Information Technology Reference
In-Depth Information
Analog transmission
Figure 6.10
How a Modem Works
Digital signals are modulated into
analog signals, which can be carried
over existing phone lines. The
analog signals are then
demodulated back into digital
signals by the receiving modem.
Digital
transmission
Digital
transmission
Modem
Modem
A cable modem can deliver network
and Internet access at up to 10Mbps.
(Source: Courtesy of D-Link
Systems, Inc.)
With a cellular modem, you can connect to other computers
while in your car, on a boat, or in any area that has cellular trans-
mission service. You can use PC memory card expansion slots
for standardized credit card-sized PC modem cards, which work
like standard modems. PC modems are becoming increasingly
popular with notebook and portable computer users.
Cable company network subscribers use a cable modem,
which has a low initial cost and can transmit at speeds up to 10
Mbps. The cable modem is always on, so you can be connected
to the Internet around the clock. Digital subscriber line (DSL)
is a family of services that provides high-speed digital data com-
munications service over the wires of the local telephone com-
pany. Subscribers employ a DSL modem to connect their
computers to this service.
Multiplexers
A
multiplexer
is a device that combines data from multiple data sources into a single output
signal that carries multiple channels, thus reducing the number of communications links
needed and, therefore, lowering telecommunications costs (see Figure 6.11). Multiplexing is
commonly used on long-distance phone lines, combining many individual phone calls onto
a single long-distance line without affecting the speed or quality of an individual call. At the
receiving end, a demultiplexer chooses the correct destination from the many possible des-
tinations and routes each individual call to its correct destination.
multiplexer
A device that combines data from
multiple data sources into a single
output signal that carries multiple
channels, thus reducing the number
of communications links needed
and therefore, lowering telecom-
munications costs.
Figure 6.11
Use of a Multiplexer to
Consolidate Data
Communications onto a Single
Communications Link
Communications link
Multiplexer
Front-end
processor
Host computer
U.S. Bancorp is a financial services holding company that provides a wide range of services
to banks, financial institutions, and government institutions. Its retail division is U.S. Bank,
the sixth largest bank in the U.S. The firm uses multiplexers to create a nationwide fiber-
optic network to connect its various communications centers.
37
