefiles	O
.	O
</s>
<s>
The	O
5ESS	B-Device
was	O
originally	O
U.S.-only	O
and	O
the	O
international	O
market	O
resulted	O
in	O
a	O
complete	O
development	O
system	O
and	O
team	O
,	O
in	O
parallel	O
to	O
the	O
U.S.	O
version	O
.	O
</s>
<s>
The	O
development	O
systems	O
were	O
Unix-based	O
mainframe	B-Architecture
systems	O
.	O
</s>
<s>
Developers	O
 '	O
desktops	O
were	O
multi-window	O
terminals	O
(	O
versions	O
of	O
the	O
Blit	B-Protocol
developed	O
by	O
Bell	O
Labs	O
)	O
until	O
the	O
mid	O
1990s	O
,	O
when	O
Sun	B-Device
workstations	I-Device
were	O
deployed	O
.	O
</s>
<s>
Developers	O
continued	O
to	O
login	O
into	O
the	O
servers	O
for	O
their	O
work	O
,	O
using	O
X11	B-Operating_System
on	O
their	O
workstations	B-Device
as	O
a	O
multi-window	O
environment	O
.	O
</s>
<s>
Source	O
code	O
management	O
was	O
based	O
on	O
SCCS	B-Device
and	O
utilized	O
"	O
#feature	O
"	O
lines	O
to	O
separate	O
source	O
code	O
between	O
releases	O
,	O
between	O
features	O
specific	O
to	O
US	O
or	O
Intl	O
,	O
and	O
the	O
like	O
.	O
</s>
<s>
Customisation	O
around	O
the	O
vi	B-Application
and	O
Emacs	B-Application
text	O
editors	O
allowed	O
developers	O
to	O
work	O
with	O
the	O
appropriate	O
view	O
of	O
a	O
file	O
,	O
hiding	O
the	O
parts	O
that	O
were	O
not	O
applicable	O
to	O
their	O
current	O
project	O
.	O
</s>
<s>
The	O
change	O
request	O
system	O
used	O
the	O
SCCS	B-Device
MR	O
to	O
create	O
named	O
change	O
sets	O
,	O
tied	O
into	O
the	O
IMR	O
(	O
initial	O
modification	O
request	O
)	O
system	O
which	O
had	O
purely	O
numeric	O
identifiers	O
.	O
</s>
<s>
The	O
system	O
is	O
administered	O
through	O
an	O
assortment	O
of	O
teletypewriter	O
"	O
channels	O
"	O
,	O
also	O
called	O
the	O
system	B-Device
console	I-Device
,	O
such	O
as	O
the	O
TEST	O
channel	O
and	O
the	O
Maintenance	O
channel	O
.	O
</s>
<s>
Typically	O
provisioning	B-Operating_System
is	O
done	O
either	O
through	O
a	O
command	B-Application
line	I-Application
interface	I-Application
(	O
CLI	O
)	O
called	O
RCV:APPTEXT	O
,	O
or	O
through	O
the	O
menu-driven	O
program	O
.	O
</s>
<s>
Most	O
service	O
orders	O
,	O
however	O
,	O
are	O
administered	O
through	O
the	O
Recent	O
Change	O
Memory	B-General_Concept
Administration	O
Center	O
(	O
RCMAC	O
)	O
.	O
</s>
