How Circuit-Switched Digital
Networks Work
Digital access equipment attached to or inside a
router calls the central office switch. ISDN
equipment uses a separate 16-megabit channel for
fast-call setup. Other circuit-switched serviced]s
use standard dialing tones.

How Circuit-Switched Digital
Networks Work
The dial-up router recognizes data with a
destination outside the LAN and initiates a call to
a destination selected from a pre-programmed
table.
Sender PC Modem

How Circuit-Switched Digital
Networks Work
The central office of every local telephone company
is a computerized switch that works with other
similar switches to route and complete a call-that's
where the term circuit switching comes from.
Analog local
Analog signal
access circuits

How Circuit-Switched Digital
Networks Work
A switching matrix connects local access lines and
long distance services on a temporary per-call
basis.

How Circuit-Switched Digital
Networks Work
The central office switch accepts the incoming call
and makes a digital connection to the destination.
A high-speed intraswitch trunk is a fiber-optic link
operating at a minimum of 45 megabits per
second.

How Circuit-Switched Digital
Networks Work
The destination could be a small LAN or a
corporate network. The destination router senses
the incoming call and makes a connection to the
LAN. The originating router strips off the Ethernet
data and sends TCP/IP packets across the LAN-to-
LAN connection. The receiving router packets the
TCP/IP packets into Ethernet. Client devices on
each LAN can access servers and services on the
other network, but with reduced speed.