This document summarizes how circuit-switched digital networks work. It discusses the key components including central office switches located in telephone company offices that route calls locally or to long-distance carriers. Dial-up routers recognize outbound data and initiate calls to internet service providers over public circuits. High-speed fiber optic links transmit data between switches at minimum speeds of 45 megabits per second. TCP/IP packets are used to transmit data across local and wide area networks with TCP/IP being the standard internet protocol.

1. How
Circuit-­‐Switched
Digital
Networks
Work
Central
Office
Switches
Dial-­‐up
router
fiber
op>cs
TCP/IP

2. Central
Office
Switches
Digital
access
equipment
aEached
to
or
inside
a
router
calls
the
central
office
switch.
A
central
office
switch
is
a
large
rack
of
equipment
located
in
a
telephone
company’s
central
office
that
switches
calls
locally
or
to
long-­‐distance
carriers
over
dedicated
circuits.

3. Dial-­‐up
router
A
dial-­‐up
router
recognizes
data
with
a
des>na>on
outside
the
LAN
and
ini>ates
a
call
to
a
des>na>on
selected
from
a
preprogrammed
table.
Dial-­‐up
router
is
an
equipment
that
access
to
internet
service
provider
and
LAN
using
a
public
circuital
net.

4. fiber
op>cs
A
highspeed
intraswitch
trunk
is
a
fiber-­‐op>c
link
opera>ng
at
a
minimum
of
45
megabits
per
second.
Refers
to
systems
that
use
op>cal
fibers.
Fiber-­‐
op>c
communica>ons
networks
have
transformed
the
world.
Barely
star>ng
in
the
late
1960s
but
gaining
serious
momentum
in
the
1980s,
the
phone
companies
began
to
replace
their
copper
long
distance
trunks
with
fiber
cable.

5. TCP/IP
The
origina>ng
router
strips
off
the
Ethernet
data
and
sends
TCP/IP
packets
across
the
LAN-­‐to-­‐LAN
connec>on.
A
communica>ons
protocol
developed
under
contract
from
the
U.S.
Department
of
Defense
to
internetwork
dissimilar
systems.
Invented
by
Vinton
Cerf
and
Bob
Kahn,
this
de
facto
Unix
standard
is
the
protocol
of
the
Internet
and
the
global
standard
for
local
area
networks
and
wide
area
networks,
the
major
excep>on
being
the
tradi>onal
networks
of
the
telephone
companies.

6. References
• IT e-­‐Words
hEp://e-­‐words.jp/
• encyclopedia
hEp://www.pcmag.com/encyclopedia/